On the mechanism of mercurial-induced permeability of the mitochondrial membrane to K+

Mangiferin, a dietary xanthone protects against mercury-induced toxicity in HepG2 cells

Mercury is one of the noxious heavy metal environmental toxicants and is a cause of concern for human exposure. Mangiferin (MGN), a glucosylxanthone found in Mangifera indica, reported to have a wide range of pharmacological properties. The objective of this study was to evaluate the cytoprotective potential of MGN, against mercury chloride (HgCl(2) ) induced toxicity in HepG2 cell line. The cytoprotective effect of MGN on HgCl(2) induced toxicity was assessed by colony formation assay, while antiapoptotic effect by fluorescence microscopy, flow cytometric DNA analysis, and DNA fragmentation pattern assays. Further, the cytoprotective effect of MGN against HgCl(2) toxicity was assessed by using biochemical parameters like reduced glutathione (GSH), glutathione-S-transferase (GST), superoxide dismutase (SOD), catalase (CAT) by spectrophotometrically, mitochondrial membrane potential by flowcytometry and the changes in reactive oxygen species levels by DCFH-DA spectrofluoremetric analysis. A significant increase in the surviving fraction was observed with 50 μM of MGN administered two hours prior to various concentrations of HgCl(2) . Further, pretreatment of MGN significantly decreased the percentage of HgCl(2) induced apoptotic cells. Similarly, the levels of ROS generated by the HgCl(2) treatment were inhibited significantly (P < 0.01) by MGN. MGN also significantly (P < 0.01) inhibited the HgCl(2) induced decrease in GSH, GST, SOD, and CAT levels at all the post incubation intervals. Our study demonstrated the cytoprotective potential of MGN, which may be attributed to quenching of the ROS generated in the cells due to oxidative stress induced by HgCl(2) , restoration of mitochondrial membrane potential and normalization of cellular antioxidant levels.

Impairment of mitochondrial function by minocycline

There is an ongoing debate on the presence of beneficial effects of minocycline (MC), a tetracycline-like antibiotic, on the preservation of mitochondrial functions under conditions promoting mitochondria-mediated apoptosis. Here, we present a multiparameter study on the effects of MC on isolated rat liver mitochondria (RLM) suspended either in a KCl-based or in a sucrose-based medium. We found that the incubation medium used strongly affects the response of RLM to MC. In KCl-based medium, but not in sucrose-based medium, MC triggered mitochondrial swelling and cytochrome c release. MC-dependent swelling was associated with mitochondrial depolarization and a decrease in state 3 as well as uncoupled respiration. Swelling of RLM in KCl-based medium indicates that MC permeabilizes the inner mitochondrial membrane (IMM) to K(+) and Cl(-). This view is supported by our findings that MC-induced swelling in the KCl-based medium was partly suppressed by N,N'-dicyclohexylcarbodiimide (an inhibitor of IMM-linked K(+)-transport) and tributyltin (an inhibitor of the inner membrane anion channel) and that swelling was less pronounced when RLM were suspended in choline chloride-based medium. In addition, we observed a rapid MC-induced depletion of endogenous Mg(2+) from RLM, an event that is known to activate ion-conducting pathways within the IMM. Moreover, MC abolished the Ca(2+) retention capacity of RLM irrespective of the incubation medium used, most likely by triggering permeability transition. In summary, we found that MC at low micromolar concentrations impairs several energy-dependent functions of mitochondria in vitro.

