The Carbohydrate Moiety of Mung Bean Vicilin

Abstract

Mung bean (Phaseolus aureus Roxb.) vicilin, purified by DEAE-cellulose chromatography, is a glycoprotein (0.2% glucosamine and 1% mannose) which contains four different polypeptides with molecular weights of 63 500, 50000, 29 500 and 24 000 in the molar ratios 1 : 5 : 1 : 1. Two of these polypeptides (50 000 and 29 500) stain positively for carbohydrate. When vicilin is digested with pronase, the carbohydrate can be recovered as a glycopeptide containing mannose and glucosamine in the carbohydrate moiety and alanine, threonine and aspartic acid (or asparagine) in the peptide moiety. The total amount of carbohydrate (1.2 %) and the size of the carbohydrate unit (12 residues) suggest that each vicilin molecule contains only one carbohydrate side chain. Data indicate that this chain is attached via a glucosamine-asparagine link. The results raise the possibility that the presence of the two small polypeptides in vicilin preparations is the result of the breakdown of the major larger one. Indirect evidence suggests that vicilin may be a tetramer of four subunits, each with a molecular weight of 50 000, and that only one subunit has a carbohydrate side chain.