INSECTICIDE AND INSECTICIDE METABOLITE INTERACTIONS WITH CYTOCHROME-P450 MEDIATED ACTIVITIES IN MAIZE

In vitro assays were used to determine if organophosphate, carbamate, and synthetic pyrethroid insecticides affected the cytochrome P450 mon ooxygenase (P450) catalyzed hydroxylation of nicosulfuron, bentazon, c innamic acid, or lauric acid in maize microsomes. All P450 activities were inhibited approximately 50% by carbaryl, and none were inhibited by permethrin. Hydroxylations of nicosulfuron, bentazon, lauric acid, and cinnamic acid were inhibited by malathion 83, 92, 38, and 0%, resp ectively. Terbufos was only moderately (36%) inhibitory of in vitro P4 50 hydroxylation of nicosulfuron. Nicosulfuron hydroxylation was more sensitive than bentazon hydroxylation to inhibition by the insecticide s, and both herbicide hydroxylations were more sensitive than lauric a cid or cinnamic acid hydroxylations to the insecticides. Since the oxi dative metabolites of terbufos were shown to be more potent inhibitors of in vivo nicosulfuron metabolism than terbufos, we examined the eff ect of terbufos-sulfone on in vivo and in vitro herbicide metabolism. Terbufos-sulfone inhibited metabolism of nicosulfuron and imazethapyr, but not bentazon, in excised corn shoots. Microsomal hydroxylation of nicosulfuron, bentazon, chlorimuron ethyl, and imazethapyr, as well a s the desulfuration of malathion, were strongly inhibited (>65%) by te rbufos-sulfone. Cinnamic acid hydroxylase appeared to be different fro m the P450(s) responsible for the pesticide metabolism as it was not i nhibited by terbufos-sulfone, However, the data also suggest that mala thion, nicosulfuron, bentazon, chlorimuron ethyl, and imazethapyr all share a P450 in common with terbufos-sulfone. Alternatively, there may be separate P450s for the metabolism of the herbicides and malathion, all of which also metabolize terbufos-sulfone. These data show that t he inhibition of P450 hydroxylation of nicosulfuron by terbufos-sulfon e can explain the injury when maize is exposed to both terbufos and ni cosulfuron. However, the insecticides that are the most potent in vitr o P450 inhibitors are not necessarily the ones that cause the most her bicide injury in the field. (C) 1996 Academic Press, Inc.