Dynamic headspace analysis of volatile metabolites from the reductive dehalogenation of trichloro- and tetrachloroethanes by hepatic microsomes.

A dynamic headspace technique was developed to facilitate the identification and quantitation of low levels of volatile metabolites produced in vitro by subcellular preparations. The method is complementary to commonly used static headspace and solvent-extraction techniques, and involves purging the compounds from microsomal suspensions with an inert gas, trapping them on a short column of adsorbant resin, and transferring the metabolites to a gas chromatograph. An apparatus was designed to facilitate the incubations and isolations of volatile compounds. Recoveries of several chlorinated hydrocarbons with boiling points in the range 12 to 186 degrees C were 85% or higher, and the recovery of vinyl chloride (boiling point -13 degrees C) was 25%. The quantitative precision of the method was determined and calibration curves were established for each metabolite, demonstrating that no discrimination occurred over a wide range of concentrations. This technique was employed to investigate the reductive metabolism of 1,1,1-trichloroethane, 1,1,2-trichloroethane, and 1,1,2,2-tetrachloroethane by rat liver microsomes. The metabolites from these substrates were 1,1-dichloroethane, vinyl chloride, and 1,2-dichloroethylene, respectively. These conversions were NADPH-dependent, occurred only under anaerobic conditions, and indicate that chloroethanes with relatively low electron affinities can be reduced slowly by microsomal cytochrome P-450. The rates of formation of vinyl chloride, 1,1-dichloroethane, and 1,2-dichloroethylene with 1.0 mM substrate were 12.5 +/- 2.0, 122 +/- 14, and 147 +/- 12 pmol/min/mg of protein, respectively. The results show that there are distinct advantages of the purge/trap method over the static headspace method for studying volatile metabolites when high sensitivity is required.