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Diquat (1,1'-ethylene-2,2'-bipyridinium ion; DQ) is a nonselective quick-acting herbicide, which is used as contact and preharvest desiccant to control terrestrial and aquatic vegetation. Several cases of human poisoning were reported worldwide mainly due to intentional ingestion of the liquid formulations. Its toxic potential results from its ability to produce reactive oxygen and nitrogen species through redox cycling processes that can lead to oxidative stress and potentially cell death. Kidney is the main target organ due to DQ toxicokinetics and redox cycling. There is no antidote against DQ intoxications, and the efficacy of treatments currently applied is still unsatisfactory. The aim of this work was to review the most relevant human and experimental findings related to DQ, characterizing its chemistry, activity as herbicide, mechanisms of toxicity, consequences of poisoning, and potential therapeutic approaches taking into account previous experience in developing antidotes for paraquat, a more toxic bipyridinium herbicide.

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Male and female C57BL/6J mice were administered diquat dibromide (DQ∙Br2) in their diets at concentrations of 0 (control), 12.5 and 62.5 ppm for 13 weeks to assess the potential effects of DQ on the nigrostriatal dopaminergic system. Achieved dose levels at 62.5 ppm were 6.4 and 7.6 mg DQ (ion)/kg bw/day for males and females, respectively. A separate group of mice was administered 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) ip as a positive control. The comparative effects of DQ and MPTP on the substantia nigra pars compacta (SNpc) and/or striatum were assessed using neurochemical, neuropathological and stereological endpoints. Morphological and stereological assessments were performed by investigators who were "blinded" to dose group. DQ had no effect on striatal dopamine concentration or dopamine turnover. There was no evidence of neuronal degeneration, astrocytic or microglial activation, or a reduction in the number of tyrosine hydroxylase positive (TH(+)) neurons in the SNpc or neuronal processes in the striatum of DQ-treated mice. These results are consistent with the rapid clearance of DQ from the brain following a single dose of radiolabeled DQ. In contrast, MPTP-treated mice exhibited decreased striatal dopamine concentration, reduced numbers of TH(+) neurons in the SNpc, and neuropathological changes, including neuronal necrosis, as well as astrocytic and microglial activation in the striatum and SNpc.

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CASE REPORT: A 37-year-old man ingested in a suicide attempt 300 mL of a diquat solution (equivalent to 60 g diquat ion). The initial diquat serum concentration was 64 microg/mL 4 hours after poisoning. The clinical course was characterized by a progressive anuria and by neurological disorders (coma and seizures). The patient died 26 hours after poisoning from refractory cardiocirculatory collapse. Extracorporeal techniques removed 1.09 g of diquat which could be considered as significant in regard to the total amount that was likely absorbed, but they did not influence the clinical outcome. There was marked renal tubular damage at autopsy and the highest diquat tissue concentration was found in the kidneys.

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A new colourimetric method is described for the quantification of diquat using a yellow-coloured derivative produced by heating diquat in alkaline solution at 80 degrees C. The absorption maximum of the yellow derivative is 420 nm and the molar absorption coefficient is 2.76 x 10(4) (0.15 in 1 microgram diquat/ml with 1 cm light path). The absorption at 420 nm shows a linear concentration dependence in the range 0.1-10 micrograms/ml and fading of the colour is about 5% after 1 h. Under the same conditions, paraquat does not produce any coloured products. The concentration of diquat in the solution containing both diquat and paraquat can be determined by the absorption of diquat derivative at 420 nm without interference from paraquat. By adding sodium dithionite to the solution the concentration of paraquat can be determined by the absorption of paraquat radicals at 600 nm without interference from diquat, because the yellow derivative does not react with dithionite. This yellow diquat derivative can be extracted completely with cyclohexanol by saturating the solution with Na2SO4. The absorption maximum in cyclohexanol shifts to 440 nm with the same molar absorbance and the same half-band width as in water. Fading of the colour is less than 5% after 24 h in cyclohexanol. Perchloric acid (3%) and trichloroacetic acid (4.5%) which are often used for deproteinization of tissue homogenates, do not inhibit production of the coloured derivative at pH 13.5 or extraction of the derivative with cyclohexanol. This method is suitable for a quick determination of small amounts of diquat in tissues, since the extraction with cyclohexanol not only concentrates the derivative rapidly but also quite efficiently eliminates the coloured substances in tissue homogenates. The detection limit of diquat is 0.02 microgram/ml for blood and 0.05 microgram/g for liver when 1 ml or 1 g is used for analysis. In three human cases of fatal intoxication, both paraquat and diquat were quantified using 50 microliters of serum. In non-toxic dosing of diquat to rats for 14 days, the diquat level was highest in the spleen followed by the kidneys.

