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 electrophoretic 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. Copyright © 1978, Wiley Blackwell. All rights reserved