Serum free bilirubin is a potential biomarker of endogenous antioxidant capacity of human vascular endothelium

Abstract

Bilirubin is a potent endogenous antioxidant with anti-inflammatory and anti-thrombotic effects. It is also in a negative correlation with the risk of cardiovascular disease, such as ischemic heart disease, hypertension, type II diabetes, metabolic syndrome and obesity. In our study, we first proved that bilirubin is not only in the serum, but also endogenously present in the endothelial cells of the vessels, which are crucial in maintaining the normal vascular function. With the development of new HPLC-Thermal Lens Spectroscopy analysis methods, we could quantitatively determine the low levels of bilirubin in endothelial cells. We found that intracellular contents of bilirubin can be increased by induction of genes for its synthesis (evidence that bilirubin is endogenous in endothelium) and/or if the content of bilirubin in the extracellular medium is increased. The intracellular antioxidative ability of bilirubin (EC50 = 11 nM) was also determined, which is in the concentration range of measured free bilirubin in serum (10–15 nM). This means that extracellular and intracellular free bilirubin concentrations are in dynamic equilibrium, providing an accessible biomarker for the assessment of the endothelial oxidative stress status.