Original contributionCatalysis of catechol oxidation by metal-dithiocarbamate complexes in pesticides
Introduction
Risk assessment for environmental toxicants, especially pesticides, is currently undergoing a refocusing to include chronic health effects from long-term, low-level exposure. This has been motivated in part by several epidemiological studies that have linked protracted occupational and residential pesticide exposure, especially dithiocarbamate (DTC)-based pesticides, with an increased risk for Parkinson’s disease (PD) 1, 2, 3. Such epidemiological associations have been buttressed by several clinical reports of parkinsonism and other deficits following high-level occupational exposure to maneb, the active ingredient of which is a manganese-DTC complex 4, 5, 6. Experimental studies have demonstrated dopaminergic neurodegeneration and motor abnormalities in mice exposed to the combination of paraquat and maneb 7, 8, 9.
DTC-based pesticides are a diverse class of compounds that are thought to act via several different mechanisms. One subset of DTC-based pesticides is the salts of either dimethyl- or diethyldithiocarbamates (DEDC) or ethylene-bis-dithiocarbamate (EBDC). Another very widely used subset of DTC-based pesticides has a metal atom coordinated with EBDC (Fig. 1); these include maneb, mancozeb, and zineb. The most recent estimate from the United States Geological Service (http://www.USGS.gov) is that 3.4 million pounds of maneb and 8.0 million pounds of mancozeb are applied annually in the United States. Zineb was withdrawn in the United States in 1991 but is still widely used throughout the world.
Toxicity from salts of DEDC or EBDC is thought to derive from rapid decomposition and release of carbon disulfide, ethylene bis-(isothiocyanate sulfide), or ethylene thiourea 10, 11, 12. Indeed, previous work has shown that these salts can influence dopaminergic neurodegeneration; they enhance 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) toxicity in vitro and in vivo 13, 14, inhibit dopamine β-hydroxylase activity 15, 16, 17, inhibit tyrosine hydroxylase activity 13, 18, and suppress glutamate uptake in cultured cells [19]. In contrast to DTC salts, maneb, mancozeb, and zineb are more stable and have not been shown to contribute to toxicity through the liberation of carbon disulfide or ethylene thiourea 12, 20, 21, 22. Thus, the contribution of maneb and similar metal-DTC-based pesticides to dopaminergic neurodegeneration remains unclear.
One hypothesis for the vulnerability of the mesostriatal dopaminergic system to degeneration is the high concentration of the neurotransmitter dopamine (DA). DA is thought to contribute to neurodegeneration by augmenting oxidative stress through two different mechanisms (Fig. 2). Paramagnetic metal-catalyzed single electron oxidation (auto-oxidation) of DA and some of its catechol metabolites generates superoxide anion and quinonoid species 23, 24. Alternatively, monoamine oxidase catalyzes oxidative deamination of the side chain of DA to generate its corresponding aldehyde and hydrogen peroxide. Experimental evidence has demonstrated that DA oxidation, most likely auto-oxidation of extravesicular DA and its catechol metabolites, contributes significantly to this region’s increased vulnerability to ischemic injury and neurotoxicity from some drugs of abuse. As assessed by oxidative damage to mitochondrial DNA, the mesostriatal dopaminergic system experiences the greatest age-related oxidative damage in human brain [25] and is also the major site of neurodegeneration in PD; in both cases DA oxidation has been proposed to be a significant contributor.
Given the proposed central role for catechol auto-oxidation in mesostriatal dopaminergic neurodegeneration, along with the epidemiological, clinical, and experimental data suggesting that DTC-based pesticides may damage this region of brain, we propose the hypothesis that DTC-based pesticides may render this region of brain more vulnerable to injury by promoting catechol auto-oxidation. A limitation of previous observational studies in patients and experimental studies in rodents is that commercial preparations of DTC-based pesticides that are only approximately 75–80% pure. Here we have tested our hypothesis by performing a structure-function analysis of the ability of synthetically pure metal-EBDC complexes contained in DTC-based pesticides to catalyze catechol auto-oxidation.
Section snippets
Materials
N-acetylcysteine (NAC), MnCl2, Cu(NO3)2, ZnCl2, Zn(DEDC)2, mushroom tyrosinase (EC 1.14.18.1), and Na(DEDC) were obtained from Sigma Chemical Corporation (St. Louis, MO, USA).
[ethylene-bis(dithiocarbamato)]manganese (II) (MnEBDC)
Ammonium ethylene-bis(dithiocarbamate) (30 mmol) was prepared by heating a solution of ethylenediamine (2.0 ml), water (40 ml), ammonium hydroxide (3.6 g), carbon disulfide (4.3 ml), and ethanol (10 ml) at 40°C for 3 h and cooling. It was added dropwise under nitrogen to a solution of manganese (II) sulfate monohydrate
Degradation of the metal-DTC complexes
Having confirmed the composition of each of the metal-DTC complexes by three independent means, we first undertook an assessment of their stability under the exact conditions used in subsequent experiments. Degradation of metal-DTC complexes was assayed by direct observation of the compounds with UV-Vis spectrophotometry as described by others [33]. The absorption maxima at approximately 250–260 nm and 280–290 nm corresponded to conjugation of the nitrogen and sulfur atoms, respectively;
Discussion
Mounting epidemiological, clinical, and experimental data suggest that DTC-based pesticides may enhance the vulnerability of the mesostriatal dopaminergic system to degenerate under some circumstances 8, 9, 14, 42, 43; however, it is noteworthy that, with the exception of massive exposures, these studies suggest that DTC-based pesticides do not act as direct dopaminergic neurotoxicants. Rather, protracted exposure to DTC-based pesticides may increase the risk of PD, or increase the dopaminergic
Acknowledgements
This work was supported by grants from the National Institutes of Health: ES10196, ES07028, and ES00267. The authors wish to express their gratitude to Drs. Kalyani Amarnath and William Valentine.
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