The efficiency of oxidative phosphorylation and the rapid control by thyroid hormone of nicotinamide nucleotide reduction and transhydrogenation in intact rat liver mitochondria

Thyroid hormone effects on mitochondrial energetics

Thyroid hormones are the major endocrine regulators of metabolic rate, and their hypermetabolic effects are widely recognized. The cellular mechanisms underlying these metabolic effects have been the subject of much research. Thyroid hormone status has a profound impact on mitochondria, the organelles responsible for the majority of cellular adenosine triphosphate (ATP) production. However, mechanisms are not well understood. We review the effects of thyroid hormones on mitochondrial energetics and principally oxidative phosphorylation. Genomic and nongenomic mechanisms have been studied. Through the former, thyroid hormones stimulate mitochondriogenesis and thereby augment cellular oxidative capacity. Thyroid hormones induce substantial modifications in mitochondrial inner membrane protein and lipid compositions. Results are consistent with the idea that thyroid hormones activate the uncoupling of oxidative phosphorylation through various mechanisms involving inner membrane proteins and lipids. Increased uncoupling appears to be responsible for some of the hypermetabolic effects of thyroid hormones. ATP synthesis and turnover reactions are also affected. There appear to be complex relationships between mitochondrial proton leak mechanisms, reactive oxygen species production, and thyroid status. As the majority of studies have focused on the effects of thyroid status on rat liver preparations, there is still a need to address fundamental questions regarding thyroid hormone effects in other tissues and species.

Direct thyroid hormone signalling via ADP-ribosylation controls mitochondrial nucleotide transport and membrane leakiness by changing the conformation of the adenine nucleotide transporter

Addition of triiodothyronine at 10 pM in vitro to hypothyroid rat liver mitochondria doubles the rate of the adenine nucleotide transporter at low ADP concentrations. Nicotinamide abolishes this effect in parallel with its inhibition of the ADP-ribosylation of an inner membrane protein identical in size to the transporter. Nicotinamide also renders euthyroid preparations indistinguishable from hypothyroid ones. A mechanism is offered to explain these findings in which it is proposed that the adenine nucleotide transporter is a true allosteric protein and that its covalent modification by ADP-ribosylation increases the stability of the less favoured externally-facing C-conformation and thus increases the proportion of transporters in this orientation: although the C-conformation is significantly more leaky to cations than the tight matrix-facing M-conformation, this enhances ADP import. This model is shown to offer an explanation not only for the transport effects of T3 but also for those of oxidative stress and ADP-ribosylation inhibitors on Ca2+, H+ and K+ transfer across the mitochondrial inner membrane. Ca2+ at 30 nM appears to stabilize the M-conformation of the transporter by a mechanism other than ADP-ribosylation.

Thyroid hormones and the creatine kinase system in cardiac cells

The paper reviews the current evidence on the role of thyroid hormones in regulating the creatine kinase energy transfer system at multiple structures in cardiac cells. 1) Thyroid hormones modulate the overall synthesis of phosphocreatine (PCr) by increasing the rate of mitochondrial oxidative phosphorylation. 2) Thyroid hormones regulate the total activity of creatine kinase and its isoenzyme distribution. In comparison with normal thyroid state (euthyroidism), hypothyroidism is characterized by decreased total creatine kinase activity owing to diminished fraction of creatine kinase. On the other hand, hyperthyroidism, while causing no change in total creatine kinase activity, leads to increased fractions of neonatal isoforms of creatine kinase, and, in case of prolonged hyperthyroidism, to decreased fraction of mitochondrial creatine kinase. The latter change is associated with partial uncoupling between mitochondrial creatine kinase and adenine nucleotide translocase reflected by decreased PCr/O ratio. 3) Hyperthyroidism leads to increased passive sarcolemmal permeability due to which the leakage of creatine along its concentration gradient occurs. As a result of (i) increased sarcolemmal permeability for creatine, (ii) uncoupling of mitochondrial PCr synthesis, and (iii) increased energy utilization rate the steady state intracellular PCr content decreases under hyperthyroidism which, in turn, increases the myocardial susceptibility to hypoxic damage. Thyroid state also modulates the protective effects of exogenous PCr on energetically depleted myocardium.

