In vitro covalent binding of the pyrethroids cismethrin,cypermethrin and deltamethrin to rat liver homogenate and microsomes

Phenobarbital-induced rat liver homogenate and microsomes were used to study covalent binding of l4C-labelled (at the alcohol moiety) cismethrin, ¹⁴C-labelled (at the alcohol and acid moieties) cypermethrin, and ¹⁴C-labelled (at the alcohol and acid moieties) deltamethrin. Covalent binding was dependent on pyrethroid concentration. With liver homogenate, inhibition of esterases by tetraethylpyrophosphate and of mitochondrial respiration by rotenone or potassium cyanide only slightly altered the covalent binding level. With microsomes, inhibition of cytochrome P-450 and mixed function oxidases by carbon monoxide and piperonyl butoxide reduced the covalent binding so far as to be nearly absent. Eighty percent inhibition of epoxide hydrolase decreased the covalent binding by 50%. The comparison of data between alcohol and acid labelling of the same pyrethroid suggested that, in vitro, the whole molecule is bound to proteins and that hydrolysis can occur afterwards. The experiments stress the role of cytochrome P-450-dependent monoxygenases in the covalent binding process.