The inhibitory mode of action of the pyridazinone herbicide norflurazon on a cell-free carotenogenic enzyme system

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The inhibition site of the phenylpyridazinone herbicide, norflurazon [SAN 9789, 4-chloro-5-(methylamino)-2-(3-trifluoromethylphenyl)-pyridazin-3(2 H)one] was determined in a cell-free carotenogenic enzyme system from a mutant strain of Phycomyces blakesleeanus (Mucoraceae). The presence of norflurazon resulted in a reduced flow of radioactivity from [2- 14C]mevalonic acid to phytoene (7,8,11,12,7′,8′,11′,12′-octahydro-ψ,ψ-carotene) and β-carotene (β,β-carotene), whereas an increased incorporation occurred in the C 30 terpenoids, squalene, and ergosterol. Furthermore, radioactivity accumulated in geranylgeranyl pyrophosphate. Since no radioactivity was found in prephytoene pyrophosphate and the radioactivity in phytoene decreased upon addition of norflurazon, this herbicide exerts its primary inhibitory action on the reaction catalyzed by phytoene synthetase. The nonbleaching phenylpyridazinone BAS 13761 [4-chloro-5-methoxy-2-phenyl-pyridazin-3(2 H)-one] did not show this effect. Other inhibitory sites of norflurazon, either on prenyl pyrophosphate synthetase or on the desaturation of phytoene, were excluded.