Study of the glycosylation of apolipoprotein H

Abstract

Apolipoprotein H is a single chain polypeptide composed of 326 amino acids highly glycosylated. Its carbohydrate content is approximately 19% of the molecular weight. We show that it is rich in sialic acid linked α(2–6) to galactose or N-acetylgalactosamine. Sialic acid is not α(2–3) linked to galactose. Galactose is β(1–4) linked to N-acetylglucosamine and β(1–3) linked to N-acetylgalactosamine. Carbohydrate O-linked chains (mainly sialic acid) are α(2–6) linked to galactose or N-acetylgalactosamine. Galactose is also organised in O-linked chains and β(1–4) linked to N-acetylglucosamine and β(1–3) linked to acetylgalactosamine. Concanavalin A lectin was used to isolate two groups of apolipoprotein H molecules bearing biantennary and truncated hybrids and high mannose and hybrid oligosaccharides. Apolipoprotein H fails to bind lysine–Sepharose. Our results thus show that it presents truncated hybrid or hybrid-type carbohydrate chains which bear few unmasked mannose residues as a terminal sugar. Biochemical analysis of carbohydrate structures conducted on single isoforms separated through IEF revealed that no specific carbohydrate complex is bound to a single isoform.