Application of L-methionine γ-lyase in chiral amino acid analysis

Here, a conventional chiral amino acid analysis method using high-performance liquid chromatography was coupled with a sample pretreatment using L-methionine γ-lyase from Pseudomonas putida ICR 3460 for the selective analysis of L-methionine and L-tryptophan. The sample was analyzed after the degradation of L-methionine with L-methionine γ-lyase, as L-methionine coelutes with L-tryptophan under the standard chiral amino acid analytical conditions used for precolumn derivatization with o-phthalaldehyde and N-acetyl-L-cysteine. The L-tryptophan in the sample was then eluted as a clearly separated peak and analyzed further. Since the L-methionine γ-lyase did not act on L-tryptophan, we were able to calculate the L-methionine or L-tryptophan concentration based on the data obtained from 2 individual runs: the sample with and without L-methionine γ-lyase pretreatment. The concentration of L-tryptophan was calculated from the data obtained from the sample with L-methionine γ-lyase pretreatment, while the concentration of L-methionine was calculated using the following equation: L-methionine concentration = {the data from the sample without L-methionine γ-lyase pretreatment}-{the data from the sample with L-methionine γ-lyase pretreatment}. Model samples containing authentic amino acids and a fermented food sample were analyzed by our method, and the calculated concentrations of L-methionine and L-tryptophan were consistently in agreement with the theoretical values.