Paraquat and diquat

Paraquat (1,1 dimethyl, 4,4 bipyridyl) is a nonselective contact herbicide. It is used almost exclusively as a dichloride salt and usually is formulated to contain surfactants. Both its herbicidal and toxicological properties are dependent on the ability of the parent cation to undergo a single electron addition, to form a free radical that reacts with molecular oxygen to reform the cation and concomitantly produce a superoxide anion. This oxygen radical may directly or indirectly cause cell death. Diquat, l,T-ethylene-2,2 -dipyridylium, is a charged quaternary ammonium compound often found as the dibromide salt. The structure of diquat dibromide and that of the closely related herbicide paraquat can be seen in Fig. 4.5. 

Following ingestion of the substance, the gastrointestinal (GI) tract is the site of initial or phase I toxicity to the mucosal surfaces. This toxicity is manifested by swelling, edema, and painful ulceration of the mouth, pharynx, esophagus, stomach, and intestine. With higher levels, other GI toxicity includes centrizonal hepatocellular injury, which can cause elevated bilirubin, and hepatocellular enzyme levels such as AST, ALT, and LDH. 

Diquat poisoning is much less common than paraquat poisoning, so that human reports and animal experimental data for diquat poisoning are less extensive than for paraquat. However, diquat has severe toxic effects on the central nervous system that are not typical of paraquat poisoning (Vanholder et al. 1981 Olson 1994). 

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Bipyridiniums. The bipyridinium herbicides (Table 2), paraquat and diquat, ate nonselective contact herbicides and crop desiccants. Diquat is also used as a general aquatic herbicide (2,296). Bipyridinium herbicides are organic cations and are retained ia the soil complex via cation exchange. They are strongly sorbed to most soils and are not readily desorbed (332). Both paraquat and diquat are not readily leached (293). 

In the field of free radicals and liver injury there is a vast body of work concerning a group of compounds that have proven to be of great value as experimental models but are of little clinical significance. The most frequently used compounds are quinones (particularly menadione), paraquat and diquat, bromobenzene, and organic hydroperoxides, particularly cumene hydroperoxide and r-butyl hydroperoxide (see Poli et al., 1989b). 

Diquat and paraquat are quaternary ammonium compounds largely used as contact herbicides and crop desiccants. When systemic absorption occurs, paraquat and diquat are rapidly distributed into the body. Paraquat primarily accumulates in the lungs and kidneys, while the highest diquat concentrations have been found in the gastrointestinal tract, liver, and kidneys (WHO, 1984). Urine is the principal route of excretion for both diquat and paraquat, which are primarily eliminated as unmodified compounds. Occupationally exposed workers can be monitored by measuring paraquat and diquat concentrations in urine samples (Table 6). Blood concentrations are useful to monitor acute poisoning cases. 

WHO (1984) Environmental Health Criteria 39 Paraquat and Diquat, World Health Organization/International Chemical Safety, Geneva. 

Lock, E.A. 1979. The effect of paraquat and diquat on renal function in the rat. Toxicol. Appl. Pharmacol. 48 327-336. 

Selypes, A., L. Nagymajtenyi, and G. Berencsi. 1980. Mutagenic and embryotoxic effects of paraquat and diquat. Bull. Environ. Contam. Toxicol. 25 513-517. 

Suleiman, S.A. and J.B. Stevens. 1987. Bipyridylium herbicide toxicity effects of paraquat and diquat on isolated rat hepatocytes. Jour. Environ. Pathol. Toxicol. Oncol. 7 73-84. 

Wong, R.C. and J.B. Stevens. 1986. Bipyridilium herbicide toxicity in vitro comparative study of the cytotoxicity of paraquat and diquat toward the pulmonary alveolar macrophage. Jour. Toxicol. Environ. Health 18 393-407. 

This technique has been used to determine the following types of organic compounds in soil polychlorobiphenyls, chlorinated insecticides, triazine herbicides, paraquat and diquat. 

