Metabolism (amino acids)

The metabolism of amino acids largely overlaps with protein metabolism. It is considered here in terms of:

• digestion
• absorption
• degradation
• synthesis

The physiological 'pool' of amino acids is in a relatively greater degree of change compared to most dietary components. Up to 100 g/day of endogenous protein is broken down; the vast majority of this is resynthesized. Protein is lost as:

• keratin in hair
• mucocutaneous secretions e.g. lysozyme in tears, plasma proteins in cuts or burns
• cellular proteins in shed keratinocytes
• stool proteins e.g. digestive enzymes which have not been reabsorbed; excessive loss is seen in the protein-losing enteropathies
• urinary protein; most is reabsorbed during passage through the renal tubules; failure of reabsorption may result in proteinuria

Amino acids within the bloodstream are taken up by specific protein carriers within the membranes of body cells. These carriers are capable of conveying amino acids against a concentration gradient. They do this by co-transport with sodium ions; the energy inherent within the sodium ion concentration gradient is used to drive the symport.