milk enzymes

8/19/2019 Milk Enzymes

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Milk enzymes

Some milk enzymes enter the alveolar milk from the mammary blood

capillaries via the intercellular fluid. Others come from the breakdown of the

mammary secretory cells. The milk enzymes, xan-thine oxidase, aldolase., and

alkaline phosphatase, are contained in the fat globule, membrane, and miik serum.

The most significant enzyme, lipase, splits triglycerides.

uman milk contains both proteolytic enzymes and protease inhibitors.!S

"mylase facilitates digestion of polysaccharides by the infant. Sulfbydryl #S$

oxidase catalyzes oxidation of S groups. %lutaihione peroxidase facilitates the

delivery of selenium to the infant. &ysozyme and peroxidase are bactericidal

Cellular components

uman milk has been called a 'live fluid' by many (and 'white blood' in

many ancient rites. )reast milk contains up to *+++ cellsm&, which have been

identified with leukocytes and enter the milk via the paracelrular pathway, pathway

.++ The cell number /s particularly high in colostrum. The cells in greatest n amber

are the macrophages, which secrete lysozyme and lactoferrin. &ymphocytes, neu-

irophii0. and epithelial cells are also present. &ymph0 cytes-produce 1g" and

interferon

2acrophages constitute a ma3or cellular component in milk compared with

levels in blood and can survive ''icier conditiort4simulating the infant5s gas-

trointestinal tract.6' )ecause they release secretory 57" in association with

phagocytosis, it is believed 588e.v play a role in host defense. 2acrophage colony-


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stimulating factor in human milk and mammary gland epithelial cells are believed to

be responsible for expansion of the macrophages m milk.

IN VOLUTION: WEAN ING

"9: ";O;TOS1S

:uring weaning, significant increases in milk protein, chloride, and sodium

sodium concentrations and decrease lactose occur when milk volumes fall below

*++ m&day. %lucose and magnesium levels are unchanged.

volume is regulated differently during weaning than during iacto-genesispfo

sentinel substance is a reliable predictor of volume in all stages, but normal ranges

of milk components during full lactation are sodium, 6 to = mmol&0 chloride, = to

!* mmoi&0 protein. = to !6 g&0 and lactose, *+ to !6+ mmo>&. alues out- 3fe

side these ranges suggest mastitis or weaning. :uring gradual weaning, between <

and ? months post partum, glucose, citrate, phosphate, and calcium levels

decrease, whereas lipitl, potassium, and magnesium increase


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of cells no longer differentiating4= :uring postlactational mammary gland

involution, most mammary epithelium dies -. and is reabsorbed

UMMA!"

1n the human, lactogenesis occurs slowly over the first few days post partum as

progesterone levels drop. Bomen experience 'milk coming in' as a feeling of

fullness between *+ and C! hours, usually corresponding to the degree of parity, with

multi-paras sensing this more Duickly than primiparas. olume of milk increases over

time for the first ! weeks, starting at less than ++ m&day and increasing to about


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6


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@ertil


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#ed$H andbook of 2ilk Fomposition. San :iego, "cademic ;ress, AA?.

Flin 9utr ?*H=, AA.


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;hysiol AC"H6C, AA+.

=?. Sherwood &2H uman prolactin. 9 Ingl G 2ed !=*HCC*, AC.

=


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A


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The biochemistry of human milk encompasses a mammoth supply of

scientific data and information, most of which has been generated since AC+. Iach

report or study adds a tiny piece to the complex puzzle of the nutrients that make up

human milk. The answers to some Duestions still elude us. " Duestion as simple as

the volume of milk consumed at a feeding remains a scientific challenge. The

methodology must be accurate, reproducible, noninvasive, and suitable for home use

night or day and must not interrupt breastfeeding. The precision analysis available

for measuring the concentration of the most minuscule of elements, however, is

remarkably accurate and reproducible in the laboratory.

"dvances in analytic methods bring greater sensitivity, resolving power, and

speed to the analysis f milk composition. ;reviously unknown and unrecognized

compounds have been detected. Be now know milk brings both nutrients and

nonnutri-tive signals to the neonate. Bith few exceptions, all milks contain the

nutrients for physical growth and development Bhen the offspring develops rapidly,

we m4 is nutrient dense0 when it develops slowly, e milk is more dilute. "ll milks

contain fat, car-hydrate, and proteins, as well as minerals, vitals, and other nutrients.

