Inhibition of metal-catalyzed oxidation of low-density lipoprotein by free and albumin-bound bilirubin.

Abstract Both free and albumin-bound bilirubin are known to scavenge peroxyl radicals in vitro. In the present work we showed that free and albumin-bound bilirubin at the physiological concentration of the bile pigment in blood plasma could greatly inhibit the metal-catalyzed oxidation of low density lipoprotein (LDL) as shown by the reduced thiobarbituric acid reactivity, smaller or no shifts in electrophoretic mobility, less apo B fragmentation and a decreased amount of cholesterol oxidation products as detected by gas chromatography. Free bilirubin (BR) was more effective in inhibiting the production of thiobarbituric acid reactive substances in iron-catalyzed LDL peroxidation as compared to the copper-catalyzed reaction up to a BR to metal molar ratio of 4:1. Above this ratio the same degree of inhibition was observed for both metal ions. It was found that serum albumin provided full protection against Cu 2+ -dependent oxidative stress only at very high protein to metal molar ratio, i.e., 30:1, that is similar to that in human plasma. Complexation of BR to albumin brought about a marked increase in the capacity of the complex to bind metal ions, particularly iron, as opposed to albumin alone. At a molar ratio of metal ion to albumin-BR of 1:1 the inhibition of lipid peroxidation was about 96% and it was almost complete at a molar ratio of 1:2. The ability of albumin-BR complex to inhibit effectively the transition metals-dependent oxidative stress could be important in the extravascular space where local concentrations of metal ions may exceed the protein binding capacity. In addition, the strong binding of iron to the albumin-BR complex may be clinically important, especially in iron loaded sera of hemochromatosis patients, where the transferrin is fully saturated with this ion and the free iron could catalyze lipid peroxidation unless bound by a metal trapping device such as the albumin-BR complex.