Effects on mitochondrial K flux of pH, K concentration, and N-ethyl maleimide

Reconstitution of transmembrane K+ transport with a 53 kilodalton mitochondrial protein

A 53 kDa protein has been purified from a Triton X-100 extract of liver mitochondrial membranes, by affinity chromatography on immobilized quinine, a K+ transport inhibitor. KCl-containing lipid vesicles reconstituted with this protein lose K+ to a medium low in K+ faster than vesicles lacking protein. With bacteriorhodopsin reconstituted in vesicles containing K+, light induces faster development of a pH gradient if the 53 kDa protein is included during vesicle preparation. This effect is like that of valinomycin, which catalyzes K+ efflux, dissipating the membrane potential arising from H+ entry. Evidence that vesicles containing the 53 kDa protein are permeable to K+, but exhibit low permeability to H+, indicates that this protein acts as a K+ uniporter.

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.

Ion permeability induction by the SH cross-linking reagents in rat liver mitochondria is inhibited by the free radical scavenger, butylhydroxytoluene

The hydrophobic, potentially SH cross-linking reagent, phenylarsine oxide (PhAsO), was found to induce K+ and Ca2+ effluxes from mitochondria and to accelerate the respiration rate in state 4. The hydrophobic monofunctional electrophilic agent, N-ethylmaleimide, does not exhibit this effect but prevents the action of PhAsO. The polar potentially SH cross-linking regents (arsenite, diamide) induce ion fluxes only in the presence of Pi. Ion fluxes induced by the SH reagents are inhibited by butylhydroxytoluene (an inhibitor of free radical reactions), and N,N'-dicyclohexylcarbodiimide, not by oligomycin. It is inferred that the induction of ion fluxes in mitochondria caused by cross-linking of two juxtaposed SH groups is related to the development of free radical reactions.

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.

Effect of quinine on mitochondrial K+ and Mg++ flux

Quinine decreases rates of unidirectional K+ flux into and out of respiring rat liver mitochondria. K+ efflux is more sensitive to quinine than K+ influx. The data are consistent with the proposal that two separate mechanisms may mediate K+ influx, only one of which is sensitive to quinine. Effects on K+ flux of the stereoisomer quinidine are similar to effects of quinine. The smaller quinuclidine causes at most a slight inhibition of K+ efflux under the same conditions. Mg++ flux exhibits a pattern of inhibition by quinine similar to that of K+ flux. Mg++ efflux is more sensitive to quinine than is Mg++ influx. These and earlier findings indicate marked similarities between liver mitochondrial transport mechanisms for K+ and Mg++.

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.

K+ transport in mitoplasts

K+ transport into mitoplasts, prepared by digitonin disruption and removal of the outer membranes from rat liver mitochondria, has been studied. Unidirectional K+ influx has been measured by means of 42K, in the presence of the respiratory substrate succinate. K+ influx is inhibited by CN-, antimycin A and dicyclohexylcarbodiimide, but is insensitive to oligomycin. A linear dependence of the reciprocal of the K+ -influx rate on the reciprocal of the external K+ concentration is observed. Under the conditions studied, the apparent Km for K+ of the transport mechanism is approx. 6 mM, while the Vmax of K+ influx is approx. 5 mu mol K+/g protein per min. The rate of K+ influx increases with increasing external pH over the range from 6.8 to 8.0. The observed kinetics, pH dependence and inhibitor sensitivity are essentially similar to previously reported characteristics of K+ transport into intact rat liver mitochondria. It is concluded that the outer mitochondrial membrane does not not have a role in controlling K+ flux into rat liver mitochondria.

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.

The transport of cations in mitochondria

The present paper has reviewed several factors related to ion transport and examined the properties of cation transport in mitochondria. The analysis suggests that: (1) The concept that a metabolically dependent electrical potential across the mitochondrial membrane plays a role in determining ion fluxes and steady-state concentrations is not justified and the data indicate that such exchanges are generally electroneutral. (2) Generally, the influx and efflux of an ion proceed by the same mechanism with at least one exception. (3) There are indications that some of the steps in transport are common to several cations. (4) The idea that carrier or ionophoric molecules are involved in cation transport has been examined in some detail together with the possible involvement of some known mitochondrial components. In particular, a model has been introduced in which local charge imbalances produced by H+ fluxes serve as the driving force of transport. The molecules of the complex are arranged in series in a tripartite arrangement including a filter or gate, a nonselective channel and an H+-transferring portion linked to either electron transport or the ATPase. Parts of this model have been introduced by other investigators. Models in which different portions of channels have differing functions have been proposed previously for other transport systems.

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.