Transport ATPase--the different modes of inhibition of the enzyme by various mercury compounds

Increased free intrasynaptosomal Ca2+ by neurotoxic organometals: distinctive mechanisms

Effects of several alkylmetals on free intrasynaptosomal Ca2+ concentration, [Ca2+]i, were studied in vitro using the fluorescent Ca2+ indicator fura-2. Neurotoxic alkylmetals methylmercury (Met-Hg), triethyllead (TEL), triethyltin (TET), and trimethyltin (TMT) (at 2.5-30 microM) increased [Ca2+]i to different degrees. Met-Hg was the most potent, elevating [Ca2+]i 100-800 nM, dose dependently and significantly more than high K+ (150 nM) or veratridine (350 nM). The effect of Met-Hg could not be inhibited with a Ca2+ channel blocker, verapamil, nor with a Na+ channel blocker, tetrodotoxin. Inhibition of the mitochondrial Ca2+ uptake in situ with rotenone + oligomycin decreased the potency of Met-Hg to elevate [Ca2+]i but did not change the resting [Ca2+]i. Met-Hg also slightly decreased synaptosomal ATP. TEL and TET elevated [Ca2+]i by 100-200 nM. The effect of TEL, but not that of TET, could be blocked with verapamil (36%) and veratridine (67%). TEL was less efficient in the presence of ouabain. Neither TEL nor TET had significant mitochondrial effects in situ contributing to [Ca2+]i. TMT increased [Ca2+]i less than TET while dimethyltin and methyltin were inactive. These results indicate that neurotoxic derivatives of alkylmetals studied increase [Ca2+]i. This occurs mainly either by nonspecific increase (Met-Hg, TET) of Ca2+ leakage through the plasma membrane and/or specific interference with the mechanisms regulating Ca2+ fluxes through the plasma membrane (TEL).

Mechanism of methylmercury cytotoxicity

Although a large number of epidemiological, clinical, and pathological studies on methylmercury intoxication have been published, these investigations have not been able to elucidate the detailed mechanisms by which the metal alkyl causes a wide variety of biological dysfunctions. Thus, the cultured cells which are free from the influence of whole body complexities, such as absorption, distribution, metabolism, etc., which complicate the interpretation of in vivo experimental results, attract the attention of many scientists who are interested in clarifying the mode of toxic action of methylmercury. The aim of this article is to review the recent studies on the toxicity of methylmercury at the cellular level and to outline the mechanisms which have been proposed to be responsible for cell injuries.

Erythrocyte phosphate release in essential hypertension

In this study erythrocyte phosphate release depended on the intracellular hydrolysis of organic phosphate esters. Total phosphate release was increased in essential hypertension, which suggests an elevated phosphate ester metabolism. Ouabain-sensitive phosphate release was decreased, and the ratio of intracellular Na+/K+ concentrations was increased, a finding consistent with a diminished Na-K-ATPase activity. Furosemide in a concentration of 1.0 mmol/L inhibited erythrocyte phosphate release by half, probably owing to nonspecific membrane effects. The combination of ouabain and furosemide reduced phosphate transfer to a higher degree than did each substance individually. Because of the nonspecific alteration of erythrocyte membrane permeability by furosemide in a concentration of 1.0 mmol/L, ouabain-insensitive, furosemide-sensitive phosphate release and ouabain-insensitive, furosemide-sensitive Na+ efflux (Na-K cotransport) must not be regarded uncritically as specific transport systems.

The effect of substrate and potassium on the inhibitory kinetics of MnCl2 on the enzyme K+-p-nitrophenyl phosphatase in rat brain

The effect of Mn2+, a divalent metal, on the enzyme K+-p-nitrophenyl phosphatase (K+-PNPPase) was studied in rat brain. The metal was found to be a moderate inhibitor of the enzyme, with an I50 of approximately 480 microM. The inhibition was pH dependent, but not temperature dependent. On measurement of the inhibition with varying concentrations of PNPP (1-5 mM), the I50 value remained constant. However, when the inhibition was measured with K+ (5-20 mM), the I50 value increased from 130 microM to 490 microM, suggesting that K+ antagonized the effect of Mn2+. In kinetic studies, Mn2+ inhibited the enzyme in a non-competitive manner with respect to PNPP. The Km remained constant (2.9), but the Vmax was decreased from 5.0 to 1.6. However, with respect to K+, the inhibition was competitive, as the concentration for half maximal activation (K0.5) increased from 1.3 to 8.9 mmol l-1 with 1 mM of MnCl2, suggesting that the apparent affinity of K+ for the enzyme was decreased. The apparent Vmax was not affected. The degree of cooperactivity (n) measured as the slope of the Hill plot remained unaltered (1.9 +/- 0.2) over the entire concentration range of MnCl2 tested.

