Phosvitin, a phosphoglycoprotein: Composition and partial structure of carbohydrate moiety

Abstract

The composition and partial structure of the carbohydrate prosthetic group of phosvitin have been investigated, using a glycopeptide preparation obtained from the pronase digest of the parent protein. The composition of the carbohydrate unit, as expressed in nearest integers, is 3 moles of mannose, 3 of galactose, 5 of N -acetyl-glucosamine, and 2 of sialic acid per mole. The structure of the oligosaccharide moiety of the glycopeptide was studied by sequential enzymic hydrolysis using neuraminidase, β -galactosidase, β-N -acetylglucosaminidase, and α -mannosidase and sequential Smith degration. Two moles of sialic acid followed by 2.4 moles of galactose were released per mole of glycopeptide when it was treated sequentially with neuraminidase and β -galactosidase. On treatment of the intact glycopeptide with β -galactosidase, 1 mole of galactose was liberated, thus showing that the carbohydrate has a branched structure in which two sialic acids and one galactose are terminal residues. In the first cycle of the sequential Smith degradation, one N -acetylglucosamine residue was oxidized and removed, in addition to 2 moles of sialic acid and 3 moles of hexose. This is followed by oxidation of 2 N -acetylglucosamines (2nd cycle of Smith degradation) and one residue each of mannose and N -acetylglucosamine (3rd cycle of Smith degradation). From these pieces of evidence and those of the enzymic studies a tentative partial structure is proposed for the carbohydrate moiety of phosvitin.