Direct detection and evaluation of conversion of D-methionine into L-methionine in rats by stable isotope methodology

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-[H-2(3)]methionine, their plasma concentrations and that of endogenous L-methionine were determined by a stereoselective GC-MS method. L-CH,]Methionine appeared rapidly after administration of D-[H-2(3)]methionine, whereas D-[H-2(3)]methionine was not detected after administration Of L-[H-2(3)]methionine. The fraction of conversion Of D-[H-2(3)]methionine into L-[H-2(3)]methionine was estimated using the area under the plasma concentration vs. time curve Of L-[H-2(3)]methionine on D-[H-2(3)]methionine administration and total clearance of L-[H-2(3)]methionine on L-[H-2(3)]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.