Types of carbamyl phosphate synthetases and subcellular localization of urea cycle and related enzymes in air-breathing walking catfish,Clarias batrachus

Abstract

The types of carbamyl phosphate synthetase (CPS), the subcellular localization of urea cycle enzymes and glutamine synthetase (GS) in liver and kidney (potential ureogenic tissues) and also the possible involvement of some extra-hepatic tissues for urea synthesis via the urea cycle were studied in an Indian ureogenic amphibious air-breathing walking catfish, Clarias batrachus. Mitochondrial CPS III [glutamine- and N-acetyl-L-glutamate (NAG)-dependent] and cytosolic CPS II (glutamine-dependent) activities were found to be present in liver, analogous to that described for two other teleosts that have CPS III activity. The same activities and subcellular localization were found in kidney. Unexpectedly, a CPS I-like (ammonia- and NAG-dependent) activity was found to be present at levels higher than the CPS III activity in the mitochondrial fraction of both liver and kidney analogous to another Indian amphibious air-breathing teleost fish, Heteropneustes fossilis. The two other urea cycle enzymes, ornithine transcarbamylase (OTC) and arginase (ARG), and GS were found to be localized primarily in the mitochondria, and argininosuccinate synthetase (ASS) and argininosuccinate lyase (ASL) in the cytosol analogous to H. Fossilis and elasmobranchs. Significant levels of urea cycle enzymes along with GS activity could also be detected in muscle, intestine and brain tissues, except ASL in muscle and ASS in brain. Our results also indicated that probably all three different types of CPS activities are also expressed in all three extra-hepatic tissues. The urea cycle–related CPS III found in invertebrates and fish is considered to be the evolutionary precursor of the urea cycle–related CPS I in ureotelic mammalian and amphibian species. Whether or not the CPS I-like activity reported here (1) is due to the presence of a separate CPS I gene in addition to a CPS III gene or (2) represents an adapted CPS III activity in C. batrachus, these results suggest that the presence of both CPS I-like and CPS III activities along with the presence of urea cycle enzymes in extra-hepatic tissues may play an important physiological adaptive role for survival of these walking catfish in higher ambient ammonia or living in air or while burrowing inside mud during habitat drying. J. Exp. Zool. 283:121–130, 1999. © 1999 Wiley-Liss, Inc.