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 t arget enzyme for chloroacetamides. As could be shown first with the green a lga Scenedesmus, the formation of verylong-chain fatty acids (VLCFAs) is se verely impaired. Subsequently, in short-term experiments, labelled malonate or stearate could be incorporated into leaf discs of cucumber, barley or l eek seedlings. While the formation of 'normal' long-chain fatty acids (up t o C18) was nor influenced, phytotoxic chloroacetamides strongly inhibited t he synthesis of VLCFAs of C20, 22 and 24, with I-50 values of 10-100nM. Inh ibition depends on the amide structure and on stereospecificity. Also cafen strole or recently developed tetrazolinones and phosphosulfonates were foun d active to inhibit fatty-acid elongation. Subsequently, a cell-free elonga se assay was developed using a microsomal preparation from leek seedlings ( Allium porrum L), [C-14]malonyl-CoA and C18, 20, or G22 acyl-CoA primer sub strates. All elongation steps were strongly affected by those phytotoxic he rbicides which were also active in vivo. The inhibitors form a tight-bindin g complex with the condensing elongase enzyme system which develops with ti me and lowers the I-50 values markedly. Apparently, a nucleophilic attack o f the inhibitor takes place at the specific target enzyme. Acyl-CoA elongat ion inhibition is correlated with growth inhibition of the intact cell. Due to the low I-50 values and the specific inhibition, we assume that impaire d VLCFA-formation is the primary phytotoxic impact of chloro-acetamides and functionally related structures. (C) 2000 Society of Chemical Industry.