Bioconversion of 2-amino acids to 2-hydroxy acids by Clostridium butyricum

Analysis by gas chromatography-mass spectrometry (GC-MS) of 24-h cultures of Clostridium butyricum type strain in synthetic BMG medium supplemented with various 2-amino acids (10 mM) revealed the presence of the corresponding 2-hydroxy acids. C. butyricum was able to bioconvert l-valine, dl-norvaline, l-leucine, dl-norleucine, l-methionine and l-phenylalanine as well as unusual 2-amino acids, i.e., l-2-aminobutyric acid, l-2-amino-4-pentenoic acid, dl-2-aminooctanoic acid, and dl-2-amino-4-phenylbutanoic acid. l-Isoleucine and cycloleucine were not converted into their corresponding 2-hydroxy acids. The bioconversion rate was maximal with dl-norvaline (6.2%). Chiral GC analysis demonstrated that only d-2-hydroxy-4-methylpentanoic acid is formed from l-leucine, indicating that the bioconversion is stereospecific, with inversion of configuration. d-Leucine and d-methionine were also converted to the corresponding 2-hydroxy acids. This observation opens new aspects in the study of C. butyricum and raises questions about the amino acid metabolism by this species.