Bilirubin degradation by uncoupled cytochrome P450. Comparison with a chemical oxidation system and characterization of the products by high‐performance liquid chromatography/electrospray ionization mass spectrometry

Abstract

Bilirubin is a protective antioxidant; however, when its conjugation and excretion are impaired, as in neonatal and hereditary jaundice, bilirubin accumulates and may cause severe neurotoxicity. Degradation of bilirubin takes place (a) on interaction with oxidative free radicals and (b) when cytochrome P450 (CYP) enzymes are uncoupled by polyhalogenated substrate analogues. The products of pathways (a) and (b) above have now been characterized by high-performance liquid chromatography/electrospray ionization mass spectrometry (HPLC/ESI-MS) and the mechanisms of fragmentation in part clarified. Oxidation of bilirubin by uncoupled CYP1A5 and by a Fe-EDTA/H2O2 system produced both biliverdin and an identical profile of dipyrrolic fragments, as detected by positive ESI-MS. A similar profile of oxidation products was found from mesobilirubin, all showing the expected increase in mass, thus providing direct evidence for fragmentation at the central methene bridge of the tetrapyrroles. Two degradation products, also detected by negative ESI-MS, were characterized as dipyrroles retaining the central bridge carbon, with one or two oxygen atom(s) bound (probably as the aldehyde and hydroperoxide derivatives). Ions compatible with propentdyopents and bilifuscins were also detected, but here the assignment was less certain. It is concluded that the first step in the oxidation of bilirubin may be hydrogen abstraction at the central methene bridge. This is followed either by loss of another hydrogen to give biliverdin, or by oxygen binding and fragmentation. Fe-EDTA/H2O2 and uncoupled CYP(Fe=O) may both initiate the reaction, the latter in an attempt to reduce the ferryl oxygen to water. These studies shed light on the CYP uncoupling mechanism and are of potential significance for the therapy of severe jaundice.