Albumin-bound bilirubin interacts with nitric oxide by a redox mechanism

Bilirubin, the final product of heme catabolism, plays a crucial role in the cellular defense against oxidative and nitrosative stress. This study investigated the interaction of albumin-bound bilirubin, the circulating form of the bile pigment, with nitric oxide (NO), a gaseous modulator involved in many physiological functions but able to induce cytotoxicity and cell death if produced in excess. A short-lived endogenous S-nitrosothiol such as S-nitroso-cysteine was used as NO donor. In PBS without chelators, bilirubin was bound to human serum albumin with an apparent affinity of 1.6 +/- 0.2 microM (n = 4). Furthermore, albumin (2-20 microM) dose-dependently increased the half-life of BR (10 microM) exposed to S-nitroso-cysteine (100 microM) of 2.4 +/- 0.4 times (n = 4). Albumin-bound bilirubin was almost completely oxidized by S-nitroso-cysteine-derived NO, and biliverdin was the major product formed; this reaction seemed to be rather specific for albumin-bound bilirubin because when free bilirubin was reacted with S-nitroso-cysteine the formation of biliverdin was significantly lower. Uric acid and reduced glutathione, two well-known plasma antioxidants, at physiological concentrations protected albumin-bound bilirubin from NO-mediated oxidation. Taken together, these data suggest that albumin-bound bilirubin maintains its ability to interact with NO also in the bloodstream counteracting extracellular nitrosative reactions.