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

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 mitochondrially bound arginase augments ureagenesis by supplying ornithine for net synthesis of citrulline, glutamate, N-acetylglutamate, and aspartate. [U-15N4]arginine was used as precursor and isolated mitochondria or liver perfusion as a model system to monitor arginine catabolism and the incorporation of 15N 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-15N4]arginine catabolism was recovered as 15N-labeled ornithine, glutamate, N-acetylglutamate, citrulline, and aspartate. The production of 15N-labeled metabolites was time- and dose-dependent. During liver perfusion, urea containing one (Um+1) or two (Um+2) 15N was generated from perfusate [U-15N4]arginine. The output of Um+2 was between 3 and 8% of total urea, consistent with the percentage of activity of matrix arginase. Um+1 was formed following mitochondrial production of [15N]glutamate from [α,δ-15N2]ornithine and transamination of [15N]glutamate to [15N]aspartate. The latter is transported to cytosol and incorporated into argininosuccinate. Approximately 70, 75, 7, and 5% of hepatic ornithine, citrulline, N-acetylglutamate, and aspartate, respectively, were derived from perfusate [U-15N4]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-acetylglutamate, citrulline, and aspartate.