Structure and Role of Carbohydrate in Freezing Point-depressing Glycoproteins from an Antarctic Fish

The linkage between the galactose and the N-acetylgalactosamine of the disaccharide attached to every threonine of the freezing point-depressing glycoproteins from the antarctic fish, Trematomus borchgrevinki, has been previously suggested to be β, 1–3 or 1–4, rather than 1–6. This paper reports some colorimetric chemical evidence favoring a 1–3 linkage. Unsubstituted chromogen is formed from the breakdown of the N-acetylgalactosamine upon β elimination with 0.05 m Na2CO3, indicating a 1–3 linkage. The linkage was proven to be of the β type by the release of the terminal galactose when small glycopeptides, obtained by hydrolysis of the glycoproteins with elastate, were incubated with β-galactosidase (Escherichia coli). Enzymatic oxidation of the C-6 hydroxyls of the galactose and N-acetylgalactosamine to the aldehydes by galactose oxidase resulted in a fully active glycoprotein. The activity was lost, however, by the conversion of these newly formed C-6 aldehydes to negatively charged groups, either by oxidation to carboxyl groups with halogen or by formation of the bisulfite addition products.