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The adsorption characteristics of paraquat and diquat onto activated carbon in vitro were discussed for the primary treatment of acute poisoning by accidental, suicidal or homicidal ingestion of paraquat containing herbicides. Paraquat was adsorbed onto activated carbon more abundantly and more rapidly in physiological saline solution than that in artificial gastric juice and distilled water. Most suitable solvent for paraquat removal by activated carbon was physiological saline solution (0.9% sodium chloride solution). No significant correlation was observed between the ability of paraquat removal and the properties of adsorbent. Paraquat was preferentially adsorbed onto activated carbon in the mixed solution. The adsorption abilities by activated carbon (the removal ratio, the amount adsorbed and the adsorption rate) for paraquat were larger than those for diquat, and it was enhanced by added sodium chloride and added magnesium sulfate. Enhancing effect for adsorption removal was proportional to the saline concentration. As addition of salts into carbon suspension enhanced the adsorption ability, it will contribute to the effective treatment of acute poisoning.

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This report describes a slight difference in the rate of decrease of serum paraquat and diquat concentrations in eight human cases of poisoning by the herbicide PreegloxL (containing paraquatCl2, 5% and diquatBr2, 7%) and the distribution of each in three autopsied cases. There was no variation between the serum concentrations of paraquat and diquat within 24 h after ingestion, but paraquat remained at a slightly higher concentration than diquat after more than 24 h. The decrease of urinary paraquat and diquat concentrations was almost the same during the 24-h determination period. In three autopsied cases, diquat concentrations in the tissues were relatively lower than those of paraquat, except in bile. Paraquat and diquat were unevenly distributed in various tissues and fluids, but the distribution patterns of each in any particular tissues were quite similar. As no difference was observed in the decrease of urinary paraquat and diquat, the much higher concentration of diquat in bile indicates that bile may be one of the effective factors in lowering the concentration of diquat in serum and in tissues of the human body long after ingestion.

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The metabolism of paraquat and diquat was studied in vitro using rat liver homogenates, and the resulting metabolites were identified. Rat liver was homogenized with three volumes of isotonic buffer, and aliquots of the homogenate were preheated in a boiling water bath for 5 min prior to use. One milliliter of a mixture including both paraquat and diquat in an isotonic buffer solution (10 micrograms ion/ml) was incubated with an equal volume of fresh or preheated homogenate for 1 to 60 min at 37 degrees C. Quantification of paraquat and diquat was carried out by high-performance liquid chromatography (HPLC). In the fresh homogenate, a gradual decrease of paraquat concentration (about a 30% decrease over 60 min of incubation) and a rapid decrease of diquat concentration (not detectable after 10 min of incubation) were observed, but the same phenomenon was not evident with the preheated homogenate. Analysis of the incubated mixture of fresh liver homogenate with paraquat and diquat revealed three unknown peaks on the HPLC chromatograms; these seemed to be breakdown products of paraquant and diquat. The products were isolated and purified from the mixture by Sep-Pak C18 cartridge extraction, HPLC, silica gel column chromatography and Sephadex LH-20 column chromatography. Analysis of the chemical structure of the purified compounds was performed by infrared spectroscopy, mass spectrometry and nuclear magnetic resonance spectroscopy. These analyses determined that paraquat-monopyridone (1',2'-dihydro-1,1'-dimethyl-2-oxo-4,4'-bipyridylium ion) was derived from paraquat, and that diquat-monopyridone (6,7-dihydro-4-oxodipyrido [1,2-a':2',1'-c] pyrazinium ion) and diquat-dipyridone (6,7-dihydrodipyrido [1,2-a:2',1'-c] pyrazine-4,9-dione) were derived from diquat. These results indicate that paraquat and diquat are metabolized by rat liver homogenate, diquat more readily so than paraquat. As the toxicity of these metabolites has been reported to be much lower than those of the parental compounds, it would seem that there is a system capable of detoxifying paraquat and diquat in rat liver.

