Non-ohmic proton conductance of the mitochondrial inner membrane in hepatocytes

Characteristics of mitochondrial proton leak and control of oxidative phosphorylation in the major oxygen-consuming tissues of the rat

Maintenance of an electrochemical proton gradient across the mitochondrial inner membrane against the significant proton permeability of the membrane accounts for 25-30% of resting oxygen consumption in hepatocytes. It has been proposed that proton leak could be a significant contributor to resting metabolic rate in mammals if it were present in other tissues. Mitochondria were isolated from the major oxygen-consuming tissues (liver, kidney, brain and skeletal muscle) of the rat. In each tissue, the mitochondria showed significant proton leak with the same characteristic non-linear dependence on membrane potential. Liver and kidney mitochondria showed similar membrane proton permeability per mg of mitochondrial protein; brain and muscle permeabilities were greater when expressed in this way. Differences in the kinetic response of the substrate oxidation and phosphorylating systems to membrane potential were observed. The substrate oxidation system was more active in kidney, brain and skeletal muscle mitochondria than in liver mitochondria per mg of mitochondrial protein. Liver and kidney phosphorylating systems were less active than brain and skeletal muscle per mg of mitochondrial protein. The control of oxidative phosphorylation was also assessed. The distribution of control in mitochondria isolated from the four tissue types was found to be similar.

Differentiation potentiates oxidant injury to mitochondria by hydrogen peroxide in Friend's erythroleukemia cells

Oxidative damage to mitochondrial functions was investigated upon non-lethal treatment with H2O2 of Friend's erythroleukemia cells induced to differentiate, in comparison with the parental cell line. Both respiration and maximal ATP synthase capacity were more severely diminished by H2O2 in induced cells. The effects were mediated by intracellular redox-active iron and OH. radicals. Specifically, the mechanisms of the selective oxidant injury to F0F1 ATP synthase observed in differentiating cells likely involved impairment of F0-F1 coupling sensitive to oligomycin. We suggest a Fenton-like reaction of H2O2 with iron ions, more available in the differentiating cells, as occurring at the surface and/or in the lipid bulk phase of the inner mitochondrial membrane, thus injuring subunits responsible for the coupling of F0F1 ATP synthase through generation in situ of the actual damaging species. Besides, we propose heme iron as the most likely candidate for such reaction in induced cells actively synthesizing heme. In accordance, pretreatment of uninduced cells with hemin made H2O2-damage qualitatively identical.

The causes and functions of mitochondrial proton leak

The non-linear relationship between respiration rate and protonmotive force in isolated mitochondria is explained entirely by delta p-dependent changes in the proton conductance of the mitochondrial inner membrane and is not caused by redox slip in the proton pumps. Mitochondrial proton leak occurs in intact cells and tissues: the futile cycle of proton pumping and proton leak accounts for 26% +/- 7% of the total oxygen consumption rate or 33% +/- 7% of the mitochondrial respiration rate of isolated hepatocytes (mean +/- S.D. for 43 rats); 52% of the oxygen consumption rate of resting perfused muscle and up to 38% of the basal metabolic rate of a rat, suggesting that heat production may be an important function in the proton leak in homeotherms. Together with non-mitochondrial oxygen consumption, it lowers the effective P/O ratio in cells from maximum possible values of 2.33 (palmitate oxidation) or 2.58 (glucose oxidation) to as low as 1.1 in liver or 0.8 in muscle. The effective P/O ratio increases in response to ATP demand; the ability to allow rapid switching of flux from leak to ATP turnover may be an even more important function of the leak reaction than heat production. The mitochondrial proton conductance in isolated mitochondria and in hepatocytes is greatly modulated by thyroid hormones, by phylogeny and by body mass. Usually the reactions of ATP turnover change in parallel so that the coupling ratio is not greatly affected. Changes in proton leak in tissues are brought about in the short term by changes in mitochondrial protonmotive force and in the longer term by changes in the surface area and proton permeability of the mitochondrial inner membrane. Permeability changes are probably caused by changes in the fatty acid composition of the membrane phospholipids.

