Post-absorptive metabolism of amino acids

摘要

Amino acids (AA) are catabolised by various tissues as they journey from absorption to the site of anabolism. Such catabolism varies between tissues and in response to nutritional, physiological and environmental factors that alter the qualitative andquantitative patterns that flow in and out of various organs. The digestive tract can oxidise most essential AA, except threonine, but for some (e.g. lysine) such catabolism only occurs from enteral sources. AA oxidation appears sensitive to dietary protein supply while low net absorption of threonine is probably linked to poorly reabsorbed endogenous secretions into the lumen. The liver regulates concentrations in arterial plasma by removing AA not used by peripheral tissues, rather than active control of post-splanchnic supply. Use of AA for muscle protein gain does not appear to be limited by the amount provided by either arterial supply or transport into the cells. Instead, there may exist a sensing mechanism that links extracellular concentrations or the relative rates of AA intracellular influx (or efflux) to regulatory components of the translation cascade. Control of this cascade involves both AA and insulin, either separately or in combination. Unlike muscle, the mammary gland appears able,under appropriate conditions, to utilise more than 90% of the AA that is supplied to and enters the cells. The efflux of AA in the udder may be less important than in muscle.