Luminal-Phase Digestion of Carbohydrates Applies Only to Starches, Because Sugars Are Digested in the Membranous Phase
The enzyme involved in luminal starch digestion is α-amylase, which is actually a mixture of several similar molecules. This enzyme arises from the pancreas in all species, as well as from
the salivary glands of some species (see Chapter 29).
Ihe α∣ 1-4] linkages of either amylose or amylopectin are attacked by α-amylase. Characteristic of luminal-phase digestion» α-amylase does not break off, or cleave, single glucose units from the ends of the chain. Rather, the starch chains are broken in their midsections, resulting in the production of polysaccharides of intermediate chain length, known as dextrins. These chains continue to be attacked until disaccharide (maltose) and trisaccharide (maltotriose) units are formed.This digestive process proceeds for amylopectin in the same way as it does for amylose, except that the α[l-6] linkages at the chain branch points of amylopectin are not hydrolyzed. Because these branch points are not hydrolyzed, branch-chain oligosaccharides, known as limit dextrins, as well as an α[l-6]-linked disaccharide, known as isomaltose, are formed (see Figure 30-7). The end result of luminal-phase carbohydrate digestion is the creation of many disaccharides, trisaccharides, and oligosaccharides from large starch molecules. These complex sugars are not hydrolyzed further in the luminal phase.