The mechanism of the reduction of cytochrome c by xanthine oxidase.

A number of substances such as oxygen, dyes, and nitrate can act as hydrogen acceptors for xanthine oxidase (1). It has now been found that cytochrome c is also reduced by this system. Since only one other enzyme has been isolated which reduces cytochrome c (2), it was of interest to study the reaction, particularly since Ball (3) has indicated that xanthine oxidase is a flavoprotein. The activity toward cytochrome c was found to parallel the oxygen activity over a 200-fold range of purification, with the same substances active in the reduction of cytochrome c as in the reaction with oxygen. Improvement of the purification procedure has resulted in a preparation of xanthine oxidase which has a Qo, 3 times higher than Ball’s best preparation, and a definite relation has been established between enzyme activity and flavin-adenine dinucleotide (FAD) content.