Metabolism of tetramethrin in houseflies and rats in vitro

Degradation of radiolabeled tetramethrin or 3,4,5,6-tetrahydrophthalimidomethyl dl-trans chrysanthemumate was tested in vitro by using abdomens of SK, lab-em-7-em, R_HOKOTA and P_y strains of houseflies and rat liver. The effect of NADH₂ and NADPH₂ on the metabolism of tetramethrin by housefly abdomen homogenate was slight, but phosphorothioates, their oxygen analogs, carbamate insecticides, NIA 16388, p-chloromercuribenzoic acid, and mercuric chloride showed marked inhibition. The enzyme activity was localized mainly in the microsomal fraction, where the major metabolites were 3,4,5,6-tetrahydrophthalimide (TPI) (a nonenzymatic reaction from N-(hydroxymethyl) 3,4,5,6-tetrahydrophthalimide, MTI) and chrysanthemumic acid. Smaller amounts of oxidized tetramethrins and chrysanthemumic acid were also produced. The cleavage of tetramethrin into MTI and chrysanthemumic acid was inhibited by such compounds as paraoxon, carbaryl, PCMB, NIA16388, and mercuric chloride. NADPH₂ or NADPH₂ plus carbon monoxide produced little effect. Similar results were obtained with rat liver microsomal fraction. It is presumed from the above findings that the cleavage is catalyzed either by a carboxyesterase or a hydrolase, and that some pyrethroids are metabolized in insects primarily through hydrolytic pathways. Metabolites from oxidative pathways (as in mammals) are formed in minor quantities.