Utilization of isomers and analogs of amino acids and other sulfur-containing compounds

Rats and chicks obtain little or no bioefficacy from the D-isomers of lysine, threonine, arginine, histidine and cystine. D-isomers of the branched-chain amino acids, i.e., leucine, isoleucine and valine (LEU, ILE, VAL) are utilized relatively well by chicks but poorly by rats. D-tryptophan is utilized efficiently by rats and pigs, but inefficiently by chicks, poults, mice and humans. The D-forms of phenylalanine (PHE), tyrosine and methionine (MET) are utilized well by both rats and chicks. Keto and hydroxy analogs of LEU, ILE, VAL, PHE and MET are utilized by both rats and chicks. The L-isomer of the alpha-hydroxy analogs of ILE and PHE has bioactivity while the D-isomer has no bioactivity. D-OH-LEU and D-OH-VAL have bioactivity, but they are less efficacious than their corresponding L-OH analogs. D-OH-MET is a more efficient precursor of L-MET than is L-OH-MET. In general, alpha-keto analogs of LEU, ILE, VAL, PHE and MET are utilized no more efficiently than the active isomeric form of the alpha-OH analogs of these amino acids. The alpha-OH and alpha-keto analogs of ILE are utilized relatively efficiently when used to replace only a portion of L-ILE in a purified amino acid diet for either rats or chicks; utilization of L-ILE analogs is poor, however, when all of the dietary L-ILE is replaced by analog material. In addition to MET and cyst(e)ine, many other dietary sulfur compounds must be considered when dealing with sulfur amino acid bioactivity. Glutathione, taurine, N-acetyl-MET, lanthionine and inorganic sulfate all have sulfur amino acid bioactivity under certain circumstances. Likewise, both oral and endogenous carnosine have histidine bioactivity, and purines, pyrimidines and urea have dispensable amino acid biosynthetic activity.