Comparative Utilization of the α-Keto and d- and l-α-Hydroxy Analogs of Leucine, Isoleucine and Valine by Chicks and Rats

Abstract

Several experiments were conducted to quantitatively evaluate the growth-promoting capacity of the isomers of the branched-chain amino acids (BCAA) and their α-hydroxy and α-keto analogs for the chick and rat. Basal chemically defined diets were formulated to be singly deficient in the BCAA under study; analogs therefore were evaluated as sources of supplemental amino acid activity. dl-Isoleucine (dl-Ile), α-keto-β-l-methylvaleric acid (KMV) and the isomers of α-hydroxy-β-methylvaleric acid (HMV) were inferior to an isomolar level of l-Ile and varied considerably in their capacity to support growth of chicks and rats. Efficacy values of the compounds for chicks (C) and rats (R) were: dl-Ile 85% (C), 58% (R); KMV 60% (C), 38% (R); l-HMV 84% (C), 65% (R); dl-HMV 49% (C), 45% (R); and d-HMV -10% (C), -2% (R). The isomers of valine (Val) and α-hydroxyisovaleric acid (HIV) had efficacy values in chicks of: dl-Val, 84%; d-Val, 72%; l-HIV, 82%; dl-HIV, 79%; d-HIV, 66%. d-Val had only marginal growth-promoting activity for rats, 16%. All Val analogs had comparable efficacy values for the rat: α-ketoisovaleric acid (KIV), 49%; l-HIV, 54%; dl-HIV, 51%; and d-HIV, 46%. dl-Leu and d-Leu, α-ketoisocaproic acid (KIC), and l-, dl- and d-α-hydroxyisocaproic acid (HIC) were all less efficacious than isomolar amounts of supplemental l-Leu in rats. Efficacy values were: dl-Leu, 78%; d-Leu, 48%; KIC, 56%; l-HIC, 58%; dl-HIC, 55%; and d-HIC, 41%. The BCAA keto analogs were not superior to the l-α-hydroxy analogs in growth-promoting capacity, and this is of great interest for the potential use of these nitrogen-free amino acid analogs in nutritional therapy.