title: The genetic control of the molybdoflavoproteins in Aspergillus nidulans. IV. A comparison between purine hydroxylase I and II.
abstract: The purine hydroxylases I and II of Aspergillus nidulans [previously called xanthine dehydrogenases I and II: Scazzocchio, Holl and Foguelman, Eur. J. Biochem. 36, 428--445 (1973)] have been studied in crude extracts. The two enzymes differ in their substrate specificities, purine hydroxylase II being able to accept nicotinate as a substrate and unable to hydroxylate xanthine. The kinetics of inhibition with allopurinol and oxypurinol are also different, the two analogues being pseudo-irreversible inhibitors of purine hydroxylase I, while allopurinol is a competitive inhibitor of purine hydroxylase II and oxypurinol shows anti-competitive inhibition. Differences in electro-phoretic mobility and molecular size are also shown. We have failed to show the formation of hybrid purine hydroxylase I/II molecules. While a common evolutionary origin of the purine hydroxylases could be postulated, the data reveal a considerable divergence.
mesh_terms: D001233:Aspergillus nidulans / Q000201:enzymology*; D005420:Flavoproteins / Q000378:metabolism; D007527:Isoenzymes / Q000235:genetics / Q000378:metabolism*; D007658:Ketone Oxidoreductases / Q000378:metabolism*; D007700:Kinetics; D008970:Molecular Weight; D008982:Molybdenum; D009539:Nicotinic Acids; D013379:Substrate Specificity; D014968:Xanthine Dehydrogenase / Q000235:genetics / Q000378:metabolism*; D014970:Xanthines
chemical_list: D005420:Flavoproteins; D007527:Isoenzymes; D009539:Nicotinic Acids; D014970:Xanthines; D008982:Molybdenum; D014968:Xanthine Dehydrogenase; D007658:Ketone Oxidoreductases