Stereoselective Metabolism of Endosulfan by Human Liver Microsomes and Human Cytochrome P450 Isoforms

Abstract

ABSTRACT:Endosulfan (6,7,8,9,10,10-hexachloro-1,5,5a,6,9,9a-hexahydro-6,9-methano-2,3,4-benzo(e)dioxathiepin-3-oxide) is a broad-spectrum chlorinated cyclodiene insecticide. This study was performed to elucidate the stereoselective metabolism of endosulfan in human liver microsomes and to characterize the cytochrome P450 (P450) enzymes that are involved in the metabolism of endosulfan. Human liver microsomal incubation of endosulfan in the presence of NADPH resulted in the formation of the toxic metabolite, endosulfan sulfate. The intrinsic clearances (CL int ) of endosulfan sulfate from ␤-endosulfan were 3.5-fold higher than those from ␣-endosulfan, suggesting that ␤-endosulfan would be cleared more rapidly than ␣-endosulfan. Correlation analysis between the known P450 enzyme activities and the rate of the formation of endosulfan sulfate in the 14 human liver microsomes showed that ␣-endosulfan metabolism is significantly correlated with CYP2B6-mediated bupropion hydroxylation and CYP3A-mediated midazolam hydroxylation, and that ␤-endosulfan metabolism is correlated with CYP3A activity. The P450 isoform-selective inhibition study in human liver microsomes and the incubation study of cDNA-expressed enzymes also demonstrated that the stereoselective endosulfan sulfate formation from ␣-endosulfan is mediated by CYP2B6, CYP3A4, and CYP3A5, and that from ␤-endosulfan is mediated by CYP3A4 and CYP3A5. The total CL int values of endosulfan sulfate formation catalyzed by CYP3A4 and CYP3A5 were consistently higher for ␤-endosulfan than for the ␣-form (CL int of 0.67 versus 10.46 l/min/pmol P450, respectively). CYP2B6 stereoselectively metabolizes ␣-endosulfan, but not ␤-endosulfan. These findings suggest that the CYP2B6 and CYP3A enzymes are major enzymes contributing to the stereoselective disposition of endosulfan.

Comparative Toxicity of Endosulfan and Fipronil Insecticides: Utilizing In Vivo and In Vitro Data

“…It is classified as a "possible human carcinogen" based on thyroid tumors in rat. ToxCast true actives for CYPs (ES: hCYP2B6 & hCYP3A4; FP: hCYP2C9) correlated well with in vivo toxicity [7,8]. Other true activities for ES and FP were for generalized liver enzyme activity, which could be predictive of liver toxicity observed in vivo.…”

“…Their action in the brain is very complex due to biotransformation to toxic metabolites that can act on GABA A R (e.g., FP sulfone [6]). Each is detoxified by P450s (CYP2B6, CYP3A4-5: ES; hCYP2C9: FP) [7,8] [9]. These well characterized in vivo effects provide a basis for comparing toxicities.…”

Metabolism of Endosulfan-α by Human Liver Microsomes and Its Utility as a Simultaneous in Vitro Probe for CYP2B6 and CYP3A4

“…Subsequently, after the current communication had been prepared for submission, a manuscript was submitted and published from another laboratory (Lee et al, 2006). Lee et al (2006) reported on the metabolism of ␣-and ␤-endosulfan isomers, whereas the present study only reports on the metabolism of the ␣-isomer.…”

“…Subsequently, after the current communication had been prepared for submission, a manuscript was submitted and published from another laboratory (Lee et al, 2006). Lee et al (2006) reported on the metabolism of ␣-and ␤-endosulfan isomers, whereas the present study only reports on the metabolism of the ␣-isomer. Although the results from the two laboratories on metabolism of endosulfan-␣ are in general agreement, the current communication extends the findings in the development of endosulfan-␣ as a simultaneous probe for CYP2B6 and 3A4 in human liver microsomes.…”

Characterization of Benidipine and Its Enantiomers' Metabolism by Human Liver Cytochrome P450 Enzymes

“…The inhibitory effects of known P450 isoform-selective inhibitors on the formation of N-desbenzylbenidipine and dehydrobenidipine were evaluated to determine the P450 isoform(s) involved in the metabolic pathway. The formation ratio of N-desbenzylbenidipine and dehydrobenidipine from benidipine racemate or enantiomers was determined from the reaction mixtures incubated in the presence or absence of known P450 isoform-selective inhibitors (Kim et al, 2006;Lee et al, 2006). The P450 isoform-selective inhibitors used were furafylline (10 M) for CYP1A2, coumarin (100 M) for CYP2A6, triethylenethiophosphoramide (5 M) for CYP2B6, montelukast (0.1 M) for CYP2C8, sulfaphenazole (10 M) for CYP2C9, S-benzylnirvanol (1 M) for CYP2C19, quinidine (10 M) for CYP2D6, diethyldithiocarbamate (10 M) for CYP2E1, ketoconazole (1 M) for CYP3A, and azamulin (5 M) for CYP3A.…”