Metabolism in Different Organs

The liver is responsible for most of the deamination of amino acids, except for the branched-chain amino acids, which are transaminated in muscle. Oxidation of amino acids is one of the main sources of energy for the liver. The liver is also the main site of gluco-neogenesis, extracting large amounts of glutamine and alanine from the plasma for this purpose. The liver is the only site of urea synthesis.

Skeletal and cardiac muscle and adipose tissue are the main sites for transamination of the branched-chain amino acids, and the resulting ketoacids are transported to the liver for oxidation. However, in fasting and diabetes the capacity of muscle to oxidize branched-chain ketoacids increases markedly. In the postabsorptive state there is a net loss of amino acids from muscle, whereas in the fed state there is net uptake, reflecting the changes in net protein deposition and loss. However, at all times there is net output of alanine and glutamine from muscle, representing the disposal of the amino groups from the branched-chain amino acids. Muscle also takes up glucose, which is metabolized to supply the carbon skeletons for alanine and glu-tamine. Thus, there is a well-recognized glucose-alanine cycle between muscle and liver (Figure 11).

The kidney is a prime site of glutamine deamidation, producing ammonium to maintain acid-base balance and regulate the pH of the urine. Glutamine also serves as a substrate for gluconeogenesis in the kidney.

Glutamine is the major energy source for the small intestine, and at least part of the glutamine is derived from the lumen of the gut. Much of the glutamine is metabolized to pyruvate, which is then transaminated and exported to the liver as alanine. Some glutamine is also converted by the gut to citrulline, which then circulates to the kidney to be converted to arginine. Glutamine is also a major energy source for lymphocytes and monocytes when the immune system is activated.

See also: Amino Acids: Chemistry and Classification; Specific Functions. Folic Acid. Inborn Errors of Metabolism: Classification and Biochemical Aspects; Nutritional Management of Phenylketonuria. Niacin. Protein: Synthesis and Turnover; Digestion and Bioavailability. Vitamin B6.

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