Nitrogen excretion and expression of carbamoyl-phosphate synthetase III activity and mRNA in extrahepatic tissues of largemouth bass (Micropterus salmoides)

Low levels of all of the enzymes required for urea synthesis via the urea cycle, including mitochondrial glutamine-and acetylglutamate-dependent carbamoyl-phosphate synthetase III (CPSase III) and cytosolic glutamine synthetase, are known to be present in liver of the teleost fish largemouth bass (Micropterus salmoides). The levels of these enzymes are higher than those in most other teleosts, but they are significantly lower than the levels present in liver of ureoosmotic elasmobranchs. The purpose of this study was to assess the physiological role of CPSase III in the context of urea synthesis in adult bass. The results showed that urea-N accounts for about 30% of the total nitrogen (ammonia-N plus urea-N) excreted under control conditions. The rate of urea-N excretion did not increase in response to exposure to 1 mM NH4Cl (3 days) or 0.25 mM NH4Cl (12 days) in the external water, except for a transient increase after a day or two of exposure. CPSase III activity in liver also did not increase in response to exposure to ammonia. Adult largemouth bass, while apparently ureogenic, are primarily ammonotelic and remain so even in the presence of relatively high concentrations of ammonia in the external environment. The total units of CPSase III activity in liver are not sufficient to account for the quantity of urea that is excreted. However, CPSase III and ornithine carbamoyltransferase (OCTase) activities were found to be present in intestinal tissue and, unexpectedly, in muscle tissue. The total units of CPSase III and OCTase in muscle, intestine, and liver appear to be sufficient to account for the observed rate of urea excretion. The sequence of CPSase III cDNA was determined, which permitted the use of ribonuclease protection assays to demonstrate the presence of CPSase III mRNA in these tissues. (C) 1998 Academic Press.