Electron spin resonance in enzyme chemistry: the mechanism of action of xanthine oxidase

Abstract

Since a comprehensive review (Beinert & Palmer 1965) of e. s. r. applications to enzyme chemistry cannot be attempted here, this discussion will be limited to a review of work on the mechanism of action of one enzyme, xanthine oxidase (Bray 1963), in the hope that this will serve to illustrate a number of points of more general interest. Clearly, the enzyme is an ideal object for study by e. s. r. It contains two transition metals and a coenzyme known to give rise to semiquinone free radicals on partial reduction. In addition, obviously its substrates could give rise to radicals as reaction intermediates. The enzyme is available in high purity (Hart & Bray, unpublished) in gramme quantities and its turnover time is of a magnitude such that full kinetic studies employing a relatively simple fast reaction technique (Bray 1964) may conveniently be carried out. In e. s. r. work on enzymes we are concerned with the nature of the species giving rise to signals and with the part the species play, along with other molecules not detectable by e. s. r. in the catalytic cycle. The aim is to characterize the individual reaction steps of the overall process.