Allopurinol and oxypurinol are hydroxyl radical scavengers

Allopurinol is a scavenger of the highly reactive hydroxyl radical ( k 2 approx. 10 9 M −1 s −1 ). One product of attack of hydroxyl radical upon allopurinol is oxypurinol, which is a major metabolite of allopurinol. Oxypurinol is a better hydroxyl radical scavenger than is allopurinol ( k 2 approx. 4 × 10 9 M −1 s −1 ) and it also reacts with the myeloperoxidase‐derived oxidant hypochlorous acid. Hence the protective actions of allopurinol against reperfusion damage after hypoxia need not be entirely due to xanthine oxidase inhibition.