INHIBITION OF XANTHINE-OXIDASE BY CYTOKININS AND RELATED SUBSTANCES

Fourteen cytokinins were tested for their inhibitory effects on xanthi ne oxidase. The enzyme, xanthine oxidase catalyses the oxidation of hy poxanthine to xanthine and of xanthine to uric acid which has lambda(m ax) of 295 nm, forming the basis for a spectrophotometric assay of the activity of xanthine oxidase. The results showed that adenine-HCl, N- 6-(2-isopentenyl)-adenine, purine and DL-dihydrozeatin displayed very potent activities (IC50 = 1.92, 10.99, 60.98 and 86.36 mu M respective ly). Their apparent inhibition constants (Ki) were 2.20, 17.99, 13.59 and 115.62 mu M, and induced competitive, uncompetitive, competitive a nd non-competitive type inhibitions respectively with respect to the s ubstrate xanthine.