The effects of methylprednisolone on oxidative phosphorylation in Concanavalin-A-stimulated thymocytes. Top-down elasticity analysis and control analysis

The glucocorticoid methylprednisolone has clinically important anti-inflammatory effects at high concentrations through unknown mechanisms. Methylprednisolone at 0.2 mg/10(7) cells inhibits respiration in Concanavalin-A(ConA)-stimulated thymocytes from rats by about 20%. We have used top-down elasticity analysis to identify the blocks of reactions within oxidative phosphorylation in thymocytes whose kinetics are significantly affected by treatment with methylprednisolone. At this concentration methylprednisolone greatly inhibited the reactions of substrate oxidation and increased mitochondrial proton leak but did not significantly affect the synthesis and and turnover of ATP by the phosphorylating system. Metabolic control analysis showed that oxygen consumption by ConA-treated thymocytes was controlled largely (0.51) by the phosphorylating system but also by proton leak (0.32) and substrate oxidation (0.17); this is similar to the distribution of control in hepatocytes, suggesting that this pattern may be general in cells. Methylprednisolone lowered control by the phosphorylating system to 0.26 and raised control by substrate oxidation to 0.37. From these results we conclude that the inhibition of respiration in ConA-stimulated thymocytes by methylprednisolone at this concentration results from an inhibition of substrate oxidation and a smaller stimulation of mitochondrial proton leak, with only a minor contribution of any effects within the phosphorylating system.

Liposomes from mammalian liver mitochondria are more polyunsaturated and leakier to protons than those from reptiles

Liposomes were prepared from phospholipids extracted from liver mitochondria of the rat (Rattus norvegicus) and an agamid lizard, the bearded dragon (Amphibolurus vitticeps) and liposome proton conductance was measured at an imposed membrane potential of 160 mV as well as the fatty acid composition of the liposomes. Despite presumed changes in fatty acid composition during liposome preparation, the mammalian liposomes had a significantly lower content of the monounsaturated oleic acid and a significantly greater content of the omega-3 polyunsaturated docosahexaenoic acid. There were significant direct correlations between the liposome arachidonic and docosahexanoic acid content and bilayer proton flux and a significant inverse correlation between liposome oleic acid content and bilayer proton flux. "Apparent valinomycin-catalysed proton flux" was significantly directly correlated with liposome docosahexaenoic acid content and inversely correlated with oleic acid content. It is suggested that the high content of long-chain polyunsaturates in the mammalian mitochondrial membrane is responsible for an increased proton leak across the mitochondrial inner membrane and thus partly responsible for the high metabolic rate in endothermic mammals compared to their ectothermic reptilian predecessors.

Comparison of mitochondrial pro-oxidant generation and anti-oxidant defenses between rat and pigeon: possible basis of variation in longevity and metabolic potential

Non-passerine birds and mammals of similar body weight have a roughly comparable metabolic rate, but the life span and the metabolic potential, i.e. the total amount of energy consumed per unit of body mass during life, is several times higher in the birds. The objective of this study was to explore the possible basis of this characteristic in the context of the predictions of the free radical hypothesis of aging. Accordingly, pigeon and rat, which have a similar body weight, were compared by examining the mitochondrial rates of O2.- and H2O2 generation and activities of superoxide dismutase, catalase and glutathione peroxidase and concentration of glutathione in the brain, heart and kidney. Compared with the rat, the rate of mitochondrial O2.- generation in the pigeon ranged between 50 and 67%, and H2O2 production between 31 and 77%. Activity of superoxide dismutase was uniformly higher and catalase activity consistently lower in the tissues of the pigeon compared with the rat. Glutathione peroxidase activity and glutathione concentration were higher in the pigeon in two out of the three organs studied, and comparable in the third organ. The magnitude of the differences between the two species was greater in the rates of O2.- and H2O2 generation than in anti-oxidant defenses. Results indicate that the relatively greater longevity and metabolic potential of the pigeon may be related to significantly lower rates of O2.- and H2O2 generation and higher overall level of anti-oxidant defenses.

Body mass dependence of H+ leak in mitochondria and its relevance to metabolic rate

The standard metabolic rate of an animal is the rate of heat production under conditions that minimize known extra requirements for energy. In tissues and cells from aerobic organisms, energy expenditure can conveniently be measured as oxygen consumption. Measurements made using isolated rat hepatocytes have shown that a significant contribution to resting oxygen consumption (and hence heat production) is made by a futile cycle of proton pumping and proton leak across the mitochondrial inner membrane. Two important factors affecting standard metabolic rate, thyroid status and phylogeny, also affect the proton permeability. A third major factor affecting standard metabolic rate is body mass. Here we show that proton leak decreases with increasing body mass in mammals. We suggest that differences in proton leak may partly explain the differences in standard metabolic rate between mammals of different mass.

