Stereoselective Metabolism of the Monoterpene Carvone by Rat and Human Liver Microsomes

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Abstract
The large amounts of carvone enantiomers consumed as food additives and in dental formulations justifies the evaluation of their biotransformation pathway. The in-vitro metabolism of R-(—)- and S-(+)-carvone was studied in rat and human liver microsomes using chiral gas chromatography. Stereoselective biotransformation was observed when each enantiomer was incubated separately with liver microsomes. 4R, 6S-(—)-Carveol was NADPH-dependently formed from R-(—)-carvone, whereas 4S, 6S-(+)-carveol was produced from S-(+)-carvone. Metabolite formation followed Michaelis-Menten kinetics exhibiting a significant lower apparent Km (Michaelis-Menten Constant) for 4R, 6S-(—)-carveol compared with 4S, 6S-(+)-carveol in rat and human liver microsomes (28.4 ± 10.6 μM and 69.4 ± 10.3 μM vs 33.6 ± 8.5 μM and 98.3 ± 22.4 μM). The maximal formation rate (Vmax) determined in the same microsomal preparations yielded 30.2 ± 5.0 and 32.3 ± 3.9 pmol (mg protein)−1 min−1 in rat liver and 55.3 ± 5.7 and 65.2 ± 4.3 pmol (mg protein)−1 min−1 in human liver microsomes. Phase II conjugation of the carveol isomers by rat and human liver microsomes in the presence of UDPGA (uridine S'-diphosphogluaronic acid) only revealed glucuronidation of 4R, 6S-(—)-carveol. Vmax for glucuronide formation was more than 4-fold higher in the rat liver compared with human liver preparations (185.9 ± 34.5 and 42.6 ± 7.1 pmol (mg protein)−1 min−1, respectively). Km values, however, showed no species-related difference (13.9 ± 4.1 μM and 10.2 ± 2.2 μM). This study demonstrated stereoselectivity in phase-I and phase-II metabolism for R-(—)- and S-(+)-carvone and might be predictive for carvone biotransformation in man.