Lecture 3-Digestion and Absorption of Macronutrients: page 5

Lecture 3 page 1 2 3 4 5

Brush Border Membrane Amino Acid Transport System

Table 2. Classification of amino acid transport systems
in brush-border membrane of the small intestine

Transport system	Substrates	Dependence on Na+ gradient	Involvement of other ions
B	Dipolar a-amino acids	Yes	None
B^0.+	Dipolar a-amino acids	Yes	None
	Basic amino acids
	Cystine
B^0.+	Dipolar a-amino acids	No	None
	Basic amino acids
	Cystine
Y⁺	Basic amino acids	No	None
IMINO	Imino acids	Yes	CI^-
P	p-Amino acids	Yes	CI^-
X-_AG	Acidic amino acids	Yes	K⁺
(From Ganapathy V, Brandsch M. Leibach FH. Intestinal transport of amino acids and peptides. In: Johnson LR, ed. Physiology of the Gastrointestinal Tract. 3^rd ed. New York: Raven Press; 1994:1773-1794.)

Basolateral-Membrane Transporters

Table 3. Classification of amino acid transport systems
in the basolateral membrane of the small intestine.

Transport system	Substrates	Dependence on Na+ gradient
A	Dipolar a-amino acids	Yes
	Imino acids
ASC	Three- and four-carbon dipolar amino acids	Yes
asc	Three- and four-carbon dipolar amino acids	No
L	Bulky, hydrophobic, dipolar amino acids	No
Y+	Basic amino acids	No
(From Ganapathy V, Brandsch M. Leibach FH. Intestinal transport of amino acids and peptides. In: Johnson LR, ed. Physiology of the Gastrointestinal Tract. 3^rd ed. New York: Raven Press; 1994:1773-1794.)

Table 4 Source and Action of Digestive Enzymes

Source of Secretion	Stimulus	Enzyme or Secretory Product	Action
Mouth Salivary glands (3 pair)	Psychic	Salivary amylase	All branched oligosaccharides and some maltose Glycogen Starch Dextrin
	Mechanical
	Chemical

Stomach Gastric glands (35,000,000)	Acetylcholine	HCI	Pespinogen-->pepsin
	Gastrin		Fe+++ --> Fe++
	Histamine		Swelling of proteins Antibacterial effect

	Lower Gastric pH	Mucus	Protective to mucosa
	Gastric irritants	Mucus	Protective to mucosa

	Vagal stim. Histamine	Pepsinogen (pepsin)	Hydrolyze peptide bonds between aromatic and dicarboxylic acids =>large polypeptides and amino acids

	Gastrin - continuous secretion, inc. with stim. to gastric secretion	Intrinsic factor	Essential for absoption of vitamin B12

Pancreas - Exocrine secretion	Secretin	Trypsinogen	Activated by enterokinase and by trypsin
	Cholecystokinin	Chymotrypsinogen	Activated by trypsin
	Acetylcholine	Procarboxypeptidase	Activated by trypsin
	Gastrin

		Endopeptidases -
		Trypsin	Protein and polypeptides -- small polypeptides
		Chymotrypsin	Protein and polypeptides -- small polypeptides

		Exopeptidases -
		carboxypeptidases A	Polypeptides with free carboxyl group -- lower peptides and aromatic amino acids
		carboxypeptidases B

		Elastase	Hydrolyzes fibrous proteins
		Collagenase	Hydrolyzes collagen
		Ribonuclease	Ribonucleic acid -- nucleotides
		Deoxyribonuclease	Deoxyribonucleic acid -- nucleotides
		a-Amylase	Starch -- dextrins and maltose
		Lipase (requires bile salts)	Fats -- monoacyglycerols, fatty acids, glycerol
		Phospholipase A	Lecithin -- lysolecithin (= removal of one fatty acid)
		Retinyl ester hydrolase	Hydrolyzes retinyl esters
		Cholesterol esterase (requires bile salts)	Free Cholesterol -- esters of cholesterol with fatty acids

Small intestine Most enzymes located in microvilli of mucosal cells	Gastrin	Aminopeptidases	Polypeptides with free amino group --> lower peptides and free amino acids
	Secretin
	Cholecystokinin	Dipeptidases	Dipeptides --> amino acids
	Vasoactive inhibitory peptide	Nucleotidase	Nucleotides --> nucleosides and H₃PO₄
	Vasoactive inhibitory peptide	Nucleosidase	Nucleosides --> purines, pyrimidines, and pentose
	Gastric inhibitory peptide	Alkaline Phosphatase	Organic phosphates --> free phosphates
	Gastric inhibitory peptide	Monoglyceride lipase	Monoglycerides -->fatty acids and glycerol
	Calcitonin	Lecithinase	Lecithin --> fatty acids, glycerol, phosphoric acid, and choline
	Glucagon	Disaccharidases
		Sucrase	Sucrose --> glucose and fructose
		Maltase	Maltose --> glucose
		Lactase	Lactose --> glucose and galactose

Gallbladder/Liver	Secretin	Bile	Emulsifies fat
	Cholecystokinin		Stabilizes emulsions
	Hepatocrinin		Neutralizes acid chyme
Source: Nutrition, An Integrated Approach, Pike and Brown, Macmillan, New York pp. 214-216

Top of page

Lecture 1:Introduction to Nutrition in Western Civilization
Lecture 2: Dietary Macronutrients, Body Fat, and Blood Lipids
Lecture 3:Digestion and Absorption of Macronutrients
Lecture 4:Basic Principles of Nutrient Metabolism
Lecture 5:Obesity
Lecture 6:Fuel Utilization During Exercise
Lecture 7:Biochemistry of Oxidant Stress in Health and Disease Antioxidants
Lecture 8:Nutrition for the 21st Century

Nutrition 101 - Natural Remedies - Weight Management - Physician Education
Vitamins & Minerals- Nutrition & Aging - Nutrition & Disease - Research