Carbohydrates in protein. 6. Studies on the carbohydrate-peptide bond in hen's-egg albumin

Abstract

2-Acetamido-3,4,6-tri-O-acetyl-l-[beta]-([alpha]-beiizylN-benzyloxycaxbonyl-L-[beta]-aspartamido)-l,2-di-deoxy-D-glucose (Ib) has been prepared. Removal of the protective groups gave 2-acetamido-1-[beta]-(L-[beta]-aspartamido)-l,2-dideoxy-D-glucose (Ia) contaminated with another substance. The cleavage of the N-glycosidic bond in N-(L-[beta]-aspartyl)-D-glucosylamine with hydrogen bromide in acetic acid to give asparagine is described. Neither the analogous compound where the sugar moiety was N-acetyl-D-glucosamine nor the egg-albumin glycopeptide gave asparagine with this reagent. The stability of the glycopeptide was compared with that of N-(L-[beta]-aspartyl)-D-glucosylamine under acid and alkaline conditions and shown to be similar. Partial acid hydrolysis yielded material behaving electrophoretically and chromatographically like 2-acetamido-l-[beta]-(L-[beta]-aspartamido)-1,2-dideoxy-D-glucose. Analysis has indicated that the compound contains aspartic acid and glucosamine and negligible amounts of mannose. Evidence is presented showing that the carbohydrate[long dash]peptide bond in egg albumin consists of a linkage of the [beta]-carboxyl group of an aspartic acid residue with the 1-amino group of 2-acetamido-l-amino-l,2-dideoxyglucose. This has been compared and contrasted with carbohydrate-peptide bonds in other glycoproteins.