Allopurinol and oxypurinol are hydroxyl radical scavengers.

Allopurinol is a scavenger of the highly reactive hydroxyl radical (k2 approx. 10(9) M-1 X 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 (k2 approx. 4 X 10(9) M-1 X 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.