Abstract
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.
Highlights
Most teleostean fishes are ammonotelic, excreting nitrogen wastes across their gills as ammonia [1, 2]
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
We report here the surprising finding that CPSase III and all other urea cycle enzyme activities are present in muscle of this alkaline lake-adapted tilapia at levels more than sufficient to account for the rate of urea excretion; in
Summary
Lake Magadi tilapia (O. a. grahami) were captured by seine net from Fish Springs Lagoon in Lake Magadi, Kenya in February 1997. For studies of urea cycle enzymes, tissues were flash-frozen in liquid nitrogen, transported to North America in a dry shipper, and kept at Ϫ70 °C until use. Extracts for measuring units/gram of tissue of urea cycle enzymes in frozen samples (Table I) were prepared and assayed as described previously [7, 12], except tissue samples were smaller (0.2 g), and the ATP-regenerating system was not used in the CPSase assay. Subcellular localization studies (Table II) of GSase, glutaminase, and arginase were conducted on-site using the isolation and assay methods previously described [7], except homogenates were centrifuged at 4 °C at 82 ϫ g for 5 min to pellet the debris fraction and the resulting supernatant at 13,800 ϫ g for 5 min to pellet the mitochondrial fraction.
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