Inhibitory effects of plant growth regulators on xanthine oxidase

Several plant hormones and analogues were tested for their inhibitory effects on xanthine oxidase. The flavoprotein enzyme, xanthine oxidase, catalyses the oxidation of hypoxanthine to xanthine and then xanthine to uric acid which has λmax 295 nm. Uric acid was thus the basis for a spectrophotometric assay of the activity of xanthine oxidase. The results showed that trans-zeatin displayed the strongest activity (IC₅₀= 23.5 μM) on xanthine oxidase inhibition, followed by indole-3-acrylic acid (IC₅₀= 136.0 μM) and then by the mixed isomers of zeatin (trans-zeatin and cis-zeatin) (IC₅₀= 198.65 μM). Trans-zeatin induced an uncompetitive inhibition of the enzyme with respect to the substrate xanthine and the apparent inhibition constant (Ki) was 5.09 μM. However, zeatin riboside was inactive. Since xanthine oxidase serum levels are increased in hepatitis, mild hepatic intoxication, tumours brain tissues, and DNA damage induced by cytotoxic agents, it is expected that trans-zeatin may be useful for the treatment of these diseases as well as gout which is caused by deposition of uric acid in the joints and oxidative damage of tissue caused by generation of superoxide union radical.