Vol. 27, Issue 5, 574-580, May 1999

Metabolism of (R)-(+)-Pulegone and (R)-(+)-Menthofuran by Human Liver Cytochrome P-450s: Evidence for Formation of a Furan Epoxide

Siamak C. Khojasteh-Bakht, Weiqiao Chen, Luke L. Koenigs, Raimund M. Peter, and Sidney D. Nelson

Department of Medicinal Chemistry, School of Pharmacy, University of Washington, Seattle, Washington

(R)-(+)-Pulegone, a monoterpene constituent of pennyroyal oil, is a hepatotoxin that has been used in folklore medicine as an abortifacient despite its potential lethal effects. Pulegone is metabolized by human liver cytochrome P-450s to menthofuran, a proximate hepatotoxic metabolite of pulegone. Expressed human liver cytochrome (CYP) P-450s (1A2, 2A6, 2C9, 2C19, 2D6, 2E1, and 3A4) were tested for their ability to catalyze the oxidations of pulegone and menthofuran. Expressed CYP2E1, CYP1A2, and CYP2C19 oxidized pulegone to menthofuran, with respective K_m and V_max values of 29 µM and 8.4 nmol/min/nmol P-450 for CYP2E1, 94 µM and 2.4 nmol/min/nmol P-450 for CYP1A2, and 31 µM and 1.5 nmol/min/nmol P-450 for CYP2C19. The human liver P-450s involved in the metabolism of menthofuran are the same as pulegone except for the addition of CYP2A6. These P-450s were found to oxidize menthofuran to a newly identified metabolite, 2-hydroxymenthofuran, which is an intermediate in the formation of the known metabolites mintlactone and isomintlactone. Based on studies with ¹⁸O₂ and H₂¹⁸O, 2-hydroxymenthofuran arises predominantly from a dihydrodiol formed from a furan epoxide. CYP2E1, CYP1A2, and CYP2C19 oxidized menthofuran with respective K_m and V_max values of 33 µM and 0.43 nmol/min/nmol P-450 for CYP2E1, 57 µM and 0.29 nmol/min/nmol P-450 for CYP1A2, and 62 µM and 0.26 nmol/min/nmol P-450 for CYP2C19.