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Amino acids as precursors of physiologically important nitrogen compounds
As already pointed out, amino acids are also used for the synthesis of important nitrogen-containing com-pounds that, in turn, play critical roles in cell, organ, and system function. In carrying out these particular roles the amino acid-derived metabolites also turn over and they need to be replaced ultimately by the nitrogen and indispensable amino acids supplied by protein intake. Estimates on the quantitative utiliza-tion of these precursor and nonproteinogenic roles of amino acids in human subjects are limited but it is possible to give some examples.
●Arginine is the precursor of nitric oxide (NO); the total amount of NO synthesized (and degraded) per day represents less than 1% of whole body arginine flux and less than 1% of the daily arginine intake.
●In contrast, the rate of synthesis and degradation of creatinine is relatively high and accounts for 10% of the whole body flux of arginine and for 70% of the daily intake of arginine.
Similarly, the synthesis and turnover of glutathione (a major intracellular thiol and important antioxi-dant, formed from glutamate, glycine, and cyste-ine) accounts for a high rate of cysteine utilization such that it greatly exceeds the equivalent of the usual daily intake of cysteine. Since continued glu-tathione synthesis involves a reutilization of endog-enous cysteine, a low intake of dietary methionine and cyst(e)ine would be expected to have an unfa-vorable influence on glutathione status and synthe-sis. This has been shown experimentally to be the case, especially in trauma patients and those suffer-ing from acquired immunodeficiency syndrome (AIDS). Because glutathione is the most important intracellular antioxidant that protects cells against damage by reactive oxygen species, this would mean that particular attention should be paid to such amino acids in nutritional therapy in these groups of patients.
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