Interaction of methylmercury chloride with cellular energetics and related processes

Upon exposure to methylmercury chloride, the whole-cell oxygen uptake by the yeast Saccharomyces cerevisiae ceases. On a fermentable carbon source, carbon dioxide continues to be evolved after respiration has stopped, indicating that fermentation is still active. Dextrose and glycerol uptake also persists until the respective processes, fermentation and respiration, are totally inhibited. Protein and nucleic acid synthesis are blocked with similar concentrations of methylmercury, while cytochrome c, the terminal component of the electron transport chain, is unaltered by the toxicant. Surprisingly, the intracellular ATP is higher in the treated cells than in the controls, although they eventually fall in response to higher concentrations of methylmercury, while cytochrome c, the terminal component of the electron transport chain, is unaltered by the toxicant. Surprisingly, the intracellular ATP is higher in the treated cells than in the controls, although they eventually fall in response to higher concentration or longer exposure. High-pressure liquid chromatography profiles show that the amounts of the other nucleotides are either unaltered or increased. The entire inhibitory process is reversible with time or fresh medium at low methylmercury concentrations. These results do not support the hypothesis expressed by several authors of an inhibition of ATP biosynthesis resulting from membrane perturbation. These data suggest that the decrease in ATP--when induced by the organomercurial--is a secondary process and is not the result of direct mitochondrial toxicity.

Effects of manganese on rat brain microsomal Mg2+-Na+-K+-ATPase: in vivo and in vitro studies

The effect of manganese on brain microsomal Mg2+-Na+K+-ATPase was examined both in vitro and in vivo. Daily intraperitoneal administration of MnCl2 . 4H2O (Mn2+, 6 mg/kg) to the rats for a period of 90 days produced 10% (P less than 0.05) inhibition in the activity of Mg2+-ATPase, and 72 and 63% increases in the contents of manganese and copper, respectively, in the microsomal fraction of brain. In in vitro studies, lower concentrations of Mn2+ activated while higher concentrations inhibited the activity of brain microsomal ATPase. Addition of equal concentrations of Mn2+ + Cu2+ (8 mM) in vitro produced 8% inhibition in the activity of Mg2+-ATPase and 83% inhibition in Na+-K+-ATPase. Free Cu2+ ions were able to antagonize the effect of Mn2+ on ATPase in vitro and inhibited the activity of Mg2+-Na+-K+-ATPase with more pronounced effect of Na+-K+-ATPase. The lack of change in the activity of Na+-K+-ATPase in the brain microsomes of rats administered manganese, in spite of a significant increase in copper, could not be explained. It is, however, evident that a manganese-induced elevation in brain copper was not responsible for initiating biochemical changes in manganese neurotoxicity.

Modification of the conformational equilibria in the sodium and potassium dependent adenosinetriphosphatase with glutaraldehyde

Glutaraldehyde treatment of electroplax membrane preparations of Na,K-ATPase leads to irreversible changes in the enzymic behavior of the protein, which are not due to modification of the active site. When the glutaraldehyde treatment is carried out in a medium containing K+ and without Na+, the "K+-modified enzyme" so produced shows the following changes in enzymic properties: The steady-state phosphorylation by ATP and the rate of ATP-ADP exchange are decreased to approximately 40% of control, while Na,K-ATPase activity decreases to approximately 15% of control. Phosphatase activity is decreased very little, but the potassium activation parameters of the reaction are changed, from K0.5 approximately equal to 5 mM and nH = 1.9 in control to K0.5 approximately equal to 0.5 mM and nH = 1 in K+-modified enzyme. KI(app) for nucleotide inhibition of phosphatase activity is increased significantly. Changes in the cation dependence of the ATPase reaction are also observed. All of these effects can be explained by assuming that the cross-linking of surface groups in protein subunits when they are in conformation E2 shifts the intrinsic conformational equilibrium of the enzyme toward E2. We considered the simplest mathematical model for the coupling between K+ binding and the conformational equilibrium, with equivalent potassium sites that must be simultaneously in the same state. If one assumes that the potassium activation of phosphatase activity in the K+-modified enzyme reflects the affinity for K+ of E2, the behavior of the phosphatase activity in the native enzyme can be fit if there are only two potassium sites, whose affinity is 80-fold higher in E2 than in E1, and the equilibrium constant for E2 in equilibrium E1 is about 250. The same sites can explain the activation of dephosphorylation during ATP hydrolysis. Independent of the model chosen, potassium ions must be required for the catalytic action of form E2 and cannot be merely "allosteric activators". The enzyme modified with glutaraldehyde in a medium containing Na+ also has interesting properties, but their rationalization is less straightforward. The Na,K-ATPase activity is inhibited more than the "partial reactions", as in the K+-modified enzyme. We suggest that this is a generally expected result of modifications of the enzyme.

