Amino acids (AA) are not only used for protein synthesis, but are precursors of a multitude of biologically active substances. Among them are, for instance, hormones and neurotransmitters like thyroxine, adrenaline, noradrenaline, acetylcholine, glutamate, and y-amino butyric acid. In order to maintain a continuous supply for synthesis of these compounds, as well as proteins, control mechanisms are crucial.
The central site of this control is the liver. Postprandially, the liver degrades a significant portion of the incoming AA, converting the N into urea for excretion (see below). Approximately a third of the AA are used for synthesis of liver and plasma proteins. Regulation of these catabolic and anabolic pathways is so tight that even a protein-rich meal leads to only a slight increase in plasma AA concentrations.
Approximately 70 % of the increase in plasma AA is due to branched-chain AA (Val, Leu, Ile) since the liver is unable to transaminate them. These AA can be transaminated in muscle, brain, and kidneys, though, making them highly significant for the metabolism of those organs. For instance, they inhibit a specific glutamine membrane transporter in skeletal muscle. The resulting increase in intracellular Gln may be a signal that initiates muscle protein synthesis.
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