Effects of thyroid hormone on mitochondrial oxidative phosphorylation

Thyroid hormone effects on the proton permeability of rat liver mitochondria

The effect of thyroid status on passive permeability of liver mitochondrial inner membrane was studied in euthyroid (EU), hyperthyroid (HYPER), hypothyroid (HYPO) rats and hypothyroid rats treated with a single dose of 3,3',5-triiodothyronine (HYPO + T3). Respiration was titrated with uncoupler and plotted against uncoupler concentration to calculate proton permeability. The passive permeability was the same in EU, HYPER, and HYPO + T3 rats, but was significantly lower in HYPO mitochondria. The State IV respiration rates were the same as the oligomycin-inhibited rats for EU, HYPO and HYPO + T3 rats, but in HYPER rats the oligomycin rate was less than the State IV rate. The data show that hypothyroid mitochondria have decreased permeability of the inner membrane which can be normalized by a single dose of T3. Hyperthyroid mitochondria show increased oligomycin-sensitive ATPase activity in State IV, a possible mechanism of the enhanced respiratory rate.

Slip and leak in mitochondrial oxidative phosphorylation

During oxidative phosphorylation by mammalian mitochondria part of the free energy stored in reduced substrates is dissipated and energy is released as heat. Here I review the mechanisms and the physiological significance of this phenomenon.

The effect of thyroid state on respiratory activities of three rat liver mitochondrial fractions

In this paper we report that three different rat liver mitochondrial fractions, differing in density, exhibit differential effects when the animals are made hypo- or hyperthyroid. The investigations have been performed by correlating the protein content, the succinic dehydrogenase behaviour and the respiratory features of the three fractions in different thyroid states with morphometric-stereologic analysis the electron micrographic level. The results indicate that the thyroid hormone influences both the mass and the functionality of the heavy (H) and light (L) fraction. In hypothyroid rats the H fraction increases (+43%) while the L fraction decreases (-32%) and their respiratory activity is drastically reduced. Adenosine triphosphate (ATP) synthesis in the H fraction is also inhibited. Triiodothyronine (T3) administration to the above animals restores the values observed in control rats. At morphometric level we note in hypothyroid rats an increase in the number of mitochondria together with a concomitant increase in the average volume of a single mitochondrion. We are inclined to explain the above results through an action exerted by T3 on a hypothetical mitochondrial cycle starting with the formation of light organelles from heavy ones.

Decreased activity of the pyruvate translocator and changes in the lipid composition in heart mitochondria from hypothyroid rats

A study of the transport of pyruvate in heart mitochondria from normal and hypothyroid rats has been carried out. Heart mitochondria from hypothyroid rats translocate pyruvate via the alpha-cyanocinnamate sensitive carrier much more slowly than do mitochondria from normal rats. Kinetic analysis of the pyruvate transport shows that the Vmax of this process is decreased while there is practically no change in the Km values. Neither a decrease in the transmembrane delta pH value nor a decrease in the total number of the pyruvate carrier molecules, titrated with labeled alpha-cyanocinnamate, account for the decreased rate of pyruvate transport. The lower activity of the pyruvate translocator in mitochondria from hypothyroid rats is associated with a parallel decrease of the rate of pyruvate supported oxygen uptake. There is, however, no difference in either the respiratory control ratios or in the ADP/O ratios between these two types of mitochondria. The heart mitochondrial lipid composition is significantly altered in hypothyroid rats. Cardiolipin, particularly, was found to decrease by around 36%. In addition the pattern of fatty acids was found to be altered in mitochondrial membranes from hypothyroid rats. It is suggested that the decreased activity of the pyruvate translocator in heart mitochondria from hypothyroid rats can be ascribed to changes in the lipid environment which surrounds the pyruvate carrier molecule in the mitochondrial membrane.

