BEHAVIOR OF N-METHYLATED ALLOPURINOLS AND RELATED 4-THIOXOPYRAZOLO [3,4-D]PYRIMIDINES TOWARDS BOVINE-MILK XANTHINE-OXIDASE

1. 1. All available N-mono- and N,N′-dimethylallopurinols and the corresponding- 4-thioxo derivatives have been tested as substrates or inibitors of bovine milk xanthine oxidase (xanthine: oxygen oxidoreductase, EC 1.2.3.2). 2. 2. None of the compounds tested revealed any inhibitory activity towards the enzyme. 3. 3. All compounds were resistant to enzymic oxidation, with the exception of 7-methylallopurinol and its 4-thioxo analog. Both these compounds were attached at position 6. 7-Methylallopurinol was oxidised nearly ten times faster than the isomeric 3-methylhypoxanthine. 4. 4. These observations can be explained by assuming that for attack at C-6, the enzyme must bind both to N-1 and N-2 in the pyrazole ring and causes tautomerisation, which places a double bond at posiiton 5,6 in the pyrimidine ring. This activation process resembles the activation of hypoxanthine. © 1979.