Aerobic and anaerobic ammonia production by fish

1. In comparison to other vertebrates, a relatively large part of energy consumption in fish is covered by protein catabolism. 2. In Teleosts, ammonia is the major component of nitrogen excretion, its production rate being directly related to the rate of protein oxidation, while urea is almost exclusively produced from nucleotides by uricolysis. 3. Aerobic ammonia excretion arises from extraction of blood ammonia by the gill. 4. Under aerobic conditions, ammonia originates mainly in the liver by transdeamination and the hydrolysis of imino groups, while an additional quantity is formed in working skeletal muscles by purine nucleotide cycling. 5. During a decline of environmental oxygen, the contribution of the liver to total ammonia production seems to be lowered, while that of skeletal muscles is increased. 6. Anaerobic ammoniogenesis seems to proceed via at least 4 different mechanisms, all occurring in goldfish: deamination of adenylates via adenylate deaminase, deamination of aspartate via the purine nucleotide cycle, breakdown of alanine to ethanol, CO2 and NH3, and oxidation of glutamate via a slowly spinning Krebs-cycle. 7. In goldfish, the combination of these mechanisms is able to sustain an anaerobic rate of ammoniogenesis being equal to that observed under normoxic conditions.