Theoretical insights into the reductive metabolism of CCl4 by cytochrome P450 enzymes and the CCl4-dependent suicidal inactivation of P450

Abstract

The anaerobic metabolism of CCl₄ by P450 enzymes was investigated using quantum chemical calculations. It was found that under anaerobic conditions, the substrate CCl₄ might undergo one or two subsequent one-electron reductions to generate different reactive metabolites, trichloromethyl radical (˙CCl₃) and dichlorocarbene (:CCl₂) respectively. Meanwhile, it was the reduced ferrous haem complex rather than the unreduced ferric haem complex that could directly achieve such reductions. Based on the formation of the former reactive metabolite, a further one-electron reduction could take place with the assistance of a proton to yield the latter reactive species, i.e., a further reductive dechloridation of ˙CCl₃ could take place via a novel S_E3 mechanism. In addition, the ˙CCl₃ species was capable of binding covalently to the meso-carbon atom of the prosthetic group, leading to the suicidal destruction of P450 enzymes. Whereas the :CCl₂ species was involved in the CCl₄-dependent reversible P450 inhibition as its hydrolysis product, CO, but was not significantly involved in the CCl₄-dependent irreversible P450 destruction. It is obvious that the reductive metabolism of CCl₄ to reactive intermediates by P450 enzymes is an essential prerequisite for its toxicity.