The rapid response of isolated mitochondrial particles to 0.1 nM-tri-iodothyronine correlates with the ADP-ribosylation of a single inner-membrane protein

Under defined conditions liver mitochondria from hypothyroid rats show an apparent lowering of the ADP/O ratio, which can be corrected by addition in vitro of 0.1 nM-tri-iodothyronine (T3). Nicotinamide prevents this restoration by hormone, lowers the ADP/O ratio of euthyroid-rat mitochondria to hypothyroid-rat values and induces T3-sensitivity in euthyroid-rat mitoplasts indistinguishable from that found with hypothyroid-rat preparations. Incorporation into the trichloroacetic-acid insoluble fraction of mitoplasts and hypothyroid-rat mitochondria of radiolabel from [adenine-14C]-NAD+ was stimulated by T3: this stimulation was abolished by nicotinamide. The findings strongly suggest that this incorporation occurs external to the matrix. Confirming the work of others, PAGE of radiolabelled mitoplasts shows alkali-labile modification of a major species of approx. 30 kDa: both nicotinamide and T3 abolish this modification. By contrast, T3 promotes incorporation of label into a single major 11 kDa species: this incorporated label is somewhat acid-labile, and the incorporation is abolished by nicotinamide. Comparative electrophoresis of purified sub-mitoplast fractions show that the 11 kDa species is in the inner membrane and absent from the matrix. The findings are consistent with a receptor-mediated ADP-ribosylation mechanism for the rapid action of T3 on mitochondria.

Physiological roles of nicotinamide nucleotide transhydrogenase

From the foregoing considerations, the energy-linked transhydrogenase reaction emerges as a powerful and flexible element in the network of redox and energy interrelationships that integrate mitochondrial and cytosolic metabolism. Its thermodynamic features make it possible for the reaction to respond readily to challenges, either on the side of NADPH utilization or on the side of energy depletion. Yet, the kinetic features are designed to prevent a wasteful input of energy when other sources of reducing equivalents to NADP are available, or to deplete the redox potential of NADPH in other than emergency conditions. By virtue of these characteristics, the energy-linked transhydrogenase can act as an effective buffer system, guarding against an excessive depletion of NADPH, preventing uncontrolled changes in key metabolites associated with NADP-dependent enzymes and calling on the supply of reducing equivalents from NAD-linked substrates only under conditions of high demand for NADPH. At the same time, it can provide an emergency protection against a depletion of energy, especially in situations of anoxia where a supply of reducing equivalents through NADP-linked substrates can be maintained. The flexibility of this design makes it possible that the functions of the energy-linked transhydrogenase vary from one tissue to another and are readily adjustable to different metabolic conditions.

The influence of nanomolar calcium ions and physiological levels of thyroid hormone on oxidative phosphorylation in rat liver mitochondria. A possible signal amplification control mechanism

Using different conditions mitochondria from hypothyroid rats can show both unchanged ADP/O ratios and lowered ADP/O ratios without evidence of uncoupling when compared with euthyroid controls. Raising the free Ca2+ concentration to around 25 nM progressively lowered the ADP/O ratio in hypothyroid but not in euthyroid mitochondria. Ruthenium Red did not alter this behaviour and further increasing the Ca2+ concentration to levels below those which stimulate State 3 respiration had no additional effect. Measurements of the free Ca2+ concentration in the mitochondrial suspending medium using a Quin 2 fluorescence assay showed that the mitochondria did not buffer the free Ca2+ at these low concentrations. At 25 nM-free Ca2+, addition of 10-13) M-T3 to hypothyroid mitochondria produced an immediate and significant increase in the ADP/O ratio without altering the free Ca2+ concentration. The hormone effect was maximal by 10(-11) M. The concentration of ATP synthetase can be estimated to lie at about 10 nM in these experiments. Hence it appears possible that a substantial amplification of the hormone signal may have taken place. Comparison with binding studies suggests that T3 may have been maximally stimulating when somewhat less than half its receptor sites had been filled. The possible mechanisms by which this receptor mediated alteration of the ADP/O ratio might be achieved are discussed.