Mitochondria as an important target in heavy metal toxicity in rat hepatoma AS-30D cells

The mechanisms of toxic effects of divalent cations of three heavy metals Hg, Cd and Cu in rat ascites hepatoma AS-30D cells cultivated in vitro were compared. It was found that the toxicity of these ions, applied in the micromolar range (10-500 microM), decreased from Hg(2+) (most toxic) to Cu(2+) (least toxic). Hg(2+) and Cd(2+) produced a high percentage of cell death by both necrosis and apoptosis, whereas Cu(2+) at concentrations up to 500 microM was weakly effective. Hg(2+) at concentration of 10 microM appeared slightly uncoupling (i.e., stimulated resting state respiration and decreased the mitochondrial transmembrane potential), whereas it exerted a strong inhibitory effect on the respiratory chain and rapid dissipation of the membrane potential at higher concentrations. Cu(2+) had inhibitory effect on cell respiration only at 500 microM concentration and after incubation of 48 h but produced a significant uncoupling effect at lower concentrations. Cu(2+) induced an early and sharp increase of intracellular production of reactive oxygen species (ROS). The action of Hg(2+) and Cd(2+) on ROS generation was biphasic. They stimulated ROS generation within the cells at low concentrations and at short incubation times but decreased ROS generation at higher concentrations and at longer incubation. It is concluded that mitochondria are an important target for toxic effects of Hg(2+), Cd(2+) and Cu(2+) in AS-30D rat hepatoma cells.

Properties of the inner membrane anion channel in intact mitochondria

The mitochondrial inner membrane possesses an anion channel (IMAC) which mediates the electrophoretic transport of a wide variety of anions and is believed to be an important component of the volume homeostatic mechanism. IMAC is regulated by matrix Mg2+ (IC50 = 38 microM at pH 7.4) and by matrix H+ (pIC50 = 7.7). Moreover, inhibition by Mg2+ is pH-dependent. IMAC is also reversibly inhibited by many cationic amphiphilic drugs, including propranolol, and irreversibly inhibited by N,N'-dicyclohexylcarbodiimide. Mercurials have two effects on its activity: (1) they increase the IC50 values for Mg2+, H+, and propranolol, and (2) they inhibit transport. The most potent inhibitor of IMAC is tributyltin, which blocks anion uniport in liver mitochondria at about 1 nmol/mg. The inhibitory dose is increased by mercurials; however, this effect appears to be unrelated to the other mercurial effects. IMAC also appears to be present in plant mitochondria; however, it is insensitive to inhibition by Mg2+, mercurials, and N,N'-dicyclohexylcarbodiimide. Some inhibitors of the adenine nucleotide translocase also inhibit IMAC, including Cibacron Blue, agaric acid, and palmitoyl CoA; however, atractyloside has no effect.

The mitochondrial aspartate/glutamate and ADP/ATP carrier switch from obligate counterexchange to unidirectional transport after modification by SH-reagents

The influence of various SH-reagents on the aspartate/glutamate carrier was investigated in the reconstituted system. When liposomes carrying partially purified carrier protein were treated with 5,5'-dithiobis(2-nitrobenzoic acid) or N-ethylmaleimide, antiport activity was strongly reduced. Several mercury compounds exerted a dual effect. They completely blocked the antiport and, in addition, induced an efflux pathway for internal aspartate. The maximum rate of this unidirectional flux was comparable to the original antiport activity. Induction of efflux always was coupled to inhibition of antiport. Efflux was neither due to unspecific leakage of proteoliposomes nor to a possible contamination by porin, but depended on active carrier protein, as elucidated by the sensitivity to proteinases and protein-modifying reagents. Besides efflux of aspartate, HgCl2 and mersalyl also induced a slow efflux of ATP from liposomes carrying coreconstituted aspartate/glutamate and ADP/ATP carrier. The two efflux activities could be discriminated taking advantage of the differential effectiveness of several inhibitors and proteinases. Although basic carrier properties were changed by the applied mercurials (Dierks, T., Salentin, A. and Krämer, R. (1990) Biochim. Biophys. Acta 1028, 281), aspartate and ATP efflux could clearly be correlated with the aspartate/glutamate and the ADP/ATP carrier, respectively. When purifying these two translocators the respective efflux activity copurified with the antiporter, thus elucidating that the two different transport functions are mediated by the same protein. These results argue for a participation of the aspartate/glutamate and the ADP/ATP carrier in the generally observed increase of mitochondrial permeability after treatment with SH-reagents.

