EFFECTS OF XANTHINE-OXIDASE ON SYNTHESIS OF 5-METHYLURIDINE BY THE RIBOSYL TRANSFER-REACTION

In the presence of xanthine oxidase, 5-methyluridine was synthesized from inosine and thymine by the ribosyl transfer reaction using purine nucleoside phosphorylase and pyrimidine nucleoside phosphorylase. The hypoxanthine formed was converted into urate via xanthine by xanthine oxidase. After inosine was completely consumed, an equilibrium state, in which 5-methyluridine, thymine, ribose-1-phosphate, and phosphate were involved, was achieved. At the equilibrium state, maximum yield of 5-methyluridine was obtained. The equilibrium constant of the reaction (K = [thymine][ribose-1-phosphate]/[5-methyluridine][phosphate]) was 0.062. Based on the equilibrium constant, the yield of 5-methyluridine depended on a ratio of initial concentrations of inosine and phosphate, on condition that the initial concentrations of inosine and thymine were the same. The yields of 5-methyluridine in the presence and absence of xanthine oxidase were 76 and 33%, respectively, when the initial concentrations of inosine, thymine, and phosphate were 5 mM each.