P450 ENZYME CYP2B CATALYZES THE DETOXIFICATION OF DIISOPROPYL FLUOROPHOSPHATE

Phenobarbital and some other enzyme-inducers are known to reduce organ ophosphate toxicity. One suggested mechanism is the induction of liver cytochrome P450 enzymes catalyzing monooxygenation reactions. The aim of the present study was to elucidate the cytochrome P450 subfamily, or P450 isoenzyme(s), participating in the detoxification of diisoprop yl fluorophosphate (DFP) in the rat. DFP resulted in a type I spectrum in liver microsomes from phenobarbital- or RP 52028-treated rats (bin ding constants 0.32 and 0.17 mu M, respectively) and in a purified P45 0 preparation enriched with CYP2B. The spectrum was reversible by mety rapone, an inhibitor of the CYP2B enzyme subfamily. The 7-pentoxyresor ufin 0-dealkylase activity was inhibited by DFP in liver microsomes fr om phenobarbital- or RP 52028-treated rats and in a reconstituted syst em containing the purified CYP2B preparation, In microsomes from pheno barbital-pretreated rats, the inhibition was of a mixed type, i,e,, co mpetitive-non-competitive (K-m = 0.5 mu M; K-i = 6 mu M). The microsom al fractions of livers from phenobarbital- or RP 52028-treated rats de toxified DFP effectively in vitro, as measured by a decrease in the DF P inhibition of cholinesterase activity. This detoxification was antag onized by metyrapone and by an antibody raised against purified CYP2B preparation. Clotrimazole, an inhibitor of P450 enzymes, inhibited the detoxification of DFP in rat liver in vivo. A genetically-modified ha mster cell line expressing CYP2B1 oxidized NADPH in the presence of DF P. No such oxidation was detected in the parent cell line. These studi es suggest that CYP2B1 metabolizes DFP and may significantly contribut e to the detoxification of this organophosphate in vivo.