Herbicide-insecticide interaction in maize: Malathion inhibits cytochrome P450-dependent primisulfuron metabolism

Abstract

Tolerance of maize to sulfonylurea herbicides such as primisulfuron has recently been reported to be impaired by the use of some organophosphorus insecticides. In an effort to elucidate the mechanism of this interaction, the effect of the insecticide, malathion, on the metabolism of primisulfuron was studied in whole plants, in excised leaves, and in a microsomal in vitro system from maize. Foliar application of malathion to 7-day-old plants had no influence on leaf uptake and translocation of primisulfuron, but caused a decrease in the rate of herbicide metabolism. In excised leaves, malathion increased the metabolic half-life of primisulfuron. In microsomal preparations, malathion inhibited cytochrome P450-dependent primisulfuron phenyl- and pyrimidinering hydroxylation. Loss of primisulfuron phenyl-ring hydroxylase activity was time-dependent, saturable with respect to malathion concentration, and attenuated in the absence of NADPH. The kinetic data suggest a mechanism-based cytochrome P450 inactivation by malathion. The oxoanalogue of malathion, malaoxon, did not influence the metabolic half-life of primisulfuron in excised leaves and was a poor inhibitor of microsomal primisulfuron hydroxylation. Neither insecticide had any effect in vitro on total microsomal cytochrome P450 content. From the present results it may be concluded that malathion affects primisulfuron tolerance of maize due to the inhibition of cytochrome P450 monooxygenases involved in herbicide metabolism.