Hydroxylation of primisulfuron by an inducible cytochrome P450-dependent monooxygenase system from maize

Abstract

Microsomes were prepared from etiolated maize seedlings and incubated with [ 14C]primisulfuron (2-[3-(4,6-bis(difluoromethoxy)-pyrimidin-2-yl)-ureidosulfonyl]-benzoic acid methylester). Two enzymatic reaction products were formed in the presence of O 2 and NADPH. Comparison on high-performance liquid chromatography with synthetic reference standards and mass spectrometry of the two in vitro metabolites revealed that [ 14C]primisulfuron was hydroxylated at two different sites, i.e., at the phenyl and at the pyrimidine ring, respectively. Both hydroxylation reactions were inhibited in vitro by tetcyclacis as well as by CO in the presence of O 2. CO inhibition was reversed by irradiation of the reaction mixture with white light. Enzyme activities were localized predominantly in the shoots. Apparent K m values for [ 14C]primisulfuron were estimated to be 137 and 47 μ M, and V max to be 427 and 261 pmol/hr/mg protein, for the hydroxylation on the pyrimidine and phenyl ring, respectively. Formation of these metabolites from [ 14C]primisulfuron was barely detectable in microsomes from germinating seedlings. However, seed treatment (0.2% w w ) with the safener, CGA 154281 (4-[dichloroacetyl]-3,4-dihydro-3-methyl-2 H-1,4-benzoxazine), increased microsomal cytochrome P450 levels twofold and dramatically stimulated in vitro [ 14C]primisulfuron phenyl- and pyrimidine-ring hydroxylation. From these data it is concluded that hydroxylation of primisulfuron in maize microsomes is catalyzed by an inducible cytochrome P450 monooxygenase system.