Biotransformation of chlorpyrifos and diazinon by human liver microsomes and recombinant human cytochrome P450s (CYP)

Abstract

1. The cytochrome P450 (CYP)-mediated biotransformation of the organophosphorothioate insecticides chlorpyrifos and diazinon was investigated. Rates of desulphuration to the active oxon metabolite (chlorpyrifos-oxon and diazinon-oxon) and dearylation to non-toxic hydrolysis products were determined in human liver microsome preparations from five individual donors and in recombinant CYP enzymes.

2. Chlorpyrifos and diazinon underwent desulphuration in human liver microsome with mean K_m = 30 and 45 μM and V_max = 353 and 766 pmol min⁻¹ mg⁻¹, respectively. Dearylation of these compounds by human liver microsome proceeded with K_m = 12 and 28 μM and V_max = 653 and 1186 pmol min⁻¹ mg⁻¹, respectively. The apparent intrinsic clearance (V_max/K_m) of dearylation was 4.5- and 2.5-fold greater than desulphuration for chlorpyrifos and diazinon, respectively.

3. Recombinant human CYP2B6 possessed the highest desulphuration activity for chlorpyrifos, whereas CYP2C19 had the highest dearylation activity. In contrast, both desulphuration and dearylation of diazinon were catalysed at similar rates, in the rank order CYP2C19 > CYP1A2 > CYP2B6 > CYP3A4.

4. Both organophosphorothioates were more readily detoxified (dearylation) than bioactivated (desulphuration) in all human liver microsome preparations. However, the role of individual CYP enzymes in these two biotransformation pathways varied according to the structure of the organophosphorothioate, which was reflected in different activation/detoxification ratios for chlorpyrifos and diazinon. Variability in activity of individual CYP enzymes may influence interindividual sensitivity to the toxic effects of chlorpyrifos and diazinon.