CARBAMYL-PHOSPHATE SYNTHETASES IN AN AIR-BREATHING TELEOST, HETEROPNEUSTES-FOSSILIS

The Indian air-breathing teleost fish Heteropneustes fossilis has been shown to have a functional urea cycle and to be able to switch from a mmoniotelic to ureotelic nitrogen metabolism when exposed to high leve ls of ammonia or air. The objective of this study was to identify the type of carbamyl phosphate synthetase (CPS) catalyzing the first step of the urea cycle in H. fossilis. Mitochondrial CPS III [glutamine- an d N-acetyl-L-glutamate (NAG)-dependent] and cytosolic CPS II (glutamin e-dependent) activities were found to be present in liver, analogous t o that described for two other teleosts that have CPS III activity. Th e same activities and subcellar localization were found in kidney. Une xpectedly, a CPS I-Like activity (ammonia- and NAG-dependent) was foun d to be present at levels higher than the CPS III activity in the mito chondrial fraction of both liver and kidney. The urea cycle-related CP S III found in invertebrates and fish is considered to be the evolutio nary precursor of the urea cycle-related CPS I in ureotelic mammalian and amphibian species. Whether or not this CPS I-like activity 1) is d ue to the presence of a separate CPS I gene in addition to a CPS III g ene or 2) represents an adapted CPS III activity in H. fossilis, these results suggest that the presence of both CPS I-like and CPS III acti vities may play an important physiological adaptive role in the tolera nce of these fish to high concentrations of external ammonia. Copyrigh t (C) 1997 Elsevier Science Inc.