Effect of hydrophobic sulphydryl reagents on the uncoupling protein and inner-membrane anion channel of brown-adipose-tissue mitochondria

The effects of three sulphydryl reagents of differing hydrophobicity (N-ethylmaleimide, N-benzylmaleimide and N,N'-o-phenylenedimaleimide) on ion permeation through the inner membrane of brown-adipose-tissue mitochondria are investigated. GDP-sensitive permeation of chloride and protons (hydroxyl ions) through the uncoupling protein is increased exponentially with time by all three reagents. With increasing hydrophobicity of the reagents, modification is enhanced and an initial inhibited state becomes apparent. Results are interpreted in terms of a two-stage modification via a non-transporting intermediate, which does not bind GDP, to a final highly conducting product. The reagents also react with a hydrophilic sulphydryl group on an independent protein to induce a GDP-insensitive pathway which allows chloride, phosphate and sulphate to cross the membrane. The use of different sulphydryl reagents allows the two pathways to be clearly distinguished.

Effect of alteration of nerve terminal Ca2+ regulation on increased spontaneous quantal release of acetylcholine by methyl mercury

Agents known to disrupt intraterminal Ca2+ buffering, N,N-dimethylamino-8-octyl-3,4,5-trimethoxybenzoate (TMB-8), 25 microM; caffeine, 7.5 mM; N,N-bis(3,4-dimethoxyphenethyl)-N-methylamine (YS035), 180 microM; ouabain, 200 microM; and dantrolene, 50 microM, were tested for the ability to alter effects of methyl mercury (MeHg) on spontaneous quantal release of acetylcholine (ACh) at the rat neuromuscular junction. In particular, we sought to determine whether any of the above agents could prevent the MeHg-induced increase of spontaneous release of ACh, an effect measured electrophysiologically as increased frequency of miniature end-plate potentials (MEPPs). MEPPs were recorded continuously from myofibers of the rat hemidiaphragm using conventional, intracellular recording techniques during pretreatment with an inhibitor of Ca2+ regulation and subsequently with the inhibitor plus MeHg (100 microM). When given alone, caffeine and ouabain, which release Ca2+ from the smooth endoplasmic reticulum and mitochondria, respectively, increased MEPP frequency in a biphasic manner. Following pretreatment, concomitant application of MeHg with caffeine or ouabain increased MEPP frequency after a brief latent period to peak values of 53 and 92 Hz, respectively. TMB-8 and dantrolene, putative inhibitors of Ca2+ release from smooth endoplasmic reticulum, differed in their effects on MEPP frequency; TMB-8 alone decreased MEPP frequency to approximately 10% of drug-free control, whereas dantrolene did not significantly alter control MEPP frequency. Subsequent concomitant application of MeHg with TMB-8 or dantrolene increased MEPP frequency to peak values of 40 and 100 Hz after 17 and 30 min, respectively. YS035, a putative inhibitor of mitochondrial uptake and release of Ca2+, decreased MEPP frequency to less than 10% of control after 15 min when given alone. Application of MeHg following YS035 pretreatment failed to increase MEPP frequency for up to 90 min. YS035 did not mask a MeHg effect by blocking postsynaptic sensitivity to ACh or preventing its release since subsequent treatment with La3+ (2 mM) after YS035 had abolished spontaneous release, increased MEPP frequency within 5 min. Thus, of the five inhibitors of nerve terminal Ca2+ regulation tested, only YS035 prevented the stimulatory action of MeHg on MEPP frequency. Results of the present study suggest that release of Ca2+ from nerve terminal mitochondria contributes to the increased MEPP frequency caused by MeHg while release of Ca2+ from smooth endoplasmic reticulum may not.

