Energy-dependent accumulation of iron by isolated rat liver mitochondria. II. Relationship to the active transport of Ca2+

Air pollutants disrupt iron homeostasis to impact oxidant generation, biological effects, and tissue injury

Air pollutants cause changes in iron homeostasis through: 1) a capacity of the pollutant, or a metabolite(s), to complex/chelate iron from pivotal sites in the cell or 2) an ability of the pollutant to displace iron from pivotal sites in the cell. Through either pathway of disruption in iron homeostasis, metal previously employed in essential cell processes is sequestered after air pollutant exposure. An absolute or functional cell iron deficiency results. If enough iron is lost or is otherwise not available within the cell, cell death ensues. However, prior to death, exposed cells will attempt to reverse the loss of requisite metal. This response of the cell includes increased expression of metal importers (e.g. divalent metal transporter 1). Oxidant generation after exposure to air pollutants includes superoxide production which functions in ferrireduction necessary for cell iron import. Activation of kinases and phosphatases and transcription factors and increased release of pro-inflammatory mediators also result from a cell iron deficiency, absolute or functional, after exposure to air pollutants. Finally, air pollutant exposure culminates in the development of inflammation and fibrosis which is a tissue response to the iron deficiency challenging cell survival. Following the response of increased expression of importers and ferrireduction, activation of kinases and phosphatases and transcription factors, release of pro-inflammatory mediators, and inflammation and fibrosis, cell iron is altered, and a new metal homeostasis is established. This new metal homeostasis includes increased total iron concentrations in cells with metal now at levels sufficient to meet requirements for continued function.

MICU1 Confers Protection from MCU-Dependent Manganese Toxicity

The mitochondrial calcium uniporter is a highly selective ion channel composed of species- and tissue-specific subunits. However, the functional role of each component still remains unclear. Here, we establish a synthetic biology approach to dissect the interdependence between the pore-forming subunit MCU and the calcium-sensing regulator MICU1. Correlated evolutionary patterns across 247 eukaryotes indicate that their co-occurrence may have conferred a positive fitness advantage. We find that, while the heterologous reconstitution of MCU and EMRE in vivo in yeast enhances manganese stress, this is prevented by co-expression of MICU1. Accordingly, MICU1 deletion sensitizes human cells to manganese-dependent cell death by disinhibiting MCU-mediated manganese uptake. As a result, manganese overload increases oxidative stress, which can be effectively prevented by NAC treatment. Our study identifies a critical contribution of MICU1 to the uniporter selectivity, with important implications for patients with MICU1 deficiency, as well as neurological disorders arising upon chronic manganese exposure.

The biological effect of asbestos exposure is dependent on changes in iron homeostasis

Functional groups on the surface of fibrous silicates can complex iron. We tested the postulate that (1) asbestos complexes and sequesters host cell iron resulting in a disruption of metal homeostasis and (2) this loss of essential metal results in an oxidative stress and biological effect in respiratory epithelial cells. Exposure of BEAS-2B cells to 50 μg/mL chrysotile resulted in diminished concentrations of mitochondrial iron. Preincubation of these cells with 200 μM ferric ammonium citrate (FAC) prevented significant mitochondrial iron loss following the same exposure. The host response to chrysotile included increased expression of the importer divalent metal transporter-1 (DMT1) supporting a functional iron deficiency. Incubation of BEAS-2B cells with both 200 μM FAC and 50 μg/mL chrysotile was associated with a greater cell accumulation of iron relative to either iron or chrysotile alone reflecting increased import to correct metal deficiency immediately following fiber exposure. Cellular oxidant generation was elevated after chrysotile exposure and this signal was diminished by co-incubation with 200 μM FAC. Similarly, exposure of BEAS-2B cells to 50 μg/mL chrysotile was associated with release of the proinflammatory mediators interleukin (IL)-6 and IL-8, and these changes were diminished by co-incubation with 200 μM FAC. We conclude that (1) the biological response following exposure to chrysotile is associated with complexation and sequestration of cell iron and (2) increasing available iron in the cell diminished the effects of asbestos exposure.

