Human intestinal sucrase-isomaltase. Identification of free sucrase and isomaltase and cleavage of the hybrid into active distinct subunits

Abstract

Sucrase-isomaltase complex and its functional subunits have been identified in homogenates of human small intestinal mucosa by use of Sephadex G-200 (superfine) chromatography aided by affinity of the isomaltase moiety for the dextran gel. The isomaltase subunit binds strongly to the gel at 4 degrees, and is eluted only after 2 column volumes; earlier recovery as a sharp peak can be achieved by raising column temperature to 37 degrees after elution of other proteins. Bio-Gel P-300 chromatography, density gradient, and equilibrium centrifugation demonstrated that the sucrase subunit (Stokes radius = 45 A, frictional ratio = 1.32, s20,w = 6.9, MW = 130,000) and the isomaltase subunit (Stokes radius = 45 A, frictional ratio = 1.30, s20,w = 6.6, MW = 120,000) are similar but unequal in size. The sucrase-isomaltase complex (Stokes radius = 70 A, frictional ratio = 1.61, s20,w = 9.8, MW = 280,000), appears to be an elongated hybrid molecule that is less symmetrical than either of itt subunits. Apparent Km and pH activity curves were indistinguishable for each enzyme whether present in the hybrid or in the free state. The sucrase-isomaltase complex, accounting for approximately 90 percent of native intestinal sucrase and isomaltase activities, was isolated and cleaved by 0.01 M beta-mercaptoethanol/6 M urea treatment into active sucrase and isomaltase subunits having biochemical characteristics identical with those of the free native moieties. Sodium dodecyl sulfate acrylamide gell electrophoresis of the complex also produced subunits having molecular weights very close to those for the active free sucrase and isomaltase moieties, indicating that each alpha-glucosidase appears to consist of a single polypeptide chain. Immunization of rabbits with pure sucrase-isomaltase complex yielded a monospecific precipitating antibody that reacted with the hybrid and the sucrase subunit, but had minimal affinity for the isomaltase subunit, providing further evidence that the sucrase-isomaltase molecule is a hybrid consisting of two distinct alpha-glucosidases.