THE UTILIZATION OF THE OPTICAL ISOMERS OF PHENYLALANINE, AND THE PHENYLALANINE REQUIREMENT FOR GROWTH

Abstract

In the use of diets containing mixtures of amino acids in place of proteins one is frequently confronted with the necessity of employing synthetic amino acids, since the latter may be more readily available than the optically active forms which occur in proteins.Therefore, it becomes of considerable practical importance, as well as of theoretical interest, to determine the relative availability in the animal organism of the enantiomorphs of the essential amino acids.Du Vigneaud et al. ( 9) and Berg (1) have shown that the optical isomers of tryptophane possess equal nutritive value for the rat.This is true also of histidine, except that the dextrorotatory form appears to be slightly less effective than the naturally occurring I(-) type (Cox and Berg (2)).Jackson and Block (4) have reported that both Z( -)-and d(+)-methionine successfully supplement diets low in methionine and cystine.This conclusion has been confirmed in unpublished experiments in this laboratory under conditions which preclude the presence of more than minute traces of either cystine or methionine in the basal ration.The present paper' is concerned with a comparison of the growth effects of I( -)-and d(+)-phenylalanine, and with a semiquantitative estimate of the minimum amount of this amino acid which is capable of inducing optimum increases in weight. EXPERIMENTALWeanling rats served as the experimental subjects throughout.These were housed in individual cages and were permitted to consume the food ad Zibitum.All amino acids were purified until they yielded correct analytical values.In the first experiments the nitrogen of the basal diet was derived from a