Small intestinal sucrase and isomaltase split the bond between glucosyl-C1 and the glycosyl oxygen.

The products of the hydrolysis of sucrose and palatinose by the sucrase-isomaltase complex from rabbit small intestine were investigated by persilylation followed by gas-liquid chromatography and mass spectrometry. If the hydrolysis is carried out in H218O, the heavy oxygen is found exclusively at the Ci of the alpha-glucopyranose formed. The 18O enrichment equals that of the incubation medium. The oxygen exchange between the monosaccharides and water is not accelerated by the sucrase-isomaltase complex. These observations show that the bond split by the sucarse and the isomaltase moiety of the complex is the one between glucosyl-Ci and the glucosyl oxygen. They agree with the mechanism proposed for these carbohydrases in the accompanying paper (Cogoli, A., and Semenza, G. (1975) J. Biol. Chem. 250, 7802-7809) involving the formation of an oxocarbonium ion.