Characterization of cytochrome p450-mediated chlorimuron ethyl hydroxylation in maize microsomes

Microsomes prepared from etiolated 3.5-day-old, naphthalic anhydride (NA)-treatrd maize coleoptiles converted chlorimuron ethyl to its 5-hydroxypyrimidine metabolite. Activity was dependent upon the presence of a reduced pyridine nucleotide and was inhibited by nitrogen or CO. Inhibition by CO was partially reversed by light. 1-Aminobenzotriazole, piperonyl butoxide, tetcyclasis, and cytochrome c also inhibited activity. Chlorimuron ethyl hydroxylase activity was induced over 30-fold by NA pretreatment. A Lineweaver-Burk plot gave an apparent Km of 83 ·M for chlorimuron ethyl and a V_max of 151 pmol/mg microsomal protein/min. Addition of some other herbicides to in vitro assays inhibited chlorimuron ethyl metabolism to varying extents. The most effective inhibitors of chlorimuron ethyl metabolism were the phenylurea herbicides chlortoluron and linuron. Chlortoluron noncompetitively inhibited chlorimuron ethyl metabolism with an apparent K_i of 66 ·M. In contrast, chlorimuron ethyl was a poor inhibitor of chlortoluron metabolism in the microsomal preparation, and at low concentrations caused stimulation of activity. These data demonstrate the occurrence of a cytochrome P450-mediated hydroxylation of chlorimuron ethyl in maize microsomal preparations. Inhibition of chlorimuron ethyl metabolism by chlortoluron and other herbicides may indicate that more than one herbicide can be metabolized by the same P450.