Mechanism of metabolic activation of chloroform by rat liver microsomes

In this investigation, we attempted to determine if reactive metabolites other than phosgene (COCl₂) are involved in the metabolic activation of chloroform (CHCl₃) in rat liver microsomes. This problem was approached by determining whether the formation of COCl₂ can account for the metabolism of CHCl₃ to covalently bound product, carbon dioxide (CO₂), and chloride ion (Cl^-). It was found that the levels of covalent binding of [¹⁴CHCl₃ and [¹⁴C]CO₂ formation decreased proportionately when [¹⁴C]COCl₂ was trapped as 2-oxothiazolidine-4-carboxylic acid by the addition of cysteine to the incubation mixture. The amount of this product corresponded closely to the sum of the decreases in covalent binding and CO₂ formation. [³⁶Cl]Chloride was formed from [³⁶Cl]CHCl₃ under the same conditions that produced COCl₂ from CHCl₃. In addition, when ¹⁴C-, ³H-, or ³⁶Cl-labelled CHCl₃ was incubated with liver microsomes under a variety of conditions, only the ¹⁴C-label was appreciably bound irreversibly to microsomal protein. These results support the view that COCl₂ is the major, if not the only, reactive metabolite formed from CHCl₃ in rat liver microsomes.