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Biliary excretion of oxidized glutathione (GSSG) is used as an index of oxidative stress. We observed a marked interanimal difference in susceptibility to diquat-induced oxidative stress. When diquat injections (120 mumol/kg, i.v.) were administered to rats, a 60-fold increase in the biliary excretion of GSSG was observed in 40% of the rats (responders); however, diquat failed to increase the biliary excretion of GSSG in 60% of the animals (nonresponders). This interanimal variation is not due to differences in the hepatic metabolism or hepatobiliary transport of GSSG, as no interanimal difference was observed in the biliary output of GSSG after administration of another oxidative stress-inducing agent, t-butyl hydroperoxide (1.4 mmol/kg, i.v.). We then examined the hepatobiliary disposition of diquat (120 mumol/kg, i.v.) using a high-performance liquid chromatography procedure to quantitate diquat in blood, liver and bile. No differences in blood or biliary concentration of diquat were noted between responders and nonresponders. However, a marked difference was observed in the hepatic concentration of diquat in responders and nonresponders. The responders exhibited a 4-fold higher hepatic diquat concentration than the nonresponders (65 or 15 nmol/g, respectively) 30 min after diquat administration. In conclusion, this study demonstrates a marked interanimal variation in the susceptibility of Sprague-Dawley rats to oxidative stress produced by diquat, which appears to be due to interanimal difference in the hepatic accumulation of diquat.

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The abilities of paraquat, diquat, and nitrofurantoin to undergo cyclic oxidation and reduction with rat microsomal systems have been assessed and compared to that of the potent redox cycler, menadione. Diquat and menadione were found to be potent redox cyclers with comparable abilities to elicit a nonstoichiometric increase in both the consumption of O2 and the oxidation of NADPH, compared to the amounts of substrate added. In contrast, paraquat and nitrofurantoin redox cycled poorly, being an order of magnitude less potent than either diquat or menadione. This was reflected in kinetic studies using lung and liver microsomes, which showed that NADPH-cytochrome P-450 reductase had a lower affinity (Km) for paraquat and nitrofurantoin than for menadione and diquat, although values of Vmax were comparable for all the substrates except nitrofurantoin, which was lower. In order to assess redox cycling of the substrates in an intact lung system, the O2 consumption of rat lung slices was measured in the presence of all four compounds. A small increase in lung slice O2 uptake was observed with paraquat (10(-5) M) in the first 2.5 hr of incubation, possibly because of redox cycling of a high intracellular concentration of paraquat resulting from active accumulation into target cells. This stimulation in O2 uptake was no longer observed when slices were incubated for a longer period or with higher paraquat concentrations (10(-4) M), possibly because of toxic effects in target cells. High concentrations of diquat (10(-5) M) had no effect on O2 consumption of lung slices.(ABSTRACT TRUNCATED AT 250 WORDS)

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An electron spin resonance (ESR) method already in use for the quantitative analysis of paraquat was applied to the analysis of diquat in blood, serum, urine, tissue homogenates and several drinks without purification of the samples. The diquat radical produced with ascorbic acid at alkaline pH was much more stable than that produced with the commonly used sodium dithionite. Radical decay in solutions covered with n-hexane was less than 5% after 60 min over a wide range of ascorbic acid concentrations. In 0.2 N NaOH solution 85% of the radicals was present even after 24 h. The limit of detection was 0.3 micrograms/ml and the required amount of sample was 0.1 ml. When both diquat and paraquat were present in a sample the diquat was first extracted with 1-butanol prior to the ESR measurement, because both species were converted to the radicals.

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The herbicide paraquat has been suggested as a causative agent for Parkinson's disease because of its structural similarity to a metabolite of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), which may induce a parkinsonism-like condition. MPTP as well as its metabolite 1-methyl-4-phenylpyridine have melanin affinity, and the parkinsonism-inducing potency of MPTP is much stronger in species with melanin in the nerve cells. Autoradiography of [3H]MPTP in experimental animals has shown accumulation in melanin-containing tissues, including pigmented neurons. In the present whole body autoradiographic study accumulation and retention was seen in neuromelanin in frogs after i.p. injection of [14C]paraquat or [14C]diquat. By means of whole body autoradiography of [14C]diquat in mice (a species with no or very limited amounts of neuromelanin) a low, relatively uniformly distributed level of radioactivity was observed in brain tissue. Accumulation of toxic chemical compounds, such as paraquat, in neuromelanin may ultimately cause lesions in the pigmented nerve cells, leading to Parkinson's disease.

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