2416. The dechlorination reaction

THE C H E M I C A L ENVIRONMENT

729

acid. The third possibly pathway is a non-enzymatic dehydrochlorination, as observed with I. This would account for the tetrachloroethylene found in the expired air, although once again the final product is probably trichloroacetic acid. A summary of the results obtained in these three recent papers is outlined in Fig. 1.

f Dichloroace?ic acid ~

I,I,2,2-TETRACHLOROETHANE

I

7 j

C02J

PENTACHLOROETHANE

T e f r a c h l ~

[b

Tr i chIo!ie thanol~Trichloro!thylene

a = Enzymatic hydrolytic dechlorination b- Non-enzymatic dehydrochlorination

I, I, 1,2- TETRACHLOROETHANE

FIG. 1

2416. The dechlorination reaction

Van Dyke, R. A. & Wineman, Catherine G. (1971). Enzymatic dechlorination. Dechlorination ofchloroethanes and propanes in vitro. Biochem. Pharmac. 20, 463. As is clear from the previous abstract, enzymatic hydrolytic dechlorination is an important metabolic step in the elimination of chloroethanes from the mammalian body. The paper cited above investigates the mechanism and kinetics of this reaction using an in oitro technique involving a6Cl-labelled chloroethanes and chloropropanes. The reaction, which requires NADPH and oxygen, was studied by incubating the labelled compounds with the 105,000 g supernatant from rat-, rabbit- and guineapig-liver homogenates, the percentage of 36C1 liberated being calculated by difference at the end of the experiment. This percentage varied widely from less than 0"5~o with 1-chloroethane, 1,1,1trichloroethane and 1,2-dichloroethane to 13.5 ~ with l,l-dichloroethane and 40.8 ~ with 1,1,2-trichloropropane. In general, however, as in the earlier studies, it was clear that the release of chlorine depended on its being attached to a carbon atom already carrying hydrogen. 1,1,2-Trichloroethane was chosen as a suitably characteristic compound and was used in further studies on the induction of the dechlorination reaction by chemical inducers and by the 105,000 g supernatant. Methoxyflurane vapour, phenobarbitone and benzo[a]pyrene were found to stimulate dechlorination, but methylcholanthrene did not have this effect. Similarly, the factor present in the supernatant was not inducible but was clearly important to the overall reaction. The authors conclude by comparing the enzyme system responsible for the dechlorination reaction with the hepatic mixed-function oxidase system active in the metabolism of a variety of compounds. In their inducibility and requirement for NADPH and oxygen the two systems are very similar, but the dechlorinating system requires some factor from the supernatant fraction for optimal activity and is apparently largely independent of cytochrome P45o. These differences, however, were not thought to be of great significance. FOOD 10/5"----1t