Human chorionic gonadotropin. II. Nature of the carbohydrate units.

Abstract

Abstract Human chorionic gonadotropin (HCG) was desialyzed by mild acid hydrolysis, reduced with mercaptoethanol, and carboxamidomethylated by iodoacetamide. The resulting alkylated product was subjected to tryptic hydrolysis. Two chromatographically homogeneous glycopeptides were obtained by gel filtration on Sephadex G-50 and ion exchange chromatography on Dowex 1. The carbohydrate composition of these glycopeptides was found to be Fuc1, Gal4, Man5, GluNAc6 and Gal2, Man5, GluNAc5. The sequence of monosaccharides in the carbohydrate chains of these highly complex units was determined by stepwise cleavage of the sugars from the nonreducing ends with specific glycosidases, such as β-galactosidase, β-N-acetylglucosaminidase, α-mannosidase, and α-l-fucosidase. HCG and desialyzed HCG were also studied with these glycosidases and neuraminidase. The carbohydrate chains in the hormone, as shown by this investigation, have the following monosaccharide sequence: N-acetylneuraminic acid [see PDF for sequence], sialic acid or fucose being at the nonreducing ends of the chains. The carbohydrate unit of each of the above glycopeptides was composed of several chains and was probably linked to the polypeptide chains by an N-acetylglucosaminyl-asparagine linkage. Preliminary studies on the alkaline degradation of HCG suggested that all galactosamine residues were linked to the polypeptide chain by N-acetylgalactosaminyl-serine linkages. Based on these studies, the tentative structures of the carbohydrate units have been proposed.