Studies in Amino Acid Utilization: I. The Dietary Utilization of Mixtures of Purified Amino Acids in Protein-Depleted Adult Albino Rats: Four Figures

Abstract

Weight-recovery of comparable pairs of protein-depleted rats was determined while the animals were fed “amino acid rations” for a 10-day period. In most instances 1 rat was offered a ration containing a mixture of 16 amino acids patterned after the amino acid composition of casein while the second received the same mixture with one indispensable amino acid omitted. In most cases the rations were reversed for a second 10-day period. In some experiments fewer amino acids were employed. The results may be listed as follows: Rats fed the amino acid ration containing 16 amino acids (ration A) gained weight rapidly, usually exhibiting as great or greater weight gains in 10 days as rats fed a ration containing an isonitrogenous quantity of casein; they also the the ration completely. Omission, singly, of histidine, lysine, tryptophane, phenylalanine, methionine, threonine, leucine, isoleucine or valine from this ration led to marked loss of weight in a 10-day period, coincident with a prompt loss of appetite as evidenced by consumption of only from one-third to one-half of the daily ration offered. The weight loss in most cases was greater than that shown by comparable rats fed a low-protein basal ration for a similar period. When, however, the missing amino acid was added to the ration the rats quickly recovered lost appetite and rapidly regained lost weight. When arginine was omitted from the ration the rats gained weight at rates approximately equal to those of similar rats receiving the complete amino acid ration and their appetites were well-maintained. No histological differences in spermatogenesis could be seen after 30 days' feeding of the respective rations. A ration containing only the 10 amino acids indispensable for rat growth, fed at a nitrogen level equivalent to that in ration A, supported a weight gain equal to or greater than that of similar rats receiving the 16 amino acids. Similarly, when only the 9 amino acids listed above were fed, an equivalent rate of weight gain was obtained. With both types of ration good appetites were maintained over a 20-day period. When 1 amino acid (lysine) was administered parenterally and the others were consumed by mouth, the weight increase was comparable to that of rats receiving the complete ration by mouth; good appetite was also maintained. When, however, salt solution was substituted for the lysine, both the animal's weight and appetite declined. The rapidity with which appetite declines in these protein-depleted rats after omission of any one of 9 indispensable amino acids from the ration suggests the development of a state of acute amino acid deficiency which can be corrected only by the addition to the ration of adequate amounts of the missing amino acid. Furthermore, no evidence of a need for an accessory polypeptide factor or factors to promote effective utilization of these amino acids was demonstrated by these experiments in which the only supplementary constituents were pure crystalline vitamins. In the biological evaluation of protein, dilution of the test-protein to the point where acute amino acid deficiency occurs prevents an adequate quantitative estimation of nutritive quality. It is suggested that ultimately it may become necessary to evaluate proteins in terms of the limiting action of each indispensable amino acid in the test-protein rather than to attempt to evaluate amino acid levels indirectly by current methods.