Antagonistic Interactions of Mefenacet with Inhibitors of Monooxygenases

Mefenacet, Monooxygenase, Safener, Antagonism, Induction The monooxygenase suicide inhibitor piperonyl butoxide (PBO) antagonizes the inhibitory activity of the herbicide mefenacet in a root regeneration test system with etiolated cut oat stems. When studying the behaviour of [l4C]mefenacet [2-(2-benzothiazolyloxy)-N-methylN-phenylacetamide] in cut oat stems that had been pretreated with PBO for 24 h after cutting, an increased rate of uptake and increased concentrations of mefenacet in the tissue were found after PBO as compared to water pretreatment. Increased uptake rates and increased rates of metabolism after PBO pretreatment, supposedly by increased monooxygenation, were also observed with the substrates diuron, 2,4-D and cinnamic acid. Mefenacet induced the same responses as PBO in all systems. The similar actions of mefenacet and PBO are interpreted to suggest that mefenacet might interfere with monooxygenase enzymes. Similar responses, i.e. stimulation of monooxygena­ tion of suitable substrates, have been reported for herbicide safeners. An environmentally con­ trolled regulatory response system is suggested to be triggered by the different classes of com­ pounds, mefenacet and similar herbicides, monooxygenase inhibitors, and safeners, and to re­ spond with increased enzyme activities in the monooxygenase and glutathione detoxification pathways.