Identification of Metabolites of the Herbicide Safener Benoxacor Isolated from Suspension-Cultured Zea mays Cells 3 and 24 h after Treatment

Abstract

The metabolic fate of the herbicide safener benoxacor was studied in suspension cultures of Zea mays (cv. Black Mexican Sweet) 3 and 24 h after treatment (HAT) with [14C]benoxacor. Following purification by reversed phase HPLC, metabolites were subjected to 1H-NMR or mass spectrometry for structural analysis. A catabolic α-hydroxyacetamide derivative was detected as well as several amino acid conjugates either containing GSH or presumably derived from GSH. A relatively abundant di(GSH) conjugate, which consisted of two GSH molecules linked to benoxacor, was biosynthesized following the apparent addition of a suitable activating group to the benzoxazine-ring C2 atom of the previously described mono(GSH) conjugate. The remaining metabolites identified were all presumably derived from either the mono(GSH) or di(GSH) conjugates of benoxacor. A disaccharide conjugate was identified as S-O-(diglycoside)GSH and was apparently formed by one or more glycosyl transferases which used the mono(GSH) conjugate as a substrate. Two additional metabolites identified were the mono- and di(γ-GluCys) conjugates of benoxacor, the formation of which was most likely the result of peptidase activity on the mono- and di(GSH) conjugates of benoxacor, respectively. An S-(S‘-Cys)GSH conjugate was also identified and was likely formed by both peptidase and dipeptidase activities on the di(GSH) conjugate. Metabolite structures and postulated pathways of their biosynthesis in vivo are presented. Keywords: Benoxacor; herbicide safener; CGA-154281; glutathione S-transferase; BMS cells; xenobiotic metabolism; xenobiotic detoxification; maize; glutathione conjugation; glutathione conjugate metabolism; dichloroacetamide, substituted benzoxazine; herbicide safener metabolism