Oligosaccharide sequences of endothelial cell surface heparan sulfate proteoglycan with affinity for lipoprotein lipase.

Abstract

Lipoprotein lipase (LpL) catalyzes the hydrolysis of triglycerides in plasma lipoproteins at the luminal surface of the vascular endothelium. This enzyme is bound via electrostatic interactions to heparan sulfate (HS). The specific endothelial cell surface HS oligosaccharide sequences that are necessary for binding of LpL to HS have not been characterized. To identify this LpL-binding oligosaccharide sequence, oligosaccharides were isolated from bovine aortic endothelial cell-derived HS and assessed for LpL binding properties. Endothelial HS chains that were isolated from endothelial total cell-associated proteoglycans were deacetylated by complete hydrazinolysis, cleaved with nitrous acid (pH 4.5), and reduced with [3H]NaBH4. The resulting fragments composed of N-sulfated glucosamine-rich oligosaccharides terminating with [3H]2,5-anhydromannitol (AManR) were chromatographed on a LpL-Sepharose column. A high affinity decasaccharide was isolated and characterized. Disaccharide analysis of this decasaccharide indicated that it yielded only the disaccharide IdceA(2-SO4)-->AManR(6-SO4) on treatment with nitrous acid at low pH. Therefore, the sequence of the LpL-binding decasaccharide is [IdceA(2-SO4) alpha 1-4GlcNSO4(6-S0(4)) alpha 1-4]4-IdceA(2-SO4) alpha 1-4AManR(6-SO4) and is distinct from those that bind antithrombin and basic fibroblast growth factor. Partial depolymerization of endothelial HS chains with hydrazine/high pH nitrous acid treatment gave rise to lipase-binding oligosaccharides larger than decasaccharide. However, further complete depolymerization of these oligosaccharides resulted in only a high affinity decasaccharide composed of repeating disaccharide units of [IdceA(2-SO4) alpha 1-4GlcNSO4(6-S0(4))]. These results indicate that the decasaccharide is the active fragment that binds to LpL with high affinity. Molecular modeling studies of the decasaccharide indicate that it presents a linear array of negatively charged sulfate groups that may adopt a favorable disposition to bind to peptide region(s) comprised of basic amino acid residues of LpL with high affinity.