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ABSTRACT:Rutaecarpine is the main active alkaloid of the herbal medicine, Evodia rutaecarpa. To identify the major human cytochrome P450 (P450) participating in rutaecarpine oxidative metabolism, human liver microsomes and bacteria-expressed recombinant human P450 were studied. In liver microsomes, rutaecarpine was oxidized to 10-, 11-, 12-, and 3-hydroxyrutaecarpine. Microsomal 10-and 3-hydroxylation activities were strongly inhibited by ketoconazole. The 11-and 12-hydroxylation activities were inhibited by ␣-naphthoflavone, quinidine, and ketoconazole. These results indicated that multiple hepatic P450s including CYP1A2, CYP2D6, and CYP3A4 participate in rutaecarpine hydroxylations. Among recombinant P450s, CYP1A1 had the highest rutaecarpine hydroxylation activity. Decreased metabolite formation at high substrate concen- Rutaecarpine is a main quinazolinocarboline alkaloid isolated from Evodia rutaecarpa (Wu-chu-yu), which has been used as a herbal medicine for the treatment of gastrointestinal disorder and headache (Liao et al., 1981;Tang and Eisenbrand, 1992). The remedy containing E. rutaecarpa is generally taken orally. The estimated dosage of rutaecarpine from ingestion of Wu-chu-yu tang, a remedy containing E. rutaecarpa, is approximately 19 mg/day (Ueng et al., 2002a). Rutaecarpine has a variety of pharmacological actions including antithrombotic, antianoxic, hypotensive, and vasorelaxant effects (Sheu, 1999). Microsomal cytochrome P450 (P450)-dependent monooxygenase plays a major role in the oxidative metabolism of xenobiotics including drugs and natural products (Guengerich, 1995;Hasler et al., 1999). Our previous reports demonstrated that rutaecarpine was a CYP1A2-selective inhibitor in human liver microsomes (Ueng et al., 2002b). Rutaecarpine was metabolized by rat liver microsomal enzymes to form 10-, 11-, 12-, and 3-hydroxyrutaecarpine (Ueng et al., 2005). Lee et al. (2004) reported that the formation rate of total rat rutaecarpine metabolites was stimulated by P450 inducers, 3-methylcholanthrene and phenobarbital, but not by acetone and dexamethasone. 3-Methylcholanthrene, phenobarbital, acetone, and dexamethasone are inducers of CYP1A, CYP2B, CYP2E1, and CYP3A, respectively, in rats (Waxman and Azaroff, 1992;Waxman, 1999). These results suggested that CYP1A and CYP2B played main roles in rat rutaecarpine metabolism. However, the main human P450 forms catalyzing rutaecarpine hydroxylations were not identified and the quantification and kinetic analyses were not reported.In human liver, CYP1A2, CYP2C9, CYP2D6, CYP2E1, and CYP3A4 are the main P450 forms responsible for drug oxidation (Guengerich, 1995). CYP1A2, CYP2C, CYP2D6, CYP2E1, and CYP3A constitute approximately 13%, 18%, 2%, 7%, and 29% of the total P450 content, respectively (Shimada et al., 1994). CYP2C9 represents about half to 75% of total CYP2C (Lasker et al., 1998). CYP3A4 is the most abundant P450 form in human liver samples. CYP1A1 and CYP1B1 are expressed in low levels in human liver, but are inducible in the liver and play import...