Air pollution particles and iron homeostasis

BACKGROUND

The mechanism underlying biological effects, including pro-inflammatory outcomes, of particles deposited in the lung has not been defined.

MAJOR CONCLUSIONS

A disruption in iron homeostasis follows exposure of cells to all particulate matter including air pollution particles. Following endocytosis, functional groups at the surface of retained particle complex iron available in the cell. In response to a reduction in concentrations of requisite iron, a functional deficiency can result intracellularly. Superoxide production by the cell exposed to a particle increases ferrireduction which facilitates import of iron with the objective being the reversal of the metal deficiency. Failure to resolve the functional iron deficiency following cell exposure to particles activates kinases and transcription factors resulting in a release of inflammatory mediators and inflammation. Tissue injury is the end product of this disruption in iron homeostasis initiated by the particle exposure. Elevation of available iron to the cell precludes deficiency of the metal and either diminishes or eliminates biological effects.

GENERAL SIGNIFICANCE

Recognition of the pathway for biological effects after particle exposure to involve a functional deficiency of iron suggests novel therapies such as metal supplementation (e.g. inhaled and oral). In addition, the demonstration of a shared mechanism of biological effects allows understanding the common clinical, physiological, and pathological presentation following exposure to disparate particles. This article is part of a Special Issue entitled Air Pollution, edited by Wenjun Ding, Andrew J. Ghio and Weidong Wu.

Wood Smoke Particle Sequesters Cell Iron to Impact a Biological Effect

The biological effect of an inorganic particle (i.e., silica) can be associated with a disruption in cell iron homeostasis. Organic compounds included in particles originating from combustion processes can also complex sources of host cell iron to disrupt metal homeostasis. We tested the postulate that (1) wood smoke particle (WSP) sequesters host cell iron resulting in a disruption of metal homeostasis, (2) this loss of essential metal results in both an oxidative stress and biological effect in respiratory epithelial cells, and (3) humic-like substances (HULIS), a component of WSP, have a capacity to appropriate cell iron and initiate a biological effect. BEAS-2B cells exposed to WSP resulted in diminished concentrations of mitochondrial (57)Fe, whereas preincubation with ferric ammonium citrate (FAC) prevented significant mitochondrial iron loss after such exposure. Cellular oxidant generation was increased after WSP exposure, but this signal was diminished by coincubation with FAC. Similarly, exposure of BEAS-2B cells to 100 μg/mL WSP activated mitogen-activated protein (MAP) kinases, elevated NF-E2-related factor 2/antioxidant responsive element (Nrf2 ARE) expression, and provoked interleukin (IL)-6 and IL-8 release, but all these changes were diminished by coincubation with FAC. The biological response to WSP was reproduced by exposure to 100 μg/mL humic acid, a polyphenol comparable to HULIS included in the WSP that complexes iron. We conclude that (1) the biological response following exposure to WSP is associated with sequestration of cell iron by the particle, (2) increasing available iron in the cell diminished the biological effects after particle exposure, and (3) HULIS included in WSP can sequester the metal initiating the cell response.

Induction of the mitochondrial permeability transition (MPT) by micromolar iron: liberation of calcium is more important than NAD(P)H oxidation

