physiology of digestion and absorption 2 professor john peters e-mail: [email protected]
TRANSCRIPT
After this lecture students, should be able to: Name the main molecular constituents of foodstuffs which can be digested in humans and
those which cannot be digested State how the small intestine is well adapted for absorption Explain how carbohydrate is digested to the monosaccharides, glucose, galactose and
fructose Appreciate how monosaccharides are transported into and out of enterocytes Provide an account of protein digestion noting the role of endo- and exo-peptidases Explain in outline how amino acids, dipeptides and tripeptides are transported into and out
of enterocytes State the problems posed by the digestion of fats and how these are overcome with bile
salts etc. State the events in the formation of small fat droplets and micelles Describe how free fatty acids and monoglycerides are absorbed in the small intestine Indicate how the absorption of free fatty acids and monoglycerides differs from that of
cholesterol Explain how chylomicrons are formed, transported and processed Explain how the absorption of Ca2+ and iron are regulated processes Outline the mechanisms that underlie the absorption of water- and fat- soluble vitamins Explain why the absorption of vitamin B12 is a special case that requires a complex series of
events
Learning Objectives
Main Constituents of Food
Carbohydrates – approx. 400 g per day - Starch (amylose and amylopectin – greater than 50% total
carbohydrate ingested) Cellulose (indigestible in humans - roughage) Glycogen Disaccharides (sucrose, lactose)
Lipids – approx. 25-160 g per day - Triacylglycerols (approximately 90% of total lipid ingested as fats
and oils) Phospholipids Cholesterol & cholesterol esters Free fatty acids Lipid vitamins
Proteins – approx. 70-100 g per day ingested, plus 35-200 g from endogenous sources e.g. digestive enzymes and dead cells from GI tract
The Small Intestine is Well Adapted for Absorption
Compared to a simple cylinder of identical dimensions surface area is increased by:
Circular folds Villi Microvilli (the brush border)
3-fold30-fold600-fold
Carbohydrate Digestion (1)
Mouth
Commences with salivary -amylase
Stomach
Continues with salivary -amylase
Small intestine (duodenum)
Pancreatic amylase (enzyme secreted and free in lumen)
Oligosaccharidases (associated with the brush border membrane of
enterocytes). Includes isomaltase and -glucosidase
Disaccharidases (associated with the brush border membrane of
enterocytes). Includes sucrase, lactase and maltase
Carbohydrate Digestion (2)Starch, glycogen
Amylase - attacks internal -1,4 glycosidic linkages (not -1,4 as in cellulose)
Maltose Maltotriose Isomaltose
Maltase(Cleaves -1,4 bond)Glucose
Isomaltase(Cleaves -1,6
bond)Glucose
Enterocyte Enterocyte
Carbohydrate Digestion (3)Sucrose
Sucrase(Cleaves -1,2 bond)
Glucose
Enterocyte
Lactose
FructoseLactase(Cleaves -1,4 bond)
Glucose
Enterocyte
Galactose
Nb. Deficiency in lactase causes the common condition lactose intolerance.
Absorption of the Final Products of Carbohydrate Digestion: Glucose, Galactose
and Fructose Glucose and galactose are absorbed by secondary active transport; fructose
by facilitated diffusion (occurs in duodenum and jejunum)
2K+
3 Na+
2 Na+
1 Glucose (or galactose)
H2O
Na+/K+ATPase
SGLT1
GLUT2
GLUT5
Glucose (or galactose)
FructoseFructose
Digestion of Proteins Stomach
HCl begins to denature proteins Pepsin cleaves proteins into peptides
Duodenum Pancreatic enzymes (trypsin, chymotrypsin) split peptide
bonds between different amino acids Brush border enzymes (aminopeptidase, carboxypeptidase,
or dipeptidase) cleave amino acid at ends of molecule, or hydrolyse dipeptide
Amino acids Dipeptides Tripeptides Oligopeptides (Some intact proteins – very few)
Final products
Protein Absorption
Passive diffusion Hydrophobic amino acids (e.g. tryptophan)
(Mostly by) active transport – against concentration gradient and also by facilitated transport in small intestine via: Brush border – at least 7 different mechanisms
o 5 are Na+-dependent co-transporters (secondary active transport)
o 2 are Na+ independent Basolateral border – at least 3 different mechanisms
o Na+ independent (facilitated transport)
Amino acids
Di- and tri-peptides via H+-dependent mechanism at brush border (co-transport)
Further hydrolysed to amino acids within the enterocyte Na+-independent systems at the basolateral border
(facilitated transport)
Na+
K+
Na+
K+
Na+Amino
acid
Aminoacid
Simplified Scheme for Amino Acid and Peptide Absorption
Secondary active transport
Na+/K+ATPase
Lumen
Interstitium
Na+
H+
H+
H+
Peptide
Peptide
Facilitated transport
Aminoacid
Hydrolysis
Aminoacid
Digestion of Lipids Mouth
Lingual lipase (little effect) Stomach
Gastric lipase (modest effect) Small intestine
Emulsification by bile Pancreatic lipase splits into fatty acids and monoglyceride
Ingested Lipids Fats / Oils – triacylglycerols (TAG) – 90 % of total Phospholipids Cholesterol and cholesterol esters Fatty acids
All are insoluble in water causing problems for digestion and absorption – only triacylglycerols are considered here.
