Effect of Carbon Source on Enzymes and Metabolites of Arginine Metabolism in Neurospora

Abstract

90024The levels of enzymes and metabolites of arginine metabolism were determined in exponential cultures of Neurospora crassa grown on various carbon sources. The carbon sources decreased in effectiveness (as determined by generation times) in the following order: sucrose, acetate, glycerol, and ethanol. The basal and induced levels of the catabolic enzymes, arginase (EC 3.5.3.1) and ornithine transaminase (EC 2.6.1.13), were lower in mycelia grown on poor carbon sources. Arginase was more sensitive to variations in carbon source than was ornithine transaminase. Induction of both enzymes was sensitive to nitrogen metabolite control, but this sensitivity was reduced in mycelia grown on glycerol or ethanol. The pools of arginine and ornithine were reduced in mycelia grown in unsupplemented medium containing poor carbon sources, but the biosynthetic enzyme ornithine transcarbamylase (EC 2.1.3.3) was not derepressed. The arginine pools were similar, regardless of carbon source, in mycelia grown in arginine-supplemented medium. The ornithine pool was reduced by growth on poor carbon sources. The rate of arginine degradation was proportional to the level of arginase in both sucroseand glycerol-grown mycelia. The distribution of arginine between cytosol and vesicles was only slightly altered by growth on glycerol instead of sucrose. The slightly smaller cytosolic arginine concentration did not appear to be sufficient to account for the alterations in basal and induced enzyme levels. The results suggest a possible carbon metabolite effect on the expression or turnover of a variety of genes for enzymes of arginine metabolism in Neurospora.The metabolism of amino acids in most organisms includes biosynthesis, degradation, and incorporation into protein. In Neurospora crassa, control of arginine metabolism includes mechanisms observed in many procaryotes but also some which appear to occur exclusively in eucaryotes (reviewed in reference 10). The better understood mechanisms include: (i) feedback inhibition by arginine of acetylglutamate kinase (EC 2.7.2.8), the second enzyme in the biosynthetic pathway (8); (ii) repression of carbamyl phosphate synthetase (EC 2.7.2.5) by arginine; (iii) induction of arginase and ornithine transaminase by arginine; and (iv) nitrogen control in which the induction of the catabolic enzymes is impaired by the end products of arginine catabolism (ammonia, glutamate, or glutamine).Two additional but poorly understood features also appear to contribute to the control of arginine metabolism in N. crassa. The first is a control system which has been termed cross pathway regulation and appears to involve a common regulatory element affecting the level of enzymes of arginine, histidine, lysine, and tryptophan metabolism (4, 5). A similar control system has been observed in Saccharomyces cerevisiae (25). The second feature, compartmentation of enzymes and metabolites, has been shown to affect arginine metabolism in N. crassa (10). In procaryotic cells, where less subcellular organization exists, most...

Energetics of vacuolar compartmentation of arginine in Neurospora crassa

“…Radioactivity in protein arginine was determined by counting a portion of this protein precipitate, when appropriate. Urea and arginine were purified by cation-exchange chromatography and analyzed as previously described (2,8).…”

Phase I clinical trial of the novel platin complex dicycloplatin: clinical and pharmacokinetic results

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