Page 271 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Digestion and Absorption of about eight different pepsins. At a pH of 2–5,
Carbohydrates and Protein these endopeptidases split off proteins at sites
where tyrosine or phenylalanine molecules
Carbohydrates provide half to two-thirds of are incorporated in the peptide chain. The pep-
the energy requirement (! p. 226). At least sins become inactive in the small intestine (pH
50% of dietary carbohydrates consist of starch 7–8). Pancreatic juice also contains pro-
(amylose and amylopectin), a polysaccharide; enzymes of other peptidases that are activated
other important dietary carbohydrates are in the duodenum (! p. 246). The endopep-
cane sugar (saccharose = sucrose) and milk tidases trypsin, chymotrypsin and elastase hy-
sugar (lactose). Carbohydrate digestion starts drolyze the protein molecules into short-chain
in the mouth (! A1 and p. 236). Ptyalin, an α- peptides. Carboxypeptidase A and B (from the
amylase found in saliva, breaks starches down pancreas) as well as dipeptidases and
Nutrition and Digestion This digestive process continues in the proxi- ing them down into tripeptides, dipeptides,
aminopeptidase (brush border enzymes) act on
into oligosaccharides (maltose, maltotriose, α
limit dextrins) in a neutral pH environment.
proteins at the end of the peptide chain, break-
and (mostly) individual amino acids. These
mal stomach, but is interrupted in the distal
the
are
absorbed
in
cleavage
products
stomach as the food is mixed with acidic gas-
tric juices. A pancreatic α-amylase, with a pH
duodenum and jejunum.
Amino acids (AA) are transported by a num-
optimum of 8 is mixed into the chyme in the
found in the kidneys (! p. 158). Neutral
continue to the final oligosaccharide stage
(without net charge) and anionic (“acid”) L-
mentioned above. The carbohydrates can be
10 duodenum. Thus, polysaccharide digestion can ber of specific carriers (! B2) similar to those
+
only absorbed in the form of monosaccharides. amino acids are transported with Na sym-
Thus, the enzymes maltase and isomaltase in- porters (secondary active transport; ! p. 28)
tegrated in the luminal brush border mem- from the intestinal lumen into mucosal cells,
brane of enterocytes break down maltose, mal- from which they passively diffuse with car-
totriose and α limit dextrins into glucose as the riers into the blood. Cationic (“basic”) L-amino
+
+
final product. As in the renal tubules acids such as L-arginine , L-lysine and L-or-
+
(! p. 158), glucose is first actively taken up by nithine are partly taken up into the entero-
+
+
the Na symport carrier SGLT1 into mucosal cytes by Na independent mechanisms, as the
cells (! A2, p. 29 B1) before passively diffusing membrane potential is a driving force for their
into the portal circulation via GLUT2, the glu- uptake. Anionic amino acids like L-glutamate –
–
cose uniport carrier (facilitated diffusion; and L-aspartate which, for the most part, are
! p. 22). The hydrolysis of saccharose, lactose, broken down in the mucosal cells, also have
+
+
and trehalose is catalyzed by other brush their own (Na and K dependent) carrier sys-
border enzymes: lactase, saccharase (sucrase) tems. Neutral amino acids use several different
and trehalase. In addition to glucose, these re- transporters.
actions release galactose (from lactose), which AA absorption disorders can be congenital and af-
is absorbed by the same carriers as glucose, fect various amino acid groups. These disorders are
and fructose, which crosses the enterocytes by often associated with defects of renal tubular reab-
passive uniporters, GLUT5 in the luminal and sorption (renal aminoaciduria, e.g. cystinuria).
GLUT2 in the basolateral membrane (! A2).
Dipeptides and tripeptides can be absorbed as
Lactase deficiency. Lactose cannot be broken down intact molecules by a symport carrier (PepT1).
+
and absorbed unless sufficient lactase is available. The carrier is driven by an H gradient (! B2),
Lactase deficiencies lead to diarrhea 1) because which in turn is generated by H secretion (ter-
+
water is retained in the intestinal lumen due to tiary active H -peptide symport, ! p. 29 B5).
+
osmotic mechanisms, and 2) because intestinal bac-
teria convert the lactose into toxic substances. Amino acids generally are much more rapidly
absorbed as dipeptides and tripeptides than as
Protein digestion starts in the stomach (! B1). free amino acids. Once they enter the cells, the
258 HCl in the stomach denatures proteins and peptides are hydrolyzed to free amino acids.
converts the three secreted pepsinogens into
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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