The role of mitochondrially bound arginase in the regulation of urea synthesis: studies with [U-15N4]arginine, isolated mitochondria, and perfused rat liver.
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-(15)N(4)]arginine was used as precursor and isolated mitochondria or liver perfusion as a model system to monitor arginine catabolism and the incorporation of (15)N into various intermediate metabolites of the urea cycle. The results indicate that approximately 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 approximately 60-70% of external [U-(15)N(4)]arginine catabolism was recovered as (15)N-labeled ornithine, glutamate, N-acetylglutamate, citrulline, and aspartate. The production of (15)N-labeled metabolites was time- and dose-dependent. During liver perfusion, urea containing one (U(m+1)) or two (U(m+2)) (15)N was generated from perfusate [U-(15)N(4)]arginine. The output of U(m+2) was between 3 and 8% of total urea, consistent with the percentage of activity of matrix arginase. U(m+1) was formed following mitochondrial production of [(15)N]glutamate from [alpha,delta-(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-acetylglutamate, 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-acetylglutamate, citrulline, and aspartate.
Nissim
Itzhak
I
Children's Hospital of Philadelphia, Division of Child Development and Rehabilitation Medicine, Department of Pediatrics, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104, USA.
Luhovyy
Bohdan
B
Horyn
Oksana
O
Daikhin
Yevgeny
Y
Nissim
Ilana
I
Yudkoff
Marc
M
D000818:Animals; D001119:Arginase / Q000378:metabolism* / Q000502:physiology*; D001120:Arginine / Q000031:analogs & derivatives* / Q000378:metabolism*; D002458:Cell Fractionation; D066298:In Vitro Techniques; D007527:Isoenzymes / Q000378:metabolism / Q000502:physiology; D008297:Male; D008930:Mitochondria, Liver / Q000737:chemistry* / Q000201:enzymology* / Q000378:metabolism; D009587:Nitrogen Isotopes; D010477:Perfusion; D051381:Rats; D017207:Rats, Sprague-Dawley; D014508:Urea / Q000378:metabolism*
D007527:Isoenzymes; D009587:Nitrogen Isotopes; D014508:Urea; D001120:Arginine; D001119:Arginase