On the Demethylation of 3-Methyl-6-Methylmercaptopurines with Hydrogen Sulfide

Influence of 8-substitutes on the oxidation of hypoxanthine and 6-thioxopurine by bovine milk xanthine oxidase

1. The influence of 8-substituents was studied on the rate of oxidation of hypoxanthine and 6-thioxopurine by bovine milk xanthine oxidase (EC 1.2.3.2). 2. An 8-methyl group does not alter the rate of oxidation of hypoxanthine materially, but an 8-phenyl substituent reduces it markedly. This is ascribed to inhibition of the tautomerisation process, responsible for substrate activation, prior to oxidation. 3. In contrast, the 8-phenyl group in 3-methyl-8-phenylhypoxanthine enhances the rate, presumably by binding to a hydrophobic site near the enzymaic center. 4. An 8-phenyl group in 6-thioxopurine markedly increases the rate of enzymaic oxidation. Probably the aromatic substituent diverts anion formation to the imidazole ring. In contrast, ionisation of 8-methyl-6-thioxopurine involves the pyrimidine moiety, thus rendering enzymic attack at position 2 more difficult.

Oxidation of N-methyl substituted hypoxanthines, xanthines, purine-6,8-diones and the corresponding 6-thioxo derivatives by bovine milk xanthine oxidase

1. The oxidation of six series of purines (hypoxanthines, xanthines, purine-6,8-diones and the corresponding 6-thioxo derivatives) by a highly purified bovine milk xanthine oxidase (EC 1.2.3.2) has been studied, using a variety of N-methyl derivatives. 2. N-Methyl substituents can either enhance or reduce enzymic rates. Enhancement is ascribed to blockade of groups which mediate unfavorable modes of binding of substrate to enzyme. Introduction of N-methyl groups can also inhibit enzymic oxidation, either by occluding essential binding groups or by preventing spontaneous or enzyme-induced tautomerisation processes, which create suitable binding sites in the substrates. 3. In all purines which are rapidly attacked by xanthine oxidase, proper attachment to the active center is mediated by the groupings (3) NH, (9) N or (3) N, (9) NH. 4. Reduced rates usually express lowered substrate affinity, which finds its expression in weak competitive inhibition of xanthine oxidation.