Inhibition of Acyl-CoA Elongation by Chloroacetamide Herbicides in Microsomes from Leek Seedlings

The inhibition of very-long-chain fatty acid (VLCFA) synthesis by chloroacetamide herbicides was studied in vivo as well as in vitro. Incorporation of [2-14C]malonate into C20 to C24 VLCFAs of etiolated leek seedlings was strongly inhibited by the chloroacetamide metazachlor (I50 ≈ 10–100 nM). The I...

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Bibliographic Details
Published in:Pesticide biochemistry and physiology 2000-05, Vol.67 (1), p.25-35
Main Authors: Schmalfuβ, Jochen, Matthes, Bernd, Knuth, Karin, Böger, Peter
Format: Article
Language:English
Subjects:
Agrochemicals products
Allium porrum
Animal, plant and microbial ecology
Applied ecology
Biological and medical sciences
chloroacetamide
Ecotoxicology, biological effects of pollution
Effects of pollution and side effects of pesticides on plants and fungi
elongase
Fundamental and applied biological sciences. Psychology
herbicide
leek
metazachlor
metolachlor
Other agrochemicals used in plant protection (herbicides, fungicides, etc.)
Phytopathology. Animal pests. Plant and forest protection
Pollution effects and side effects of agrochemicals on crop plants and forest trees. Other anthropogenic factors
Pollution effects. Side effects of agrochemicals
very-long-chain fatty acid
VLCFA
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Summary:The inhibition of very-long-chain fatty acid (VLCFA) synthesis by chloroacetamide herbicides was studied in vivo as well as in vitro. Incorporation of [2-14C]malonate into C20 to C24 VLCFAs of etiolated leek seedlings was strongly inhibited by the chloroacetamide metazachlor (I50 ≈ 10–100 nM). The I50 values of inhibition of incorporation into each VLCFA decreased with their chain length [I50(20:0) ≈ 1 μM, I50(22:0) ≈ 100 nM, I50(24:0) ≈ 10 nM]. With a microsomal preparation from leek seedlings an in vitro elongase assay to quantify chloroacetamide activity was developed using [2-14C]malonyl-CoA and acyl-CoA (16:0-CoA to 22:0-CoA) as substrates. Under our assay conditions the I50 values of inhibition by metazachlor of each step of acyl-CoA elongation were determined to be 0.5 μM (for elongation of 16:0-CoA), 1.7 μM (18:0-CoA), 0.7 μM (20:0-CoA), and 0.5 μM (22:0-CoA). Furthermore, it could be shown that inhibition increased with decreasing concentrations of acyl-CoA primer substrate. Using the chiral chloroacetamide metolachlor acyl-CoA elongation was only inhibited with the herbicidally active S-enantiomer while the R-enantiomer had no influence. As demonstrated by 20:0-CoA elongation the I50 value decreased about 10-fold after preincubation of the enzyme preparation at higher temperatures. Furthermore, enzyme activity could not be recovered by dilution of the enzyme–inhibitor complex. These findings strongly support the hypothesis that a covalent binding of metazachlor to its target may occur. From this study the mode of action of herbicidal chloroacetamides is disclosed as a strong inhibition of VLCFA synthesis resulting from (a) an inhibition of each elongation step in series, (b) the increase of that inhibition with the decrease of acyl-CoA primer substrate concentration, and (c) the tight binding of the inhibitor to the target enzyme.
ISSN:0048-3575
1095-9939
DOI:10.1006/pest.2000.2473