Uncouplers fungicide

Dinitrophenol is a member of the aromatic family of pesticides, many of which exhibit insecticide and fungicide activity. DNP is considered to be highly toxic to humans, with a lethal oral dose of 14 to 43mg/kg. Environmental exposure to DNP occurs primarily from pesticide runoff to water. DNP is used as a pesticide, wood preservative, and in the manufacture of dyes. DNP is an uncoupler, or has the ability to separate the flow of electrons and the pumping of ions for ATP synthesis. This means that the energy from electron transfer cannot be used for ATP synthesis [75,77]. The mechanism of action of DNP is believed to inhibit the formation of ATP by uncoupling oxidative phosphorylation. 

The mode of action of this group of compounds is diverse and depends on the parental compound. The fungicidal activity of most phenols depends on their ability to uncouple oxidative phosphorylation and therefore prevent the production of ATP which is required for growth. Other substituted aromatics reduce growth rates by reacting with the amino or sulfhydryl groups of essential metabolic compounds. 

The fungicidal activity of the various phenols depends on their ability to uncouple oxidative phosphorylation and thus prevent the incorporation of inorganic phosphate into ATP without effecting electron transport. The result of this inhibition is cell death due to the lack of energy for cellular metabolism. 

A final class of uncouplers are compounds that transport protons across the membrane, leading to dissipation of the transmembrane proton gradient, and hence removing the proton-motive force that drives ATP synthase [25-27]. These are the only uncouplers of real significance so far as fungicide discovery is concerned. 

In some cases the lack of selectivity can be beneficial. For example, the primary use of dinocap (3) is as a fungicide for control of powdery mildews, but it is also acaricidal and is used to suppress the populations of various mites [33], In a similar way fluazinam (4), one of the most selective fungicidal uncouplers, has been shown to control some mites [34]. 

Several toxic effects of fungicidal uncouplers have been observed, but it is not clear whether these are directly related to the uncoupling mode of action or to other, compound-specific mechanisms. Some uncouplers have a very high acute toxicity, which is almost certainly a result of the mode of action. This has been exploited in the case of bromethalin (6), which is used as a rodenticide. Another toxic effect that has been observed for several distinct structural types of uncoupler, and hence is likely to be a direct result of this mode of action, is oedema of the central nervous system [48]. Direct interaction with the myelin sheath membrane is probably responsible for this toxicity. 

The development of resistance is a major issue for many classes of fungicide, including those that inhibit respiration. However, uncouplers appear to be less susceptible to the onset of resistance than many other fungicides. For example, a study of resistance development in Sphaerotheca Jidiginea in cucumber greenhouses observed no resistance to dinocap, despite it having been used for over 30 years [60]. This was in contrast to other classes of fungicide, such as benzimidazoles, where resistance had developed over a much shorter period. Even under artificial selection pressure, no resistance to dinocap was seen [61]. 

Clearly, from the discussion above, many different classes of chemistry could theoretically satisfy the physicochemical requirements for uncoupling. This is also the case in practice, with a wide range of different chemical types showing fungicidal (and wider agrochemical) effects. Table 13.4.1 lists a selection of the most important of these, along with some key references. 

The dinitrophenols were the first group of uncouplers to be commercialized, and representatives of this group have found use as herbicides, insecticides/ acaricides and fungicides. However, several have been superseded due to their toxicity and lack of selectivity. 

Further exploration of this area of chemistry by Ishihara resulted in the discovery that arylaminopyridines are also potent uncouplers [98]. These compounds possess moderate acaricidal activity, but are better fungicides than the diarylamines. Optimization of this class of chemistry resulted in the discovery of fluazinam (4) [151]. Not only is fluazinam one of the most potent uncouplers known [99, 126], it also shows a high level of reactivity with thiols, the chlorine atom on the highly electron-deficient phenyl ring being readily displaced [126,... 

Fluazinam s unique combination of properties has resulted in its worldwide commercialization by Ishihara and Syngenta under various trade names, including Shirlan and Frowncide, and it is by far the most commercially important of the fungicidal uncouplers discovered to date. 

Antimycin A, an antibiotic produced by Streptomyces spp. that is used as a fungicide against fungi that are parasitic on rice. It inhibits complex III (ubiquinone —> cytochrome c reduction). It inhibits the oxidation of both malate and succinate, as both require complex III, and the addition of the uncoupler has no effect. 

Inhibition of oxidative phosphorylation (e.g., by trisubstituted tin fungicides) results in limited oxygen uptake with lower ATP formation. The effects of fatigue and weakness are similar to the effects of oxidative uncouplers, but there is no fever. 

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