Safrole is a natural plant constituent, found in sassafras oil and certain other essential oils.The carcinogenicity of safrole is mediated through 1'-hydroxysafrole formation, followed bysulfonation to an unstable sulfate that reacts to form DNA adducts. To identify the maincytochrome P450 (P450) involved in human hepatic safrole 1'-hydroxylation (SOH), wedetermined the SOH activities of human liver microsomes and
Escherichia coli membranesexpressing bicistronic human P450s. Human liver (
n = 18) microsomal SOH activities were inthe range of 3.5-16.9 nmol/min/mg protein with a mean value of 8.7 ± 0.7 nmol/min/mg protein.In human liver (
n = 3) microsomes, the mean
Km and
Vmax values of SOH were 5.7 ± 1.2 mMand 0.14 ± 0.03
mol/min/nmol P450, respectively. The mean intrinsic clearance (
Vmax/
Km)was 25.3 ± 2.3
L/min/nmol P450. SOH was sensitive to the inhibition by a CYP2C9 inhibitor,sulfaphenazole, and CYP2E1 inhibitors, 4-methylpyrazole and diethyldithiocarbamate. Theliver microsomal SOH activity showed significant correlations with tolbutamide hydroxylation(
r = 0.569) and chlorzoxazone hydroxylation (
r = 0.770) activities, which were the modelreactions catalyzed by CYP2C9 and CYP2E1, respectively. Human CYP2C9 and CYP2E1showed SOH activities at least 2-fold higher than the other P450s. CYP2E1 showed an intrinsicclearance 3-fold greater than CYP2C9. These results demonstrated that CYP2C9 and CYP2E1were the main P450s involved in human hepatic SOH.