The reduction of nicotinamide N-oxide by xanthine oxidase.

The following evidence has led to the conclusion that xanthine oxidase, with its usual electron donors, is capable of reducing nicotinamide N-oxide to nicotinamide. 1. The purest enzyme fraction from hog liver which catalyzes the reduction has the spectrum of a metalloflavoprotein and has xanthine oxidase activity. 2. The enzyme responsible for the reduction cofractionates with hog liver xanthine oxidase. 33 Milk xanthine oxidase also catalyzes the reduction of nicotinamide N-oxide. 4. The electron donors for the reduction are known substrates of xanthine oxidase. 5. Both milk and liver xanthine oxidases exhibit separate pH optima for the reductions dependent on reduced diphosphopyridine nucleotide and xanthine. 6. Xanthine oxidase activity and nicotinamide N-oxide reduction activity are destroyed by heat at the same rate. 7. Each activity decreases to the same extent in the livers of rats maintained on protein-deficient diets. 8. Xanthine oxidase activity and xanthine-dependent nicotinamide N-oxide reduction exhibit parallel inhibition by cyanide. It is proposed that xanthine oxidase might be responsible for other N-oxide reductions.