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Vitamin B6: Effects of Deficiency, andMetabolic and Therapeutic Functions

Krishnamurti Dakshinamurti, Shyamala Dakshinamurti,and Michael P. Czubryt

AbstractThe vitamin B6 vitamers include pyridoxine, pyridoxal, and pyridoxamine, aswell as their phosphorylated forms such as pyridoxal phosphate, which is a keycoenzyme for a surprising variety of enzymes involved in myriad aspects ofmetabolism. Vitamin B6 also contributes to the synthesis of many neurotransmit-ters. Given this widespread role, it is not surprising that vitamin B6 deficiency caninduce many negative effects including convulsive seizures in infants, develop-mental delay, hypertension, and susceptibility to atherosclerosis. Conversely, theadministration of vitamin B6 vitamers, or the manipulation of vitamer-boundenzymes, has shown promise against cancer, parasitic diseases such as malaria,and Parkinson’s disease. In this chapter, we examine the critical and broad roleplayed by vitamin B6 vitamers and their coenzymes in metabolism, with a focuson the detrimental effects of deficiency and their therapeutic potential.

K. DakshinamurtiDepartment of Biochemistry and Medical Genetics, St. Boniface Hospital Albrechtsen ResearchCentre, Winnipeg, MB, Canadae-mail: [email protected]

S. DakshinamurtiDepartments of Pediatrics and Physiology, University of Manitoba, Biology of Breathing Group,Manitoba Institute of Child Health, Winnipeg, MB, Canadae-mail: [email protected]

M.P. Czubryt (*)Department of Physiology and Pathophysiology, University of Manitoba, Institute ofCardiovascular Sciences, St. Boniface Hospital Albrechtsen Research Centre, Winnipeg, MB,Canadae-mail: [email protected]

# Springer International Publishing AG 2017V.R. Preedy, V.B. Patel (eds.), Handbook of Famine, Starvation, and NutrientDeprivation, https://doi.org/10.1007/978-3-319-40007-5_81-1

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https://doi.org/10.1007/978-3-319-40007-5_81-1

KeywordsVitamin B6 • Vitamers • Pyridoxal phosphate • Coenzyme • Neurological func-tion • Seizure • Neuroprotection • Immunity • Diabetes

List of Abbreviations5-HT SerotoninAADC L-Aromatic amino acid decarboxylaseAGE Advanced glycation end productCBS Cystathionine β-synthaseDA DopamineDOPA DihydroxyphenylalanineGABA ɣ-Aminobutyric acidGABA-T GABA transaminaseGAD Glutamic acid decarboxylaseGc GlucocorticoidNAS N-AcetylserotoninNE NorepinephrineODC Ornithine decarboxylasePL PyridoxalPLP Pyridoxal phosphatePM PyridoxaminePN Pyridoxine

ContentsIntroduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3PLP-Dependent Enzymes and Antioxidant Actions of PLP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4Assay, Sources, Bioavailability, and Requirement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Vitamin B6 Deficiency: Primary and Secondary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6Neurobiology of Vitamin B6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6Neuroendocrinology of Vitamin B6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Experimental Vitamin B6 Deficiency in the Neonatal Rat: Seizures and Vitamin B6 . . . . . . . . . . 9Pyridoxine: Dependency Seizures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10Neuroprotection by Vitamin B6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10Vitamin B6 and Hypertension . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11Pyridoxal Phosphate, Calcium Channels, and Cardiovascular Function . . . . . . . . . . . . . . . . . . . . . . . . 11Cardiovascular Complications of Diabetes Mellitus: Inhibition of Advanced Glycation End ProductFormation by Pyridoxamine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13Vitamin B6: Gene Expression and Anticancer Effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14Vitamin B6 and Immunity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15Toxicity of Pyridoxine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17Pyridoxal Phosphate Enzymes as Drug Targets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17Policies and Protocols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18Dictionary of Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18Summary Points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

2 K. Dakshinamurti et al.

Introduction

The term vitamin B6 refers to a group of naturally occurring pyridine derivativesincluding pyridoxine (PN, pyridoxol), pyridoxal (PL), and pyridoxamine (PM). Theirphosphorylated derivatives are also included in this group, generically referred to asvitamin B6 vitamers. PN specifically refers to the alcohol form, PL to the aldehydeform, and PM to the amine form. These forms of the vitamin B6 vitamers could beconverted to the key coenzymatic form pyridoxal phosphate (PLP) through the actionsof an oxidase, a kinase, and a phosphatase (Fig. 1). Pyridoxal kinase activity increasesduring brain maturation. The activity of this enzyme in the red blood cells of African-Americans is approximately 50% lower than that of Caucasian Americans. An inverserelationship between the activity of the kinase and the concentrations of brain PLP andbrain monoamines has been reported. Unbound PLP is hydrolyzed by an alkalinephosphatase. Much of the PLP in liver and muscle is protein bound, thus regulating theconcentration of active, unbound PLP. There are more than 140 PLP-dependentenzymatic reactions distributed in all organisms comprising diverse groups such asthe oxidoreductases, transferases, hydrolases, lyases, and isomerases. About 1.5% ofthe genes of prokaryotes encode PLP enzymes which participate in the metabolism ofamino acids, carbohydrates, and lipids indicating the versatility of PLP-dependentenzymes. A detailed account has been provided (Dakshinamurti 1990) and recentlyreviewed (Dakshinamurti and Dakshinamurti 2014).

Kinase

Pyridoxine

NH3C

CH2OHHOCH2OH

Pyridoxal

NH3C

CH2OHHOCHO

Pyridoxamine

NH3C

CH2OHHOCH2NH2

Pyridoxinephosphate

NH3C

CH2OPO3H2HOCH2OH

Pyridoxalphosphate

NH3C

CH2OPO3H2HOCHO

Pyridoxaminephosphate

NH3C

CH2OPO3H2HOCH2NH2

Kinase KinasePhosphatase Phosphatase Phosphatase

Pyridoxinephosphate

oxidase

Pyridoxinephosphate

oxidase

Fig. 1 Interconversion between vitamin B6 vitamers. Pyridoxine, pyridoxal, and pyridoxamine arephosphorylated by a common pyridoxal kinase. The resulting phosphorylated forms of pyridoxinephosphate and pyridoxamine phosphate can be converted to pyridoxal phosphate by pyridoxinephosphate oxidase