The organization of milk composition includes lipids in emulsified globules F+ateci with

a membrane, colloidal dispersions of eins as micelles, and the remainder as a true

solution.?A "t no other time in life is a single food adeDuate as the sole source of

nutrition. The discussion in this chapter is limited to infor-mation perceived as

immediately useful to the clinician. Fonsiderable detail and species variability are

overlooked to help focus attention on details directly influencing management.

Ixtensive and exhaustive reviews are referenced to provide the reader with easy


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access to greater detail and validation of the general conclusions reported here.

uman milk is not a uniform body fluid but a secretion of the mammary gland of

changing composition #@ig. *-$. @oremilk differs from hindmilk. Folostrum differs

from transitional and mature milks. 2ilk changes over time of day and as time goes

by. "s concentrations of protein, fat, carbohydrates, minerals, and cells differ,

physical properties such as osmolarity and p change. The impact of changing

composition on the physiology of the infant gut is beginning to be appreciated. 2any

constituents have dual roles, not only nutrition but infection protection, immunity, or

a host of other effects.

The more than !++ constituents of milk include a tremendous array of molecules

whose descriptions continue to be refined as Dualitative and Duantitative laboratory

techniDues are perfected. Eesolution of lipid chemicals has advanced dramatically in

recent years, but new carbohydrates and proteins have been identified as well. Some

of the compounds identified may well be intermediary products in the process.

That occurs within the mammary cells and may be only incidental in the final

product.!


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specificity!H

. Fonstituents specific to both organ and species #e.g., most proteins and lipids$!. Fonstituents specific to organ but not to species #e.g., lactose$

6. Fonstituents specific to species but not to organ #e.g., albumin, some

immunoglobulins$

NO!MAL VA!IATION IN /UMAN MIL'

1n defining the constituents of human milk, it is important to recognize that

the composition varies with the stage of lactation, the time of day, the sampling time

during a given feeding, maternal nutrition, and individual variation. 2any early

interpretations of the content of human milk were based on spot samples or even

pooled samples from multiple donors at different times and stages of lactation.

Samples obtained by pumping may.

The variation in the fat content has received some attention. @at content

changes during a given feeding, increasing at the end of the feeding. @at content rises

from early morning to midday0 the volume increased from two to five times, as

reported in early studies when feedings were controlled. 2ultiple studies in different

countries and different decades, summarized by Gackson and coworkers+= reveal that

some of the variation is related to other factors. :emand feeding #Thai mothers in

A==$ has a different circadian variation than scheduled feeding #L.S. mothers in

A6!$ #@ig. *-6$. 1n the later part of the first year of lactation, the fat content

diminishes. Bork done by "tkinson and associates6 and confirmed by other

investigators has shown that the nitrogen content of the milk of mothers who deliver

prematurely is higher than that of those whose pregnancies reach full term. @or a

given volume of milk, the premature infant would receive !+U more nitrogen than


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the full-term infant if each were fed his or her own mother5s milk. Other constituents

of milk produced by mothers who deliver prematurely have also been studied.

"n additional consideration in reviewing information available on the levels of

various constituents of milk is the techniDue used to derive the data. 1n AC?,

ambraeus=6 reported that, there was less protein in human milk than originally

calculated. The present techniDues of immunoassay measure the absolute amounts,

whereas earlier figures were derived from calculations based on measurements of the

nitrogen content. "bout !?U of the nitrogen in human milk is nonprotein nitrogen.

Fow milk has only ?U nonprotein nitrogen.

" ma3or concern about variation in content of human milk is related to the

mother5s diet. 2aternal diet is of particular concern when the mother is malnourished

or eais an unusually restrictive diet. 2alnourished mothers have approximately the

same proportions of protein, fat and carbohydrate as well-nourished mothers, bat

they produce less milk. &evels of water-soluble vitamins, such as ascorbic acid,

thiamin, and vitamin ) p, are Duickly affected by deficient diets. '@rom a nutritional

perspective, infancy is a critical and vulnerable period. "t no other stage in life is a

single food adeDuate as a sole source of nutrition,' writes ;icciano.CC This results

from the immaturity of the tissues and organs involved in the metabolism of

nutrients, which limits the ability to respond to nutrition excesses and deficiencies.

5The system is species-specific and depends on the presence of the self-contained

enzymes and gancls to facilitate digestion at the proper stage while preserving

function #such as slg"$. 1t continues to facilitate absorption and utilization.

milk enzymes

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