Lipid digestion of TAG by lipases In Stomach
Heat and movements in stomach mix food with gastric lipase which begins digestion and forms an emulsion
In duodenum Pancreatic lipase - main lipid digestive enzyme
Aided by bile salts from gall bladder
HCO3- in pancreatic juice neutralises stomach acid - provides suitable pH for
optimal enzyme action
o Hydrolysis initially slow due to largely separate
aqueous/lipid interface
o As hydrolysis proceeds, rate increases due to
fatty acids produced acting as surfactants
breaking down lipid globules aiding
emulsification
o Emulsified fats ejected from stomach to
duodenum
H2CO C (CH2)16 CH3
O
HCO C (CH2)16 CH3
O
H2CO C (CH2)16 CH3
O
1
2
3
Triglyceride
O
Gastric lipase + H2O
H2CO C (CH2)16 CH3
HCO C (CH2)16 CH3
O
CH2OH
Diglyceride
CH3 (CH2)16 COOH
Free Fatty acid (stimulates CCK
release from duodenum and
secretion of pancreatic lipase)
+
Role of Bile Salts (1) Bile salts secreted in bile from the gall bladder in response to CCK act as
detergents to emulsify large lipid droplets to small droplets
Failure to secrete bile salts results in: Lipid malabsorption - steatorrhoea (fat in faeces) Secondary vitamin deficiency due to failure to absorb lipid vitamins
Bile salts are amphipathic -
Hydrophilic (projects from surface of droplet
NH
CH2O
O H
O HOH
COO-
Hydrophobic (adsorbs onto droplet)
Large fat droplet
NH
CH2O
OH
OHOH
COO
NH
CH2 O
OH
OH OH
COO
NH
C H2
O
OH
OH
OH
CO
O
N H
CH2
O
OH
OH
OH
CO
O
_
__
_
NH
CH2O
OH
OHOH
COO
NH
CH2 O
OH
OH OH
COO
NH
C H2
O
OH
OH
OH
CO
O
N H
CH2
O
OH
OH
OH
CO
O
_
__
_
NH
CH2O
OH
OHOH
COO
NH
CH2 O
OH
OH OH
COO
NH
C H2
O
OH
OH
OH
CO
O
N H
CH2
O
OH
OH
OH
CO
O
_
__
_NH
CH2O
OH
OHOH
COO
NH
CH2 O
OH
OH OH
COO
NH
C H2
O
OH
OH
OH
CO
O
N H
CH2
O
OH
OH
OH
CO
O
_
__
_
NH
CH2O
OH
OHOH
COO
NH
CH2 O
OH
OH OH
COO
NH
C H2
O
OH
OH
OH
CO
O
N H
CH2
O
OH
OH
OH
CO
O
_
__
_ NH
CH2O
OH
OHOH
COO
NH
CH2 O
OH
OH OH
COO
NH
C H2
O
OH
OH
OH
CO
O
N H
CH2
O
OH
OH
OH
CO
O
_
__
_
NH
CH2O
OH
OHOH
COO
NH
CH2 O
OH
OH OH
COO
NH
C H2
O
OH
OH
OH
CO
O
N H
CH2
O
OH
OH
OH
CO
O
_
__
_NH
CH2O
OH
OHOH
COO
NH
CH2 O
OH
OH OH
COO
NH
C H2
O
OH
OH
OH
CO
O
N H
CH2
O
OH
OH
OH
CO
O
_
__
_NH
CH2O
OH
OHOH
COO
NH
CH2 O
OH
OH OH
COO
NH
C H2
O
OH
OH
OH
CO
O
N H
CH2
O
OH
OH
OH
CO
O
_
__
_
Bile salts
Increased surface areafor action of lipase
Role of Bile Salts (2) Bile salts increase surface area for attack by pancreatic lipase, but block
access of the enzyme to the lipid with the hydrophobic core of the small droplets
Problem solved by colipase, an amphipathic polypeptide secreted with lipase by the pancreas – binds to bile salts and lipase allowing access by the latter to tri- and di-glycerides
Small droplet
Bile saltTriglyceride
Lipase
Colipase
Digestion by Pancreatic Lipase Produces 2-Monoglyceride and Free Fatty Acids
H2CO C (CH2)16 CH3
O
HCO C (CH2)16 CH3
O
H2CO C (CH2)16 CH3
O
1
2
3
Triglyceride
Pancreatic lipase + 2H2O
CH2OH
HCO C (CH2)16 CH3
O
CH2OH
2-monoglyceride
+
CH3 (CH2)16 COOH
CH3 (CH2)16 COOH
Free fatty acid
Free fatty acid
The Final Products of Lipid Digestion are Stored in, and Released From, Mixed
Micelles
Hydrophobic core
Fatty acid
Bile saltMonoglyceride
Cholesterol
Phospholipid
Lipid Absorption (1)
Transfer between mixed micelles and the apical membrane of enterocytes entering by the cell by passive diffusion
Free Fatty acids and monoglycerides
Fatty acidsMonoglycerides
Short chain (i.e. 6 carbon) and medium (i.e. 8-12 carbon ) fatty acids diffuse through the enterocyte, exit through the basolateral membrane and enter the villus capillaries
Long chain fatty (i.e. 12 carbon) fatty acids and monoglycerides are resynthesized to triglycerides in the endoplasmic reticulum and are subsequently incorporated into chylomicrons
Lipid Absorption (2) – Chylomicron Formation
