Role of Xanthine Oxidase in the Reduction of Resazurin by Raw Milk

Summary The addition to raw milk of a variety of substrates for xanthine oxidase resulted in an almost immediate reduction of resazurin. When no substrate was added, doubling the concentration of xanthine oxidase in raw milk by the addition of purified enzyme did not cause a significant increase in the rate of resazurin reduction, confirming the conclusion that the unavailability of substrate limits the reducing activity of this enzyme. Milk from two cows which were very low in xanthine oxidase did show an increased reduction rate on the addition of the purified enzyme, but the final rate was still much too slow to have any bearing on the commercial grade of the milk. The pasteurization of milk destroyed the ability to reduce resazurin but had little influence on the activity of xanthine oxidase. The addition to milk of folic acid, a very effective competitive inhibitor for xanthine oxidase, failed to slow the rate of resazurin reduction signiflcantly. All of these factors point to the fact that xanthine oxidase does not play a significant role in the reduction of resazurin by raw milk.