Sensitivity of mitochondrial Mg++ flux to reagents which affect K+ flux

Effects on Mg++ transport in rat liver mitochondria of three reagents earlier shown to affect mitochondrial K+ transport have been examined. The sulfhydryl reactive reagent phenylarsine oxide, which activates K+ flux into respiring mitochondria, also stimulates Mg++ influx. The K+ analog Ba++, when taken up into the mitochondrial matrix, inhibits influx of both K+ and Mg++. The effect on Mg++ influx is seen only if Mg++, which blocks Ba++ accumulation, is added after a preincubation with Ba++. Thus the inhibition of Mg++ influx appears to require interaction of Ba++ at the matrix side of the inner mitochondrial membrane. Added Ba++ also diminishes observed rates of Mg++ efflux but not K+ efflux. This difference may relate to a higher concentration of Ba++ remaining in the medium in the presence of Mg++ under the conditions of our experiments. Pretreatment of mitochondria with dicyclohexyl-carbodiimide (DCCD), under conditions which result in an increase in the apparent Km for K+ of the K+ influx mechanism, results in inhibition of Mg++ influx from media containing approximately 0.2 mM Mg++. The inhibitory effect of DCCD on Mg++ influx is not seen at higher external Mg++ (0.8 mM). This dependence on cation concentration is similar to the dependence on K+ concentration of the inhibitory effect of DCCD on K+ influx. Although mitochondrial Mg++ and K+ transport mechanisms exhibit similar reagent sensitivities, whether Mg++ and K+ share common transport catalysis remains to be established.

Interactions of mitochondrial inhibitors with methylmercury on spontaneous quantal release of acetylcholine

The interaction of methylmercury (MeHg) with various inhibitors of mitochondrial function (dinitrophenol, 50 microM; dicoumarol, 100 microM; valinomycin, 20 microM; and ruthenium red, 20 microM) on spontaneous quantal release of acetylcholine was tested at the neuromuscular junction of the rat. The objective was to determine whether these mitochondrial inhibitors blocked the MeHg-induced increase of spontaneous release of acetylcholine, an effect measured electrophysiologically as increased miniature endplate potential (MEPP) frequency. MEPPs were recorded from myofibers of the rat hemidiaphragm using conventional, intracellular microelectrode recording techniques. When given alone, all four inhibitors increased MEPP frequency from resting levels of 1-2/sec (Hz) to approximately 10-60 Hz after a latency which ranged from 5 to 30 min. MEPP frequency subsequently returned to control levels. Subsequent concomitant application of MeHg (100 microM) with dinitrophenol, dicoumarol, or valinomycin increased MEPP frequency sharply to peak values of 40-60 Hz after 15-20 min. MEPP frequency subsided to pre-MeHg levels 10 min later. The time course and peak MEPP frequency elicited by MeHg after pretreatment with these uncouplers were similar to results obtained in preparations treated with MeHg alone. Ruthenium red, a putative specific inhibitor of the Ca2+ uptake uniporter in mitochondria, increased MEPP frequency to 12 Hz after 8.5 min when given alone. MEPP frequency returned to control levels approximately 10 min later. Subsequent application of MeHg and ruthenium red for up to 80 min failed to increase MEPP frequency. The inability of MeHg to increase MEPP frequency in ruthenium red-treated preparations was not due to depletion of acetylcholine nor to block of postjunctional receptors by ruthenium red since subsequent treatment with La3+ (2 mM) increased MEPP frequency to 12.5 Hz within 10 min. Thus, ruthenium red blocked the stimulatory effect of MeHg on MEPP frequency while uncouplers of oxidative phosphorylation and a K+ ionophore did not. The results with ruthenium red are consistent with the proposal that MeHg may block mitochondrial uptake of Ca2+ or promote its release, leading to an increased free cytosolic Ca2+ concentration which in turn stimulates spontaneous release of acetylcholine.

