No abstract

Manganese ethylene-bis-dithiocarbamate (Mn-EBDC) is the major active element of maneb, a pesticide linked to parkinsonism in certain individuals upon chronic exposure. Additionally, it has been shown to produce dopaminergic neurodegeneration in mice systemically coexposed to another pesticide, 1,1¢-dimethyl-4,4¢-bipyridinium (paraquat). Here, we described a rat model in which selective dopaminergic neurodegeneration was produced by delivering Mn-EBDC directly to the lateral ventricles. After establishing this model, we tested whether Mn-EBDC provoked dopamine efflux in the striatum, a well-known phenomenon produced by the mitochondrial inhibitor 1-methyl-4-phenylpyridinium (MPP + ), the active metabolite of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) that causes parkinsonism in humans, as well as in some animals. Finally, we investigated whether Mn-EBDC directly inhibited mitochondrial function in vitro using isolated brain mitochondria. Our data demonstrated that Mn-EBDC induced extensive striatal dopamine efflux that was comparable with that induced by MPP + , and that Mn-EBDC preferentially inhibited mitochondrial complex III. As mitochondrial dysfunction is pivotal in the pathogenesis of Parkinson's disease (PD), our results support the proposal that exposure to pesticides such as maneb, or other naturally occurring compounds that inhibit mitochondrial function, may contribute to PD development.

1,4-Dicarbonyl compounds, which include 2,5-hexanedione and recently discovered endogenous 4-ketoaldehydes (levuglandins, isoketals, and neuroketals), exhibit severe toxicity. The key step in the toxicity of these compounds is their reaction with the lysyl residues of proteins to form pyrrole adducts. To screen for effective scavengers of these toxic compounds, we determined the reaction rates of pyrrole formation for a series of primary amines with a model 4-ketoaldehyde, 4-oxopentanal (OPA). We found pyridoxamine (PM) to react extremely rapidly, with a second-order rate constant at physiological pH being approximately 2300 times faster than that of Nalpha-acetyllysine. The extreme reactivity of PM was unique to 1,4-dicarbonyls, as its reactions with methylglyoxal and 4-hydroxy-2(E)-nonenal were much slower and only slightly faster than with Nalpha-acetyllysine. The phenolic group of PM was found to be essential to its high reactivity, and the rate constant for pyrrole formation with OPA exhibited a maximum at pH 7.5, close to the second pKa of PM. We therefore propose a mechanism involving transfer of the phenolic proton to the carbonyl of the initially formed hemiacetal, which facilitates subsequent nucleophilic attack and ring closure. Only 1,4-dicarbonyls are likely to participate in the proposed mechanism, thereby conferring unique sensitivity of this class of compounds to scavenging by PM.

No abstract

Abstract-Sudden cardiac death attributable to ventricular tachycardia/fibrillation (VF) remains a catastrophic outcome of myocardial ischemia and infarction. At the same time, conventional antagonist drugs targeting ion channels have yielded poor survival benefits. Although pharmacological and genetic models suggest an association between sodium (Na ϩ ) channel loss-of-function and sudden cardiac death, molecular mechanisms have not been identified that convincingly link ischemia to Na ϩ channel dysfunction and ventricular arrhythmias. Because ischemia can evoke the generation of reactive oxygen species, we explored the effect of oxidative stress on Na ϩ channel function. We show here that oxidative stress reduces Na ϩ channel availability. Both the general oxidant tert-butyl-hydroperoxide and a specific, highly reactive product of the isoprostane pathway of lipid peroxidation, E 2 -isoketal, potentiate inactivation of cardiac Na ϩ channels in human embryonic kidney (HEK)-293 cells and cultured atrial (HL-1) myocytes. Furthermore, E 2 -isoketals were generated in the epicardial border zone of the canine healing infarct, an arrhythmogenic focus where Na ϩ channels exhibit similar inactivation defects. In addition, we show synergistic functional effects of flecainide, a proarrhythmic Na ϩ channel blocker, and oxidative stress. These data suggest Na ϩ channel dysfunction evoked by lipid peroxidation is a candidate mechanism for ischemia-related conduction abnormalities and arrhythmias.