The mitochondrial permeability transition (MPT) plays an important role in cell death. The MPT is triggered by calcium and promoted by oxidative stress, which is often catalyzed by iron. We investigated the induction of the MPT by physiological concentrations of iron. Isolated rat liver mitochondria were initially stabilized with EDTA and bovine serum albumin and energized by succinate or malate/pyruvate. The MPT was induced by 20μM calcium or ferrous chloride. We measured mitochondrial swelling, the inner membrane potential, NAD(P)H oxidation, iron and calcium in the recording medium. Iron effectively triggered the MPT; this effect differed from non-specific oxidative damage and required some residual EDTA in the recording medium. Evidence in the literature suggested two mechanisms of action for the iron: NAD(P)H oxidation due to loading of the mitochondrial antioxidant defense systems and uptake of iron to the mitochondrial matrix via a calcium uniporter. Both of these events occurred in our experiments but were only marginally involved in the MPT induced by iron. The primary mechanism observed in our experiments was the displacement of adventitious/endogenous calcium from the residual EDTA by iron. Although artificially created, this interplay between iron and calcium can well reflect conditions in vivo and could be considered as an important mechanism of iron toxicity in the cells.

Fe(2+) induces a transient Ca(2+) release from rat liver mitochondria

Isolated mitochondria loaded with Ca(2+) and then exposed to Fe(2+) show a transient release of Ca(2+). The magnitude of this response depends on the Ca(2+) loading and the kinetics of the response depends on the concentration of added Fe(2+). We investigated the Fe(2+)-induced Ca(2+) release mechanism by measuring mitochondrial Ca(2+) uptake in the presence of Fe(2+). The presence of Fe(2+) inhibits Ca(2+) uptake two times. Since mitochondria can cycle Ca(2+) across their inner membrane, the suppression of Ca(2+) uptake, but not release, results in an elevation of the extramitochondrial Ca(2+), thereby varying the steady state. The transient release of Ca(2+) initially observed from mitochondria appears to occur via the electroneutral 2H(+)/Ca(2+)-exchange mechanism, since it can be markedly decreased by cyclosporin A and does not involve lipid peroxidation. When Fe(2+) accumulation is completed, reuptake of released Ca(2+) into mitochondria resumes. Finally, we propose that Fe(2+) either inhibits Ca(2+) entry at the uniporter or is transported by it into the matrix.

Transport of calcium by mitochondria

The identification of intramitochondrial free calcium ([Ca2+]m) as a primary metabolic mediator [see Hansford (this volume) and Gunter, T. E., Gunter, K. K., Sheu, S.-S., and Gavin, C. E. (1994) Am. J. Physiol. 267, C313-C339, for reviews] has emphasized the importance of understanding the characteristics of those mechanisms that control [Ca2+]m. In this review, we attempt to update the descriptions of the mechanisms that mediate the transport of Ca2+ across the mitochondrial inner membrane, emphasizing the energetics of each mechanism. New concepts within this field are reviewed and some older concepts are discussed more completely than in earlier reviews. The mathematical forms of the membrane potential dependence and concentration dependence of the uniporter are interpolated in such a way as to display the convenience of considering Vmax to be an explicit function of the membrane potential. Recent evidence for a transient rapid conductance state of the uniporter is discussed. New evidence concerning the energetics and stoichiometries of both Na(+)-dependent and Na(+)-independent efflux mechanisms is reviewed. Explicit mathematical expressions are used to describe the energetics of the system and the kinetics of transport via each Ca2+ transport mechanism.

Ca2+ extrusion across plasma membrane and Ca2+ uptake by intracellular stores

The aim of this review is to summarize the various systems that remove Ca2+ from the cytoplasm. We will initially focus on the Ca2+ pump and the Na(+)-Ca2+ exchanger of the plasma membrane. We will review the functional regulation of these systems and the recent progress obtained with molecular-biology techniques, which pointed to the existence of different isoforms of the Ca2+ pump. The Ca2+ pumps of the sarco(endo)plasmic reticulum will be discussed next, by summarizing the discoveries obtained with molecular-biology techniques, and by reviewing the physiological regulation of these proteins. We will finally briefly review the mitochondrial Ca(2+)-uptake mechanism.

