Human Biochemistry, Volume 1 - Murray R. 1993

Protein and Amino Acid Metabolism
Catabolism of Amino Acid Nitrogen
Amino Acid Turnover in the Postabsorptive State

Maintaining steady-state concentrations of plasma-circulating Amino Acids between meals (for example, overnight after dinner) depends on the balance between amino acid supply from endogenous protein reserves and their utilization across various Tissues. Muscle tissue accounts for the generation of over 50% of the total free amino acid pool, whereas the Liver houses The Urea Cycle Enzymes necessary for eliminating the End products of Nitrogen METABOLISM from the Organism. Thus, Muscles and the liver play a pivotal role in determining circulating amino acid levels and their turnover rates.

Muscles

More than 50% of $\alpha$-amino nitrogen released from muscle tissue is accounted for by Alanine and glutamine. Along with the release of significant amounts of $\alpha$-amino acids from the muscle, small quantities of Serine, Cysteine, and glutamate are continuously supplied to the muscle from the Circulatory system.

Liver and Intestines

The Liver and intestines (Internal Organs) continuously take up large amounts of alanine and glutamine from the plasma—specifically, those amino acids that are predominantly released from muscle tissue. The liver is the primary site of alanine uptake, whereas the intestinal tissue utilizes glutamine. In the intestines, the majority of the amino groups of glutamine are released from the tissue as alanine or in the form of ammonia. Like muscles, internal organs also take up serine.

Kidneys

The kidneys serve as the primary source of serine and, in addition, supply alanine in small but significant amounts. The kidneys take up glutamine, Proline, and Glycine from the circulatory system.

There is a fairly good correlation between the release of Most amino acids from Introduction/39.html">Muscles and their uptake by internal organs.

Brain

Among the amino acids taken up by the brain, valine is predominant. The capacity of the rat brain to oxidize branched-chain amino acids (leucine, isoleucine, and valine) is at least 4 times greater than that of muscles and liver. Although significant amounts of these Amino acids are released from muscle tissue in the postabsorptive state, they are not taken up by the liver, suggesting that the brain is their primary site of utilization.

Fig. 30.10 illustrates the overall pattern of Amino acid metabolism in the postabsorptive state. Free amino acids, primarily alanine and glutamine, enter the circulatory system from muscle tissue. Alanine, which appears to be the major transport form of nitrogen in plasma, is taken up mainly by the liver. Glutamine is taken up by the intestine and kidneys—in both organs, the major part of it is converted into alanine. Glutamine is also the primary source of ammonia excreted by the kidneys. The kidneys are the main source of serine, which is taken up by various tissues, including the liver and muscles. Branched-chain amino acids, especially valine, are released from muscles and taken up predominantly by the brain.

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Fig. 30.10. Interorgan Amino Acid metabolism in a healthy human after the completion of intestinal absorption. The key role of alanine in The amino acid flux from muscles and intestine, as well as its uptake by the liver, is shown. (Reproduced with permission of the author from the review by Felig P.: Amino acid metabolism in man. Annu. Rev. Biochem. 1975:44:937. © 1975 by Annual Reviews. Inc.)

Alanine serves as a key precursor for glucose derived from protein, i.e., it is a glucogenic amino acid (Fig. 30.11). In the liver, The rate of glucose synthesis from alanine and serine is much higher than that from Other Amino Acids. The capacity of the liver for Gluconeogenesis from alanine is remarkably high; it does not reach saturation even at an alanine concentration of 9 mM, which is 20–30 times higher than its physiological level. The predominance of alanine among the $\alpha$-amino acids released from muscle tissue reflects the synthesis of alanine in muscles via Transamination involving Pyruvate.