Human plasma trans-sialidase causes atherogenic modification of low density lipoprotein.

In earlier studies we have found that incubation of low density lipoprotein (LDL) with autologous blood plasma-derived serum leads to a loss of sialic acid from lipoprotein particles. In this study we demonstrated that sialic acid removed from LDL was transferred to glycoconjugates of lipoproteins, glycoproteins and sphingolipids of human serum. This showed that human serum contained the trans-sialidase activity. Gel-filtration chromatography of human blood serum demonstrated the presence of trans-sialidase activity in lipoprotein subfractions as well as in lipoprotein-deficient serum. Trans-sialidase (about 65 kDa) was isolated from lipoprotein-deficient serum using affinity chromatography carried out with Neu5Acalpha2-8Neu5Ac-sepharose FF-6. Optimal pH values for the trans-sialidase were 3.0, 5.0 and 7.0. Calcium and magnesium ions stimulated the enzyme activity at millimolar concentrations. Isolated enzyme can remove sialic acid from LDL, IDL, VLDL, and HDL particles (in decreasing rate order). Serum trans-sialidase transferred sialic acid from glycoconjugates of plasma proteins (fetuin, transferrin) and gangliosides (GM3, GD3, GM1, GD1a, GD1b). Sialylated glycoconjugates of human blood erythrocytes also served as substrate for serum trans-sialidase. We have found that sialic acid can also be removed from N- and O-linked glycans, sialylated Le(x) and Le(a), oligosialic acids, and sphingolipid carbohydrate chains. The rate of sialic acid release decreased in the following order: alpha2,6>alpha2,3>>alpha2,8. Transferred molecule of sialic acid can form alpha2,6, alpha2,3 and to a lesser degree alpha2,8 linkage with galactose, N-acetyl-galactosamine or sialic acid of acceptors. The glycoconjugates of erythrocytes, lipoprotein particles, plasma proteins, neutral sphingolipids and gangliosides may serve as acceptors of transferred sialic acid. Trans-sialidase-treated native LDL becomes desialylated and then can induce cholesteryl ester accumulation in human aortic intimal smooth muscle cells. Thus, trans-sialidase may be involved in the early stages of atherogenesis characterized by foam cell formation.