The effect of ferric iron complex on isolated rat liver mitochondria. I. Respiratory and electrochemical responses

Addition of iron(III)-gluconate complex to isolated rat liver mitochondria resulted in an increased iron content of mitochondria. Iron was accumulated through a relatively fast process (maximal uptake in less than 2 min incubation) by an energy-independent mechanism. The in vitro iron overload of mitochondria was associated with enhancement in the oxygen consumption, which was due to the induction of lipoperoxidative processes catalyzed by iron. It was found that a concentration of iron as low as 0.1 mM elicits a consistent production of malondialdehyde in mitochondria. Concomitant with the induction of lipoperoxidation a progressive fall in the mitochondrial membrane potential was observed. The occurrence of energy-consuming processes as a consequence of iron addition, and particularly the enhancement of endogenous Ca2+ cycling across the membrane, was suggested as the cause of the membrane potential drop.

Relevance of ferritin-binding sites on isolated mitochondria to the mobilization of iron from ferritin

Iron can be released from ferritin and utilized by isolated rat liver mitochondria for the synthesis of heme. Mobilization of iron from ferritin is initiated by the binding of ferritin to the mitochondria in an manner compatible with binding sites or receptors for ferritin on the mitochondria. The binding completes rapidly, it is independent of temperature, saturable, reversible and enhanced by K+ and Mg2+. The amount of ferritin binding sites is approx. 0.8 pmol/mg mitochondrial protein, and the affinity constant is 6.4 . 10(6)M-1. The binding kinetics correlate well with the functional features of the ferritin-mitochondrial interaction: i.e. mobilization of iron from ferritin followed by insertion of the iron into heme. The results support the concept of ferritin as a possible donor of iron to the mitochondria.

Studies on the efflux of metalloporphyrin from isolated rat liver mitochondria. Effect of respiratory substrates and metabolic inhibitors

Intramitochondrially synthesized Co-deuteroporphyrin is released to the incubation medium at a rate inversely correlated to the energy state of the mitochondria; i.e. the rate of efflux increases when substrate is depleted, respiration inhibited or the mitochondria are uncoupled. The efflux of Co-deuteroporphyrin from mitochondria remains low as long as the residual membrane potential is above one-third that of maximally energized mitochondria. Globin enhances the efflux of Co-deuteroporphyrin not only from mitochondria depleted of substrates [Husby & Romslo (1980) Biochem. J. 188, 459-465], but also from maximally energized mitochondria. The results provide further evidence for a co-operative mechanism between the mitochondria and their surroundings for the mobilization of metalloporphyrin from mitochondria.

Studies on the efflux of metalloporphyrin from rat liver mitochondria. Effect of K+ and other cations

The mechanism by which metalloporphyrins synthesized within the mitochondria escape to the incubation medium was studied in isolated rat liver mitochondria. In a low-ionic-strength sucrose medium, the efflux of metalloporphyrins is markedly decreased when K+ (greater than 10 mM) is added. The effect of K+ is not dependent on the energy state of the mitochondria and it can in part be abolished by adding globin, but not albumin. K+ also decreases the uptake of porphyrins by the mitochondria and thereby the rate of synthesis of metalloporphyrins. Qualitatively similar results are found with Na+, Li+, Mg2+ and Ca2+. Quantitatively, however, the efficiency of cations to inhibit the release of metalloporphyrins decreases in the order: Mg2+ greater than Ca2+ greater than K+ greater than Li+ greater than Na+. Co-protoporhyrin behaves essentially as Co-deuteroporphyrin. The results provide further evidence that the efflux of metalloporphyrins from the mitochondria depends on haem-binding ligands of the suspending medium and also on the ionic strength of the incubation medium.

Reduction of exogenous FMN by isolated rat liver mitochondria. Significance to the mobilization of iron from ferritin

When FMN is added to rat liver mitochondria or mitoplasts it is reduced at a rate of approx. 0.2 nmol . min-1 . mg-1 protein. Sonicated mitochondria do not reduce exogenous FMN. The reduction depends on drainage of reducing equivalents from the respiratory chain at the level of ubiquinone. The net production of reduced FMN is detectable only at oxygen concentrations less than 4-5 muM. The mitochondrial ubiquinol-FMN oxidoreductase provides a mechanism for the coupling of FMN-reduction to the reductive mobilization of iron from ferritin. The results are discussed in relation to the role of ferritin as a donor of iron to the mitochondria.

