Parathion Activation Enzymes in the Fat Body Microsomes of the American Cockroach1

Abstract

Activation of parathion was studied as an example of microsomal oxidation by using the fat body enzymes of 3- to 4-month-old female American cockroaches, Periplaneta americana (L.). Activation was estimated manometrically by inhibition of house fly, Musca domestica L., head cholinesterase. The anticholinesterase product was identified as para-oxon. Purified parathion gave no inhibition of the cholinesterase when assayed in low light. No activation was detected with fat body homogenates from late instar nymphs. The activation enzymes were present primarily in a subcellular fraction that resembled mammalian liver microsomes. The enzymes required oxygen and NADPH₂ for activation. NADH₂ was a less effective cofactor. Oxidized forms of the cofactors and tetrahydrofolic acid were without effect. The optimum pH for the stability of the enzymes was 7 and that for the activation reaction was 7.5. Loss of para-oxon owing to the microsomes was prevented by pretreating them with para-oxon. Heated microsomes contained a heat-stable factor that supported the reaction. The enzymes were apparently inactivated by the activation reaction. Possible causes were discussed. Activation was inhibited by WARF antiresistant, sesamex, and other insecticide synergists. The mode of inhibition was examined. A nonenzymatic model reaction also was studied.