Question

Nitrogenous waste excreted by tilapia

Answer 1

Answer:

Explanation:

The tilapia of Lake Magadi (pH 10), Oreochromis alcalicus grahami, excrete all nitrogen wastes as urea and possess the ornithine urea cycle (OUC), unlike the majority of teleost fish. It was proposed that liver OUC enzymes could be induced in the freshwater ammoniotelic tilapia Oreochromis niloticus in response to environmental or dietary manipulations that stimulate urea excretion in other animals. Elevated water ammonia (1 mM NH₄Cl) inhibited ammonia excretion over the first 12 h and increased urea excretion rates three- to fivefold over 7 d. The OUC was not functional in control fish, as carbamoyl phosphate synthetase III activity was not detectable and ornithine carbamoyltransferase activity was relatively low. No changes in OUC enzyme activities were detected in NH₄Cl-exposed fish, but the activity of the uricolytic enzyme allantoicase was twofold higher. Exposure to pH 10 water had only a transient effect on nitrogen excretion, and OUC enzymes and allantoicase activities were unchanged. The rate of urea and ammonia excretion increased with an increase in the dietary protein content, but OUC or uricolytic enzymes were not induced. These results demonstrate that O. niloticus, unlike Lake Magadi tilapia, do not recruit the hepatic OUC, even under environmental or dietary conditions that enhance urea excretion. Rather, uricolysis and argininolysis appear to be the major routes for ureagenesis.