ABSTRACT
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 carbohydrases for mucosal digestion are produced by epithelial cells, bound to surface membranes, 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 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 trehalose, an a-1,1-linked dimer of glucose, into two molecules of glucose.
