Bleaching herbicides inhibit carotene biosynthesis in photosynthetic organisms. The interaction of norflurazon [4-chloro-5-methylamino-2-(3-trifluoromethylphenyl)-pyridazin-3(2H)one] with its target enzyme phytoene desaturase, has been characterized by enzyme-kinetic studies. A Lineweaver-Burk plot showed a non-competitive m anner for norflurazon inhibition. Binding of norflurazon to phytoene desaturase was reversible as dem onstrated by complete replacem ent of bound 14 C-labeled herbicide by unlabeled norflurazon. In a linear Dixon plot the k i value for norflurazon was determ ined as 0.09 μм . With the in vitro system from Anacystis used in this study it was possible to perform structure-activity studies with selected m -phenyl-substituted pyridazinones. A linear relationship between inhibitory properties of these compounds and their lipophilicity could be established.
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Sandmann et al. (1989) studied this question.