DMD#10439 INHIBITION OF THE HUMAN LIVER MICROSOMAL AND HUMAN CYTOCHROME P450 1A2 and 3A4 METABOLISM OF ESTRADIOL BY DEPLOYMENT-RELATED AND OTHER CHEMICALS

A list of non-standard abbreviations: DEET-N,N-diethyl-m-toluamide E2-estradiol TST-testosterone HLM-human liver microsomes CYP-cytochrome P450 HPLC-high performance liquid chromatography This article has not been copyedited and formatted. The final version may differ from this version. DMD#10439 Abstract Cytochrome P450s (CYPs) are major catalysts in metabolism of xenobiotics and endogenous substrates such as estradiol (E 2). It has previously been shown that E 2 is predominantly metabolized in humans by CYP1A2 and 3A4 with 2-hydroxyestradiol (2-OHE 2) the major metabolite. This study examines effects of deployment-related and other chemicals on E 2 metabolism by human liver microsomes (HLM) and individual CYP isoforms. Kinetic studies using HLM, CYP3A4, and CYP1A2 demonstrated similar affinities (K m) for E 2 with respect to 2-OHE 2 production. V max and CL int values for HLM are 0.32 nmol/min/mg protein and 7.5 µ l/min/ mg protein, those for CYP3A4 are 6.9 nmol/min/nmole CYP and 291 µ l/min/ nmol CYP and those for CYP1A2 are 17.4 nmol/min/nmole CYP and 633 µ l/min/ nmol CYP. Phenotyped HLM use demonstrated that individuals with high levels of CYP1A2 and CYP3A4 have the greatest potential to metabolize E 2. Preincubation of HLM with a variety of chemicals, including those used in military deployments, resulted in varying levels of inhibition of E 2 metabolism. The greatest inhibition was observed with organophosphorus compounds, including chlorpyrifos and fonofos, with up to 80% inhibition for 2-OHE 2 production. Carbaryl, a carbamate pesticide, and naphthalene, a jet fuel component, inhibited ca. 40% of E 2 metabolism. Preincubation of CYP1A2 with chlorpyrifos, fonofos, carbaryl, or naphthalene resulted in 96, 59, 84, and 87% inhibition of E 2 metabolism, respectively. Preincubation of CYP3A4 with chlorpyrifos, fonofos, deltamethrin or permethrin resulted in 94, 87, 58 and 37% inhibition of E 2 metabolism. Chlorpyrifos inhibition of E 2 metabolism is shown to be irreversible. This article has not been copyedited and formatted. The final version may differ from this version.