Urea synthesis in the African lungfishProtopterus dolloi-hepatic carbamoyl phosphate synthetase III and glutamine synthetase are upregulated by 6 days of aerial exposure
Author:
Chew Shit F.1, Ong Tan F.1, Ho Lilian1, Tam Wai L.2, Loong Ai M.2, Hiong Kum C.2, Wong Wai P.2, Ip Yuen K.2
Affiliation:
1. Natural Sciences, National Institute of Education, Nanyang Technological University, 1 Nanyang Walk, Singapore 637616, Republic of Singapore 2. Department of Biological Sciences, National University of Singapore, 10 Kent Ridge Road, Singapore 117543, Republic of Singapore
Abstract
SUMMARYLike the marine ray Taeniura lymma, the African lungfish Protopterus dolloi possesses carbamoyl phosphate III (CPS III) in the liver and not carbamoyl phosphate I (CPS I), as in the mouse Mus musculus or as in other African lungfish reported elsewhere. However,similar to other African lungfish and tetrapods, hepatic arginase of P. dolloi is present mainly in the cytosol. Glutamine synthetase activity is present in both the mitochondrial and cytosolic fractions of the liver of P. dolloi. Therefore, we conclude that P. dolloi is a more primitive extant lungfish, which is intermediate between aquatic fish and terrestrial tetrapods, and represents a link in the fish-tetrapod continuum. During 6 days of aerial exposure, the ammonia excretion rate in P. dolloi decreased significantly to 8-16% of the submerged control. However, there were no significant increases in ammonia contents in the muscle, liver or plasma of specimens exposed to air for 6 days. These results suggest that (1) endogenous ammonia production was drastically reduced and (2)endogenous ammonia was detoxified effectively into urea. Indeed, there were significant decreases in glutamate, glutamine and lysine levels in the livers of fish exposed to air, which led to a decrease in the total free amino acid content. This indirectly confirms that the specimen had reduced its rates of proteolysis and/or amino acid catabolism to suppress endogenous ammonia production. Simultaneously, there were significant increases in urea levels in the muscle (8-fold), liver (10.5-fold) and plasma (12.6-fold) of specimens exposed to air for 6 days. Furthermore, there was an increase in the hepatic ornithine-urea cycle (OUC) capacity, with significant increases in the activities of CPS III (3.8-fold), argininosuccinate synthetase + lyase(1.8-fold) and, more importantly, glutamine synthetase (2.2-fold). This is the first report on the upregulation of OUC capacity and urea synthesis rate in an African lungfish exposed to air. Upon re-immersion, the urea excretion rate increased 22-fold compared with that of the control specimen, which is the greatest increase among fish during emersion-immersion transitions and suggests that P. dolloi possesses transporters that facilitate the excretion of urea in water.
Publisher
The Company of Biologists
Subject
Insect Science,Molecular Biology,Animal Science and Zoology,Aquatic Science,Physiology,Ecology, Evolution, Behavior and Systematics
Reference46 articles.
1. Anderson, P. M. (1980). Glutamine- and N-acetylglutamate-dependent carbamoyl phosphate synthetase in elasmobranchs. Science208,291-293. 2. Anderson, P. M. (2001). Urea and glutamine synthesis: environmental influences on nitrogen excretion. In Fish Physiology, vol. 20, Nitrogen Excretion (ed. P. A. Wright and P. M. Anderson), pp. 239-277. New York:Academic Press. 3. Anderson, P. M. and Little, R. M. (1986). Kinetic properties of cyanase. Biochemistry25,1621-1626. 4. Anderson, P. M. and Walsh, P. J. (1995). Subcellular localization and biochemical properties of the enzyme carbamoyl phosphate synthetase and urea synthesis in the batrachoidid fishes, Opsanus beta, Opsanus tau and Porichthys notatus.J. Exp. Biol.198,755-766. 5. Anderson, P. M., Broderius, M. A., Fong, K. C., Tsui, K. N. T.,Chew, S. F. and Ip, Y. K. (2002). Glutamine synthetase expression in liver, muscle, stomach and intestine of Bostrichyths sinensis in response to exposure to a high exogenous ammonia concentration. J. Exp. Biol.205,2053-2065.
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