Biochemical basis and efficacy of using L-Aminoacid precursors in animal nutrition
Most amino acids (AA) are made via fermentation synthesis, although some are made via chemical synthesis (e,g. DL-Met) or from chemical extraction processes (e.g. L-Cys). Chemical synthesis generally results in the DL-racemic mixture (1:1 ratio of D- and L-isomer) while fermentative synthesis and chemical extraction generally yield the L-isomer. Dietary AA or AA precursors must be converted to the L-AA in the animal body in order for prorein synthesis to procced. The aim of this review is to present the biochemical basis and efficacy of the use of precursors of L-aminoacids in animal nutrition. D-forms of AA that have biological efficacy are converted to L-isomers via a two-step reaction sequence involving oxidation to the keto analogue amd then transamination of the keto analogue to the L-AA. For biological efficacy, hydroxy-substituted AA analogues must first be enzymatically oxidized to the keto analogue, and then transaminated to the L-AA. Relative bioavailability (RBV) of AA isomers, analogues and precursors is expressed as growth efficacy percentages of the L-isomer, which is assumed in all cases to represent 100% RBV. The α-keto analogue of Lys is not formed in the principal pathway of Lys degration. D-Lys and α-keto analogue of Lys have no biological efficacy fo animals. Thr is catabolized initially via dehydratase, dehydrogenase or aldolase reactions, and none of these reaction sequences yields the α-keto analogue of Thr. Hence, D-Thr has no biological efficacy. D-Trp is utilized well by pigs and rats. Pigs obtain good activity from D-Trp, with values ranging from 70% to 100%. The D-isomer of Met is utilized well as an L-Met precursor except in humans. The efficacy of D-Met for pig growth is 100%. The keto analogue of cysteine is not produced in metabolism, and thus neither the keto analogue of cysteine nor D-cysteine have L-cysteine activity. The α-hydroxy analogue of Met (OH-Met) is an important commercial product. This compound is made chemically and therefore it is a 1:1 mixture of D- and L-OH-Met. Two separate enzymes are necesary to convert DL-Met to the α-keto analogue of Met, a dehydrogenase for D-OH-Met and an oxidase for L-OH-Met. The keto analogue of Met is then transaminated to L-Met. Table 2 gives DL-Met as having 80% molar efficacy in chicks, but 100% molar efficacy in young pigs. Neither chicks nor rats could use D-Arg as a precursor fof L-Arg. The α-keto analogue of Arg is not formed in metabolism. The α-keto analogue of His is not formed in His degradation. Thus, D-His would not be expected to have bioactivity as a precursor for L-His. D-Leu is copletely efficacious in chicks as an L-Leu precursor. Both D and L-isomers of the OH-analogue of Leu are well utilized by chicks. D-OH-Val is utilized at 70% efficiency in chicks, while the keto analogue of Val (α-ketoisovaleric acid) and L-OH-Val are utilized at 80% efficiency. D-isoleucine and L-alloisoleicine have no biological activity. D-alloisoleucine, however, has an efficacy value of 60% for chicks. With L-OH-isoleucine, chicks obtain 85% bioactivity from this compound, but D-OH-isoleucine has no bioafficacy for chicks. D-phenylalanine and D-tyrosine are utilized well by both chicks and rats.