Specific modification of gastric K+-stimulated ATPase activity by thimerosal

Treatment of hog gastric microsomes with the sulfhydryl reagent, thimerosal (ethylmercurithiosalicylate), produced differential effects on the K+-ATPase and the K+-stimulated p-nitrophenylphosphatase activities. For example, exposure to 2 mM thimerosal for 3 min severely reduced the activity of K+-stimulated ATPase, while K+-p-nitrophenylphosphatase activity was enhanced 2- to 3-fold. Higher concentration of thimerosal, or longer incubation times, also led to inhibition of K+-p-nitrophenylphosphatase. The activated state of p-nitrophenylphosphatase could be sustained by a 20-fold, or greater, dilution of treated membranes, and could be reversed by reduction of membrane SH groups by exogenous thiols. Significant activation of K+-p-nitrophenylphosphatase was not produced by p-chloromercuribenzene sulfonate, p-chloromercuribenzoate or mersalyl; however, ethyl mercuric chloride had qualitatively similar activity effects as thimerosal. Kinetics of K+-p-nitrophenylphosphatase for thimerosal-treated membranes were altered as follows: V increased; Km for p-nitrophenylphosphate unchanged for Ka for K+ increased. ATP, which is a potent inhibitor of K+-p-nitrophenylphosphatase activity in native membranes (KI approximately 200 microM). These data suggest that there are multiple SH groups which differentially influence the gastric K+-stimulated ATPase activity. Defined treatments with thimerosal are interpreted as an uncoupling of the K+-stimulated phosphatase component of the enzyme (for which p-nitrophenylphosphatase is a presumed model reaction). Such differential modifications can be usefully applied to the study of partial reactions of the enzyme and their specific role in the related H+-transport reaction.

Comparison of the effects of metals on cellular injury and lipid peroxidation in isolated rat hepatocytes

Various mechanisms, including increases in lipid peroxidation, have been proposed to account for metal-induced cellular injury. By comparing several metals in the same cell population, it is possible to determine whether a correlation exists between ability to produce cell injury and ability to alter parameters pertaining to a particular mechanism. Of particular interest in this study was the relation between metal-induced cytotoxicity and increases in lipid peroxidation. The effects of Cr, Mn, Zn, Ni, Pb, Se, V, Fe, Cd, Hg, Cu, at final concentrations of 1-1000 microM, on the viability of isolated hepatocytes were therefore examined by assessing the loss of intracellular K+ and aspartate aminotransferase (AST). Simultaneously, the ability of the metals to induce lipid peroxidation, as measured by an increase in thiobarbituric acid (TBA) reactants, was assessed. Hg and Cu required the lowest concentration to produce cellular injury, while Cd produced less dramatic changes in cell viability and Fe at 1000 microM produced only a small decrease in intracellular K+. The largest absolute increases in lipid peroxidation were found in the presence of V, followed by Fe and Hg, with Cd and Se causing the smallest increase in TBA reactants. These observations suggest that the lipid peroxidation associated with Cd and Hg is not necessarily responsible for the loss of cell viability induced by these two metals.

Inhibition of adenosine triphosphatases by gold

Inhibition of adenosine triphosphatase (ATPase) by chlorauric acid (Au3+) and gold sodium thiomalate (Au+) was studied in dog brain and kidney and in human kidney enzyme preparations. Au3+ indiscriminately affected ouabain-sensitive (Na+ + K+-dependent) ATPase and ouabain-insensitive (Mg2+-dependent) ATPase with concentrations for 50% inhibition (I50) approximately 10(-6) M. The I50 of Au3+ for Na+ + K+ ATPase was several-fold higher in homogenates than in microsomal fractions. The enzyme was protected by bovine serum albumin. Although Au3+ and Au+ were equipotent against Mg2+ ATPase, Au+ inhibited Na+ + K+ ATPase 2 to 3 times more effectively than did Au3+. The inhibitory action of Au3+ (but not Au+) was potentiated by ascorbic acid, suggesting reduction of Au3+ to Au+ by ascorbic acid. The fractional inhibition of Na+ + K+ ATPase by Au3+ or Au+ was not affected by changing concentrations of NaCl, KCl, MgCl2, ATP, and MgATP. Decreasing pH from 8.0 to 6.8 enhanced both Au+ and Au3+ inhibition. We conclude that gold is one of the most potent nonspecific of Na+ + K+ ATPase, with characteristics differing from other metallic inhibitors of this enzyme system.

Protective effect of phospholipids in methylmercury inhibition of hydroxybutyrate deshydrogenase

The inhibitory effect of methylmercury on rat liver mitochondrial D 3-hydroxybutyrate deshydrogenase--an enzyme of the inner membrane matrix, which requires lecithin as a cofactor and has thiol residues in the active site--has been investigated. Using a partially purified enzymatic extract, methylmercury inhibition of the reactivation of the apodeshydrogenase by liposomes of lecithin has been studied as a function of lecithin concentration in the incubation medium. Partial reactivation has been observed at a concentration 3 times higher than that needed to reactivate the control. The present studies support the hypothesis that phospholipids are implicated in the mechanism of methylmercury inhibition.