Determination of D- and L-enantiomers of methionine and [2H3]methionine in plasma by gas chromatography-mass spectrometry

A method for the stereoselective determination of D- and L-enantiomers of both methionine and [(2)H3]methionine in rat plasma was developed using gas chromatography-mass spectrometry with selected-ion monitoring (GC-MS-SIM). DL-[(2)H7]Methionine was used as analytical internal standard to account for losses associated with the extraction, derivatization and chromatography. The amino acids were purified by cation-exchange chromatography using BondElut SCX cartridge and derivatized with HCl in methanol to form methyl ester followed by subsequent N-acylation with optically active (+)-alpha-methoxy-alpha-trifluoromethylphenylacetyl chloride to form diastereomeric amide. Quantification was performed by SIM of the molecular-related ions of the diastereomers on the chemical ionization mode. Endogenous L-methionine concentrations in 50 microl of rat plasma were measured with relative intra- and inter-day precision of 4.0 and 6.3%, respectively. The intra- and inter-day reproducibility in the amounts of D- and L-[(2)H3]methionine determined were in good agreement with actual amount added.