The Genetic Control of the Molybdoflavoproteins in Aspergillus nidulans IV. A Comparison between Purine Hydroxylase I and II

Abstract

The purine hydroxylases I and I1 of Aspergillus niduluns [previously called xanthine dehydrogenases I and 11: Scazzocchio, Holl and Foguelman, Eur. J. Biochem. 36, 428-445 (2973)l have been studied in crude extracts. The two enzymes differ in their substrate specificities, purine hydroxylase 11 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 I1 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 1/11 molecules. While a common evolutionary origin of the purine hydroxylases could be postulated, the data reveal a considerable divergence.

Completing the purine utilisation pathway of Aspergillus nidulans

“…ANID_06573 was not identified in the genetic screen but is most probably the orthologue of the K. aerogenes ureF gene (Lee et al, 1992). hxnS encodes a nicotinate hydroxylase which accepts hypoxanthine but not xanthine as substrate (Lewis et al, 1978;Mehra and Coughlan, 1984). Its transcription is induced by nicotinate, but also by nitrogen starvation (Scazzocchio, 1973), thus it may play a physiological role in purine utilisation and/or recycling under these conditions.…”

The nadA gene of Aspergillus nidulans, encoding adenine deaminase, is subject to a unique regulatory pattern

“…However, this elevation of enzyme activity seems to be due to a post-transcriptional regulation, as the steady-state level of the AAH1 messenger is apparently unchanged (Deeley, 1992). The genetics, biochemistry and regulatory features of purine utilisation have been thoroughly studied in A. nidulans (Darlington et al, 1965;Darlington and Scazzocchio, 1967;Scazzocchio and Darlington, 1968;Lewis et al, 1978;see Scazzocchio, 1994, for a review). The genes coding for three purine transporters and for the enzymes involved in all the metabolic steps between hypoxanthine and allantoin have been cloned, they regulation studied at the level of mRNA steady state levels (Gorfinkiel et al, 1993;Diallinas et al, 1995;Glatigny and Scazzocchio, 1995;Amrani et al, 1999;Ribard et al, 2003;Cecchetto et al, 2004;Cultrone et al, 2005Cultrone et al, , 2007.…”

The AzgA Purine Transporter of Aspergillus nidulans

“…The above results suggest that uric acid is the physiological inducer of the azgA gene. This is the case for every other gene of the purine catabolism pathway tested to date, other purines inducing through their conversion to uric acid (43)(44)(45)(46). To establish this unequivocally, we determined the azgA mRNA steady state levels in an hxB20 xanA1 double mutant.…”