Phospholipid synthesis
Apolipoprotein(ApoB-48)
Cholesterolesters
Central lacteal
Carried in lymph vessels to systemic
circulation (subclavian vein) via the thoracic
duct
Exocytosis
Monoglyceride
Free fatty acid
Triglyceride synthesis
Chylomicron
Nascentchylomicron
Endoplasmic reticulum
Lipid Absorption (3) – Chylomicron Processing
Chylomicron enters systemic circulation into the subclavian vein via the thoracic duct and distributed to tissues
Chylomicron triglyceride metabolised in capillaries (particularly muscle and adipose tissue) by lipoprotein lipase present on endothelial cells
Free fatty acids and glycerol released initially bind to albumen and are subsequently taken up by tissues
Remainder of chylomicron is a chylomicron remnant, enriched in phospholipids and cholesterol
Chylomicron remnant undergoes endocytosis by hepatocytes – cholesterol released to:o be storedo secreted unaltered in bileo oxidised to bile salts
Lipid Absorption (4) – Cholesterol Absorption
Once thought to be passive (similar to free fatty acids and monoglycerides)
Now appreciated to be mainly due to transport by endocytosis in clatherin coated pits by Niemann-Pick C1-like 1 (NPC1L1) protein
Ezetimibe binds to NPC1L1, prevents internalization, and thus cholesterol absorption. Used in conjunction with statins in hypercholesterolaemia
Absorption of Ca2+
Occurs by passive (i.e. paracellular; whole length of small intestine) and active (i.e. transcellular; mainly duodenum and upper jejunum) transport mechanisms
With [Ca2+] in chyme 5 mM absorption is mainly active
Active Ca2+ absorption is regulated by 1,25-dihydroxyvitamin D3 (calcitriol) and parathyroid hormone (increases 1,25-dihydroxyvitamin D3 synthesis)
Ca2+-ATPase (PMCA1) – expression increased by 1,25-dihydroxyvitamin D3
Sodium/calcium exchanger (NXC1)
Ca2+ channel (TRPV6) – expression increased by 1,25- dihydroxyvitamin D3
Ca2+
(high lumenal Ca2+)
Ca2+
(low lumenal Ca2+)
Ca2+-calbindin-D
Ca2+
Ca2+
3Na+
Ca2+
(high lumenal Ca2+)
Absorption of Iron Iron – important constituent
of haemoglobin, myoglobin, many enzymes
12-15 mg ingested daily – only 3-10 % absorbed (female more than male)
Divalent metal transporter 1 (DMT1)
Ferroportin (negatively regulated by the hormone hepcidin released from liver when body iron levels are high) – major control on iron absorption
Haem carrier protein 1
Haem
Fe3+Fe2+
Fe2+ Fe3+
(Vit C)
Haem oxidase
Fe2+
Apoferratin+
Ferratin(storage form of iron)
Fe2+
+Transferrin
Transferrin-Fe2+
e.g. haemoglobin synthesis
Absorption of Vitamin B12 (cobalamin) Present in minute amounts in the diet (5-15 g day – daily requirement
approximately 6 g per day, hence efficient and selective absorption required
Vitamin B12 ingested in food
Salivary glands secrete haptocorin
Stomach acid releases vitamin B12 from food
Haptocorin binds vitamin B12 released in stomach
Stomach parietal cells release intrinsic factor
Pancreatic proteases digest haptocorin in small intestine, vitamin B12 released
Vitamin B12 binds to intrinsic factor in small intestine
Vitamin B12-intrinsic factor complex absorbed in terminal ileum by endocytosis
Absorption of Vitamins
Fat soluble vitamins (i.e. A, D, E and K)
Incorporated into mixed micelles
Usually passively transported into enterocytes
Incorporated into chylomicrons, or VLDLs
Distributed by intestinal lymphatics
Water soluble vitamins (i.e. B vitamins (but not B12), C, H
o Vitamin C – the Na+-dependent vitamin C transporters (SVCT1 and 2)
Transport processes in the apical membrane are similar to those described for monosaccharides, amino acids and di- and tri-peptides
o Vitamin H – the Na+-dependent multivitamin transporter (SMVT)
For example:
o Vitamin B9 – the Na+-independent proton-coupled folate transporter 1; FOLT)