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The effects of the xanthine derivative drugs, nonsteroidal anti-inflammatories and allopurinal on the milk xanthine oxidase were studied at pH 8.2 pyrophosphate buffer, ´23.5 t; under aerobic condition. The results are summarized as follows;Caffeine, theobromine and theophylline, as the xanthine derivative drugs, are slightly effective on the oxidation of xanthine by the milk xanthine oxidase. Aspirin, diclofenac sodium, ibuprofen and naproxen, as the nonsteroidal anti-inflammatories, are a little effective on the oxidation of xanthine by the milk xanthine oxidase. It has been reported that allopurinol is quickly metabolized to oxypurinol by the xanthine oxidase, however allopurinol does not act as a substrate on the milk xanthine oxidase. Then, it has been reoported that allopurinol acts as a competitive inhibitor at low concentration and as a non-competitive inhibitor at high concentration, however allopurinol inhibits strongly milk xanthine oxidase from oxidizing xanthine and is seemed specific inhibition profile. This is thought to bring about suicide reaction. It seems that milk xanthine oxidase is inhibited by the excessive xanthine used as substrate. It can be known that the crude milk xanthine oxidase, not purified accurately, is also used in case of the studies on characters of the milk xanthine oxidase with the determination ´method of milk xanthine oxidse activity by HPLC. Then, this method is demonstrated as a practically useful means of the studies of enzyme characters of the milk xanthine oxidase.
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