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J Biol Chem, Vol. 274, Issue 42, 29858-29861, October 15, 1999
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From the The tilapia fish Oreochromis alcalicus
grahami from Kenya has adapted to living in waters at pH 10.5 by
excreting the end product of nitrogen metabolism as urea rather than as
ammonia directly across the gills as occurs in most fish. The level of activity in liver of the first enzyme in the urea cycle pathway, carbamoyl-phosphate synthetase III (CPSase III), is too low to account
for the observed high rates of urea excretion. We report here the
surprising finding that CPSase III and all other urea cycle enzyme
activities are present in muscle of this species at levels more than
sufficient to account for the rate of urea excretion; in addition, the
basic kinetic properties of the CPSase III appear to be different from
those of other known type III CPSases. The sequence of the CPSase III
cDNA is reported as well as the finding that glutamine synthetase
activity is present in liver but not in muscle. This unusual form of
adaptation may have occurred because of the apparent impossibility of
packaging the needed amount of urea cycle enzymes in liver.
Department of Biochemistry and Molecular
Biology, University of Minnesota-Duluth, Duluth, Minnesota 55812, the § Rosenstiel School of Marine and Atmospheric Science,
University of Miami, Miami, Florida 33149-1098, the
¶ Department of Biology, McMaster University, Hamilton, Ontario
L8S4K1, Canada, and the Department of Zoology and Physiology,
University of Wyoming, Laramie, Wyoming 82071
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