Emerging aspects of amino acid metabolism. Where do we go from here?

The mechanisms are discussed by which the body maintains nitrogen balance and constancy of the amount and pattern of body proteins. A distinction is made between effectors, such as hormones, and regulators. Free amino acid concentrations, which are maintained remarkably constant, may play an important role. The key step in the regulation of urea production could be the stimulation of acetyl glutamate synthesis by amino acids entering the liver from the portal tract and the consequent activation of carbamoyl-phosphate synthetase I. Both protein synthesis and breakdown are regulated to some extent by amino acid supply. It is probable that the effects of diet are exerted mainly on the turnover of visceral proteins. Evidence is presented for the existence of a substantial pool of protein that turns over by lifetime kinetics. Such a pool could complicate the interpretation of data obtained in kinetic studies.