title: Behavior of N-methylated allopurinols and related 4-thioxopyrazolo [3,4-d]pyrimidines towards bovine milk xanthine oxidase.
abstract: 1. All available N-mono- and N,N'-dimethylallopurinols and the corresponding 4-thioxo derivatives have been tested as substrates or inhibitors of bovine milk xanthine oxidase (xanthine: oxygen oxidoreductase, EC 1.2.3.2). 2. None of the compounds tested revealed any inhibitory activity towards the enzyme. 3. All compounds were resistant to enzymic oxidation, with the exception of 7-methylallopurinol and its 4-thioxo analog. Both these compounds were attacked at position 6. 7-Methylallopurinol was oxidised nearly ten times faster than the isomeric 3-methylhypoxanthine. 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 position 5,6 in the pyrimidine ring. This activation process resembles the activation of hypoxanthine.
mesh_terms: D000493:Allopurinol / Q000031:analogs & derivatives* / Q000494:pharmacology; D000818:Animals; D002417:Cattle; D008892:Milk / Q000201:enzymology*; D013056:Spectrophotometry, Ultraviolet; D013329:Structure-Activity Relationship; D014969:Xanthine Oxidase / Q000037:antagonists & inhibitors / Q000378:metabolism*
chemical_list: D000493:Allopurinol; D014969:Xanthine Oxidase