ConA induced changes in energy metabolism of rat thymocytes

The influence of ConA on the energy metabolism of quiescent rat thymocytes was investigated by measuring the effects of inhibitors of protein synthesis, proteolysis, RNA/DNA synthesis, Na+K(+)-ATPase, Ca(2+)-ATPase and mitochondrial ATP synthesis on respiration. Only about 50% of the coupled oxygen consumption of quiescent thymocytes could be assigned to specific processes using two different media. Under these conditions the oxygen is mainly used to drive mitochondrial proton leak and to provide ATP for protein synthesis and cation transport, whereas oxygen consumption to provide ATP for RNA/DNA synthesis and ATP-dependent proteolysis was not measurable. The mitogen ConA produced a persistent increase in oxygen consumption by about 30% within seconds. After stimulation more than 80% of respiration could be assigned to specific processes. The major oxygen consuming processes of ConA-stimulated thymocytes are mitochondrial proton leak, protein synthesis and Na+K(+)-ATPase with about 20% each of total oxygen consumption, while Ca(2+)-ATPase and RNA/DNA synthesis contribute about 10% each. Quiescent thymocytes resemble resting hepatocytes in that most of the oxygen consumption remains unexplained. In contrast, the pattern of energy metabolism in stimulated thymocytes is similar to that described for Ehrlich Ascites tumour cells and splenocytes, which may also be in an activated state. Most of the oxygen consumption is accounted for, so the unexplained process(es) in unstimulated cells shut(s) off on stimulation.

The mechanism of the increase in mitochondrial proton permeability induced by thyroid hormones

Three possible mechanisms by which different levels of thyroid hormones in rats might cause the observed sevenfold change in the apparent proton permeability of the inner membrane of isolated liver mitochondria were investigated. (a) Cytochrome c oxidase was isolated from the livers of hypothyroid, euthyroid and hyperthyroid rats and incorporated into liposomes made with soya phospholipids. There was no difference between the proton current/voltage curves of the three types of vesicles. The hormonal effects, therefore, were not an inherent property of the enzymes, and were not due to different coupling of electron flow through the enzyme to proton transport. (b) The surface area of the mitochondrial inner membrane was shown by three different assays to be greater by a factor of between two and three in mitochondria from hyperthyroid animals than in mitochondria from hypothyroid animals; euthyroid controls were intermediate. This difference in surface area of the inner membrane explains less than half of the difference in apparent proton permeability. (c) The proton permeability of liposomes prepared from phospholipids extracted from mitochondrial inner membranes of hyperthyroid rats was three times greater than the proton permeability of those from hypothyroid rats; euthyroid controls were intermediate. This suggests, first, that the proton permeability of the phospholipid bilayer is an important component of the proton permeability in intact mitochondria and, second, thyroid hormone-induced changes in the bilayer are a major part of the mechanism of increased proton permeability. Such changes may be due to the known differences in fatty acid composition of mitochondrial phospholipids in different thyroid states. Thus we have identified two mechanisms by which thyroid hormone levels in rats change proton flux/mass protein in isolated liver mitochondria: a change in the area of the inner membrane/mass protein and a change in the intrinsic permeability of the phospholipid bilayer.

ConA induced changes in energy metabolism of rat thymocytes

The influence of ConA on the energy metabolism of quiescent rat thymocytes was investigated by measuring the effects of inhibitors of protein synthesis, proteolysis, RNA/DNA synthesis, Na+K(+)-ATPase, Ca(2+)-ATPase and mitochondrial ATP synthesis on respiration. Only about 50% of the coupled oxygen consumption of quiescent thymocytes could be assigned to specific processes using two different media. Under these conditions the oxygen is mainly used to drive mitochondrial proton leak and to provide ATP for protein synthesis and cation transport, whereas oxygen consumption to provide ATP for RNA/DNA synthesis and ATP-dependent proteolysis was not measurable. The mitogen ConA produced a persistent increase in oxygen consumption by about 30% within seconds. After stimulation more than 80% of respiration could be assigned to specific processes. The major oxygen consuming processes of ConA-stimulated thymocytes are mitochondrial proton leak, protein synthesis and Na+K(+)-ATPase with about 20% each of total oxygen consumption, while Ca(2+)-ATPase and RNA/DNA synthesis contribute about 10% each. Quiescent thymocytes resemble resting hepatocytes in that most of the oxygen consumption remains unexplained. In contrast, the pattern of energy metabolism in stimulated thymocytes is similar to that described for Ehrlich Ascites tumour cells and splenocytes, which may also be in an activated state. Most of the oxygen consumption is accounted for, so the unexplained process(es) in unstimulated cells shut(s) off on stimulation.

