The inhibition of xanthine oxidase by flavonoids and related compounds.

Investigations of the inhibition of the activity of xanthine oxidase in vitro have yielded a number of substances which have been reported to exert marked inhibitory actions at low concentrations. Among these are p-aminophenol in its oxidized form (l), pyrogallol (a), bromoacetophenone (3)) 6-pteridylaldehyde (4)) and 6-hydroxymethylpterin (5). An examination of the structures of these compounds reveals that they have as a common characteristic the possession of hydroxyl or carbonyl groups. These groups, at least in the case of the pteridines, appear to be necessary for the inhibitory action. It has been reported, for example, that blocking the aldehyde group in 6-pteridylaldehyde (4) causes a loss of inhibitory action. These facts suggested that compounds of the flavonoid type, which are rich in hydroxyl groups and are known to form quinonoid compounds in biological systems (6), might prove to be effective as inhibitors of xanthine oxidase.