Nutritional and metabolic responses to arginine deficiency in carnivores.

The metabolic basis for the high dietary arginine requirement of the cat appears to be primarily the low activity of the enzyme pyrroline-5-carboxylate synthase (P5C synthase) in the intestinal mucosa. P5C synthase is required for de novo production of glutamyl-gamma-semialdehyde, the immediate precursor for the synthesis of ornithine from glutamate. The next enzyme in ornithine synthesis, ornithine amino-transferase, in the cat intestinal mucosa shows low activity, which provides an additional barrier to ornithine and citrulline formation. It is suggested that the low activities of these enzymes corroborate other evidence that indicates that the cat evolved as a strict carnivore. The dog has a requirement for arginine intermediate between the cat and the rat, which is consistent with the dog having an omnivorous diet during its evolution. It is suggested that during periods of fasting, depletion of urea cycle intermediates in the cat results in some conservation of nitrogen while maintaining urea cycle enzymes at a relatively high level. However, after ingestion of animal protein (and arginine) the urea cycle of cats is capable of rapidly responding to the ammonia load, which rises from the deamination of amino acids. By this method of regulation the cat can respond rapidly to short-term fluctuations in protein intake.