Uncouplers herbicides

Dinitrophenols had been used as insecticides since 1892 but it was not until the 1930s that their value as herbicides was discovered and 4,6-dinitro-o-cresol (DNOC) was introduced. The trouble with dinitrophenols was their toxicity to all living organisms that respire. Their mode of action is through the uncoupling of oxidative phosphorylation, an effect that leads to a rapid death of any organism that comes into contact with the chemical, including the operator. 

Herbicides that inhibit the photochemical reactions of isolated chloroplasts have been called routinely inhibitors of the Hill reaction. This has been done primarily for convenience and because, for many years, their action was evaluated under nonphosphorylating conditions, frequently with ferricyanide as the electron acceptor. In the past few years, more sophisticated studies have been conducted with herbicides and more is known about their differential actions. Consequently, Moreland and Hilton (2) separated herbicidal inhibitors of the photochemically induced reactions into the following classes (a) electron transport inhibitors, (b) uncouplers, (c) energy transfer inhibitors,... 

Uncouplers. Uncouplers dissociate electron transport from photophosphorylation. Both noncyclic and cyclic phosphorylation are inhibited, but electron transport reactions are either unaffected or stimulated. Because uncouplers relieve the inhibition of electron transport imposed by energy transfer inhibitors, they are considered to act at a site closer to the electron transport chain than the site of phosphate uptake. In Figure 2, they are shown (site 2) as dissipating some form of conserved energy represented as on the noncyclic and cyclic ATP-gener-ating pathways. Perfluidone is the only herbicide identified to date that functions as a pure uncoupler at pH 8.0 (2). Compounds that uncouple photophosphorylation also uncouple mitochondrial oxidative phosphorylation. 

Herbicides that act as inhibitory uncouplers are dinitro-phenols, N-phenylcarbamates, acylanilides, halogenated benzoni-triles, substituted imidazoles, substituted benzimidazoles, bromofenoxim, substituted 2,6-dinitroanilines, pyridinols, and substituted 1,2,4-thiadiazoles (2). 

As pointed out previously, most herbicides have been discovered using random screening programs rather than from an applied rational approach to herbicide design, target, and synthesis. A few attempts to rationally design herbicides chose Inhibition of sites of photophosphorylation uncouplers (284). glycol ate oxidase (285). oxidation of Indoleacetic acid (lAA) by peroxidase (286). and secondary plant metabolism, I.e., phenlyalanine ammonia-lyase (2SZ)... 

Phenoxy herbicides (2,4-D, mecoprop, dichlorprop) are used to control broad-leaved weeds. Ingestion causes nausea, vomiting, p5rrexia (due to uncoupling of oxidative phosphorylation), hyperventilation, hypoxia and coma. Their elimination is enhanced by urine alkalinisation. Organochlorine pesticides, e.g. dicophane (DDT), may cause convulsions in acute overdose. Treat as for status epilepticus. 

Many herbicides act by damaging the photosynthetic chain. Atrazine damages tlie quinone binding area on PSII paraquat takes electrons from ferredoxin and generates (lethal) hydrogen peroxide. Uncouplers like DNP (obviously) work in chloroplasts as well as mitochondria. 

Ioxynil is a more important uncoupler that is widely used as an herbicide. It acts in both mitochondria and chloroplasts. Bromoxynil is similar to the ioxynil, but has bromine instead of iodine substitutions. 

The biochemical action of arsenic herbicides is primarily interference in phosphorus metabolism. They probably kill plants be the uncoupling of oxidative phosphorylation and by the blocking of the enzymes containing suifhydryl groups. 

A large number of commercial herbicides interfere with electron transport and ATP production in isolated chloroplasts and mitochondria (1.). These herbicides can be divided into two groups electron transport inhibitors and inhibitory uncouplers ( 1, 2, The dimethylphenylureas, substituted uracils, s-triazines, and pyridazinones have been classified as electron... 

The atebrin/phosphorylation ratio (Table II, last column) relates inhibition of photophosphorylation to dissipation of the postulated energized state (A pH) of the thylakoid membrane. A low ratio suggests that the two responses are correlated. However, the high ratios obtained for ioxynil and propanil suggest that these two herbicides act as electron transport inhibitors rather than as uncouplers. 

Mitochondrial Responses. The herbicides referred to as inhibitory uncouplers were so named because at low molar concentrations they satisfy most, if not all, of the criteria established for uncouplers of oxidative phosphorylation. However, at higher molar concentrations, they also inhibit mitochondrial electron transport ( 1). ... 

Dose/response curves were developed from traces such as shown in Figure 3B for dinoseb. For comparative purposes, the concentration of compound required to increase the rate of ferricyanide reduction to twice that of the no-herbicide control rate are shown in the last column of Table VI. Uncouplers such as FCCP accelerate the rate of ferricyanide reduction, presumably by shuttling protons across the membrane in response to the electrical potential generated by the reduction of ferricyanide by ferrocene (28). In this study, FCCP was the most effective compound. The two phenolic herbicides (dinoseb and ioxynil) were more active than propanil and chlorpropham. Among the carbanilates, 3-CHPC and... 

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