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

A C Boon; C L Hawkins; J S Coombes; K H Wagner; A C Bulmer

doi:10.1016/j.freeradbiomed.2015.05.031

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

Free Radic Biol Med. 2015 Sep:86:259-68. doi: 10.1016/j.freeradbiomed.2015.05.031. Epub 2015 Jun 6.

Authors

A C Boon¹, C L Hawkins², J S Coombes³, K H Wagner⁴, A C Bulmer⁵

Affiliations

¹ Heart Foundation Research Centre, Menzies Health Institute Queensland, Griffith University, Gold Coast, QLD 4222, Australia. Electronic address: c.boon@griffith.edu.au.
² Heart Research Institute, Newtown, Sydney, NSW 2042, Australia; Sydney Medical School, University of Sydney, Sydney, NSW 2006, Australia.
³ School of Human Movement Studies, University of Queensland, St Lucia, QLD 4072, Australia.
⁴ Department of Nutritional Science, University of Vienna, Vienna 1090, Austria.
⁵ Heart Foundation Research Centre, Menzies Health Institute Queensland, Griffith University, Gold Coast, QLD 4222, Australia. Electronic address: a.bulmer@griffith.edu.au.

PMID: 26057938
DOI: 10.1016/j.freeradbiomed.2015.05.031

Abstract

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) are 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 (≤250µM) 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 MPO/H2O2/Cl(-)-induced chloramine formation. Albumin-bound UCB efficiently and specifically (3.9-125µM; P<0.05) scavenged taurine, glycine, and N-α-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 50µM, 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.

Keywords: Antioxidants; Bile pigments; Chloramines; Free radicals; Heme oxygenase; Myeloperoxidase; Protein oxidation.

MeSH terms

Animals
Bilirubin / pharmacology
Bilirubin / physiology*
Case-Control Studies
Chloramines / metabolism*
Female
Gilbert Disease / blood*
Gilbert Disease / enzymology
Lipid Peroxidation
Male
Malondialdehyde / metabolism
Peroxidase / physiology*
Protective Factors
Rats, Gunn

Substances

Chloramines
Malondialdehyde
Peroxidase
Bilirubin