Vitamin B6: Effects of Deficiency, and Metabolic and Therapeutic Functions 3

PLP-Dependent Enzymes and Antioxidant Actions of PLP

PLP is the active coenzymatic form of vitamin B6 and is the most versatile of all thecoenzymatic form of vitamins found in all organisms. PLP-dependent enzymes areincluded in five of the six enzyme classes as defined by the Enzyme NomenclatureCommittee of the International Union of Biochemistry and Molecular Biology. Theyact as coenzymes in enzymatic reactions involved in synthesis, degradation, andinterconversion of amino acids, acting as oxidoreductases, transferases, hydrolases,lyases, and isomerases. Based on the fold type of the structural superfamilies, foldtype I is the largest group, comprising the aspartate aminotransferase family. Foldtype II is the tryptophan synthase family. Alanine racemase is fold type III, whereasthe D-amino acid aminotransferase is type IV. PLP-dependent enzymes are classifiedinto three groups depending on the site of elimination and replacement of thesubstituents. Reactions occurring at the α-carbon atom include transaminase,racemases of α-amino acid, amino acid α-decarboxylases, and enzymes catalyzingthe condensation of glycine and the 2-β cleavage of β-hydroxy amino acids such asδ-aminolevulinic acid synthase, serine hydroxyl methylase, and sphingosine syn-thetase. Reactions occurring at the β-carbon atom of the substrate include serine andthreonine dehydratase, cystathionine synthetase, tryptophanase, and kynureninase.Reactions occurring at the ɣ-carbon include homoserine dehydratase and ɣ-cystathionase.

Glycogen phosphorylase, another PLP-dependent enzyme, catalyzes the first stepin the degradation of glycogen. The physiological role of phosphorylase in skeletalmuscle is as an energy source, as approximately 2% of total soluble protein ofmuscle tissue is the enzyme phosphorylase. This enzyme is under regulatory controlwith AMP and IMP being activators and ATP and ADP being inhibitors. Theinvolvement of the phosphate group rather than the carbonyl group is a novel featureof the role of PLP in the phosphorylase reaction. The phosphate group of PLPfunctions in the form of dianion as a proton donor-acceptor. PLP has an importantrole in maintaining the quaternary structure and conformation of phosphorylase. Areservoir function for PLP in muscle phosphorylase is also indicated.

Hydrogen sulfide (H2S) is produced endogenously in organs such as the heart andthe central nervous system. This endogenous production of H2S depends upon PLPenzymes such as cystathionine β-synthase (CBS), cystathionine ɣ-lyase, and 3-mercaptopyruvate sulfotransferase. CBS is highly expressed in the hippocampusand the cerebellum. Along with nitric oxide and carbon monoxide, H2S forms a triadof gaseous signaling molecules in the body. H2S is involved in the regulation ofintracellular signaling molecules such as protein kinase A, receptor tyrosine kinase,and mitogen kinase and in oxidative stress signaling. The L- and T-type calciumchannels as well as potassium and chloride channels are regulated by H2S. Therelease and function of neurotransmitters such as ɣ-aminobutyric acid (GABA),N-methyl-D-aspartate, glutamate, and catecholamines require H2S as a signal.Thus, H2S has a number of neuronal functions.


Recent evidence attests to the antioxidant properties of vitamin B6 vitamers,comparing favorably to the well-established antioxidant vitamins such as ascorbicacid and the tocopherols. PN and PM inhibit superoxide radicals and prevent lipidperoxidation, protein glycosylation, and Na+,K+-ATPase activity. Hyperglycemia-induced oxidative stress is a significant cause of diabetic complications. PLP inhibitsthis oxidative stress as well as lipid peroxidation and protein oxidation. PN has avery high level of quenching of hydroxyl radicals.

Assay, Sources, Bioavailability, and Requirement

Much of the data currently available on the total vitamin B6 content of foods is basedon microbiological methods using the growth of Saccharomyces uvarum (ATCC9080). Enzymatic and radioenzymatic methods have been used for the determinationof PLP. The most commonly used methods are based on ion exchange or paired ionreverse-phase high-performance liquid chromatography with post-column derivati-zation (Dakshinamurti and Stephens 1969; Sharma and Dakshinamurti 1992a;Deitrick et al. 2001).

Vitamin B6 vitamers and their phosphorylated derivatives are present in mostfoods. Glycosylated forms of PN are present in plant-based foods. PL and PLP arethe major forms present in animal-based foods. The vitamin B6 content of selectedfoods has been reported (Lecklem 2001). Vitamin B6 vitamers and their phosphor-ylated derivatives are photosensitive. There is a loss of vitamin activity due to foodprocessing techniques such as heat sterilization. This loss was responsible for theepidemic of seizures caused by vitamin B6 deficiency in infants fed such formuladiets (Coursin 1954).

The absorption of vitamin B6 occurs after the hydrolysis of the phosphorylatedand glycosylated forms in the lumen of the intestine. There is a specialized Na+-dependent carrier-mediated system for the absorption of PN (Said 2004). Onceabsorbed, there is interconversion of the various forms of vitamin B6 vitamers.The concentrations of PL and PLP in the erythrocyte are 2.6- and 1.8-fold higherthan in blood plasma due to the higher affinity of PL to hemoglobin than to albumin.Similarly, PLP synthesized in the erythrocytes is bound with higher affinity tohemoglobin than to albumin. The kinase, oxidase, and transaminase are all presentin the erythrocytes. In the muscle, vitamin B6 is present mostly as PLP bound toglycogen phosphorylase.

The physiological requirement for vitamin B6 depends on one’s age, sex, bodysize, extent of physical activity, and protein intake in the diet. Oral contraceptivedrug use is associated with many clinical side effects that are normally associatedwith pregnancy. The requirements for vitamin B6 in women during pregnancy andlactation and of adolescents during the rapid phase of growth are high. The currentdietary allowance (RDA) recommendations are set at 2.0 mg for adult males andfemales, 0.9 mg for children between 4 and 6 years of age, and 1.2 mg for children in


the age group 7–10 years (National Research Council, Dietary Reference IntakeTables, The National Academies, Washington, D.C. 2005).

Vitamin B6 Deficiency: Primary and Secondary

Impairment of somatic growth and a pellagra-like dermatitis are reported in allvitamin B6-deficient animals. Anemia is reported in most vitamin B6-deficientspecies. Ataxia, hyperacusis, hyperirritability, impaired alertness, abnormal headmovements, and convulsions are seen in a variety of vitamin B6-deficient speciesincluding humans. The widespread occurrence of vitamin B6 deficiency-inducedconvulsive seizures in infants receiving heat-sterilized proprietary milk formulae hasbeen referred to earlier (Coursin 1954). Treatment of these infants with PN resultedin a marked improvement in the wave form and normalization of the amplitude andfrequency of their EEG.

