Towards the primary target of chloroacetamides –new findings pave the way

This review reports on research of the last ten years to find the primary target enzyme for chloroacetamides. As could be shown first with the green alga Scenedesmus, the formation of very‐long‐chain fatty acids (VLCFAs) is severely impaired. Subsequently, in short‐term experiments, labelled malonate or stearate could be incorporated into leaf discs of cucumber, barley or leek seedlings. While the formation of ‘normal’ long‐chain fatty acids (up to C18) was not influenced, phytotoxic chloroacetamides strongly inhibited the synthesis of VLCFAs of C20, 22 and 24, with I₅₀ values of 10–100 nM . Inhibition depends on the amide structure and on stereospecificity. Also cafenstrole or recently developed tetrazolinones and phosphosulfonates were found active to inhibit fatty‐acid elongation. Subsequently, a cell‐free elongase assay was developed using a microsomal preparation from leek seedlings (Allium porrum L), [¹⁴C]malonyl‐CoA and C18, 20, or C22 acyl‐CoA primer substrates. All elongation steps were strongly affected by those phytotoxic herbicides which were also active in vivo. The inhibitors form a tight‐binding complex with the condensing elongase enzyme system which develops with time and lowers the I₅₀ values markedly. Apparently, a nucleophilic attack of the inhibitor takes place at the specific target enzyme. Acyl‐CoA elongation inhibition is correlated with growth inhibition of the intact cell. Due to the low I₅₀ values and the specific inhibition, we assume that impaired VLCFA‐formation is the primary phytotoxic impact of chloroacetamides and functionally related structures. © 2000 Society of Chemical Industry