Stimulation of Adenosine Triphosphatase Activity of Mitochondria and Submitochondrial Particles by Arsenate

Neurotoxic Mechanism of Arsenic: Synergistic Effect of Mitochondrial Instability, Oxidative Stress, and Hormonal-Neurotransmitter Impairment

Arsenic toxicity which is now a global concern is predicted to affect more than 200 million people. Chronic arsenic exposure conduce carcinogenicity, hepatotoxicity, and neurotoxicity. Here we have reviewed numerous epidemiological and experimental reports related to arsenic toxicity to explore its neurotoxicity mechanism. Penetrability of this metalloid through blood-brain barrier makes it a potent neuro-toxicant by inducing mitochondrial membrane instability and calorie exhaustion. It directly affects the cortex, cerebellum region, and specially microglial cells by the induction of a variety of pro-inflammatory cytokines like TNF-α, IL-6, etc. Pro-apoptotic signaling and the caspase activation by arsenic initiate large-scale tissue damage. Severe diminution of the antioxidant enzymes like superoxide dismutase, catalase, and GPx increases the tissue damage by reactive oxygen and nitrogen species. Hormonal imbalance and neurotransmitter dysregulations make the neural damage and synergism of so many toxic effects create nonresponsive neural control over multiple organs. That enhances the peripheral major organ damage besides direct arsenic effects on these organs. There is motor and cognitive dysfunction which may initiate Parkinsonism- and Alzheimer's-like symptoms. Our present analysis is helpful for the therapeutic studies on arsenic or other heavy metal associated neurological dysfunction.

Modulation of F0F1-ATP synthase activity by cyclophilin D regulates matrix adenine nucleotide levels

Cyclophilin D was recently shown to bind to and decrease the activity of F(0)F(1)-ATP synthase in submitochondrial particles and permeabilized mitochondria [Giorgio V et al. (2009) J Biol Chem, 284, 33982-33988]. Cyclophilin D binding decreased both ATP synthesis and hydrolysis rates. In the present study, we reaffirm these findings by demonstrating that, in intact mouse liver mitochondria energized by ATP, the absence of cyclophilin D or the presence of cyclosporin A led to a decrease in the extent of uncoupler-induced depolarization. Accordingly, in substrate-energized mitochondria, an increase in F(0)F(1)-ATP synthase activity mediated by a relief of inhibition by cyclophilin D was evident in the form of slightly increased respiration rates during arsenolysis. However, the modulation of F(0)F(1)-ATP synthase by cyclophilin D did not increase the adenine nucleotide translocase (ANT)-mediated ATP efflux rate in energized mitochondria or the ATP influx rate in de-energized mitochondria. The lack of an effect of cyclophilin D on the ANT-mediated adenine nucleotide exchange rate was attributed to the ∼ 2.2-fold lower flux control coefficient of the F(0)F(1)-ATP synthase than that of ANT, as deduced from measurements of adenine nucleotide flux rates in intact mitochondria. These findings were further supported by a recent kinetic model of the mitochondrial phosphorylation system, suggesting that an ∼ 30% change in F(0)F(1)-ATP synthase activity in fully energized or fully de-energized mitochondria affects the ADP-ATP exchange rate mediated by the ANT in the range 1.38-1.7%. We conclude that, in mitochondria exhibiting intact inner membranes, the absence of cyclophilin D or the inhibition of its binding to F(0)F(1)-ATP synthase by cyclosporin A will affect only matrix adenine nucleotides levels.

Acute arsenical myopathy: morphological description

We describe the histological findings of the muscle in a case of acute voluntary massive arsenic intoxication resulting in severe rhabdomyolysis. The main features on muscle biopsy were perifascicular hypercontracted fibers, myofibrillar disruption, mitochondrial abnormalities and abundant cytoplasmic vacuoles containing lipids.

