Amino Acids in Nitrogen Metabolism with Particular Reference to the Role of Methionine

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Abstract
A marked reduction in the quantity of nitrogen excreted in the urine by standardized rats partially depleted of their bodily reserves of protein follows the introduction of whole eggs into a protein-free ration. This effect may be ascribed to the amino acid content of the supplement. It has been shown that the influence of an equivalent quantity of nitrogen supplied in the form of the 10 essential amino acids is as marked as that of eggs providing 3.5% of dietary protein. Depressions in the excretion of urinary nitrogen lead to a marked sparing of body tissue. The validity of the concept, “body nitrogen spared,” has been tested by relating changes in the nitrogen balance in the 2 metabolism periods to changes in body weight. Cystine, choline, and all of the essential amino acids except phenylalanine, valine, and tryptophane exert some action in sparing body nitrogen. Of the group, methionine is the most powerful in this respect. No single amino acid, however, can be very important quantitatively in sparing body tissue in the partially depleted animal, in view of the complex demands of the body for the synthesis of its structural and functional components. Egg proteins and a mixture of the essential amino acids are much more effective in this respect than is any individual amino acid, as indicated by the quantities of body nitrogen spared and by restoration of weight loss when the nitrogen-poor ration was fed. Analyses of whole carcasses, liver, and muscle of adult rats suggest that the effect of methionine in nitrogen metabolism is specific. Continued loss in body weight, the lack of change in the total nitrogen content of the carcass, the constancy of the ratio of methionine nitrogen to total nitrogen of the carcass, and the increment in hepatic tissue when methionine is fed to the depleted animal point to the possibility that this amino acid does not act in the general maintenance of body tissues but in the synthesis of functional proteins and important metabolites.