Inhibitors of acetolactate synthase

A great number of herbicides that work through the inhibition of acetolactate synthase (ALS) have been commercialized. They belong to four chemical groups sulfonylureas (23), triazolopyrimidines (2), imidazolinones (5), and pyrimidinyloxybenzoic analogues (3). (The number of active ingredients in parentheses is taken from The Pesticide Manual.) Also in this case, potent herbicides were developed (e.g., chlorsulfuron) before the site of action was found. 

Chemical structures found in acetolactate synthase inhibitors from commercial herbicides 

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In AChE-based biosensors acetylthiocholine is commonly used as a substrate. The thiocholine produced during the catalytic reaction can be monitored using spectromet-ric, amperometric [44] (Fig. 2.2) or potentiometric methods. The enzyme activity is indirectly proportional to the pesticide concentration. La Rosa et al. [45] used 4-ami-nophenyl acetate as the enzyme substrate for a cholinesterase sensor for pesticide determination. This system allowed the determination of esterase activities via oxidation of the enzymatic product 4-aminophenol rather than the typical thiocholine. Sulfonylureas are reversible inhibitors of acetolactate synthase (ALS). By taking advantage of this inhibition mechanism ALS has been entrapped in photo cured polymer of polyvinyl alcohol bearing styrylpyridinium groups (PVA-SbQ) to prepare an amperometric biosensor for... 

It is not yet clear whether such factors will delay the rate of appearance of other types of resistance e.g. the multiple resistances to grass-killing herbicides in wheat, or to inhibitors of acetolactate synthase, where the fitness of resistant biotypes may be higher. 

Application of Global Sequence Similarity to Find an Inhibitor of Acetolactate Synthase. Acetolactate synthase (ALS) Is the site of action of sulfonylurea, Imldazollnone, and trlazolo pyrimidine herbicides (10-14). Their mode of Inhibition and binding sites on ALS were ambiguous, because (1) these herbicides bear no obvious similarity In their chemical structures to those of ALS substrates (pyruvate and acetolactate), cofactors (thiamine pyrophosphate, FAD, and Mg ) and effectors (valine, Isoleuclne, and leucine) and (2) they Inhibit ALS In a mode too complex to be analyzed. 

Inhibitors of acetolactate synthase Inhibitors of chitin synthesis... 

Inhibitors of Acetolactate Synthase (ALS/AHAS) The enzyme acetolactate synthase (ALS) plays in plants an essential role in branched-chain amino acid biosynthesis. In the pathway leading to valine and leucine, ALS catalyzes the formation of 2-acetolactate from two pyruvate molecules, and in the pathway to isoleucine the formation of 2-acetohydroxybutyrate from 2-ketobutyrate and pyruvate. Due to this double function the enzyme is also called with a more general term aceto-hydroxyacid synthase. ALS is inhibited by several groups of herbicides, mainly the sulfonylureas (SUs), imidazolinones (IMIs), triazolopyrimidines (TPs), pyrimidinylthiobenzoates(PTBs) and sulfonylaminocarbonyltriazolinone (SCTs) (see Chapter 2.1, M. E. Thompson). 

The best characterized extraneous site ir bitors are the many chemical classes of herbicidal inhibitors of acetolactate synthase. These have been extensively reviewed elsewhere (i, 8,9,52,56,81,92). Extraneous site inhibitors are also exemplified by four classes of grass selective herbicides the aryloxyphenoxypropionic acids represented by diclofop, the cyclohexanediones typified by clethc m (for a recent review see (57)), the triazinediones (82) and the perhydroindolizinediones (83) (Figure 7). The following discussion is limited to the first two classes. Both classes of herbicides are potent, reversible inhibitors of acetyl-CoA carboxylase (ACC) from susceptible plants, the putative rate limiting enzyme in lipid biosynthesis. With wheat... 

Second, a key enzyme or receptor in the pathway should be identified as the target. It is best to select enzymes whose products are important for several functions in the species. Cellular response to such a metabolic blockade should also be considered (e.g., cascading effects). Often end-product limitation results in more metabolites entering the pathway. After sufficient substrate accumulation, catalysis may occur even in the presence of an inhibitor (10). However, accumulation of toxic intermediates would prevent tTTTs cellular response and lead to death. Again using sulfonylureas as an example, acetolactate synthase is a common enzyme in the pathway for two essential amino acids rather than just one. Also, inhibition of acetolactate synthase leads to high levels of a-ketobutyrate which is thought to have deleterious effects (11). 

Comparison of Acetyl-CoA-Carboxylase (ACC) Inhibitors with Acetolactate Synthase (ALS) Inhibitors The response patterns from four chemically-unrelated herbicides are compared in Figure 4 The structural formulas of the compounds are given in Figure 6 These treatments have been carried out with different amounts of the active ingredients Therefore, one should not look at the bars too quantitatively the increase or decrease of metabolites is what is important ... 

What are the criteria for regarding a compound as a TS analog The observation that the binding affinity of an inhibitor is greater than that of a substrate, i.e., X, < XM, is insufficient as many potent inhibitors bind differently to an enzyme than the substrate examples are methotraxate, inhibiting dihydrofolate reductase (DHFR) X] = 0.15 pM (Werkheiser, 1961), and sulfonyl urea herbicides, inhibiting acetolactate synthase (ALS) at picomolar levels. 

Gaeddert, J.W., D.E. Peterson, and M.J. Horak (1997). Control and cross-resistance of an acetolactate synthase inhibitor-resistant Palmer amaranth (Amaranth palmeri) biotype. WeedTechnol., 11 132-137. 

Hall, L.M., K.M. Stromme, G.P. Horsman, and M.D. Devine (1998). Resistance to acetolactate synthase inhibitors and quinclorac in a biotype of false cleavers (Galium spurium). Weed Sci., 46 390-396. 

Shaner, D.L. and R.G. Lym (1991). Mechanism of resistance to acetolactate synthase/acetohydroxyacid synthase inhibitors. Proc. West. Soc. Weed Sci., 44 122-125. 

Acetolactate synthase (ALS, EC 4.1.3.18) is the first common enzyme in the biosynthetic route to the branched chain amino acids, valine, leucine and isoleucine. It is the primary target site of action for at least three structurally distinct classes of herbicides, the imidazolinones (IM), sulfonylureas (SU), and triazolopyrimidines (TP) (Figure 1). SU and IM were discovered in greenhouse screening programs whereas TP was subsequently targeted as a herbicide. Numerous substitution patterns can be incorporated into the basic structure of all three classes of herbicides to provide crop selectivity as well as broad spectrum weed control. This is amply demonstrated in the seven products based on SU and four based on IM already in the market. A number of others are in various stages of development. The rapid success of ALS inhibitors as herbicidal products has attracted a world-wide research commitment. Not since the photosystem II... 

Inhibits ALS activity by 50%. A good correlation exists between the herbicidal activity of sulfonylureas and their ability to inhibit acetolactate synthase (2). This in vitro assay using the target enzyme along with the three-dimensioFTI structure of the enzyme should aid in the generation of a substantial data base that can be used to design potent inhibitors. 

This has changed dramatically in recent years, as is apparent throughout the herbicide chapters of this book. Of particular note are the surprising speed of resistance development to acetolactate synthase inhibitors (Mazur al., Gressel, this volume), and the emergence of multiply resistant ryegrass and blackgrass biotypes noted earlier. 

Both compounds are inhibitors of the acetolactate synthase enzyme, also known as aceto hydroxy acid synthase (AHAS) and are classified in group B by the Herbicide Resistance Action Committee HRAC. Table 2.6.1 gives the physicochemical properties of 1 and 2. 

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