Nitrogenous Substrates: Nucleic Acids to Amino Excretion

Nitrogen is often the most limiting nutrient during growth and reproduction because nitrogenous compounds (proteins, nucleic acids) are the basis for the structure and function of animals. Proteins consist of 20 common amino acids, of which approximately half cannot be synthesized by the animal and so must be supplied by the diet or by microbial synthesis in the digestive tract. The quality of food protein is therefore determined by its amino acid composition; high-quality food proteins (high biological value) closely match those synthesized by the animal. Amino acids are deaminated before the carbon is used for energy or glucose production. The N released from amino acid catabolism is potentially toxic. Ammonium is highly toxic but easily excreted by fish into the surrounding water. Urea and uric acid are non-toxic forms of waste N that are produced by terrestrial animals. Herbivores that consume low-N diets can recycle waste N by degrading urea or uric acid with microbes in the digestive tract. Nitrogen recycling minimizes the loss of fecal and urinary N, which reduces the requirement for N.