Enzyme-Kinetic Studies on the Interaction of Norflurazon with Phytoene Desaturase

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 manner for norflurazon inhibition. Binding of norflurazon to phytoene desaturase was reversible as demonstrated by complete replacement of bound 14C-labeled herbicide by unlabeled norflurazon. In a linear Dixon plot the k, value for norflurazon was determined as 0.09 (j.m . 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 lipo­ philicity could be established.