The administration of triiodothyronine to rats results in a lowering of the mitochondrial membrane potential in isolated hepatocytes

Although thyroid status has been shown to influence the magnitude of the membrane potential in isolated rat-liver mitochondria, there is variation in the reported size and direction of the thyroid hormone-induced change relative to the normal state. Measurement of the mitochondrial membrane potential in intact hepatocytes isolated from hyperthyroid and euthyroid rats reveals that hyperthyroidism results in a decrease of approximately 30 mV in the magnitude of this potential relative to that in the euthyroid state. As well, the magnitude of the plasma membrane potential of hepatocytes from hyperthyroid rats is reduced by 6 mV compared with that in cells from euthyroid rats. The thyroid hormone-induced decrease in these potentials may reflect reported changes in the lipid composition of the membranes.

An extended dynamic model of oxidative phosphorylation

The presented model based on an earlier one (Korzeniewski, B. and Froncisz, W. (1989) Studia Biophys. 132, 173-187) simulates concentration changes in time of chemical compounds and thermodynamic forces during respiration of cell suspension in a closed chamber. A set of differential equations solved numerically describes the utilization of oxygen up to anaerobiosis and the behaviour of the system after a sudden pulse of oxygen. Flux control coefficients for most important reactions (enzymes) of oxidative phosphorylation were calculated. A good qualitative and (when a direct comparison is possible) quantitative agreement with experimental results can be observed. The following conclusions can be drawn from the simulation: (1) Wilson's steady state model is not in contradiction with sharing of the control over the respiration between some steps and displacement of the ATP/ADP carrier from equilibrium. (2) The overshoot characteristics of the delta microH+ time-course after reoxygenation can be explained without using the lag-phase kinetics of ATP-synthetase. (3) A 'hot region' (sharp changes of many parameters) can be distinguished when the oxygen concentration approaches zero; only cytochrome oxidase is clearly sensitive on oxygen concentration in all its range. (4) Control over oxidative phosphorylation is shared mainly between inputs of the system (ATP utilization and substrate dehydrogenation) and the proton leak.

On the nature of the mitochondrial proton leak

Respiring mitochondria have a significant passive permeability to protons; the mechanism of this proton leak is unknown. Several putative mechanisms were tested. Mitochondrial permeability to small sugars was unaffected by energization, suggesting that there is no significant dielectric breakdown at high membrane potential. Mitochondria are argued to have a proton permeability that is 6 to 8 orders of magnitude higher than the permeability to other cations, suggesting that the proton leak is probably not via a simple pore or membrane defect. 15-30% of the proton leak of freshly prepared mitochondria was extractable with bovine serum albumin and is probably due to fatty acids. Little if any of the proton leak appears to be due to cycling of ions other than protons, or to be associated with the functional activity of the proton pumps. The mitochondrial proton leak shares several properties with the proton permeability of pure phospholipid bilayers, suggesting that they share the same mechanism, although the leak through the bilayer in mitochondria may be modified by the presence of proteins.

The contribution of the leak of protons across the mitochondrial inner membrane to standard metabolic rate

This paper presents and assesses the hypothesis that the proton leak across the mitochondrial inner membrane is an important contributor to standard metabolic rate, and that increases in the amount of mitochondrial inner membrane may be important in causing changes in proton leak and in the standard metabolic rate. The standard metabolic rate of an animal is known to be a function of body mass, phylogeny and thyroid status, and is largely attributed to the metabolically active internal organs. The total area of mitochondrial inner membrane in these organs correlates well with standard metabolic rate over a wide range of body masses in both ectotherms and endotherms. In hepatocytes isolated from rats, proton leak across the mitochondrial inner membrane accounts for about 30% of the resting oxygen consumption, and the distribution of control over respiration suggests that changes in mitochondrial inner membrane surface area will be accompanied by significant changes in the proton leak. This change in the leak will result in significant changes in resting oxygen consumption, but changes in ATP demand may also have a role to play in determining resting respiration rate. Extrapolation of these results to other tissues and other animals suggests that the hypothesis has the potential to explain a substantial proportion of the variation in standard metabolic rate with body mass, phylogeny and thyroid status. However, in most cases the quantitative contribution of proton leak compared to cellular ATP turnover has yet to be experimentally determined.