Stimulation of K+ flux into mitochondria by phenylarsine oxide

The dithiol-reactive reagent phenylarsine oxide causes a pH-dependent stimulation of unidirectional K+ flux into respiring rat liver mitochondria. This stimulation is diminished by subsequent addition of either the dithiol 2,3-dimercaptopropanol or the monothiol 2-mercaptoethanol. In contrast, uncoupling by phenylarsine oxide is reversed by 2,3-dimercaptopropanol but not by 2-mercaptoethanol. The data suggest separate sites of interaction of phenylarsine oxide with mechanisms of K+ entry and ATP synthesis. Stimulatory effects of mersalyl and phenylarsine oxide on K+ influx are not additive. Thus PheASO and mersalyl may affect K+ influx at a common site. Pretreatment of the mitochondria with DCCD, which inhibits K+ influx, fails to alter sensitivity to PheAsO or mersalyl. Thus the DCCD binding site associated with the K+ influx mechanism appears to be separate from and independent of the sulfhydryl group(s) which mediate stimulation of K+ influx by PheAsO and mersalyl. PheAsO, like mersalyl, also increases the rate of unidirectional K+ efflux from respiring mitochondria. The combined presence of PheAsO plus mersalyl causes a greater stimulation of K+ efflux than is observed with either reagent alone.

Evidence for the existence of an inner membrane anion channel in mitochondria

Mitochondria normally exhibit very low electrophoretic permeabilities to physiologically important anions such as chloride, bicarbonate, phosphate, succinate, citrate, etc. Nevertheless, considerable evidence has accumulated which suggests that heart and liver mitochondria contain a specific anion-conducting channel. In this review, a postulated inner membrane anion channel is discussed in the context of other known pathways for anion transport in mitochondria. This anion channel exhibits the following properties. It is anion-selective and inhibited physiologically by protons and magnesium ions. It is inhibited reversibly by quinine and irreversibly by dicyclohexylcarbodiimide. We propose that the inner membrane anion channel is formed by inner membrane proteins and that this pathway is normally latent due to regulation by matrix Mg2+. The physiological role of the anion channel is unknown; however, this pathway is well designed to enable mitochondria to restore their normal volume following pathological swelling. In addition, the inner membrane anion channel provides a potential futile cycle for regulated non-shivering thermogenesis and may be important in controlled energy dissipation.

Ba2+ uptake and the inhibition by Ba2+ of K+ flux into rat liver mitochondria

Rapid uptake of Ba2+ by respiring rat liver mitochondria is accompanied by a transient stimulation of respiration. Following accumulation of Ba2+, e.g. at a concentration of 120 nmol per mg protein, the mitochondria exhibit reduced rates of state 3 and uncoupler-stimulated respiration. ADP-stimulated respiration is inhibited at a lower concentration of Ba2+ than is required to affect uncoupler-stimulated respiration, suggesting a distinct effect of Ba2+ on mechanisms involved in synthesis of ATP. Ba2+, which has an ionic radius similar to that of K+, inhibits unidirectional K+ flux into respiring rat liver mitochondria. This effect on K+ influx is observable at concentrations of Ba2+, e.g. 23 to 37 nmol per mg protein, which cause no significant change in state 4 or uncoupler-stimulated respiration. The accumulated Ba2+ decreases the measured Vmax of K+ influx, while having little effect on the apparent Km for K+. The inhibition of K+ influx by Ba2+ is seen in the presence and absence of mersalyl, an activator of K+ influx. In contrast, under the conditions studied, Ba2+ has no apparent effect on the rate of unidirectional K+ efflux. These data are consistent with the idea that K+ may enter and leave mitochondria via separate mechanisms.

