Human Biochemistry, Volume 1 - Murray R. 1993

Protein and Amino Acid Metabolism
Catabolism of Amino Acid Nitrogen
Interorgan Amino Acid Metabolism Following a Meal

Following the intake of a protein-rich meal, Amino Acids are released from Internal Organs into the bloodstream, with branched-chain compounds predominating (Fig. 30.12). Valine, isoleucine, and leucine account for at least 60% of the total amino acids entering the systemic Circulation (as opposed to the portal system), even though these amino acids comprise only about 20% of the total amino acid content in dietary Proteins. As amino acids are released from internal organs postprandially, they are actively taken up by Muscles, with branched-chain amino acids being the primary targets. During the first postprandial hour, branched-chain amino acids constitute at least 50% of the amino acids extracted by Muscle tissue. After 2–3 hours, the proportion of these amino acids delivered to peripheral Tissues reaches 90–100%. The branched-chain amino acids oxidized in muscle in response to food intake likely serve as the primary donor of amino groups utilized in the amination of Pyruvate to form Alanine.

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Fig. 30.11. Glucose-alanine cycle. Alanine is synthesized in muscle via the Transamination of pyruvate derived from glucose; it is then released into the bloodstream and taken up by the Liver. In the liver, the carbon Skeleton of alanine is converted back into glucose, which enters the circulation, is transported to the muscle, and is subsequently used for alanine synthesis. (Reprinted with permission from Felig P.: Amino acid METABOLISM in man. Annu. Rev. Biochem. 1975:44:938. © 1975 by Annual Reviews, Inc.)

Fig. 30.12. Overview of interorgan amino acid metabolism immediately following a meal.

Thus, branched-chain amino acids play a specialized role in Nitrogen metabolism both during the postabsorptive state, when they serve as an energy source for the Brain, and immediately postprandial, when they are preferentially taken up by muscle tissue. In muscle, they appear to function as a crucial source of both energy and nitrogen.