| Abstract: | The radioisotopic method used to assay acetylglutamate kinase (EC 2.7.2.8) of Neurospora crassa has been shown to detect two distinct enzymatically catalyzed reactions. The enzymes were separated by differential centrifugation into a cytosolic activity and an organellar activity. Both activities required ATP and were thermal-labile. The cytosolic activity was insensitive to inhibition by arginine and formed a stable reaction product in the absence of hydroxylamine. The organellar activity had an absolute requirement for hydroxylamine in order to form a stable reaction product. The product of the cytosolic activity was separated from acetylglutamate hydroxamate (the product of the organellar activity) and was identified as the cyclic amide pyroglutamate by cation exchange chromatography. The organellar activity has been implicated in arginine biosynthesis by the following criteria: it was completely and specifically inhibited by arginine concentrations as low as 200 microM; its level was elevated 2-fold in a mutant strain with derepressed levels of arginine biosynthetic enzymes; and it was absent in an arginine auxotrophic strain (the cytosolic activity was present). The organellar activity co-sedimented with mitochondria during isopycnic gradient centrifugation. The metabolic problems posed by a mitochondrial location of a feedback-sensitive enzyme and the cytosolic location of its effector are discussed. |
