Microsomal oxidation of the herbicides EPTC and acetochlor and of the safener MG-191 in maize

Microsomal fractions isolated from shoot and root tissues of 4-day-old etiolated seedlings of maize (Zea mays L., DeKalb XL72AA, and DeKalb XL67) were used to study the metabolism of EPTC, acetochlor, and MG-191. XL72AA maize is tolerant to EPTC, whereas XL67 maize is susceptible. Acetochlor is phytotoxic to both hybrids and treatment with MG-191 protects XL72AA and XL67 maize from acetochlor and EPTC injury. In vitro assays with [14C]EPTC, [14C]acetochlor, and [14C]MG-191 as substrates revealed that microsomal preparations from shoots and roots of both maize hybrids mediate the formation of a single metabolite from each substrate. TLC analysis showed that EPTC was converted to its sulfoxide (Rf = 0.17) and MG-191 to an unknown metabolite (Rf = 0.16). Both of these reactions were dependent on oxygen but did not require NADPH as a cofactor. Acetochlor was converted to an unknown metabolite (Rf = 0.45) and the reaction required both oxygen and NADPH. Pretreatment with EPTC enhanced microsomal EPTC-metabolizing activity in shoots and roots of XL72AA maize. Pretreatment with acetochlor enhanced microsomal acetochlor-metabolizing activity in shoots and roots of XL67 maize. Pretreatment with MG-191 enhanced MG-191-metabolizing activity as well as acetochlor-metabolizing activity but depressed EPTC-metabolizing activity in shoot and root microsomal preparations from both hybrids. Pretreatment with naphthalic anhydride enhanced microsomal activity metabolizing EPTC and acetochlor in shoots and roots of both maize hybrids. Pretreatment with piperonyl butoxide did not affect microsomal activity metabolizing EPTC, acetochlor, or MG-191 in shoots and roots of both maize hybrids. The cytochrome P450 content of microsomal preparations from shoots of both maize hybrids was 5-fold greater than that detected in their roots. Cytochrome P450 levels in shoots of XL72AA maize were 1.4-fold higher than those in shoots of XL67 maize. Acetochlor and EPTC induced difference spectral responses approximating "Type I" binding to maize cytochrome P450. MG-191 did not induce any binding spectrum. These results suggest that a maize cytochrome P450-dependent monooxygenase may be involved in the oxidative metabolism of acetochlor. The sulfoxidation of EPTC may be dependent on cytochrome P450, but it does not seem to be mediated by a monooxygenase.