The rapid alteration by tri-iodo-L-thyronine in vivo of both the ADP/O ratio and the apparent H+/O ratio in hypothyroid-rat liver mitochondria

Mitochondria from the livers of thyroidectomized rats have a lowered ADP/O ratio, which can be restored to normal within 15 min after intravenous injection of a near-physiological dose of tri-iodothyronine. Thyroidectomy lowered the measured delta pH, which appears to be compensated by a rise (not statistically significant) in the membrane potential, so that the protonmotive force is unaltered. A simple simulation technique is described for use in estimating H+/O ratios by the oxygen-pulse technique, which circumvents the problem that this ratio can be seriously underestimated because of re-uptake of protons from the bulk phase by the mitochondria before their expulsion is complete. By this procedure the H+/O ratio of hypothyroid mitochondria is shown to be lowered by the same factor as the ADP/O ratio, and both these ratios are very rapidly restored in parallel by hormone administration. Although these findings could be consistent with a proposal that tri-iodothyronine rapidly modulates by some mechanism the efficiency of the respiratory-chain-linked proton pumps, the kinetic properties of the proton exchange suggest that the bulk-phase protons measured may not reflect faithfully those that drive the ATP synthetase.

Effect of hypothyroidism on the cytosolic free Ca2+ concentration in rat hepatocytes during rest and following stimulation by noradrenaline or vasopressin

The mean resting concentration of cytosolic free Ca2+ [( Ca2+]i) in parenchymal liver cells, as determined with the intracellular Ca2+ indicator quin2, was lowered by about 30% in hypothyroidism (0.17 microM vs. 0.27 microM in normal cells). The [Ca2+]i level in hypothyroid cells at 10 s following stimulation by noradrenaline (1 microM) was about 64% lower than in normal cells (0.33 microM vs. 1.0 microM). The response to noradrenaline in hypothyroid cells was slower in onset (significant at 5 s vs. 3 s in euthyroid cells), and the maximum of the initial [Ca2+]i increase was reached later (14 s vs. 8 s in normal cells). In hypothyroid hepatocytes the initial increase was followed by a slow but prolonged secondary increase in [Ca2+]i. With vasopressin similar results were found. Chelation of extracellular Ca2+ with EGTA immediately prior to stimulation had no effect on the initial [Ca2+]i increase. Treatment with T3 in vivo (0.5 micrograms/100 g body weight daily during 3 days) completely restored the basal and stimulated [Ca2+]i in hypothyroid cells. The half-maximally effective dose of noradrenaline was the same in euthyroid and hypothyroid liver cells (1.8 X 10(-7) M). Hypothyroidism had no significant effect on the number of alpha 1-receptors determined by [3H]prazosin labeling in crude homogenate fractions, while the Kd for [3H]prazosin was 21% lower than in the euthyroid group. These results show that thyroid hormone has a general stimulating effect on intracellular Ca2+ mobilization by Ca2+-mobilizing hormones, probably at a site distal to the binding of the agonist to its receptor. The results also support our idea that thyroid hormone may control metabolism during rest and activation, at least partially, by altering Ca2+ homeostasis.

Regulation of respiration and ATP synthesis in higher organisms: hypothesis

The present view on the regulation of respiration and ATP synthesis in higher organisms implies only Michaelis-Menten type kinetics and respiratory control as regulatory principles. Recent experimental observations, suggesting further regulatory mechanisms at respiratory chain complexes, are reviewed. A new hypothesis is presented implying regulation of respiration and ATP synthesis in higher organisms mainly via allosteric modification of respiratory chain complexes, in particular of cytochrome c oxidase. The allosteric effectors, e.g., metabolites, cofactors, ions, hormones, and the membrane potential are suggested to change the activity and the coupling degree of cytochrome c oxidase by binding to specific sites at nuclear coded subunits. Recent results on the structure and activity of cytochrome c oxidase, supporting the hypothesis, are reviewed.

