Comparative utilization of the isomers of phenylalanine and phenyllactic acid by chicks and rats

A series of experiments was conducted to assess quantitatively the growth-promoting capacity of the isomers of phenylalanine (Phe) and phenyllactic acid (PLA). The slope-ratio procedure was used to determine percent efficacy relative to l-Phe. Addition of the isomers of Phe to a Phe-deficient chemically-defined diet produced linear growth and feed efficiency responses. Only the l-isomer and racemic mixture of PLA elicited a gain or gain:feed response. Efficacy estimates for male chicks were: dl-Phe, 90.1%; d-Phe, 74.6%; l-PLA, 70.1%; dl-PLA, 5.9%. The comparative utilization of d-Phe, phenypyruvic acid (PPA) and l-PLA by chicks was also evaluated when these compounds were fed to male chicks as the sole source of dietary Phe. PPA activity, 84.8%, was slightly greater than that of either d-Phe, 75.3%, or l-PLA, 67.4%. Efficacies of l-PLA and dl-PLA were 46.1 and 28.0%, respectively, d-PLA had no growth-promoting activity for the rat.