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Pathways for urea production during early life of an air-breathing teleost, the African catfish Clarias gariepinus Burchell
1 Department of Zoology, University of Bergen, Allègaten, 41, N-5007 Bergen, Norway,
2 Department of Zoology, University of Guelph, Guelph, Ontario, Canada N1G 2W1 and
3 Fish Culture and Fisheries Group, Wageningen Institute of Animal Sciences, Wageningen University, PO Box 338, 6700 AH Wageningen, The Netherlands
*Author for correspondence at present address: Institute of Aquaculture Research, N-6600 Sunndalsøra, Norway (e-mail: bendik.terjesen{at}akvaforsk.nhl.no)
Accepted March 27, 2001
Embryos and larvae of the African catfish Clarias gariepinus excrete significant quantities of urea. The present study focused on the potential urea-generating pathways during early development of this teleost; uricolysis, argininolysis and the ornithine–urea cycle (OUC). Uricase, allantoinase, allantoicase and ureidoglycollate lyase of the uricolytic pathway were expressed in all early life stages and in adult liver of C. gariepinus. Uricase activity increased in starved larvae compared with yolk-sac larvae. The key regulatory enzyme of the teleost OUC, carbamoyl phosphate synthetase III (CPSase III), was expressed predominantly in muscle of developing C. gariepinus larvae and showed negligible activity in the absence of its allosteric effector N-acetyl-L-glutamate. CPSase III and ornithine carbamoyl transferase activities increased in fed larvae compared with starved larvae. In contrast to the early developmental stages, adult C. gariepinus expressed only low and variable levels of CPSase III, suggesting that, under the experimental conditions employed, OUC expression is influenced by developmental stage in this species. The data indicate that early C. gariepinus life stages express the enzymes necessary for urea production by uricolysis, argininolysis and the OUC, and this may explain why urea tissue levels and urea excretion rates are substantial during the early development of this air-breathing teleost.
Key words: Clarias gariepinus, African catfish, embryo, larva, urea, excretion, uricolysis, ornithine–urea cycle, carbamoyl phosphate synthetase III, arginine.
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