Dietary carbohydrates include monosaccharides, disac-charides, and polysaccharides. Starch is a glucose-containing polysaccharide. Amylose, which constitutes 10 to 20% of dietary starch, is a long, straight chain of a-1, 4-glucosyl units. Amylopectin, which composes 80 to 90% of dietary starch, also has the straight chain, but with some a-1, 6-branching linkages. Glycogen is similar to amylopectin with more branching linkages. Amylopectin and amylose are of plant origin, whereas glycogen is of animal origin. Dietary fiber (nonstarch polysaccharides) is 50 to 80% cellulose, 20% hemicellulose, and 10 to 50% lignin.
Ingested starch is first attacked by salivary a-amylase in the mouth. Because the optimal pH of this enzyme is 6.7, its activity is inhibited by the acidic gastric juice when food enters the stomach. In the lumen of the small intestine, both the salivary and the pancreatic a-amylase act on starch. The hydrolytic products are a mixture of oligosaccharrides: maltose (disaccharide), maltotriose (trisaccharide), and a-dextrins. These products of luminal carbohydrate digestion cannot be absorbed by the mucosa, but must be further degraded into monosaccharides through mucosal (membranous) digestion. Specific car-bohydrases for mucosal digestion are produced by epithelial cells, bound to surface membrane, and transported to the tip of the brush border. Some of these membrane-bound enzymes have more than one substrate: a-dextrinase (isomaltase) and maltase hydrolyze maltose, maltotriose, and a-dextrins into glucose. Sucrase breaks down sucrose into glucose and fructose, as well as maltose and maltotriose into glucose. Lactase hydrolyzes lactose to glucose and galactose. Trehalase breaks down treha-lose, a a-1, 1-linked dimer of glucose, into two molecules of glucose.
Glucose, galactose, and fructose are absorbed by the mature enterocytes lining the upper third of the intestinal villi. Absorption takes place in the duodenum and jejunum and is usually complete before the chyme arrives at the ileum. Glucose and galactose are initially transported into the enterocyte against their concentration gradient by a Na+-dependent glucose transporter located in the apical brush border and then released into the blood by a facilitated sugar transporter (GLUT 2) located on the basolateral membrane. Fructose is passively absorbed by a Na+-independent brush border fructose transporter and then released out of the enterocyte into the blood by GLUT 2. Because of the simultaneous active transport of Na+, the absorption of glucose and galactose is very rapid and efficient compared with that of fructose, which is determined by its concentration gradient from gut to blood. The monosaccharides absorbed into the blood of intestinal capillaries are drained into the portal vein. In ruminants with well-developed rumen, most of ingested starch is digested and absorbed as fermentative products in the forestomach. Typically, very little monosaccharides are released and absorbed in the small intestine.
Protein digestion starts in the stomach and lasts one to two hours. Gastric mucosa secretes, in the form of inactive proenzymes, three proteases: pepsin A (pepsin II), pepsin B (parapepsin I), and gastricsin (pepsin I). The inactive pepsinogens and progastricsin are activated by gastric hydrochloric acid. Gastricsin and pepsins hydrolyze the bonds between aromatic amino acids and a second amino acid to yield polypeptides of very diverse size. Chymosin (rennin), a milk-clotting enzyme with limited general proteolytic activity, is found in the stomach of young animals. The gastric proteases have two pH optima, one at pH 2 and the other near 3.5. When the chyme enters the duodenum and mixes with the alkaline bile, pancreatic juice, and duodenal secretions, the pH of the chyme rises to 6.5. Because this pH is out of their optimal range, the proteolytic activity of the gastric proteases is thus terminated in the duodenum.
The macromolecular proteins and polypeptides formed by the gastric digestion are hydrolyzed in the lumen and on the mucosal surface of the small intestine. The pancreas secretes two groups of inactive proenzymes into the duodenum. One group acts at interior peptide bonds in the peptide molecules and is called endopep-tidases: trypsinogen, chymotrypsinogen, and proelastase. The other group hydrolyzes the amino acids at the carboxyl and amino ends of the polypeptides and is termed exopeptidases: procarboxypeptidases A and B. Trypsinogen is activated to trypsin in the duodenum by enterokinase, a duodenal brush border enzyme. The trypsin then activates more trypsinogen autocatalytically and the other pancreatic proenzymes. Luminal digestion produces amino acids and considerable amounts of oligo-peptides. Further hydrolysis of oligopeptides occurs at the mucosal membrane by a wide array of brush border and cytoplasmic peptidases in the enterocyte. Oligopep-tides of more than three amino acids are broken down extracellularly by brush border peptidases. Tripeptides and dipeptides are hydrolyzed by both brush border and cytoplasmic peptidases or are absorbed intact and transported into circulation.
The products of luminal and mucosal digestion of protein are transported across the brush border membrane and into the enterocytes by specialized transport mechanisms. There are two Na+-independent facilitated transport systems for neutral amino acids and cationic or basic amino acids. At least four different Na+-dependent active systems exist to transport most neutral amino acids, proline and hydroxyproline, phenylalanine and methionine, and two acidic amino acids. Cysteine and cystine are taken up by a different Na+-dependent system. A substantial amount of protein is absorbed into the enterocytes as intact dipeptides or tripeptides. The peptide transport process is distinct from transport systems for free amino acids. The absorbed amino acids are transported out of enterocytes by two Na+-dependent and three Na+-independent transport systems and enter the hepatic portal blood. Considerable amounts of dipeptides and tripeptides also enter the portal blood.
Dietary fats are composed primarily of water-insoluble triglycerides, some phospholipids, sterols, and sterol esters. Fat digestion is initiated in the stomach, where fats are warmed to body temperature, and subjected to intense mixing, agitating, and sieving action of the distal stomach. Fat globules are broken up into droplets and pass into the small intestine for further emulsification and eventual enzymatic hydrolysis. Although fat digestion is mainly dependent on pancreatic enzymes secreted into the duodenum, gastric lipase also plays a major role in the hydrolysis of triglycerides in the stomach of young animals.
Digestion of triglycerides in the lumen of the small intestine involves the emulsification of lipid droplets released from the stomach by bile acids and phospho-lipids, the hydrolysis of emulsified particles by the combined action of pancreatic lipase and colipase as well as cholesterol esterase and phospholipase, and the formation of micelles that are small water-soluble aggregations of bile acids and the end-products of lipid digestion (fatty acids, monoglycerides, and so forth). The micelles diffuse through the gut lumen to the brush border of the mucosal cells and allow the lipids to diffuse across the apical membrane of the enterocyte and into the cell. Inside the enterocyte, the long-chain monoglycerides are reesterified with the long-chain fatty acids to diglycerides, which are further esterified to triglycerides. The resynthesized triglycerides are then associated with cholesterol, cholesterol esters, phospholipids, and various apoproteins to form chylomicrons, which are in turn transported across the intestine and into the lymph. The short-chain fatty acids, however, pass directly into the portal blood without being esterified. In the fowl, the products of fat digestion are absorbed directly into the blood rather than the lymph in the mammal.
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