Presence of CuZn superoxide dismutase in human serum lipoproteins

It has previously been demonstrated that CuZn-superoxide dismutase (SOD) is secreted by several human cell lines. This suggests that the circulating enzyme derives from both hemolysis and peripheral tissues as a result of cellular secretion. In the present report, we evaluated the presence of CuZn-SOD in human serum lipoproteins by both enzyme-linked immunosorbent assay and Western blot analysis of immunoprecipitated lipoprotein samples. The distribution of CuZn-SOD activity among the different lipoprotein fractions was also determined by the xanthine/xanthine oxidase method. The results demonstrated that CuZn-SOD is noticeably present in serum lipoproteins and mainly in low and high density lipoproteins (LDL and HDL). Moreover, experiments performed by incubating CuZn-SOD with a lipid emulsion and subsequent separation of the lipid fraction by ultracentrifugation showed that this enzyme associates in a saturable manner with lipids. The CuZn-SOD bound to LDL and HDL could exert a physiological protective role against oxidative damage of these lipoprotein classes that carry out a crucial role in the cholesterol transport.