The Role of Mitochondrially Bound Arginase in the Regulation of Urea Synthesis

Abstract

The main goal of the current study was to elucidate the role of mitochondrial arginine metabolism in the regulation of N-acetylglutamate and urea synthesis. We hypothesized that arginine catabolism via mitochondri-ally bound arginase augments ureagenesis by supplying ornithine for net synthesis of citrulline, glutamate, N-acetylglutamate, and aspartate. [U-15 N 4 ]arginine was used as precursor and isolated mitochondria or liver perfusion as a model system to monitor arginine catab-olism and the incorporation of 15 N into various intermediate metabolites of the urea cycle. The results indicate that 8% of total mitochondrial arginase activity is located in the matrix, and 90% is located in the outer membrane. Experiments with isolated mitochondria showed that 60-70% of external [U-15 N 4 ]arginine ca-tabolism was recovered as 15 N-labeled ornithine, gluta-mate, N-acetylglutamate, citrulline, and aspartate. The production of 15 N-labeled metabolites was time-and dose-dependent. During liver perfusion, urea containing one (U m1) or two (U m2) 15 N was generated from perfusate [U-15 N 4 ]arginine. The output of U m2 was between 3 and 8% of total urea, consistent with the percentage of activity of matrix arginase. U m1 was formed following mito-chondrial production of [ 15 N]glutamate from [,-15 N 2 ]ornithine and transamination of [ 15 N]glutamate to [ 15 N]aspartate. The latter is transported to cytosol and incorporated into argininosuccinate. Approximately 70, 75, 7, and 5% of hepatic ornithine, citrulline, N-acetyl-glutamate, and aspartate, respectively, were derived from perfusate [U-15 N 4 ]arginine. The results substantiate the hypothesis that intramitochondrial arginase, presumably the arginase-II isozyme, may play an important role in the regulation of hepatic ureagenesis by furnishing ornithine for net synthesis of N-acetylgluta-mate, citrulline, and aspartate.