Signs of vitamin B6 deficiency caused by a primary dietary deficiency are rare inthe developed world. However, many conditions are recognized in which a deficientcondition is caused due to increased requirements or poor availability of the vitaminbecause of formation of inactive complexes between the vitamin and various drugs.

Vitamin B6 deficiency has been recognized in pregnant women based on thetryptophan load test, as well as on the determination of vitamin B6 vitamer levels.When maternal vitamin B6 levels are low, the PLP levels of cord blood are signif-icantly decreased. Premature infants have very low levels of plasma PLP at birth(Reinken and Mangold 1973). The use of oral contraceptive drugs is associated withclinical side effects that are similar to those associated with pregnancy and arerelated to hormone-induced changes in tryptophan metabolism.

A functional deficiency of vitamin B6 has been recognized in uremic patientsreceiving hemodialysis (Dobbelstein et al. 1974). Resin-based phosphate binderslead to a greater loss of water-soluble vitamins. Isonicotinic acid hydrazide has beenused for a long time in the treatment of pulmonary tuberculosis. Its use has beenassociated with signs of vitamin B6 deficiency, and supplementation with 50 mg PNreversed this. Another antituberculosis drug, cycloserine, has similar effects on thevitamin B6 status of patients receiving this drug. Penicillamine is used in thetreatment of Wilson’s disease and also for cystinuric patients to prevent the forma-tion of urinary cysteine stones. These patients are prone to epileptic seizures whichare corrected by the administration of PN (Smith and Gallagher 1970; Cohen 1969).

Neurobiology of Vitamin B6

The crucial role played by vitamin B6 in the nervous system arises out of the fact thatthe putative neurotransmitters dopamine (DA), norepinephrine (NE), serotonin (5-HT), and GABA, as well as taurine, sphingolipids, and polyamines, are synthesizedby PLP-dependent enzymes. There is considerable variation in the affinities of thevarious apoenzymes for the cofactor PLP. In view of this, there is a differential


susceptibility of various PLP enzymes to a decrease of PLP during vitamin B6

depletion in animals and humans. The decarboxylation of glutamic acid, 5-hydroxytryptophan, and ornithine is considerably decreased in all vitamin B6-defi-cient species (Fig. 2).

The enzyme L-aromatic amino acid decarboxylase (AADC) lacks substratespecificity and is considered to be involved in the synthesis of catecholamines andserotonin. There are many differences in the optimal conditions for enzyme activityincluding kinetics, affinity for PLP, activation and inhibition by specific chemicals,and differences in regional distribution of dihydroxyphenylalanine (DOPA) and 5-hydroxytryptophan decarboxylase activities. Nonparallel changes in brain mono-amines in vitamin B6-deficient rats have been reported (Siow and Dakshinamurti1986; Dakshinamurti et al. 1976). Brain levels of serotonin were significantlydecreased during vitamin B6 deficiency, whereas DA and NE were not affected.

The neurotransmitter GABA is present almost exclusively in the nervous systemof invertebrates and vertebrates. GABA is formed from glutamic acid through theaction of glutamic acid decarboxylase (GAD) and is catabolized by GABA trans-aminase (GABA-T) to yield succinic semialdehyde. Both GAD and GABA-T arePLP enzymes. GABA is an inhibitory neurotransmitter, whereas glutamic acid isexcitatory. GABA, GAD, and GABA-T are localized in areas of the brain that areinhibitory in function. In addition to the involvement of GABA in the etiology ofconvulsive seizures, abnormalities in the GABA-ergic neuronal pathways contribute

PyridoxalPhosphate

5-HTP-DC

AADCDOPA-DC

SEROTONIN

DOPAMINE

GABA

MELATONIN Sleep

Altered balance between inhibitory

GABAergic and excitatory GLUergicneurotransmission

Overt seizures or

threshold to convulsants

NOREPINEPHRINE

Thermoregulation

Cardiovascular function

Behaviour

Hypothalamus-pituitary end organ

systems

Nociception

Vitamin B6 GAD

Fig. 2 Physiological consequences of vitamin B6 depletion. Depletion of vitamin B6 results in lossof the coenzyme form of pyridoxal phosphate/PLP, in turn decreasing synthesis of GABA andserotonin. These alterations have wide-ranging effects on normal physiology (gray boxes). Keyenzymes are in red text


to other CNS disorders such as depression, anxiety, and panic disorders. During amoderate deficiency of vitamin B6, there are biologically significant decreases in theactivities of the PLP-dependent GAD-65 isoform and AADC (5-HTP-DC) leadingto decreases in neurotransmitters GABA and 5-HT. Under these conditions, DOPAdecarboxylase is not affected, resulting in no change or even an increase in cate-cholamines in the nervous system. Decreases in brain serotonin are implicated inphysiological changes such as decreased deep-body temperature and altered sleeppattern with a decrease in deep slow-wave and REM sleep (Dakshinamurti 1982).

Neuroendocrinology of Vitamin B6

The hypothalamus is one of the areas of the brain of vitamin B6-deficient rats thatexhibits significant decreases in PLP and serotonin compared with vitamin B6-replete controls. There is no decrease in the content of dopamine and norepinephrine.The concept of the regulatory role of the hypothalamus through the neurotransmit-ters is generally accepted. The hypothalamus of normal animals has high concen-trations of both serotonin and dopamine, which are antagonistic in their effects onpituitary hormone regulation (Dakshinamurti et al. 1988).

In determining the focus of the biochemical lesion leading to the hypothyroidstate in the vitamin B6-deficient animal, various possibilities such as primary with adefective thyroid gland, secondary with a defective pituitary thyrotroph, or tertiarywith a defective hypothalamus were considered. The results are consistent with ahypothalamic type of hypothyroidism in the vitamin B6-deficient animal, caused bya specific decrease in hypothalamic serotonin levels (Dakshinamurti et al. 1986).

