Characterization of bilirubin inhibitory properties in free radical oxidation reactions

The rate constant of the reaction between bilirubin (BR), the end product of heme catabolism, and alkylperoxyl radical was estimated by the chemiluminescence method in the model reaction of ethyl benzene induced oxidation. The constant was determined to be 1.5.10(4) M-1.s-1. The data on the decrease in the absorbance at 450 mn (as a result of BR oxidation) give the same rate constant. A marked antioxidant action of BR, which was comparable to that of ionol (4-methyl-2,6-di-tert-butyl-phenol), was demonstrated in the methyl oleate oxidation model. The results indicate that each molecule of BR breaks more than two chains of oxidation. Oxidation of BR results in the formation of colourless reaction products without significant amounts of biliverdin being formed. BR oxidation products, hypothetically, dipyrrole fragments of the tetrapyrrole BR molecule, also demonstrate the radical trapping activity (the rate constant, 0.5.10(4) M-1.s-1). Our data show that BR is a very efficient peroxyl radical scavenger and can protect membrane lipids from oxidation by these radicals. It is suggested that elevation of the BR level in different physiological and pathological states can be considered to be a protective reaction.