Ketamine is used clinically as a racemic mixture of optical isomers that differ in their analgesic properties and psychomimetic effects. Administered individually, or together as the racemate, ketamine enantiomers differ in their hepatic clearance and duration of anesthetic effect. S(+) ketamine exhibits a greater clearance and faster anesthetic recovery compared to the racemate and a greater clearance compared to R(-) ketamine. Ketamine undergoes extensive hepatic metabolism, primarily via N-demethylation to norketamine, yet little is known about the human metabolism of ketamine enantiomers. The purpose of this investigation therefore was to characterize ketamine racemate and enantiomer metabolism by human liver and to test the hypothesis that differences in hepatic ketamine enantiomer metabolism can account for observed differences in ketamine enantiomer pharmacokinetics. Ketamine N-demethylation by microsomes from three human livers was measured by gas chromatography-mass spectrometry. At ketamine concentrations typically achieved during anesthesia (5 muM), the rate of S(+) ketamine demethylation was 20% greater than that of R(-) ketamine and 10% greater than that of the racemate (P <.05). At all ketamine concentrations, the rate of racemate demethylation was less than the sum of the rates for the individual enantiomers, reflecting a metabolic enantiomeric interaction whereby one ketamine enantiomer inhibits the metabolism of the other enantiomer. N-demethylation of racemic ketamine and each enantiomer was catalyzed by two apparent enzymes, a high affinity-low capacity enzyme (K(m1) 30-50 muM, V(max1) 2-6 nmoles.min-1.nmole-1) and a low affinity-high capacity enzyme (K(m2) 600-800 muM, V(max2) 9-15 nmoles.min-1.nmole-1). At therapeutic concentrations, ketamine metabolism is catalyzed predominantly by one enzyme (the low K(m) enzyme), permitting calculation of the in vitro clearance parameter V(max1)/K(m1). In vitro clearances of ketamine enantiomers and the racemate were excellent predictors of the relative in vivo clearances. These results demonstrate relative enantiomeric selectivity in human liver ketamine metabolism, which accounts for observed differences in the clinical pharmacokinetics of racemic ketamine and ketamine enantiomers.