Production of d-Branched-Chain Amino Acids by Lactic Acid Bacteria Carrying Homologs to Isoleucine 2-Epimerase of .

Isoleucine 2-epimerase (ILEP) is a novel branched-chain amino acid racemase isolated from . In this study, we examined production of free d-branched-chain amino acids such as d-valine, d-leucine, and d--isoleucine, using lactic acid bacteria carrying homologs to ILEP. Twelve selected strains of lactic acid bacteria were grown at optimal growth temperatures and accumulation of d-branched-chain amino acids in the medium was monitored in exponential, early stationary, and stationary phases. To analyze the d-branched-chain amino acids, enantiomers in the medium were initially converted into diastereomers using pre-column derivatization with -phthaldialdehyde plus -isobutyryl-l-cysteine. The resultant fluorescent isoindole derivatives were analyzed on an octadecylsilyl stationary phase using ultra-high performance liquid chromatography. The analyses revealed that the seven following lactic acid bacteria carrying homologs showing 53-60% amino acid sequence identity to the ILEP accumulate d-branched-chain amino acids: and produce d-valine, d-leucine, and d--isoleucine; subsp. , and subsp. accumulate d-leucine and d--isoleucine; and and produce d--isoleucine. These results suggest that d-branched-chain amino acids are produced by a variety of lactic acid bacteria species, particularly those carrying homologs to the ILEP.