Energy-dispersive X-ray analysis of the mitochondria of sideroblastic anaemia

Energy-dispersive X-ray analysis has been performed on marrow sideroblasts obtained from 10 patients with sideroblastic anaemias or erythroleukaemia (six primary refractory sideroblastic anaemia, two pyridoxine-responsive, one secondary sideroblastic anaemia, two erythroleukaemia). Irrespective of the nature of the disorder associated with the presence of sideroblasts, X-ray analysis of siderotic mitochondria consistently revealed the presence of iron and phosphorus with the average Fe/P intensity ratio measuring 1.4-1.5. Other elements variably detected within siderotic mitochondria included calcium, lead, potassium and zinc. Variation in the presence of these latter elements was detected not only between different patients, but also within different samples taken at different times from a single patient and even among different cells of the same sample. Despite the detection of lead in siderotic mitochondria of a significant number of patients (five out of seven), there was no clinical evidence of lead toxicity. The elemental composition of the intramitochondrial deposits in sideroblasts was distinct from that of ferritin or haemosiderin and probably consists of ferric phosphate, possibly, ferric orthophosphate (FePO4).

Uptake of protoporphyrin IX by isolated rat liver mitochondria

Rat liver mitochondria accumulate protoporphyrin IX from the suspending medium into the inner membrane in parallel with the magnitude of the transmembrane K+ gradient (K+in/K+out). Only protoporphyrin IX taken up in parallel with the transmembrane K+ gradient is available for haem synthesis. Coproporphyrins (isomers I and III) are not taken up by the mitochondria. The results support the suggestion by Elder & Evans [(1978) Biochem. J. 172, 345-347] that the prophyrin to be taken up by the inner mitochondrial membrane belongs to the protoporphyrin(ogen) IX series. Protoporphyrin IX at concentrations above 15 nmol/mg of protein has detrimental effects on the structural and functional integrity of the mitochondria. The relevance of these effects to the hepatic lesion in erythropoietic protoporphyria is discussed.

Studies on the efflux of metalloporphyrin from rat-liver mitochondria. Effect of albumin, globin, haemin and haemoglobin

The mechanism by which metalloporphyrins escape from mitochondria has been studied in isolated rat-liver mitochondria using Co-deuteroporphyrin as the model compound. During the first 10--15 min of incubation the efflux is about 10% of the total amount of Co-deuteroporphyrin synthesized. The efflux then increases to a second steady-state leve of 25--35% after 30--45 min of incubation. The efflux is inversely correlated to the energy state of the mitochondria. Globin at concentrations less than 0.4 mumol/l enhances the efflux of Co-deuteroporphyrin, but has no effect on the degree of energy coupling or on the rate of Co-deuteroporphyrin synthesis. The effect of globin can be competitively inhibited by adding haemin. Haemin (0.5--1.0 mumol/l) when added to the medium in the absence of globin reduces the efflux of Co-deuteroporphyrin by 20--30%, but has no effect on the metal-chelatase activity. Neither albumin nor haemoglobin increases the efflux of Co-deuteroporphyrin from intact mitochondria. The results suggest that the efflux of metalloporphyrin is regulated in part by the energy state of the mitochondria and in part by the presence of metalloporphyrin-binding ligants and unattached haemin in the incubation medium.

Improved renal allograft survival after blood transfusion: a nonspecific, erythrocyte-mediated immunoregulatory process?

Phagocytosis of altered red blood cells in vitro by mononuclear phagocytic cells is followed by profound depression of bystander lymphocytes' responses to antigen. Rapid endocytosis of transfused erythrocytes in vivo may transiently impair mononuclear phagocytic cell function, resulting in immunological unresponsiveness as observed in vitro. Transplantation at this time would be predicted to benefit from this attenuation of recipient immunocompetence, resulting in improved graft survival.

