31] Antioxidant activities of bile pigments: Biliverdin and bilirubin

Abstract

This chapter discusses the antioxidant activities of bile pigments—biliverdin and bilirubin. Biliverdin and bilirubin are potent scavengers of singlet oxygen. The methods described in the chapter apply to the measurement of the peroxyl radical scavenging activity of biliverdin and the various biological forms of bilirubin. Most of the methods are applicable to testing the antioxidant activity of other compounds. The method is based on the thermal decomposition of an azo compound under aerobic conditions whereby peroxyl radicals are produced at a known and constant rate. In the presence of phospholipids containing polyunsaturated fatty acids (LH), the peroxyl radical will abstract a hydrogen atom to give rise to a lipid radical, thus initiating the chain reaction of lipid peroxidation. Because the peroxyl radical-mediated oxidation of polyunsaturated fatty acids initially results in quantitative formation of the corresponding lipid hydroperoxides, the extent of oxidation can be followed simply by measuring the accumulation of hydroperoxides. Any compound possessing peroxyl radical scavenging activity will compete with LH for the lipid peroxyl radical, resulting in inhibition of the chain reaction and hence inhibition of formation of lipid hydroperoxides. The results show that both biliverdin and conjugated bilirubin efficiently scavenge hypochlorous acid and act synergistically with vitamin E in protecting lipid membranes from peroxidation initiated within the lipid phase, this add further credit to the notion that the two bile pigments function as important physiological antioxidants.