Osmotica, dimethyl sulfoxide, parahydroxymercuribenzoate, and cyanide change the period of the circadian clock in the pulvini of Phaseolus coccineus L

Several substances change the period length of the circadian oscillator in the laminar pulvinus of Phaseolus coccineus L. if offered continuously via the transpiration stream to the isolated leaves in continuous light and constant temperature. The osmotica mannitol and PEG 6000 (polyethylene glycol) lengthen the circadian period from 27.9 hr (control) to about 29.5 hr. The dose-response of both substances reveal saturation curves. DMSO (dimethyl sulfoxide) lengthens the circadian period much more strongly than the osmotica. The dose-response curve reflects a curvilinear dependence of the period length from the DMSO concentration. PHMB (p-hydroxymercuribenzoate) and NaCN shorten the circadian period. The dose-response curve for PHMB is a saturation curve; saturation is reached at 80 microM PHMB with a period length of about 25.7 hr. We conclude from these results that the period of the circadian oscillator in the Phaseolus pulvinus is not directly homeostatically regulated but influenced by the intracellular milieu which can be changed by affecting membrane permeability, active transport systems or other regulatory systems within the cells.

Activation of potassium ion transport in mitochondria by cadmium ion

Low levels of Cd2+ (1-5 microM) produce rapid swelling of mitochondria, which is respiration-dependent and uncoupler-sensitive. No cation requirement is apparent, since the swelling occurs in a medium containing only sucrose and the respiratory substrate. The swelling is inhibited by ruthenium red, suggesting that this effect of Cd2+ requires its entry into mitochondria. In medium containing 9 mM K+, addition of Cd2+ along with ruthenium red increases the rate of K+ influx threefold. In the presence of K+, Rb+ or Li+, but not of Na+, addition of Cd2+ produces first efflux of H+ into the medium followed by discharge of the pH gradient or uncoupling. Only the latter effect is inhibited by ruthenium red, showing that the efflux and influx of H+ are independent reactions. The H+ efflux appears to be an antiport response to the induced K+ entry. Its activation by Cd2+ is similar to the known effect of p-chloromercuriphenyl sulfonate. The H+ influx or uncoupling appears to result from binding of Cd2+ to some matrix-facing membrane site, perhaps the dithiol group on coupling factor B, and may relate to apparent permeability changes associated Cd2+-induced swelling.

Some effects of dibutylchloromethyltin chloride and other reagents on mitochondrial K+ flux

Respiration-dependent K+ fluxes across the limiting membranes of isolated rat liver mitochondria, measured by means of 42K, are stimulated by the oxidative phosphorylation inhibitor dibutylchloromethyltin chloride (DBCT). A lack of effect of Cl- concentration indicates that the stimulation of K+ flux by DBCT is not attributable to Cl-/OH- exchange activity. The mercurial mersalyl was previously shown to stimulate respiration-dependent K+ influx. The combined presence of mersalyl plus DBCT results in a greater stimulation of K4 influx than is caused by either DBCT or mersalyl alone. The oxidative phosphorylation inhibitor oligomycin, which alone has no effect on respiration-dependent K+ influx, enhances the stimulatory effect of mersalyl on K+ influx. The data are consistent with, although not proof of, a direct interaction of the K+ transport mechanism with the mitochondrial energy transduction apparatus.

Effect of mersalyl on mitochondrial Mg++ flux

The mercurial mersalyl has little effect either on rapid Mg++ binding by isolated rat liver mitochondria or on the total Mg++ content of these organelles measured after 0.75 min of incubation at 20 degrees C. The data do not support the previous suggestion that the increased permeability to K+ of mitochondria treated with mersalyl results from release of endogenous Mg++. An increased pH-dependence of unidirectional Mg++ flux into respiring rat liver mitochondria is suggested to arise indirectly from inhibition by mersalyl of pH shifts associated with exchanges of endogenous phosphate. In addition, mersalyl appears to have a stimulatory effect on Mg++ influx. Mersalyl also increases the average rate of unidirectional efflux of endogenous Mg++. The stimulatory effects of mersalyl on Mg++ flux are similar to, although quantitatively less than, the previously reported effects of mersalyl on mitochondrial K+ flux.