Metabolism of Herbicides by Cytochrome P450 in Corn

Abstract

Cytochrome P450 monooxygenases (P450) have long been considered a potentially important enzyme system for the detoxification of herbicides in corn and other crop plants. Only recently has herbicide metabolism by P450 in corn microsomes been conclusively demonstrated. Fourteen herbicides in six chemical families have been shown to be metabolized by P450 in corn. P450 is now considered of equal importance to glutathione S-transferases for the metabolism of herbicides in corn. P450 metabolism of herbicides in corn is characterized as an inducible enzyme activity which primarily catalyses hydroxylations and demethylations of the herbicide substrates. Induction of the herbicide metabolizing activity is specific, with only slight changes in the total P450 level accompanying several-fold induction of herbicide metabolism. Turnover rates for herbicide metabolism can equal those for drug metabolism. Microsomes isolated from both shoots and roots metabolize herbicides, although the majority of activities are most fully characterized in microsomes isolated from shoots of young etiolated seedlings. Many questions concerning the P450 metabolism of herbicides in corn remain. These include determining how many P450s are involved in herbicide metabolism and how their levels are regulated. Evidence is gathering that, for a number of the herbicides presently known to be metabolized by P450 in corn, there may be relatively few or even one P450 which metabolizes all the herbicide substrates.