Studies on the utilization of ferritin iron in the ferrochelatase reaction of isolated rat liver mitochondria

The utilization of ferritin as a source of iron for the ferrochelatase reaction has been studied in isolated rat liver mitochondria. 1. It was found that isolated rat liver mitochondria utilized ferritin as a source of iron for the ferrochelatase reaction in the presence of succinate plus FMN (or FAD). 2. Under optimal experimental conditions, i.e., approx. 50 micromol/1 FMN, 37 degrees C, pH 7.4 and 0.5 mmol/l Fe(III) (as ferritin iron), the release process, as shown by the formation of deuteroheme, amounted to approx. 0.5 nmol iron/min per mg protein. 3. The release process could not be elicited by ultrasonically treated mitochondria, lysosomes, microsomes or cytosol, i.e., the release of iron from ferritin was due to mitochondria and was a function of the in situ orientation of the mitochondrial inner membrane. 4. The release of iron from ferritin by the mitochrondria might be of relevance not only for the in situ synthesis of heme in the hepatocyte, but also with respect to the mechanism(s) by means of which iron is mobilized for transport to the erythroid tissue.

Ferritin phenotypes: structure and metabolism

Recent advances in our understanding of the biochemistry of ferritin have provided new insights into its role in iron metabolism. Findings of multiple structural forms in many tissues may have important consequences for ferritin's function and metabolism. This article reviews the molecular basis of apoferritin heterogeneity and discusses mechanisms operating in the phenotypic expression of ferritin in normal and malignant cells.

Transferrin and iron uptake by isolated rat liver mitochondria

Isolated rat liver mitochondria accumulate iron from the suspending medium when [59Fe]transferrin is used as a model compound. The accumulation proceeds by two different mechanisms, i.e. by an energy-independent and an energy-dependent (uncoupler sensitive) mechanism, which have different time, pH, and temperature dependencies. The energy-dependent accumulation, which is inhibited by ruthenium red and sulphydryl reagents, reaches a saturation level of approx. 30 pmoles iron/mg protein during 30 min incubation. The energy-independent accumulation of iron-transferrin reveals no saturation kinetics, it is inhibited neither by ruthenium red nor by N-ethylmaleimide, and it proceeds linearly for at least 90 min. With [125I]transferrin as a model compound, quantitatively the energy-independent accumulation is as reported for [59Fe]transferrin. There is, however, no energy-dependent accumulation of [125I]transferrin. The results indicate that the energy-dependent accumulation of [59Fe]transferrin represents a process by which mitochondria accumulate iron from transferrin.

Energy-dependent accumulation of iron by isolated rat liver mitochondria. IV. Relationship to the energy state of the mitochondria

1. The energy-dependent accumulation of iron by isolated rat liver mitochondria, respiring on endogenous substrates, is strongly dependent on the efficiency of energy coupling in the respiratory chain as measured by respiratory control with ADP and the endogenous energy dissipation. The accumulation reached a saturation level at respiratory control with ADP values (with succinate as the substrate) of approx. 4.0. 2. In the presence of exogenous substrate, the energy-dependent accumulation of iron was markedly reduced, primarily due to binding of iron as carboxylate complexes having less favourable constants than the iron (III)-sucrose complex(es). 3. The effect of added ATP was at least 2-fold, i.e. that of providing energy and that of chelating iron. When the mitochondria respired on endogenous substrate, the energy-dependent accumulation of iron increased at low concentrations of ATP, whereas higher concentrations (greater than 50 mu M) gradually inhibited the uptake. 4. Energization of the mitochondria by the generation of an artificial K-+ gradient across the inner membrane with valinomycin in a K-+-free medium increased the energy-dependent accumulation iron.

