Physiological Effects of Bilirubin: Protection from Protein Oxidation, Kidney Dysfunction and Regulation of Hepatic Lipid Metabolism

Abstract

Clinical evidence indicates that hyperbilirubinaemic individuals with Gilbert’s syndrome (GS) are at reduced risk of developing cardiovascular and chronic kidney disease. This thesis consists of five manuscripts which review and explore various mechanisms whereby unconjugated bilirubin (UCB) may prevent cardiovascular disease (CVD) and chronic kidney disease (CKD). The first study of this thesis followed and extended upon the candidate’s Master of Medical Research program. Forty-four age, gender and body mass index matched Gilbert’s syndrome (GS) and healthy controls were recruited and blood was analysed for lipid parameters and plasma antioxidants/oxidative stress status. Individuals with GS had elevated unconjugated bilirubin (UCB), reduced thiol and glutathione concentrations compared to controls. Oxidative stress biomarkers including oxidised glutathione, protein carbonyl and oxidised low-density lipoprotein concentration were significantly reduced in GS and were negatively correlated with UCB concentrations. To better characterise bilirubin’s ability to inhibit atherogenesis based upon its antioxidant capacity, study two investigated the susceptibility of plasma to myeloperoxidase induced oxidation was tested in hyperbilirubinaemic humans and rodents. Plasma with exogenous UCB supplementation (15.6-250 µM) inhibited HOCl (100 µM) and (100 nM)-hydrogen peroxide (H2O2; 50-100 µM) induced chloramine formation in a dose dependent manner. Chloramine formation was significantly reduced in GS plasma and Gunn rat serum and which was negatively correlated with UCB concentrations. Chloramine decomposition, including protein carbonyl and malondialdehyde formations were significantly reduced in a dose-dependent manner. This study suggested that UCB could inhibit protein and lipid oxidation induced by the formation of biologically relevant radicals/oxidants in vitro.