Partial amino acid sequences around the essential carboxylate in the active sites of the intestinal sucrase-isomaltase complex.

A carboxylate group occurs in each of the active sites of the intestinal sucrase-isomaltase complex and can be labeled selectively by [3H]conduritol-B-epoxide (Quaroni, A., Gershon, E., and Semenza, G. (1974) J. Biol. Chem. 249, 6424-6433). After peptic digestion of the labeled, denatured, reduced, and cyanoethylated enzyme three radioactive peptides were isolated. The peptide originating from the sucrase subunit had the following sequence: Ile-Asp-Met-Asn-Gln-Pro-Asn-Ser-Ser; the other two, deriving from the isomaltase subunit, had the sequences: Asn-Gly2-Gln-Ile-Asp-Met and Ile-Asp-Met. The radioactive label was in each case bound to the beta carboxyl group of an aspartic acid residue.