The pineal gland transduces photoperiodic information and has a role in thetemporal organization of various metabolic, physiological, and behavioral processes.Melatonin is the major secretory product of the pineal gland. In the pinealocytetryptophan is hydroxylated to 5-hydroxytryptophan and decarboxylated to yieldserotonin which is converted to N-acetylserotonin (NAS). NAS is converted tomelatonin by hydroxyl-indole-O-methyltransferase. Melatonin synthesis is stimu-lated by β-adrenergic postganglionic sympathetic fibers from the superior cervicalganglion, which are stimulated in the dark. Melatonin levels in tissues and bodyfluids show both circadian and seasonal rhythms. Pineal levels of 5-HT and 5-hydroxyindoleacetic acid are significantly lower in the vitamin B6-deficient rat,and treatment of the deficient rats with PN restored the levels of 5-HT, NAS, andmelatonin to the levels seen in vitamin B6-replete animals (Viswanathan et al. 1988).

The secretion of prolactin is regulated by both stimulatory and inhibitory factorsof hypothalamic origin. Evidence based on the administration of serotonin pre-cursors, agonists or antagonists, intraventricular administration of serotonin, andelectrical stimulation of the raphe nucleus indicates that the central serotonergicprojections to the hypothalamus are involved in the stimulation of prolactin secre-tion. Administration of PN to vitamin B6-deficient rats results in a significantincrease in plasma prolactin (Sharma and Dakshinamurti 1994).


The vitamin B6 status of the individual has significant effects on the centralproduction of serotonin and GABA, neurotransmitters that control pain perception,anxiety, and depression. High-dose PN, through its effects on neurotransmitters, hasa favorable impact on dysphoric mental states (McCarty 2000; Russo et al. 2003).

Experimental Vitamin B6 Deficiency in the Neonatal Rat: Seizuresand Vitamin B6

It was the general belief that if the nutrition of the mother was adequate forconception and maintenance of pregnancy, the intrauterine mechanisms for activetransport and concentration would supply the necessary nutrients for the normaldevelopment of the unborn child. In view of this, it was of interest to produce andcharacterize vitamin B6 deficiency in the very young rat. Sperm-positive femaleHoltzman rats were maintained on a vitamin B6-supplemented diet during the firstweek of gestation and then divided into two groups. One was continued on the B6-supplemented diet, and the other was fed a vitamin B6-deficient diet until thedelivery of the pups and also during the nursing period. There was a small butsignificant difference in the body weight of the pups between the two groups.Deficient pups had a significant decrease in brain PLP content. Related to this wasthe occasional finding, among the vitamin B6-deficient group, of pups with sponta-neous convulsions that became noticeable at about 3–4 days of postnatal age. Thesefits were characterized by a high-pitched scream followed by generalized convul-sions of a few seconds’ duration and repeated many times within a 1–3-min timeperiod. The motility, perception, and alertness of the deficient neonates were inferiorto those of the control pups. This was the first report of the production of congenitalPN deficiency (Dakshinamurti and Stephens 1969). In view of the high mortality ofthe deficient pups, they could not be used in studies on the development of thecentral nervous system. In a further study, female rats were fed a vitamin B6-deficientdiet from the first postpartum day and the pups were fed the deficient diet from theday they were weaned until they were 5–6 weeks of age. The effects of deficiency onvarious electrophysiological parameters were examined. The bursts of high-voltagespikes during spontaneous EEG activity, as well as the spontaneous convulsionsobserved, reflect the decrease in cerebral GABA concentration in the deficient rats.The more complicated changes in cortical auditory evoked potentials in the vitaminB6-deficient rats are the result of the retardation of normal ontogenetic developmentof the central nervous system of these rats (Stephens et al. 1971).

The thalamus acts as a relay station for various peripheral and central inputs to thecerebral cortex. Hence, the electro-responsiveness of thalamic ventroposterior lateralneurons in normal control and vitamin B6-deficient adult rats in response to localadministration of convulsants such as picrotoxin or pentylene tetrazole was studied.The extent of neuronal recovery following intrathalamic administration of eitherGABA or PN, or systemic administration of PN, was assessed using computerizedEEG analysis. The results demonstrated an antiepileptic effect of exogenouslyapplied GABA and PN on the thalamic ventroposterior lateral neuron. Neuronal


recovery following PN administration is related to synthesis of GABA throughactivation of GAD (Sharma and Dakshinamurti 1992b; Sharma et al. 1994).

Pyridoxine: Dependency Seizures

PN dependency has been recognized as an inborn abnormality. Infants, generallysoon after birth, have seizures that are resistant to the commonly used antiepilepticdrugs and respond only to pharmacological doses of PN. It is a rare autosomalrecessive disorder. In view of the prevalence of atypical variants of this disorder, it isgenerally under-recognized. A PN-dependent condition has to be considered in allchildren with intractable epilepsy up to 3 years of age (Gospe 2002). The unusualrhythmic in utero movements reported retrospectively by some mothers mightrepresent fetal seizures (Clayton 2006). At present there is no biochemical test toconfirm PN-dependent seizures: clinical diagnosis is the only mode of recognition.Response to PN monotherapy and recurrence of seizures upon withdrawal oftreatment is the only confirmatory test. Such testing is fraught with difficulty dueto ethical considerations. PN dependency is distinct from the PN-deficient state ofinfants reported earlier (Coursin 1954).

The intravenous administration of 50–100 mg PN results in a dramatic cessationof seizures in affected children. In some cases, the dose might be as high as 500 mg.The pharmacological requirement of PN is for life, and the required dose level canbe titrated. An untreated PN-dependent condition results in delay in achieving mile-stones, developmental defects, as well as permanent brain damage (Alkan et al. 2004).

Recent studies indicate that increasing the dose of PN in PN-dependent childrenwithout seizures could improve their IQ, indicating a role for PN in normal braindevelopment (Baxter 2003). Autopsy studies on PN-dependent seizure patients showelevated glutamate and decreased GABA levels in the frontal and occipital cortices(Alkan et al. 2003). Of the two isoforms of GAD, GAD-65 is PLP-dependent, anddefective binding of PLP to the apoenzyme is suggested to be the cause of thedecreased synthesis of GABA in these patients. There are other seizure conditions inwhich PN therapy finds a place. Infantile spasm in combination with diffuseelectroencephalographic abnormalities is referred to as “West syndrome.” Mentalretardation is associated with this condition. Following reports of beneficial effects,initial treatment with high doses of PN is the established therapy in some Europeancountries and in Japan (Pietz et al. 1993).

