The proposed target enzyme for benzoylcyclohexanedione herbicides, 4-hydroxyphenylpyruvate dioxygenase (HPPD) was purified from etiolated maize seedlings with a purification factor of 105. Enzyme activity was measured by detection of carbon dioxide formed from radiolabelled substrate. The enzyme has a pH optimum of 7.3 and an apparent molecular mass of 43 kDa, similar to that of the mammalian liver enzyme. Activity needs the presence of a reducing system glutathione/dichlorophenol indophenol or ascorbate and catalase. Surprisingly, a commercial catalase preparation of low specific activity-generally used for the enzyme assay-showed HPPD activity which was separable from the catalase activity on a gel filtration column. According to kinetic studies with purified maize HPPD, experimental herbicides from the family mentioned were strong competitive inhibitors of the plant enzyme in nanomolar range with Ki values of 5 and 15 nM for 2-(2-nitro-4-chlorobenzoyl)-5-(2-methoxyethyl) cyclohexane-1,3-dione and 2-(2-chloro-4-methanesulfonylbenzoyl)-cyclohexane-1,3-dione (SC-0051 ; sulcotrione), respectively.
Summary:
The effect of l‐aminobenzotriazole (ABT) and EPTC mixtures on plant growth was investigated in greenhouse experiments by presowing application in sand. The concentration of ABT for 50% shoot growth inhibition (I50) was about 1 mM in the monoctyledons investigated. Both subtoxic and toxic doses of ABT were antagonists of EPTC in maize (Zea mays L.) However, subtoxic doses of ABT were syner‐gists of EPTC in wheat (Triticum aestivum L.), barley (Hordeum vulgare L.) and oat (Avena sativa L.). No correlation between ABT and EPTC sensitivity of monocotyledonous plants was found. Some of the C‐ and N‐substituted ABT derivatives (e.g. N‐methyl or N‐benzyl‐ABT) completely reversed the effect of toxic dose of EPTC in maize. These results are considered as indirect evidence for the hypothesis that oxidation by cytochrome P‐450 enzymes is the biochemical target of EPTC and is involved in the mode of action of EPTC safeners.
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