Hydroxylation and dechlorination of chlorinated paraffins by human cytochrome P450 monooxygenases and prediction of its metabolic sites

Chlorinated paraffins (CPs) are a mixture of chlorine-substituted n-alkanes detected in the environment, wildlife, and humans and toxic to organisms. CPs are metabolized by cytochrome P450 (CYP, P450) monooxygenases contained in human hepatic microsomes. However, studies on the metabolism of CPs by human P450s and in silico analyses to reveal the structural basis of CP metabolism are limited. Therefore, this study showed that enzymes in human hepatic microsomes hydroxylated and dechlorinated CPs with 11, 14, and 17 carbon chains when NADPH was added to the reaction mixture. The metabolic activities of human P450s decrease as the carbon chain length of the CPs increases. The use of P450 inhibitors indicates that human CYP3A4 and CYP4A11 are involved in CP metabolism. Human CYP3A4 catalyzes the hydroxylation and dechlorination of CPs, and its activity decreases with increasing CP carbon chain lengths. Using in silico tools, 3,4,6,7,9,10-hexachloroheptadecane was predicted to be preferentially metabolized at non-terminal carbons by human CYP3A4 and at terminal carbons by human CYP4A11. This may be due to differences in the substrate-binding cavity shapes between CYP3A4 and CYP4A11. This approach will help us to understand the structural basis of CP metabolism by human P450s.