Bilirubin scavenges chloramines and inhibits myeloperoxidase-induced protein/lipid oxidation in physiologically relevant hyperbilirubinemic serum

Hypochlorous acid (HOCl), an oxidant produced by myeloperoxidase (MPO), induces protein and lipid oxidation, which is implicated in the pathogenesis of atherosclerosis. Individuals with mildly elevated bilirubin concentrations (i.e., Gilbert syndrome; GS) arc protected from atherosclerosis, cardiovascular disease, and related mortality. We aimed to investigate whether exogenous/endogenous unconjugated bilirubin (UCB), at physiological concentrations, can protect proteins/lipids from oxidation induced by reagent and enzymatically generated HOCl. Serum/plasma samples supplemented with exogenous UCB (<= 250 mu M) were assessed for their susceptibility to HOCl and MPO/H2O2/Cl- oxidation, by measuring chloramine, protein carbonyl, and malondialdehyde (MDA) formation. Serum/plasma samples from hyperbilirubinemic Gunn rats and humans with GS were also exposed to MPO/H2O2/Cl- to: (1) validate in vitro data and (2) determine the relevance of endogenously elevated UCB in preventing protein and lipid oxidation. Exogenous UCB dose dependently (P<0.05) inhibited HOCl and H2O2/Cl--induced chloramine formation. Albumin bound UCB efficiently and specifically (3.9-125 mu M; P<0.05) scavenged taurine, glycine, and N-alpha-acetyllysine chloramines. These results were translated into Gunn rat and GS serum/plasma, which showed significantly (P<0.01) reduced chloramine formation after MPO-induced oxidation. Protein carbonyl and MDA formation was also reduced after MPO oxidation in plasma supplemented with UCB (P<0.05; 25 and 50 mu M, respectively). Significant inhibition of protein and lipid oxidation was demonstrated within the physiological range of UCB, providing a hypothetical link to protection from atherosclerosis in hyperbilirubinemic individuals. These data demonstrate a novel and physiologically relevant mechanism whereby UCB could inhibit protein and lipid modification by quenching chloramines induced by MPO-induced HOCl. (C) 2015 Elsevier Inc. All rights reserved.