Oxidative damage to biological macromolecules has been implicated in a number of diseases. Much interest has focused on how non-proteinaceous, low-molecular weight antioxidants prevent oxidative damage to lipids, while comparatively little is known about protein antioxidation. Here we show that bilirubin (BR), the end-product of heme catabolism, when bound to bovine serum albumin (BSA), is oxidised by hydroxyl ('OH), hydroperoxyl (HO2), and Superoxide anion (Oxxx 2) radicals to so far mostly uncharacterised products. The initial oxidation rates of BSA-bound BR decreased in the order OH ⪢ HO 2 ⪢ O−. 2 BR protected its carrier protein from oxidative damage inflicted by OH radicals. This protective action included a reduction in the OH-mediated cleavage of BSA, conversion of Trp into kynurenine and formation of ‘bityrosine-specific’ fluorescence. BR also strongly inhibited OH-mediated formation of protein carbonyls, whereas ascorbate and Trolox (a water-soluble analogue of vitamin E) were much less effective. These results support an antioxidant-protective function of BR and point towards significant differences in the efficacies of various antioxidants in the prevention of oxidative damage to lipids and proteins.
