Incorporation of Amino Acids into Other Compounds

Tabje.2.7 lists some of the important products made from amino acids, directly or in part. The list is not inclusive and is meant to highlight important compounds in the body that depend upon amino acids for their synthesis. Amino acids are also used for the synthesis of taurine ( 20, 24, 25), the "amino acid-like" 2-aminoethanesulfonate found in far higher concentrations inside skeletal muscle than any amino acid ( 10). Glutathione, another important sulfur-containing compound (26, 27), is composed of glycine, cysteine, and glutamate.

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Table 2.7 Important Products Synthesized from Amino Acids

Carnitine (28, 29) is important in the transport of long-chain fatty acids across the mitochondrial membrane before fatty acids can be oxidized. Carnitine is synthesized from e-W,W,M-trimethyllysine (TML) (30). TML synthesis from free lysine has not been demonstrated in mammalian systems; rather TML appears to arise from methylation of peptide-linked lysine. The TML is released when proteins containing the TML are broken down ( 30). TML can also arise from hydrolysis of ingested meats. In contrast to 3-methylhistidine, TML can be found in proteins of both muscle and other organs such as liver ( 31). In rat muscle, TML is about one-eighth as abundant as 3-methylhistidine. Using comparisons of 3-methylhistidine to TML concentration in muscle protein and rates of 3-methylhistidine release in the rat ( 32), Rebouche estimated that protein breakdown in a rat would release about 2 pmol/day of TML, which could be used for the estimated 3 pmol/day of carnitine synthesized (30). These calculations suggest that carnitine requirements can be met from synthesis from TML from protein plus the carnitine from dietary intake.

Amino acids are the precursors for a variety of neurotransmitters that contain N. Glutamate may be an exception in that it is both a precursor for neurotransmitter production and is a primary neurotransmitter itself (33). Glutamate appears important in a variety of neurologic disorders from amyotrophic lateral sclerosis to Alzheimer's disease (34). Tyrosine is the precursor for catecholamine synthesis. Tryptophan is the precursor for serotonin synthesis. A variety of studies have reported the importance of plasma concentrations of these and other amino acids upon the synthesis of their neurotransmitter products; most commonly cited relationship is the increase in brain serotonin levels with administration of tryptophan.

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