Antagonism of the D-Forms of the Essential Amino Acids Toward the Promotion of Growth by D-Histidine

Growth in rats fed diets containing normal levels of the L-amino acids plus glycine, is only moderately retarded by replacing the L-histidine with its D-isomer. Tests were made of the simultaneous replacement with double quantities of the DL-isomers of either the group of essential amino acids whose D-isomers are poorly invertible (lysine, threonine, isoleucine, leucine, and valine) or of the other members of the group whose D-isomers are readily invertible (methionine, tryptophan, phenylalanine, and arginine). Such replacement in either group markedly impaired the response to the D-histidine diet, but only slightly that to the L-histidine diet. Doubling the L-amino acids in either group induced less retardation in the D-histidine diets. D-Lysine and D-threonine were largely, but not completely, responsible for the markedly retarded growth produced in the D-histidine diets which contained the poorly invertible group of amino acids in the DL-form. Replacement by its D-isomer of L-methionine, L-phenylalanine, L-tryptophan, or L-arginine also retarded the response to the D-histidine diet. Differences in percentage of the D-amino acid fed in the diet and in its susceptibility to attack by D-amino acid oxidase were probably responsible for the differences in degree of retardation noted.