Studies in Amino Acid Utilization

Abstract

Protein-depleted rats can synthesize new plasma protein and erythrocytes on a diet in which the sole source of amino nitrogen is a mixture of crystalline amino acids. The mixture includes tryptophane, lysine, methionine, histidine, threonine, valine, leucine, isoleucine, phenylalanine, and arginine, in relative proportions equivalent to those found by chemical analysis of casein. Small amounts of 6 non-essential amino acids were added to bring the nitrogen level to the equivalent of 9% protein (N × 6.25). Moreover, synthesis occurs to an extent comparable to that produced by a diet containing casein at an equivalent nitrogen level. With synthetic diets identical to the above, but lacking one of the above mentioned amino acids other than arginine there is poor diet consumption and no significant gain in serum protein, just as with the complete absence of protein from the diet. The same is true concerning the production of erythrocytes. On a synthetic amino acid diet entirely lacking arginine a rat can synthesize proteins and erythrocytes to an extent equalling that produced by casein or amino acid mixtures at equivalent nitrogen levels and containing all essential amino acids including arginine. Furthermore, the first mentioned 9 amino acids alone can act as a nitrogen source capable of inducing the formation of serum proteins to an extent comparable to that found with the mixture of 16 amino acids (ration A or “synthetic casein”) or the natural protein. Therefore, it can be said that 9 amino acids only are indispensable in the hypoproteinemic rat for the construction of serum protein and erythrocytes.