Energy-dependent accumulation of iron by isolated rat liver mitochondria. V. Effect of factors controlling respiration and oxidative phosphorylation

1. Depending on the metabolic state, the addition of iron(III)-sucrose induces an inhibition or a stimulation of the respiration rate when added to isolated rat liver mitochondria. 2. Under conditions identical to those used in the accumulation studies (Romslo, I. and Flatmark T. (1973) Biochim. Biophys. Acta 305, 29-40), the ferric complex induces a decrease in the oxygen uptake concomitant to an oxidation of cytochromes c (+c1) and a(+a3). These results suggest that ferric iron is reduced to ferrous iron by the respiratory chain prior to or simultaneously with its energy-dependent accumulation. 3. On the other hand, the addition of iron(III)-sucrose induces a stimulation of respiration in State 4 and State 3 provided Mg-2+ is present in the suspending medium. In contrast to Ca-2+, iron stimulates State 4 respiration in a cyclic process only within narrow concentration limits; at concentrations of iron above 100 mu M the respiration remains in the activated state until anaerobiosis. The stimulation of State 4 respiration is more pronounced with succinate than with NAD-linked substrates, a difference which partly may be attributed to a stimulation of the succinate dehydrogenase complex. 4. The stimulation of respiration by iron is approx. 3 times higher in State 3 than in State 4 and this difference can be attributed to a stimulation of the adenine mucleotide exchange reaction in State 3 with a concomitant increase in the rate of oxidative phosphorylation, although the P/O ration is slightly diminished.

Magnetic resonance studies on the mitochondrial divalent cation carrier

Measurements of water proton spin relaxation enhancements (epsilon) can be used to discriminate high-affinity binding of Mn-2+ or Gd-3+ to biological membranes, from low-affinity binding. In rat liver mitochondria, epsilon b values of approx. 11 are observed upon binding of Mn-2+ to the inner membrane, while internal or low-affinity binding remains invisible to this technique. Energy-driven Mn-2+ uptake by liver mitochondria results in the subsequent decay of epsilon. Comparison of epsilon with the initial velocity of Mn-2+ uptake in rat liver mitochondria reveals a linear correlation, which holds at all temperatures between 0 degrees C and 40 degrees C, regardless of the mitochondrial protein concentration. Consequently, enhancement appears to reflect the binding of Mn-2+ to the divalent cation pump. Binding of Mn-2+ to blowfly flight muscle also results in substantial epsilon, which is associated with the glycerol-1-phosphate dehydrogenase instead of divalent cation transport. Consequently, no decay in epsilon due to uptake occurs after Mn-2+ is bound. Lanthanide ions are also bound and transported by mitochondria. Addition of Gd-3+ to pigeon heart or rat liver mitochondria results in epsilon b approximately equal to 5-6, which decays with similar kinetics in both systems. The uptake velocity of Gd-3+ in rat liver mitochondria is about 1/6 the rate with which Mn-2+ is transported. Lanthanides also diminish epsilon due to the addition of Mn-2+, and greatly retard the Mn-2+ uptake kinetics. The presence of carbonylcyanide-p-trifluoromethoxyphenylhydrazone depresses epsilon upon addition of Mn-2+ or Gd-3+ and also uncouples energy-driven uptake. On the other hand, prolonged anaerobic incubation in the presence of antimycin and rotenone exhausts the mitochondria of their energy stores, blocks the uptake of Mn-2+, but does not affect epsilon significantly. Evidently, the uncoupler-induced disappearance of divalent cation binding sites is not the result of "de-energization". Measurements of epsilon at several NMR frequencies indicate a correlation time (tau b) for carrier-bound Mn-2+ in rat liver mitochondria between 20 ns and 4 ns as one varies the temperature between 10 degrees C and 30 degrees C. The 13 Kcal/mole activation energy for tau b suggests that the 11 ns time constant at room temperature represents the movement of the Mn-11-carrier comples. On the other hand, tau b is probably approx. 100 times too short to represent the rotational motion of a carrier protein. Apparently, Mn-2+ binds to a small arm of the carrier which moves independent