Spectroelectrochemical study of biliverdin produced by the electrooxidation of bilirubin

Abstract

For a long time now bilirubin (BR) in mammals has attracted considerable attention. This is because its structure and metabolic process are closely connected to human health and also because it is involved in important aspects of the catabolic chemistry of heme. Hemoglobin generally is considered to be catabolized into heme first. The methyne (=CH-) bridge of heme is oxidized, and broken to release CO, resulting in biliverdin (BV). Under enzymatic action, it is reduced into BR[1,2]. BR is liposoluble in physiological conditions; in aqueous solution, including physiological conditions, the solubility is very small. Therefore the chemical properties of BR and its analogs are usually investigated in organic solvent. The electrooxidation process of BR was first studied by Van Norman. He proposed that BR can be oxidized to BV irreversibly in dimethylformamide (DMF) [3]. Later, Slifstein and Ariel [4], Moussa et al. [5] and Pradko et al. [6] studied the mechanism of the oxidation of BR into BV, using different electrochemical and spectroscopic techniques. It is known that BV can be reduced into BR by the action of BV reductase in the body. However, no reports on the mechanism of the electrochemical reduction process of BV have been published. In this note, the electroreduction process of BV produced by the electrooxidation of BR is investigated using spectroelectrochemical techniques. Rapid scan techniques are used to demonstrate that a reactive intermediate is involved in the process. Also, the rate constant of the subsequent chemical reaction is determined.