Inhibitory effects of methylxanthines on the activity of xanthine oxidase

The $\text{in vitro}$ and $\text{in vivo}$ effects of three methylxanthines caffeine, theophylline and theobromine on the activity of the enzyme xanthine oxidase (EC 1.2.3.2.) was investigated with a view to understand their biochemical action. The studies revealed all the three methylxanthines to be inhibitors of the milk xanthine oxidase activity and the inhibition was found to be competitive in nature. The preincubation studies indicated a greater inhibition of the enzyme with the methylxanthines. Excessive amount of the substrate (2.5 × 10^?4M) resulted in progressive inhibition of the enzyme activity. Low concentrations of methylxanthines exerted a definite inhibitory effect on the xanthine oxidase activity at lower substrate concentrations. At higher concentrations of the substrate, the inhibitory effect due to the same concentration of methylxanthines did not produce any added inhibition of the enzyme activity to that produced by the substrate alone. However, added inhibition by high concentrations of methylxanthines was detectable even when the enzyme activity was markedly inhibited by higher concentrations of the substrate. The $\text{in vivo}$ administration of methylxanthines caused a significant inhibition of the xanthine oxidase activity in lungs, kidneys, heart and brain of rats. Consequently, the level of uric acid in the tissues of the drug treated animals was also found to be reduced.