Effect of thyroid hormone on intracellular Ca2+ mobilization by noradrenaline and vasopressin in relation to glycogenolysis in rat liver

The relation between Ca2+ efflux, Ca2+ mobilization from mitochondria and glycogenolysis was studied in perfused euthyroid and hypothyroid rat livers stimulated by Ca2+-mobilizing hormones. Ca2+ efflux, induced by noradrenaline (1 microM) in the absence or presence of DL-propranolol (10 microM) from livers perfused with medium containing a low concentration of Ca2+ (approx. 24 microM), was decreased by more than 50% in hypothyroidism. This correlated with an equal decrease of the fractional mobilization of mitochondrial Ca2+, which could account for 65% of the difference between the net amounts of Ca2+ expelled from the euthyroid and hypothyroid livers. With vasopressin (10 nM) similar results were found, suggesting that hypothyroidism has a general effect on mobilization of internal Ca2+. In normal Ca2+ medium (1300 microM), however, the effect of vasopressin on net Ca2+ fluxes and phosphorylase activation was not impaired in hypothyroidism, indicating that Ca2+ mobilization from the mitochondria in this case plays a minor role in phosphorylase activation. The alpha 1-adrenergic responses of Ca2+ efflux, phosphorylase activation and glucose output, glucose-6-phosphatase activity and oxygen consumption in hypothyroid rat liver were completely restored by in vivo T3 injections (0.5 micrograms per 100 g body weight, daily during 3 days). Perfusion with T3 (100 pM) during 19 min did not influence hypothyroid rat liver oxygen consumption and alpha 1-receptor-mediated Ca2+ efflux. However, this in vitro T3 treatment showed a completely recovered alpha 1-adrenergic response of phosphorylase and a partly restored glucose-6-phosphatase activity and glucose output. The results indicate that thyroid hormones may control alpha 1-adrenergic stimulation of glycogenolysis by at least two mechanisms, i.e., a long-term action on Ca2+ mobilization, and a short-term action on separate stages of the glycogenolytic process.

Effects of thyroid hormone on mitochondrial oxidative phosphorylation

In order to locate sites of action of thyroid hormone on mitochondrial oxidative phosphorylation we have used an experimental application of control analysis as previously described [Groen, Wanders, Westerhoff, Van der Meer & Tager (1982) J. Biol. Chem. 257, 2754-2757]. Rat-liver mitochondria were isolated from hypothyroid rats or from hypothyroid rats 24 h after treatment with a single dose of 3,3',5-triiodothyronine (T3). The amount of control exerted by four different steps on State-3 respiration with succinate as respiratory substrate was quantified by using specific inhibitors. The hormone treatment resulted in an increase in the flux control coefficient of the adenine nucleotide translocator, the dicarboxylate carrier and cytochrome c oxidase and a decrease in the flux control coefficient of the bc1-complex. The results of this analysis indicate that thyroid hormone treatment results in an activation of the bc1-complex and of at least one other enzyme, possibly succinate dehydrogenase. Measurement of the extramitochondrial ATP/ADP ratio at different rates of respiration (induced by addition of different amounts of hexokinase in the presence of glucose and ATP) showed that the adenine nucleotide translocator operates at a higher (ATP/ADP)out after T3 treatment, which supports previous reports on stimulation of this step by thyroid hormone.

The influence of thyroid hormone on the degree of control of oxidative phosphorylation exerted by the adenine nucleotide translocator

Impaired phosphorylation efficiency in liver mitochondria from hypothyroid rats is paralleled by a defect in adenine nucleotide transport. Both of these lesions can be corrected within 15 min by a near-physiological dose of triiodo-L-thyronine. Measurement of the control strength of the translocator shows, however, that this step has a smaller share of the control for oxidative phosphorylation after thyroidectomy and that this is unaltered after 15 min by replacement therapy. Rapid control by triiodothyronine is thus exerted elsewhere than at this transfer and the effects of hormone on the translocator are likely to be indirect.