Neuroprotection by Vitamin B6

Domoic acid, a rigid structural analog of glutamate, is a neuroexcitant. It wasidentified as the toxic contaminant of cultivated mussels responsible for the outbreakof acute food poisoning characterized by gastrointestinal and neurological symptoms(Teitelbaum et al. 1990). Acute intrahippocampal administration of picomole


amounts of domoic acid led to EEG epileptiform seizure discharge activity. Domoicacid was 125 times more potent than kainic acid, a well-known neuroexcitant. Localadministration of GABA or PN attenuated the seizure activity (Dakshinamurti et al.1991). Following domoic acid injection, GABA levels decreased significantly invarious brain regions. The direct application of GABA to the hippocampus of ratsexhibiting domoic acid-induced seizure activity resulted in suppression of spikedischarges. PN had a similar, but slower effect (Dakshinamurti et al. 1993). It hasbeen reported that serotonin functions in the stabilization of brain regional GABA-ergic neurons and in seizure control. The neuroprotective action of PN flows from itseffect on the synthesis of both GABA and serotonin (Dakshinamurti et al. 2003). PNtreatment, in association with the histidine deacetylase inhibitor sodium butyrate,significantly restored the age-related reduction in memory function. The restorativepotential of PN on ischemic damage in the hippocampal CA1 region of Mongoliangerbils has been established (Yoo et al. 2012).

Vitamin B6 and Hypertension

Hypertension is one of the major causes of chronic illness in western societies, whereabout 20–30% of the adult population have some degree of blood pressure elevation.For the majority of patients, the underlying cause has not yet been recognized andthe condition is referred to as “essential hypertension.” Various animal models havebeen used to study this hypertensive state. The moderately vitamin B6-deficient rathas been introduced as an additional animal model to study hypertension (Paulose etal. 1988; Dakshinamurti and Lal 1992; Dakshinamurti and Dakshinamurti 2001).The possibility that the reversible hypertension in these animals was related tosympathetic stimulation was studied. The concentrations of both epinephrine andnorepinephrine in plasma were threefold higher compared to controls. The turnoverof norepinephrine in the hearts of deficient hypertensive rats was threefold higher ascompared to controls. Treatment with PN returned both blood pressure and cate-cholamine levels to normal within 24 h, indicating that the lesion could be at thelevel of neurotransmitter regulation. Serotonergic neurotransmission in the centralnervous system controls a wide variety of functions such as blood pressure, emo-tional behavior, endocrine secretion, and perception of pain and sleep. It is possiblethat the decrease in neuronal serotonin and the consequent changes in its receptors,particularly 5-HTA, may cause hypertension in B6 deficiency. The 5-HT1A receptoragonists all have acute hypotensive effects in this rat model.

Pyridoxal Phosphate, Calcium Channels, and CardiovascularFunction

The end result of centrally mediated sympathetic stimulation is an increase inperipheral resistance which is reflected in elevation of both resting and stimulatedvascular tone in the resistance arteries of the moderately vitamin B6-deficient


hypertensive rat. Elevated peripheral resistance is the hallmark of hypertension asseen in all models of hypertension. The increase in the tone of caudal artery segmentsfrom the hypertensive rat is calcium-dependent. The decrease in the tone followingthe addition to the medium of the calcium channel antagonist, nifedipine, indicatesthat increased peripheral resistance resulting from increased permeability of smoothmuscle plasma membrane to Ca2+ might be central to the development of hyperten-sion in the vitamin B6-deficient rat. Calcium influx occurs through plasma mem-brane Ca2+ channels that are voltage-operated or receptor-mediated. Voltage-sensitive Ca2+ channels open upon depolarization of the cell membrane, resultingin an inward movement of calcium ions. ATP is an extracellular nucleotide thatmediates its effect via plasma membrane-bound P2 receptors. The slow L-typechannel is the major pathway by which Ca2+ enters the cell during excitation forinitiation and regulation of the force of contraction of cardiac and skeletal muscle.Vascular smooth muscle also contains L-type channels.

The possibility that in the vitamin B6-deficient rat a higher concentration ofcytosolic-free Ca2+ might be responsible for the higher tension in the vascularsmooth muscle was studied. In the vitamin B6-deficient hypertensive rat, radio-labeled 45Ca2+ influx into the vascular smooth muscle was increased to twice thatof the controls. The KCl-induced [Ca2+] increase was significant in cardiomyocytesisolated from vitamin B6-deficient hypertensive rats. A single injection of vitamin B6

(10 mg/kg body weight) to the deficient animal completely reversed the KCl-induced changes in [Ca2+] due to vitamin B6 deficiency. Similar results wereobtained in other diet-induced hypertensive animal models, as well as in geneticallyhypertensive animal models such as the Zucker obese rat (Lal and Dakshinamurti1993, 1995). In further studies, the possibility that PN or more particularly PLPcould directly modulate the cellular calcium uptake process was studied. BayK8644, a dihydropyridine-sensitive calcium channel agonist, stimulated calciumentry into artery segments from control rats. PLP dose-dependently reduced theBay K8644-stimulated calcium uptake by control artery segments (Lal et al. 1993;Dakshinamurti et al. 1998). The basal uptake of 45Ca2+ by caudal artery segmentsfrom vitamin B6-deficient hypertensive rats was at least twice the uptake by caudalartery segments from normal rats. The in vitro direct antagonism indicates thepossibility that the calcium channel agonist Bay K8644, the calcium channel antag-onist nifedipine, and PLP might all act at the same site on the calcium channel.

In a further study, the effect of PLP on the ATP-induced contractile activity of theisolated rat heart and the ATP-mediated increase in intracellular [Ca2+] in freshlyisolated adult rat cardiomyocytes, as well as on the specific binding of ATP to thecardiac sarcolemmal membrane, were examined to determine if PLP is an effectiveantagonist of ATP receptors in the myocardium. The infusion of ATP caused animmediate increase (within seconds) in LVDP, +dP/dt and –dP/dt. This effect wascompletely blocked in hearts pretreated with PLP for 10 min. The specificity of theeffect of PLP was established (Wang et al. 1999).

Studies in humans have identified an independent association between lowplasma vitamin B6 concentration and a higher risk of coronary artery disease(Chang et al. 1999). In addition to the role of vitamin B6 in atherosclerosis, other


potential explanations include the role of PLP in platelet aggregation and theassociation between low PLP and inflammatory markers (Friso et al. 2005). Lowplasma PLP concentration was inversely associated with major markers of inflam-mation and independently associated with cardiovascular disease risk (Friso et al.2004). Our observations on the role of PLP in both major calcium channels for theinflux of extracellular calcium might proffer a viable biochemical explanation for theassociation between low PLP concentration and the risk for cardiovascular disease(Fig. 3). The increase in [Ca2+] in cardiomyocytes might contribute to heart dys-function and increased myocardial infarction and explain the beneficial effect ofvitamin B6 in patients with hypertension and myocardial infarction (Dakshinamurtiet al. 2000; Dhalla et al. 2000).

