Subcellular localization of two glutamine‐dependent carbamoyl‐phosphate synthetases and related enzymes in liver of Micropterus salmoides (largemouth bass) and properties of isolated liver mitochondria: Comparative relationship with elasmobranchs

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Abstract
Two different glutamine‐dependent carbamoyl‐phosphate synthetase (CPSase) activities are preset in tissues of Squalus acanthias (spiny dogfish shark, a representative elasmobranch) (Anderson, Biochem. J. 261:523–529, 1989). CPSase III (acetylglutamate‐dependent) activity is present in liver mitochondria; the function of this enzyme is related to urea synthesis for the purpose of osmoregulation, and the properties are analogous to the classical ammonia and acetylglutamate‐dependent CPSase I in liver mitochondria of ureotelic species (except that the nitrogen‐donating substrate is glutamine). CPSase II activity is present in spleen and other extrahepatic tissues, but is apparently absent from liver; the function of this enzyme is related to pyrimidine nucleotide biosynthesis, and the properties are analogous to mammalian CPSase II activities (independent of acetylglutamate, inhibited by UTP, and activated by 5‐phosphoribosyl 1‐pyrophosphate). Glutamine synthetase in shark liver is localized in the mitochondria, but the spleen glutamine synthetase is localized in the cytosol. Liver of largemouth bass (Micropterus salmoides, a freshwater teleost) also has CPSase III activity; however, unlike shark, aspartate carbamoyltransferase activity is also present in bass liver. Consequently, a major objective of this study was to establish if two different glutamine‐dependent CPSase activities are present in a single tissue (liver). Two different glutamine‐dependent CPSase activites were found to be present in bass liver, a mitochondrial CPSase III and a cytosolic CPSase II. The CPSase II activity, in contrast to the shark spleen CPSase II, appears to be associated with aspartate carbamoyltransferase and dihydro‐orotase activities, as found in mammalian species. As in shark, ornithine carbamoyltransferase, arginase, and phosphoenolpyruvate carboxykinase activities in bass liver are localized in the mitochondria, but, in contrast to shark, glutamine synthetase activity is localized in the cytosol. The properties of isolated, actively respiring bass liver mitochondria were investigated for comparison with previously reported properties of shark liver mitochondria. Respiration of bass mitochondria is supported by succinate and glutamate plus malate, but not by β‐hydroxybutyrate, palmitoyl‐CoA, citrate, or glutamine, all of which serve as excellent substrates for dogfish shark liver mitochondria. In contrast to shark mitochondria, bass mitochondria are not capable of ketone body formation and, of particular interest, do not catalyze citrulline formation from glutamine. CPSase III activity was not present in two other species of teleost fishes closely related to largemouth bass. The occurrence and function of CPSase III in teleost fishes are discussed.
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