Xanthine oxidase. V. Differential inhibition of the reduction of various electron acceptors.

Milk xanthine oxidase can catalyze the reduction of oxygen, cytochrome c, nitrate, ferricyanide, and various quinones and dyes by many aldehydes and purines. When acting upon its substrates, this enzyme can also initiate the aerobic oxidation of sulfite (1, 2) and induce the chemiluminescence of lucigenin and of luminol (3, 4). The reduction of cytochrome c by milk xanthine oxidase has been the subject of conflicting reports. Thus, Horecker and Heppel (5) found that oxygen was essential to this reaction, whereas Morel1 (6) reported that cytochrome c reduction was faster in the absence of oxygen. The requirement for oxygen was confirmed by Weber, Lenhoff, and Kaplan (7) who proposed that hydrogen peroxide generated by the enzyme is the immediate reductant of cytochrome c. Mackler, Mahler, and Green (8) reported that inorganic phosphate is specifically required for the reduction of cytochrome c and that removal of molybdenum from the enzyme caused a loss of this activity, but other workers (9, 10) have been unable to repeat these findings. Continued interest in the pathways of electron transport within this complex enzyme prompted a systematic analysis of the influence of a series of inhibitors on the reduction of various electron acceptors by the enzyme in the presence of its substrates. The results of these studies permit an estimate of the minimal number of sites of electron egress from xanthine oxidase.