Cardiovascular Complications of Diabetes Mellitus: Inhibition ofAdvanced Glycation End Product Formation by Pyridoxamine

The reducing sugar glucose, which is elevated in the diabetic condition, can modifyproteins through condensation of its aldehyde group with the ε-amino group oflysine residues, forming a Schiff base. This is referred to as the Maillard reaction andis dependent on the concentration of glucose, being exacerbated in the diabeticcondition. The Schiff bases isomerize to intermediate ketosamine Amadori productsincluding glycated hemoglobin (HbA1c). Over a period of weeks and months,

Calcium channels(voltage- or

receptor-mediated)

Pyridoxal Phosphate

Ca2+ influx

Serotonin

Nucleus tractus solitariia-agonists

Central sympathetic outflow

Peripheral vasculartone

Total vascularresistance

Cardiovasculardisease

Fig. 3 PLP-mediated mechanisms leading to attenuation of hypertension. Adrenergic stimulationleads to increased peripheral vascular tone, increased total vascular resistance, and an increase insusceptibility to cardiovascular disease such as hypertension. Pyridoxal phosphate induces seroto-nin-mediated central sympathetic outflow, but also inhibits both voltage- and receptor-mediatedcalcium channels, reducing calcium influx and attenuating the increase in vascular tone, which mayprovide the mechanism for PLP-mediated cardioprotection


glucose-independent reactions including rearrangement, condensation, dehydration,polymerization, and fragmentation lead to a host of metabolites referred to asadvanced glycation end products (AGEs). Lipid peroxidation of polyunsaturatedfatty acids similarly produces advanced lipoxidation end products.

Histopathological evidence attests to the accumulation of AGEs in a variety oftissues such as the renal cortex, glomerular mesangium, and basement membrane.Evidence for the role of AGEs in vascular complications has been presented (Ahmedand Thornalley 2007). Experimentally, the injection of AGE precursors or AGE-modified proteins induces vascular damage similar to that seen in the diabeticcondition. Various studies indicate that hyperglycemia is the most significant factorin the onset and development of vascular complications of diabetes, of whichchronic kidney disease is a major component. Inhibition of the renin-angiotensinsystem with angiotensin-converting enzyme inhibitors and/or angiotensin II receptorblockers is the main pharmacotherapy for chronic kidney disease. In addition, theyalso block the formation of reactive carbonyl precursors of AGE as well as inducedreactive oxygen species generation (Thomas et al. 2005). Of the various compoundsthat inhibit or correct individual steps in the pathophysiological sequence leading tovascular complications of diabetes, two stand out: PM and benfotiamine. PM is amember of the vitamin B6 vitamer group. Benfotiamine is a lipid-soluble vitamin B1

analog. Treatment with high-dose PM improved the urinary albumin/creatine ratiosand fasting serum triglyceride and 3-deoxyglucosone. PM also prevents the accu-mulation of Nε-(carboxymethyl) lysine, nitrotyrosine, TGF-β1, and laminin-β1 inthe kidney. The therapeutic potential of ACE inhibitors has been tested in clinicaltrials (Brenner et al. 2001). Studies using various animal models indicate that PM iseffective in the treatment of diabetic nephropathy, retinopathy, and neuropathy(Chang et al. 2009; Chen and Francis 2012). The function of PM is not related toits role as a member of the vitamin B6 vitamers. PM, in combination therapy, mightfind a use in the treatment of a wide range of chronic diseases in which oxidativestress, inflammation, and tissue damage lead to a chemical modification of proteins(Metz et al. 2003).

Vitamin B6: Gene Expression and Anticancer Effect

In rats fed a diet adequate in vitamin B6, the fraction of total PLP found in the nucleiof liver cells was 21%, and this increased to 39% in rats fed a vitamin B6-deficientdiet indicating a conservation of the vitamin in the nuclear compartment duringdeficiency, a situation analogous to the distribution of biotin in biotin-replete andbiotin-deficient animals (Dakshinamurti 1997). PLP in the cell nucleus is proteinbound. Cells grown in the presence of 5 mM PN have a decreased glucocorticoid-dependent induction of enzymes. Vitamin B6 regulates the transcriptional activationof human glucocorticoid receptors in HeLa cells. This modulatory role on transcrip-tion is not restricted to the glucocorticoid receptor, but extends to all members of thesteroid hormone receptorsuper family, leading to a decreased transcriptionalresponse to various steroid hormones (Oka et al. 1995). PLP modulates gene


expression through its influence on the functional interaction between the steroidhormone receptors and transcription factor NFI (Davis and Cowing 2000).

The growth of B16 melanoma in vitro was inhibited by 5 mM PN or PL.Treatment of mice with PL (0.5 g/kg body weight) reduced the growth of bothnew and established B16 melanoma. PL supplementation was shown to reduce cellproliferation and DNA synthesis in both estrogen-dependent and estrogen-indepen-dent mammary carcinoma cell lines (Komatsu et al. 2003). The growth of MH-134hepatoma cells transplanted into C3H/He mice was significantly reduced by theadministration of large amounts of PN to mice. High dietary intake of vitamin B6 hasbeen shown to suppress herpes simplex virus type 2-tranformed cell-induced tumorgrowth in BALB/c mice (Jansen et al. 1999). Epidemiological studies indicate areduced risk of lung and colorectal cancer in older men ingesting high doses ofvitamin B6 (Hartman et al. 2001; Larsson et al. 2010).

PLP was reported to be a strong inhibitor of DNA polymerases α and ε from aphylogenetically wide range of organisms, from protists, plants, insects, and fish tomammals. These polymerase classes are related to DNA replication. Treatment withpharmacological doses of vitamin B6 suppressed the expression of the cell prolifer-ation-related genes c-myc and c-fos in colon epithelium of mice treated withazomethane. PLP has been shown to inhibit DNA topoisomerases I and II, whichare needed for strand separation, replication, and recombination. Inhibition of DNAtopoisomerase arrests the cell cycle and induces apoptosis. PLP has been shown tobe an effective inhibitor of many enzymes that have binding sites for phosphate-containing substrates of effectors, including RNA polymerase, reverse transcriptase,and DNA polymerase (Matsubara et al. 2003). Based on a meta-analysis of prospec-tive studies, an inverse association between blood PLP levels and the risk ofcolorectal cancer has been reported (Larsson et al. 2010).

