Interactions between bilirubin and reactive oxygen species in liver microsomes and in human neutrophil granulocytes

Bilirubin protects polyunsaturated fatty acids from lipid peroxidation, thus preventing damage by reactive oxygen species to cell membranes and proteins. On the other hand, such reactive oxygen species may contribute to the degradation and elimination of bilirubin. We therefore examined the interactions between bilirubin and reactive oxygen species. Bilirubin is decomposed in microsomes via a NADPH-independent process. This reaction appears to be mediated by H2O2 or by the hydroxyl radical since it is stimulated by exogenous H2O2 and by cytochrome P450 inducers, which increase H2O2 production in microsomes, and is inhibited by the hydroxyl radical scavenger sodium benzoate. These results suggest that cytochrome P450 may act as a peroxidase or as a Fenton catalyst in bilirubin degradation. On the other hand, bilirubin inhibits the NADPH consumption of microsomes as well as the NADPH oxidase activity of human neutrophil granulocytes and the resulting superoxide formation in these cells. This effect on superoxide concentration may be partially due to direct interaction between superoxide and bilirubin, since bilirubin reduces the superoxide concentration in a xanthine oxidase system. Bilirubin degradation is inhibited by superoxide dismutase suggesting that bilirubin may be oxidized in this system by the superoxide radical. The bilirubin-induced reduction in superoxide concentration in the supernatant of granulocytes suggests that hyperbilirubinemia may compromise immune function.