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-hyd roxypyrimidine metabolite. Activity was dependent upon the presence of a reduced pyridine nucleotide and was inhibited by nitrogen or CO. In hibition by CO was partially reversed by light. 1-Aminobenzotriazole, piperonyl butoxide, tetcyclasis, and cytochrome c also inhibited activ ity. Chlorimuron ethyl hydroxylase activity was induced over 30-fold b y NA pretreatment. A Lineweaver-Burk plot gave an apparent K-m of 83 m u M for chlorimuron ethyl and a V-max of 151 pmol/mg microsomal protei n/min. Addition of some other herbicides to in vitro assays inhibited chlorimuron ethyl metabolism to varying extents. The most effective in hibitors of chlorimuron ethyl metabolism were the phenylurea herbicide s chlortoluron and linuron. Chlortoluron noncompetitively inhibited ch lorimuron ethyl metabolism with an apparent K-i of 66 mu M. In contras t, chlorimuron ethyl was a poor inhibitor of chlortoluron metabolism i n the microsomal preparation, and at low concentrations caused stimula tion of activity. These data demonstrate the occurrence of a cytochrom e P450-mediated hydroxylation of chlorimuron ethyl in maize microsomal preparations. Inhibition of chlorimuron ethyl metabolism by chlortolu ron and other herbicides may indicate that more than one herbicide can be metabolized by the same P450. (C) 1995 Academic Press, Inc.