Trehalase breaks down trehalose to generate glucose. Lactase acts on the milk sugar lactose to release glucose and galactose. As the α-1,6 glycosidic bond is present only at branch points in α-limit dextrins, its hydrolysis by isomaltase results in debranching of α-limit dextrins after which maltase-glucoamylase and sucrase act on the resultant maltose and other linear malto-oligosaccharides to generate free glucose. The isomaltase component of the enzyme is selective for the α-1,6 glycosidic bond present in α-limit dextrins. 38 The sucrase component of the enzyme is responsible for the digestion of sucrose into glucose and fructose, and also for the digestion of maltose into glucose.

However, though the enzyme is initially synthesized as a single polypeptide that gets inserted into the BBM, it is subsequently cleaved by pancreatic proteases into 2 subunits, one with sucrase activity and the other with isomaltase activity. Sucrase-isomaltase is a bifunctional enzyme with 2 catalytic sites (i.e., sucrase and isomaltase) that reside in different parts of the same protein. Maltase-glucoamylase hydrolyzes maltose and malto-oligosaccharides to generate free glucose. At least 4 enzymes are involved in membrane digestion: maltase-glucoamylase, sucrase-isomaltase, lactase, and trehalase all of them are integral proteins in the BBM with their catalytic sites exposed to the luminal surface of the membrane so that their respective intraluminal substrates have access to the active site.

The products of the luminal digestion of starch and glycogen by salivary and pancreatic amylases, along with the disaccharides sucrose and lactose present in diet form the substrates for membrane digestion, which occurs on the external surface of the BBM of the intestinal absorptive cells ( Fig. Mark Feldman MD, in Sleisenger and Fordtran's Gastrointestinal and Liver Disease, 2021 Membrane Digestion

Hydrophobic amino acids in membrane