Formation of carbonyl chloride in carbon tetrachloride metabolism by rat liver in vitro.

In order to identify intermediates of CCl4 metabolism, whole, suitably fortified rat liver homogenates were incubated with 14CCl4 in the presence and absence of "pools" of unlabeled suspected intermediates. In the presence of NADH or NADPH, incorporation of radioactivity was rapid and substantial in CO2, lipid, protein, and the acid-soluble fraction. It was not influenced by the presence of large pools of unlabeled chloroform or formate, thus excluding these substances as obligatory intermediates. However, when incubated with L-cysteine, radioactivity incorporation in the acid-soluble fraction was almost doubled, and about one-third of the radioactivity of this fraction was identified as 2-oxothiazolidine 4-carboxylic acid. This substance is formed chemically by condensation of cysteine with carbonyl chloride and has been identified previously by others as a product of chloroform metabolism by liver microsomes in the presence of L-cysteine. Based on current knowledge of CCl4 metabolism, the following aerobic pathway is envisioned: microsomal cleavage to Cl- and .CCl3 and oxidation of the latter to the unstable intermediate, Cl3COH, which loses HCl to yield COCl2. COCl2 is likely to be the major source of CO2 from CCl4 but is probably not the intermediate that binds to lipid and protein. The addition of glutathione had no effect on CCl4 metabolism in rat liver homogenate, suggesting that glutathione S-transferases, which catalyze other dehalogenation reactions, do not play a role in CCl4 metabolism.