Bilirubin as an antioxidant in micelles and lipid bilayers: Its contribution to the total antioxidant capacity of human blood plasma

The antioxidant capacities, antioxidant activities, kinh, and stoichiometric factors, n, of water-soluble derivatives of bilirubin (BR), BR-human serum albumin (BR-HSA), and BR-ditaurate disodium conjugate (BRC) were determined in aqueous/lipid dispersions of sodium dodecyl sulfate (SDS) micelles/methyl linoleate and in bilayers of dilinoleoylphosphatidylcholine (DLPC) during initiation by water-soluble azo-bis-amidinopropane dihydrochloride (ABAP). The inhibition rate constants for BRC and BR-HSA were similar in micelles (kinh ≈ 1.3 × 104 M− 1 s− 1), where n ≈ 2, whereas the kinh for BR-HSA dropped by ½ in bilayers. The dimethyl ester of bilirubin (BRDE) gave a kinh only one-tenth that of the vitamin E analog, pentamethylhydroxychroman (PMHC) in SDS micelles/methyl linoleate when initiated by lipid-soluble azo-bis-2,4-dimethylvaleronitrile (DMVN). Biliverdin hydrochloride (BVHCl) was NOT an effective peroxyl radical-trapping agent in the micellar phase during initiation by ABAP or DMVN containing methyl linoleate but it inhibited oxygen uptake in the aqueous phase. Both BRC and BR-HSA extended the total radical antioxidant parameter (TRAP) of human blood plasma and their contribution to TRAP was in the range of 5–10% of the natural TRAP of blood plasma, depending on the BR content determined in the blood plasma.