Electrophoretic and isotachoelectrophoretic techniques are gaining in popularity in soil analysis with applications to polyaromatic hydrocarbons, polychlorobiphenyls, tetrahydrothiophene and triazine herbicides, Paraquat and Diquat and growth regulators. Other lesser-used techniques include spectrophotometric methods (five determinants), spectrofluorimetric methods (two determinants), luminescence methods (one determinant), titration methods (one determinant), thin-layer chromatography (five applications), NHR spectroscopy (two applications) and enzymic immunoassays (one determinant). 

Khan [181] has described a method for determining Paraquat and Diquat in soils involving catalytic dehydrogenation of the herbicide followed by gas chromatography and also a pyrolytic method [182]. 

Stransky [107] used isotachophoresis to determine Paraquat and Diquat in soils in amounts down to 10pg kgy1. 

Olson KR (1994) Paraquat and diquat. In Olson KR et al. (eds), Poisoning and drug overdose, 2nd ed. Appelton and Lange, Norwalk CT, pp. 245-246 Ordish G (2007) History of agriculture Beginnings of pest control. In Encyclopedia Britannica. 

Gage JC Toxicity of paraquat and diquat aerosols generated by a size-selective cyclone Effect of particle size distribution. Br J Ind Mei 25 304-314, 1968... 

The quaternary ammonium compounds paraquat and diquat are widely used non-selective contact herbicides, which are extremely toxic to humans. Fee et al. [112] established an HPLC-MS-MS procedure for the determination of these herbicides in whole blood and urine using ethyl paraquat as internal standard. After extraction with Sep-Pak C18 cartridges, analytes were separated using ion pair chromatography with heptafluorobutyric acid in 20 mM ammonium acetate and acetonitrile gradient elution. Detection was carried out in ESI MS-MS SRM mode. Using similar separation and detection conditions, paraquat, diquat, difenzoquat, and a number of structurally related quaternary nitrogen muscle relaxants (see Section 20.2.1.3) were determined in whole blood by Ariffin and Anderson [113]. 

Many aryl- and heteroaryl-substituted derivatives of the ir-deficient heterocyclic compounds have been prepared and some of them are of considerable commercial importance, for example as pesticides (paraquat and diquat) and as pharmaceuticals. 

Cyclic photophosphorylation is also a highly energetic reaction. The bipyridyliums, paraquat and diquat (Figure 2.2), divert the electron flow of cyclic photophosphorylation (photosystem I). The capture of an electron from the chlorophyll reduces the herbicide and the reduced herbicide reacts with oxygen to form superoxide. Superoxide produces hydrogen peroxide within the chloroplast and these two compounds interact to form hydroxyl radicals in the presence of an iron catalyst. Hydroxyl radicals are very damaging and lead to the destruction of the cellular components leading to rapid plant death. 

Strengths of the method include its ability to quantitatively recover neutral, nonvolatile solutes such as glucose and its exceptional recovery of aromatic cations and humic acids on Chromosorb T. The exceptional affinity of aromatic cations for porous PTFE suggests its use to recover pollutants such as paraquat and diquat. The affinity of humate for porous PTFE should also be studied more intensively to learn whether other humic and fulvic acids, or fractions thereof, are adsorbed. 

Memory aid In general, weak organic acids readily diffuse across a biological membrane in an acidic environment, and organic bases can similarly diffuse in a basic environment. This is illustrated quite well in Table 6.2 for the chemical in rat intestine. There are the usual exceptions to the generalizations concerning ionization and membrane transport, and some compounds, such as pralidoxime (2-PAM), paraquat, and diquat, are absorbed to an appreciable extent even in the ionized forms. The mechanisms allowing these exceptions are not well understood. 

Cannard and Criddle [195] have described a rapid pyrolysis-gas chromatography method for the simultaneous determination of Paraquat and Diquat in pond and river waters in amounts down to 0.00 lppm. 

Several systematic studies of the driving force dependence of the rate of forward and back ET in type 1 dyads (see Fig. 1) were carried out during the past decade. As might be expected, the type 1 dyads used in these investigations consist of covalently linked assemblies of metal complexes and organic quenchers used in early studies of bimolecular photoinduced ET reactions. Thus, the type 1 dyads consist of polypyridine Ru(II) complexes linked to pyridinium acceptors such as paraquat and diquat (quatemized 2,2 -bipyridine). 

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