Oxaliplatin in combination with 5-fluorouracil is standard treatment for meta-static colorectal carcinoma. Peripheral sensory neuropathy is the dose-limiting sideeffect of this treatment. PN administration reduces oxaliplatin-induced neurotoxicity,thus allowing for more effective and less toxic treatment of such cancers. Thepreventive effect of vitamin B6 on tumorigenesis might also derive from its strongantioxidant action (Garg and Ackland 2011; Jain and Lim 2001). These observationshighlight the potential use of pharmacological doses of vitamin B6 in cancer therapy.

Vitamin B6 and Immunity

There is overwhelming evidence for the requirement of vitamin B6 in antibodyproduction. The thymus of vitamin B6-deficient rats is depleted of lymphocytes,and deficient animals had impaired antibody formation following exposure tovarious antigens. Cell-mediated immunity is also impaired in vitamin B6-deficientrats. A decline in immune response is a concomitant of the aging process in animalsand humans with the most significant effect on cell-mediated immunity. PN supple-mentation led to lymphocyte proliferative response to both T- and B-cell antigens(Kwak et al. 2002). Diseases such as uremia and arthritis are associated with


immunological abnormalities. Treatment of uremic patients with pharmacologicaldoses of PN results in a significant increase in lymphocyte reactivity. Plasma PLP issignificantly lower in patients with rheumatoid arthritis compared to controlsmatched for age, gender, race, and weight (Roubenoff et al. 1995). The plasmalevels of PLP inversely correlated with production of tumor necrosis factor byunstimulated peripheral blood mononuclear cells.

Glucocorticoids (Gc) are widely used in the treatment of inflammatory andautoimmune states. Long-term Gc use is associated with severe side effects. VitaminB6 does not interfere with Gc action in immune cells while selectively inhibiting theunwanted side effects of Gc receptor-dependent transactivation in nonimmune cells(Bamberger et al. 2004). PL and PLP suppress the expression of cytokine genes inmacrophages, indicating the anti-inflammatory activity (Zhang et al. 2016).

Vitamin B6 deficiency is widely prevalent among HIV-infected persons (Baum etal. 1991). The relationship of PLP to the activation of CD4 T cells by antigen-presenting cells has been studied. CD4 T cells are the mediators in the initiation andcontinuation of the immune response causing autoimmune diseases and allogenictransplant rejection. The CD4 glycoprotein is the characteristic surface receptor of allhelper T cells. The extracellular part of the CD4 molecule is comprised of fourdomains, D1 to D4. CD4 binds to major histocompatibility complex MHC class IIthrough the D1 and D2 domains. PLP binds very tightly to the D1 domain of CD4and thus interferes with the CD4-MHC II interaction. Non-incorporation of CD4 intothe activation complex could lead to T-cell apoptosis. The tight association of D1and PLP would prevent protein-protein interaction of CD4 itself, its dimerization,and the interaction of the dimer with other molecules on the T-cell surface leading toapoptosis.

PLP has been shown to be an anion channel blocker in a variety of cells (Korchaket al. 1980). It has been suggested that PLP might have a role in the treatment ofautoimmunity and in transplant rejection (Namazi 2003). As the interaction of HIVgp120 and CD4 occurs through the D1 domain, PLP may have an anti-HIVeffect aswell. High concentrations of PLP inhibit viral coat protein envelope glycoproteinbinding and infection of CD4 T cells by isolates of HIV-1 in vitro (Guo et al. 1994).Thus, PLP might function as an immune stimulator, not only by increasing CD4 T-cell count but also by protecting uninfected CD4 T cells from infection by HIV-1(Salhany and Stevenson 1996). These effects of PLP are seen at concentrations in therange 50–70 mM. The rapid hydrolysis of PLP by tissue nonspecific alkalinephosphatase is a problem in maintaining such high concentrations of PLP. Levam-isole, an antihelminthic drug, is an inhibitor of alkaline phosphatase. Thus a com-bination of high PLP dose along with this inhibitor might maintain the requiredtissue concentration of PLP to afford protection of uninfected CD4 T cells againstHIV-1.


Toxicity of Pyridoxine

Concern about the toxicity of PN was associated with the use of Bendectin(doxylamine plus PN) by pregnant women and the subsequent occurrence of birthdefects in the offspring. Later studies have ruled out any teratogenic effect of PN(Check 1979). Concern about its toxicity resurfaced after reports of reversiblesensory neuropathy in persons ingesting gram quantities of PN for long periodsextending to years. It is significant to note that the reported sensory neuropathy wasreversible after stopping of the ingestion of these large amounts. This indicates nopermanent structural damage to the nervous system. High-dose PN ingestion overseveral years has been in vogue for the treatment of various clinical conditions suchas homocysteinemia, PN-dependent seizures, autism, and Down syndrome (Rimlandet al. 1978). There have been no adverse effects associated with these treatments. Adose of 500 mg/day PN for up to 2 years was not associated with neuropathy(Bendich and Cohen 1990).

Pyridoxal Phosphate Enzymes as Drug Targets

PLP enzymes have been targets of therapeutic intervention. In Parkinson’s disease,abnormal pulsatile stimulation of the striatal dopamine receptors leads todysregulation of genes and proteins in the downstream neurons and, consequently,alterations in the neuronal firing patterns. This results in the motor complicationassociated with Parkinson’s disease, and the treatment aim is to restore normaldopaminergic transmission of striatal synapse. Levodopa slows the progression ofParkinson’s disease.

The PLP-binding enzyme ornithine decarboxylase (ODC) is the rate-limiting stepin polyamine biosynthesis. Polyamines are DNA-binding agents regulating itsconformation. Mammalian ODC is a downstreammediator of myc-regulated reactionsand is upregulated in proliferating cells. It is implicated as an oncogene in multipletypes of tumors. Inhibition of ODC suppresses tumor development. Hence, ODC is apromising anticancer target. Inhibitors of ODC such as difluoromethylornithine arebeing tested as anticancer agents (Wu et al. 2011; Seiler 2003).

