The sialylation of plasma lipoproteins

Sialic acids are a family of amino sugars that are commonly found as terminal oligosaccharide residues on glycoproteins and glycolipids. Plasma lipoproteins 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 clearance for some apolipoproteins. The sialic acid content of individual apolipoproteins can vary in response to physiological conditions while the sialic acid content of individual sialylated glycolipids (gangliosides) is constant. Thus, the sialic acid content of plasma lipoproteins can differ considerably as a result of (1) variations in the sialylation of their apolipoprotein constituents, (2) variations in their content of sialylated apolipoproteins 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 made between sialic acid and charge (very low density lipoprotein), lipoprotein solubility, receptor binding and uptake, and interactions with vascular matrix (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 oligosaccharides will aid in determining the enzymes responsible for their sialylation. Manipulations of the sialylation of apolipoproteins and of the quantity of apolipoproteins and gangliosides on lipoproteins will be useful methods in determining the role of lipoprotein sialic acid in the development of atherosclerosis.