31P NMR studies of the intact perfused rat heart: a novel analytical approach for determining functional-metabolic correlates, temporal relationships, and intracellular actions of cardiotoxic chemicals nondestructively in an intact organ model

Intact hearts isolated from adult male, Sprague-Dawley rats were perfused under standardized conditions in an apparatus designed for use in a high-resolution nuclear magnetic resonance (NMR) spectrometer system. Myocardial phosphate metabolite concentrations (ATP, PCr, Pi, and phosphomonoesters) and intracellular pH were determined sequentially at timed intervals coincident with the functional assessments of the intact heart by phosphorus-31 (31P) NMR spectroscopic methods. Myocardial functional and metabolic parameters were unaffected by sustained control perfusion (2 hr). The negative inotropic actions of cadmium were associated with significant changes in the chemical environment of inorganic phosphate (Pi) within the cells. This initial cellular response to cadmium, which correlated with the onset and magnitude of the contractile disturbances, appeared to represent the formation of an acidic, intracellular Pi pool (pH, 6.0). This pH compartment reached a steady state during the period in which maximal changes in contractile function were manifested, and before cellular ATP and PCr concentrations were altered. These findings are consistent with the interpretation that the functional deficits caused by cadmium originated primarily from changes in the chemical environment experienced by intracellular metabolites, rather than changes in the amounts of cellular high energy substrates. In contrast, the time-dependent negative inotropic effects of arsenate were proportional to the loss of cellular ATP stores. Intracellular pH was not affected in these hearts. A distinctive metabolic finding associated with the cardiotoxicity of arsenate was the time-dependent accumulation of previously undetected phosphate metabolites in the arsenate-treated hearts. Efforts to chemically identify these metabolites proved inconclusive; however, existing evidence suggests the possibility that these phosphorus-containing compounds may be arsenophosphate derivatives of naturally occurring cellular metabolites. The present findings provide experimental evidence demonstrating that toxicologic assessments in an intact organ model are feasible using whole organ 31P NMR spectroscopic methods and that meaningful, new insights regarding the biochemical mechanisms responsible for the cardiotoxic actions of xenobiotic agents can be obtained by this analytical approach.

Pyruvate metabolism after in vivo exposure to oral arsenic

This study investigated altered pyruvate metabolism after prolonged oral arsenic exposure. Male rats were given access to deionized drinking water containing 0, 40 or 85 ppm sodium arsenate (As5+) for 3 weeks. Respiration studies with mitochondria isolated from treated animals indicated decreased state 3 respiration (with ADP) and decreased respiratory control ratios (RCR) for pyruvate/malate-mediated respiration, but not for succinate-mediated respiration, as compared to control respiration values. In addition, pyruvate dehydrogenase activity was measured, in both liver and intestine, before and after Mg-activation in vitro. After 3 weeks, the effects of arsenic at the highest dose level were pronounced on the basal pyruvate dehydrogenase activity (before activation) as well as the total pyruvate dehydrogenase (after activation). The inhibition of pyruvate dehydrogenase activity both before and after Mg-activation suggests an arsenic effect on mitochondrial pyruvate metabolism which, in part, involves inhibition of pyruvate decarboxylase. Evidence is also presented which may indicate an arsenic effect on the kinase and/or phosphatase which regulate pyruvate dehydrogenase activity.

Intracellular effects of chronic arsenic administration on renal proximal tubule cells

Arsenic is one of the more common toxic elements in the environment. The kidney accumulates this element and plays a major role in its metabolism and excretion. Mitochondria have been found in vitro to be highly sensitive to the toxicity of this element. Combined oxygen electrode and electron microscopic studies were conducted on kidneys of rats exposed to arsenate in the drinking water at concentrations of 40, 85, or 125 ppm for 6 weeks to evaluate in vivo mitochondrial toxicity. Decreased state 3 respiration and respiratory control ratios were observed in kidneys of rats given the 85 and 125 ppm dose levels. Ultrastructural alterations, which consisted of swollen mitochondria and increased numbers of dense autophagic lysosome-like bodies, were confined to proximal tubule cells of these same animals. This study places renal arsenate mitochondrial toxicity into an in vivo context and points to the value of using complementary techniques for assessing the subacute or chronic toxicity of environmental agents.

Seed dormancy: breaking by uncouplers and inhibitios of oxidative phosphorylation

When 2,4-dinitrophenol and carbon dioxide were applied together to dormant seeds of Trifolium subterraneum L. (subterranean clover), 2,4-dinitrophenol did not disturb the breaking of dormancy which carbon dioxide usually induces in legume seeds. On the contrary, on its own, it promoted germination in a substantial proportion of seeds; a similar effect was produced by other uncouplers or inhibitors of oxidative phosphorylation.