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Nutritional Methodology

Direct Detection and Evaluation of Conversion of D-Methionine into L-Methionine in Rats by Stable Isotope Methodology¹

Department of Pathophysiology, School of Pharmacy, Tokyo University of Pharmacy and Life Science, 1432–1 Horinouchi, Hachioji, Tokyo 192-0392, Japan

²To whom correspondence should be addressed. E-mail: hasegawa{at}ps.toyaku.ac.jp.

The stereoselective kinetics of methionine enantiomers in rats was investigated to evaluate the fraction that converted from D-methionine to the L-enantiomer using a stable isotope methodology. After bolus i.v. administration of D- or L-[²H₃]methionine, their plasma concentrations and that of endogenous L-methionine were determined by a stereoselective GC-MS method. L-[²H₃]Methionine appeared rapidly after administration of D-[²H₃]methionine, whereas D-[²H₃]methionine was not detected after administration of L-[²H₃]methionine. The fraction of conversion of D-[²H₃]methionine into L-[²H₃]methionine was estimated using the area under the plasma concentration vs. time curve of L-[²H₃]methionine on D-[²H₃]methionine administration and total clearance of L-[²H₃]methionine on L-[²H₃]methionine administration, and that fraction was >90%. This result demonstrates that almost all i.v. administered D-methionine is converted into the L-enantiomer in vivo.

KEY WORDS: • D-methionine • chiral inversion • GC-MS • stable isotope • D-amino-acid oxidase