Implications of the cellular transport step for amino acid metabolism.

e do well to place the physicochemical W step of amino acid transport on our metabolic diagrams, wherever it intervenes, on a par with the more purely chemical steps we assemble on such charts. This recommendation does not mean, as some suppose, that we merely insert the step of intestinal absorption between digestion and intermediary metabolism; it means that in addition we insert the step of the uptake and the modulated release of amino acids by the hepatocyte, along with the hepatic interconversions of amino acids and other hepatic contributions to their metabolism. It means we should consider the step of uptake and release by muscle along with muscle metabolism, the transfer across the bloodbrain barrier with brain metabolism, and the step of transfer into the renal epithelial cell from the blood plasma with amino acid uptake from the proximal tubular lumen and with the release phases of the same two transports. We are at a stage of the study of amino acid metabolism where we deal not merely with the aggregate effects of the four possibilities (synthesis, access, interconversion and degradation of amino acids) but we also need to consider the relative fluxes of all such events. Thus, we become able to recognize how dominant are some of the possible fluxes (e.g., of alanine from muscle to liver in gluconeogenesisl) and how small are some others. Also, we are able to know the steady state concentrations