Comparative Utilization of the Isomers of Phenylalanine and Phenyllactic Acid by Chicks and Rats

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Abstract
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, 36.9%; d-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 d-Phe and l-PLA were similar whether fed as supplements to a Phe-deficient diet or when fed as the sole source of dietary Phe activity. Isomers of Phe and PLA were also fed as the sole dietary source of Phe to growing rats. dl-Phe (80.4%) and d-Phe (68.1%) were well utilized by the rat. Efficacies of l-PLA and dl-PLA were 46.1 and 28.0%, respectively. d-PLA had no growth-promoting activity for the rat.