Enzymatic Oxidation of Bilirubin

The following agents were found to oxidize bilirubin in vitro: hemoglobin and horse-radish peroxidase (both with hydrogen peroxide), cytochrome c, xanthine oxidase, and an insoluble oxidase, present in brain and other tissues. Kinetic constants were determined. The process with hemoglobin was inhibited competitively by two product molecules. The insoluble oxidase from brain was present in mitochondria. The supernatant fraction contained an inhibitor. The oxidase was inactive in the absence of salt and was unspecifically activated by a number of salts, the activity depending upon ionic strength, irrespective of which ions were present. Reaction products included biliverdin and a yellow, diazo-negative, polar pigment with the same oxidation level as bilirubin.