The sialylation of plasma lipoproteins

Sialic acids are a family of amino sugars that are commonly found as termin al oligosaccharide residues on glycoproteins and glycolipids. Plasma lipopr oteins are sialylated on their apolipoprotein and glycolipid constituents. The function of sialic acid on apolipoproteins is not completely understood but has been associated with secretion, lipid-binding, and plasma clearanc e for some apolipoproteins. The sialic acid content of individual apolipopr oteins can Vary in response to physiological conditions while the sialic ac id content of individual sialylated glycolipids (gangliosides) is constant. Thus, the sialic acid content of plasma lipoproteins can differ considerab ly as a result of (1) variations in the sialylation of their apolipoprotein constituents, (2) variations in their content of sialylated apolipoprotein s and gangliosides, and (3) modifications of the sialic acid on lipoprotein constituents while circulating in plasma. The significance of sialic acid on lipoproteins is not fully understood although associations have been mad e between sialic acid and charge (very low density lipoprotein), lipoprotei n solubility, receptor binding and uptake, and interactions with vascular m atrix (low density lipoprotein and Lp(a)) and with cholesterol efflux (high density lipoprotein). Further studies identifying sites of sialylation on apolipoproteins and characterizing the structures of sialylated oligosaccha rides will aid in determining the enzymes responsible for their sialylation . Manipulations of the sialylation of apolipoproteins and of the quantity o f apolipoproteins and gangliosides on lipoproteins will be useful methods i n determining the role of lipoprotein sialic acid in the development of ath erosclerosis. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.