Chloroacetamide mode of action, I: Inhibition of very long chain fatty acid synthesis in Scenedesmus acutus

Herbicidal chloroacetamides cause a very sensitive inhibition of fatty acid incorporation into an insoluble cell wall fraction of Scenedesmus acutus. The molecular basis was investigated in more detail. After incubation of the algae with [C-14]oleic acid and saponification, the remaining pellet was solubilized and fractionated consecutively with chlor oform/methanol, phosphate buffer, amylase, pronase, and finally with dioxan e/HCl. By acid hydrolysis in dioxane a part of the cell wall residue was so lubilized showing inhibition of exogenously applied oleic acid and other la belled precursors such as stearic acid, palmitic acid, and acetate. After e xtraction of this dioxane-soluble subfraction with hexane, HPLC could separ ate labelled metabolites less polar than oleic acid. Their formation was co mpletely inhibited by chloroacetamides, e.g. 1 mu M metazachlor. This effec t was also observed with the herbicidally active S-enantiomer of metolachlo r while the inactive R-enantiomer had no influence. These strongly inhibite d metabolites could be characterized by radio-HPLC/MS as very long chain fa tty acids (VLCFAs) with a carbon chain between 20 and 26. Incubating a meta zachlor-resistant cell line of S. acutus (Mz-1) with [C-14]oleic acid, VLCF As could not be detected in the dioxane/ HCl-subfraction. Furthermore, comp aring the presence of endogenous fatty acids in wildtype and mutant Mz-1 th e VLCFA content of the mutant is very low, while the content of long chain fatty acids (C16-18) is increased, particularly oleic acid. Obviously, the phytotoxicity of chloroacetamides in S. acutus is due to inh ibition of VLCFA synthesis. The resistance of the mutant to metazachlor has a bearing on the higher amount of long chain fatty acids replacing the mis sing VLCFAs in essential membranes or cell wall components.