ABSTRACT
Several of the macromolecules necessary for life, in particular nucleic acids and proteins, contain nitrogen. These macromolecules are continuously turned over through degradation to nucleotides and amino acids, and then re-synthesised. The majority of the nitrogenous end-products are initially in the form of ammonia which is highly toxic, and necessitates efficient excretion or detoxification. Excretory nitrogen metabolism interacts closely with water homeostasis. Most teleosts are ammonotelic, excreting predominately ammonia as their nitrogenous end-product. Several other nitrogenous compounds have been detected during excretion measurements of fishes, including amino acids, peptides, uric acid, creatine, creatinine, and trimethylamines. Amino acids, a major source of ammonia following deamination, occupy a pivotal position in the nitrogen metabolism of animals. Ammonia is usually the immediate end-product of nitrogen metabolism in aquatic vertebrates.
