INHIBITION OF METAL-CATALYZED OXIDATION OF LOW-DENSITY-LIPOPROTEIN BYFREE AND ALBUMIN-BOUND BILIRUBIN

Both free and albumin-bound bilirubin are known to scavenge peroxyl ra dicals in vitro. In the present work we showed that free and albumin-b ound 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, les s apo B fragmentation and a decreased amount of cholesterol oxidation products as detected by gas chromatography. Free bilirubin (BR) was mo re effective in inhibiting the production of thiobarbituric acid react ive substances in iron-catalyzed LDL peroxidation as compared to the c opper-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 Cu2+-dependent oxidative stress only at very high protein to metal mo lar ratio, i.e., 30:1, that is similar to that in human plasma. Comple xation of BR to albumin brought about a marked increase in the capacit y 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 compl ete at a molar ratio of 1:2. The ability of albumin-BR complex to inhi bit 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 imp ortant, especially in iron loaded sera of hemochromatosis patients, wh ere the transferrin is fully saturated with this ion and the free iron could catalyze lipid peroxidation unless bound by a metal trapping de vice such as the albumin-BR complex.