Kinetic studies of electron transport reactions at low temperatures in xanthine oxidase.

Samples of rapidly frozen xanthine oxidase reduced with xanthine have been warmed between ?78°C and ?50°C. EPR measurements of oxidation — reduction processes at these temperatures have revealed a new EPR signal which appears to be a disulfide radical involved in xanthine hydrolysis. Other EPR signal changes indicate that at pH 6.5 enzyme reduction by xanthine is rate limiting and at pH 8.5 or higher that some step following enzyme reduction is rate limiting. Evidence is presented for the lack of anaerobicity in most rapid freeze apparatus, the oxygen entering the samples during rapid freeze quenching in isopentane.