Unconjugated bilirubin inhibits the oxidation of human low density lipoprotein better than trolox

Oxidative modification of human low density lipoprotein (LDL) has been implicated in plaque formation in blood vessels leading to atherogenesis. Conversely, there is increasing evidence that prevention of LDL oxidation reduces the incidence of coronary artery disease. Here, we have compared the effect of unconjugated bilirubin (Bu) and Trolox (a vitamin E analogue) on the oxidation of LDL after treatment with Cu2+ under defined conditions. We observed that Bu, at or near the normal serum level (i.e. 17 microM) effectively inhibits oxidation of LDL, while it takes at least 500 microM Trolox to achieve a similar effect. This means that, on a per mole basis, Bu is > 20 times more effective than Trolox in preventing LDL oxidation. The oxidation of LDL was assessed by agarose gel electrophoresis. This was further corroborated by assaying the malondialdehyde formed upon reacting the presumptive peroxidation product(s) of LDL with thiobarbituric acid. Thus, we have directly verified that Bu and, less so, Trolox, can each prevent the oxidative damage of LDL in vitro. Our result supports the contention that Bu as an endogenous antioxidant can prevent LDL oxidation and hence reduce the risk of atherogenesis.