PLP-dependent enzymes are potential targets for developing antiparasitic drugs.There are several PLP enzymes with high prevalence among the protozoan species.Enzymes of the sulfur-containing amino acid metabolic pathway are significant interms of their distribution between hosts and their disease-causing parasites. Theenzymes of the forward transsulfuration pathway are present in protozoan parasites,but lacking in their host. Another PLP enzyme, methionine ɣ-lyase, is important tothe parasite for the production of propionic acid for energy metabolism and for thedegradation of sulfur-containing amino acids. Host mammalian cells lack thisenzyme, and hence it is a target for drug development against infection by pro-tozoans such as Trichomonas vulgaris and Entamoeba histolytica. The halogenated


methionine analog S-trifluoromethyl-L-homocysteine and its amide derivative aretoxic to the parasite and have potential as antiparasitic drugs (Sato et al. 2010).

Of the parasitic diseases, the most devastating is malaria, with a worldwideannual death toll of more than a million people. It is transmitted by the mosquito.Xanthurenic acid is required for gametogenesis and fertility of the parasite Plasmo-dium falciparum. It is synthesized in the tryptophan degradative pathway throughthe PLP-dependent enzyme kynurenine aminotransferase. Hence, this enzyme is atarget for the development of antimalarial drugs (Muller et al. 2009). Africansleeping sickness is another widespread epidemic caused by Trypanosoma brucei,which is transmitted by flies. ODC, a key enzyme in the pathway for the synthesis ofpolyamines, is a target enzyme for drug development. Alpha-difluoromethylornithine, an irreversible inhibitor of ODC, has been developed forthe treatment of sleeping sickness (Krauth-Siegel et al. 2007).

Policies and Protocols

The prevalence of frank clinical symptoms of vitamin B6 deficiency is rare inadvanced countries. However, a relative deficiency is present in pregnant womenand women on oral contraceptive steroids. Vitamin B6-dependent seizures in infantsand children, and the use of pyridoxine for their treatment, are well-recognized.However, newer information on the protective role of pharmacological doses ofvitamin B6 in the treatment and amelioration of a variety of chronic conditions needsto be disseminated, and policies in this regard are useful. RDA levels do not have anyrelevance to the protective role of vitamin B6 in hypertension, cardiovasculardiseases, nephropathy, retinopathy, neuropathy, and dysfunction of the immunesystem and as an adjuvant in cancer treatment. The required therapeutic doses aremuch higher. There is no toxicity associated with these dose levels. Given theseconsiderations, hospital-level policies to consider vitamin B6-based treatment asfrontline or adjuvant treatment for such conditions should be evaluated.

Dictionary of Terms

Primary vitamin B6 deficiency A deficiency in vitamin B6 that is due toa lack of availability in the diet. Primaryvitamin B6 deficiency is relatively rare inthe developed world.

Pyridoxal phosphate-dependentenzymes

An enzyme whose activity dependsupon the presence of pyridoxal phos-phate as a coenzyme. Over 140 pyri-doxal phosphate-dependent enzymesare known, and they play critical rolesin the metabolism of amino acids, car-bohydrates, and lipids.


Pyridoxine dependency A condition characterized by a require-ment, typically lifelong, for administra-tion of high levels of pyridoxine far inexcess of normal daily requirements.Pyridoxine dependency can result in epi-leptic seizures that can only be alleviatedby the provision of pyridoxine.

Secondary vitamin B6 deficiency A deficiency in vitamin B6 that occursdespite an adequate dietary supply. Asecondary vitamin B6 deficiency canoccur in specific situations of vitaminB6 loss, such as uremic dialysis patients,in whom water-soluble vitamin loss canbe significant.

Vitamer Chemical compounds that exhibit theactivity of a particular vitamin. Thesecompounds typically have a similarchemical structure and can be used invitamin-deficient individuals to providevitamin activity.

Summary Points

• Pyridoxine, pyridoxal, and pyridoxamine and their phosphorylated derivativesare vitamers of the vitamin B6 group. Pyridoxal phosphate is the most versatile ofall coenzymatic forms of vitamins.

• There are over 140 PLP-dependent enzymes and they are present in all organisms.PLP enzymes participate in the metabolism of amino acids, carbohydrates, andlipids.

• Primary deficiency of vitamin B6 is rare in developed countries. However,pregnant women and women on oral contraceptive steroids have a relativevitamin deficiency even on a normal intake of vitamin B6. A functional deficiencyof vitamin B6 is also recognized in uremic patients and patients treated with drugssuch as isonicotinic acid hydrazide, cycloserine, and penicillamine.

• Vitamin B6 has a crucial role in neurobiology as the putative neurotransmitters,GABA, catecholamines, and serotonin are products of PLP-dependentdecarboxylases. There are nonparallel changes in the catecholamines and seroto-nin in vitamin B6 deficiency leading to a significant neurotransmitter imbalance.

• Maternal vitamin B6 deficiency has a significant impact on neuronal developmentof the offspring. Congenital deficiency leads to seizures in the neonate andimpaired neuronal development.


• Pyridoxine dependency is recognized as an inborn abnormality in children havingintractable epilepsy which is not responsive to anticonvulsant therapy andresponds only to pharmacological doses of pyridoxine.

• Pyridoxine affords neuroprotection against neuroexcitants such as domoic acid.This neuroprotection flows from its effect on the syntheses of GABA andserotonin.

• Vitamin B6 deficiency is also associated with hypertension. Various animalmodels of hypertension respond to pharmacological doses of pyridoxine. Inaddition to its effect on serotonergic neurotransmission, PLP is a regulator ofcellular calcium channels affecting both the voltage-mediated and the ATP-mediated Ca2+ channel transporters. High-dose pyridoxine is effective in thetreatment of hypertension and cardiovascular diseases.

• Hyperglycemia is the most significant factor in the onset and development ofvascular complications of diabetes leading to chronic kidney disease. Treatmentwith high-dose pyridoxine is effective in the treatment of nephropathy, retinop-athy, and neuropathy.

• Vitamin B6 regulates the transcriptional activation induced by steroid hormonereceptors. Large doses of pyridoxine have an effect on tumorigenesis. PLPfunctions as an immune stimulator and protects uninfected CD4 T cells frominfection by HIV-1.

• High doses of pyridoxine up to 500 mg/day over years have been used in thetreatment of several chronic human conditions with no adverse effects.

• Pyridoxal phosphate-dependent enzymes such as ornithine decarboxylase arepotential targets for developing antiparasitic drugs.

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