the maintenance of steady-state concentrations of circulating plasma amino acids between meals depends on the net balance between release from endogenous protein stores and utilization by various tissues. Muscle generates over half of the total body pool of free amino acids, and liver is the site of the urea cycle enzymes necessary for disposal of excess nitrogen. Muscle and liver thus play major roles in maintaining circulating amino acid levels.

Figure 1 summarizes the postabsorptive state. Free amino acids, particularly alanine and glutamine, are released from muscle into the circulation. Alanine is extracted primarily by the liver, and glutamine is extracted by the gut and the kidney, both of which convert a significant portion to alanine. Glutamine also serves as a source of ammonia for excretion by the kidney. the kidney provides a major source of serine for uptake by peripheral tissues, including liver and muscle.

Fig1. Interorgan amino acid exchange in normal postabsorptive humans. The key role of alanine in amino acid output from muscle and gut and uptake by the liver is shown.

Branched-chain amino acids, particularly valine, are released by muscle and taken up predominantly by the brain.

Alanine is a key gluconeogenic amino acid (Figure 2). the rate of hepatic gluconeogenesis from alanine is far higher than from all other amino acids. the capacity of the liver for gluconeogenesis from alanine does not reach saturation until the alanine concentration reaches 20 to 30 times its normal physiologic level. Following a protein-rich meal, the splanchnic tissues release amino acids (Figure 3) while the peripheral muscles extract amino acids, in both instances predominantly branched-chain amino acids. Branched-chain amino acids thus serve a special role in nitrogen metabolism. In the fasting state, they provide the brain with an energy source, and post prandially they are extracted predominantly by muscle, having been spared by the liver.

Fig2. The glucose-alanine cycle. Alanine is synthesized in muscle by transamination of glucose-derived pyruvate, released into the bloodstream, and taken up by the liver. In the liver, the carbon skeleton of alanine is reconverted to glucose and released into the bloodstream, where it is available for uptake by muscle and resynthesis of alanine.

Fig3. Summary of amino acid exchange between organs immediately after feeding.

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مواضيع ذات صلة

Amino Acid Oxidases Remove Nitrogen as Ammonia

Biosynthesis of Urea

ATP & Ubiquitin-Dependent Degradation

Biosynthesis of the Nutritionally Nonessential Amino Acids

Nutritionally Essential & Nutritionally Nonessential Amino Acids

Cholesterol Synthesis, Transport, & Excretion: Clinical Aspects

Cholesterol is Excreted from The Body in The Bile as Cholesterol or as Bile Acids

Cholesterol is Transported Between Tissue in Plasma Lipoproteins

The Cholesterol Balance in Tissues is Tightly Regulated

Cholesterol Synthesis is Controlled by Regulation of HMG-CoA Reductase

Cholesterol is Biosynthesized From Acetyl-CoA

Brown Adipose Tissue Promotes Thermogenesis