Inhibition of the Human Liver Microsomal and Human Cytochrome P450 1A2 and 3A4 Metabolism of Estradiol by Deployment-Related and Other Chemicals

Cytochromes P450 (P450s) are major catalysts in the metabolism of xenobiotics and endogenous substrates such as estradiol (E₂). It has previously been shown that E₂ is predominantly metabolized in humans by CYP1A2 and CYP3A4 with 2-hydroxyestradiol (2-OHE₂) the major metabolite. This study examines effects of deployment-related and other chemicals on E₂ metabolism by human liver microsomes (HLM) and individual P450 isoforms. Kinetic studies using HLM, CYP3A4, and CYP1A2 showed similar affinities (K_m) for E₂ with respect to 2-OHE₂ production. V_max and CL_int values for HLM are 0.32 nmol/min/mg protein and 7.5 μl/min/mg protein; those for CYP3A4 are 6.9 nmol/min/nmol P450 and 291 μl/min/nmol P450; and those for CYP1A2 are 17.4 nmol/min/nmol P450 and 633 μl/min/nmol P450. Phenotyped HLM use showed that individuals with high levels of CYP1A2 and CYP3A4 have the greatest potential to metabolize E₂. Preincubation of HLM with a variety of chemicals, including those used in military deployments, resulted in varying levels of inhibition of E₂ metabolism. The greatest inhibition was observed with organophosphorus compounds, including chlorpyrifos and fonofos, with up to 80% inhibition for 2-OHE₂ production. Carbaryl, a carbamate pesticide, and naphthalene, a jet fuel component, inhibited ca. 40% of E₂ metabolism. Preincubation of CYP1A2 with chlorpyrifos, fonofos, carbaryl, or naphthalene resulted in 96, 59, 84, and 87% inhibition of E₂ metabolism, respectively. Preincubation of CYP3A4 with chlorpyrifos, fonofos, deltamethrin, or permethrin resulted in 94, 87, 58, and 37% inhibition of E₂ metabolism. Chlorpyrifos inhibition of E₂ metabolism is shown to be irreversible.