their stability to the hepatic carboxylesterases (Soderlund and Casida, 1977). However, it apparently has little effect on metabolism and elimination in vivo.The difference between the cis and trans isomers of cypermethrin in the amounts retained in the fat is of interest. In terms of lipophilic character, there is little difference between the two isomers, and experiments in mice (Hutson et al., 1981) show that the amount reaching fat is similar with both isomers. The difference lies in the rate of release from this tissue. It is possible that hydrolysis of the ester bond occurs in the fatty tissue, perhaps catalyzed by a lipase. This hydrolysis would be expected to proceed much more rapidly with the trans isomer than with the cis isomer.The rapid elimination of cypermethrin from rats is due primarily to the efficient cleavage of the ester bond giving rise to polar metabolites which are further oxidized and conjugated before excretion. The structural elucidation of these metabolites and their mechanisms of formation will be described in a subsequent paper. ACKNOWLEDGMENT We thank P. A. Harthoorn of the Shell Biosciences Laboratory, Sittingboume, for the synthesis of the various radiolabeled forms of cypermethrin.
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