Inhibitory activity of flavonols toward the xanthine oxidase enzyme

The spectrophotometric methods of analysis adopted for the study of the inhibition of the enzyme, xanthine oxidase, are very different as regards the time required for the assay. With a view to studying the inhibitory effect of flavonoids, we examined the stability and the inhibitory activity of quercetin and myricetin in relation to environment and time. The results show that, in a buffer solution of pH 7.6 at 20-degrees-C, these flavonols undergo transformation in accordance with the environment and that their transformation products have a lower inhibitory capacity towards xanthine oxidase. The method that assesses the initial rate of formation of uric acid is therefore that which affords the most reliable results regarding the activity of flavonols; accordingly, this was the method used to determine the kinetic parameters of the enzymatic inhibition exerted by quercetin and myricetin.