Altered relationship between protonmotive force and respiration rate in non-phosphorylating liver mitochondria isolated from rats of different thyroid hormone status

We have determined the relationship between rate of respiration and protonmotive force in oligomycin-inhibited liver mitochondria isolated from euthyroid, hypothyroid and hyperthyroid rats. Respiration rate was titrated with the respiratory-chain inhibitor malonate. At any given respiration rate mitochondria isolated from hypothyroid rats had a protonmotive force greater than mitochondria isolated from euthyroid controls, and mitochondria isolated from hyperthyroid rats had a protonmotive force less than mitochondria isolated from euthyroid controls. In the absence of malonate mitochondrial respiration rate increased in the order hypothyroid less than euthyroid less than hyperthyroid, while protonmotive force increased in the order hyperthyroid less than euthyroid less than hypothyroid. These findings are consistent with a thyroid-hormone-induced increase in the proton conductance of the inner mitochondrial membrane or a decrease in the H+/O ratio of the respiratory chain at any given protonmotive force. Thus the altered proton conductance or H+/O ratio of mitochondria isolated from rats of different thyroid hormone status controls the respiration rate required to balance the backflow of protons across the inner mitochondrial membrane. We discuss the possible relevance of these findings to the control of state 3 and state 4 respiration by thyroid hormone.

Control of respiration in non-phosphorylating mitochondria is shared between the proton leak and the respiratory chain

We measured the relationship between rate of respiration and membrane potential in isolated mitochondria titrated with malonate (to inhibit the electron transport chain) or with uncoupler (to increase the proton conductance of the inner membrane). We used the flux control summation and connectivity theorems of metabolic control theory to calculate the control over non-phosphorylating respiration exerted by the respiratory chain (and associated reactions) and by the leak of protons across the inner membrane. At 37 degrees C the flux control coefficient of the leak over respiration was 0.66; the flux control coefficient of the chain over respiration was 0.34. At 25 degrees C the values were 0.75 and 0.25 respectively. We argue that the basis for previous conclusions that all the control is exerted by the proton leak under similar conditions is invalid.

Effect of hyperthyroidism on the transport of pyruvate in rat-heart mitochondria

A comparative study of the transport of pyruvate in heart mitochondria from normal and triiodothyronine-treated rats has been carried out. It has been found that the rate of carrier-mediated (alpha-cyanocinnamate-sensitive) pyruvate uptake is significantly enhanced in mitochondria from triiodothyronine-treated rats as compared with mitochondria from control rats. The kinetic parameters of the pyruvate uptake indicate that only the Vmax of this process is enhanced whilst there is no change in the Km value. The enhanced rate of pyruvate uptake is not dependent on the increase of the transmembrane delta pH value (both mitochondria from normal and triiodothyronine-treated rats exhibit the same delta pH value) neither does it depend on the increase of the pyruvate carrier molecules (titration of these last with alpha-cyanocinnamate gives the same total number of binding sites). the pyruvate-dependent oxygen uptake is stimulated by 35-40% in mitochondria from hyperthyroid rats when compared with mitochondria from control rats. There is, however, no difference in either the respiratory control ratios or in the ADP/O ratios between these two types of mitochondria. The heart mitochondrial phospholipid composition is altered significantly in hyperthyroid rats; in particular, negatively charged phospholipid such as cardiolipin and phosphatidylserine were found to increase by more than 50%. Minor alterations were found in the pattern of fatty acids with an increase of 20:4/18:2 ratio. It is suggested that the changes in the kinetic parameters of pyruvate transport in mitochondria from hyperthyroid rats involve hormone-mediated changes in the lipid composition of the mitochondrial membranes which in turn modulate the activity of the pyruvate carrier.

Hypothyroidism in rats does not lower mitochondrial ADP/O and H+/O ratios

We investigated reports that mitochondria isolated from hypothyroid rats have decreased ADP/O and H+/O ratios. We observed no decrease in the H+/O ratio in mitochondria from hypothyroid rats, in the presence of either 2% (w/v) fatty-acid-free bovine serum albumin or 100 nM free Ca2+. The ADP/O ratio in mitochondria isolated from hypothyroid rats in the presence of 2% fatty-acid-free bovine serum albumin was measured. Under normal experimental conditions we found no decrease in the ADP/O ratio, relative to that measured for littermate controls. At the low concentrations of mitochondrial protein used in the previously reported studies, the ADP/O ratio of mitochondria from hypothyroid rats was decreased, whereas that for control rats was only slightly decreased. The difference between the ADP/O ratios measured for mitochondria form hypothyroid rats and from control rats under these conditions was eliminated by inhibition of endogenous adenylate kinase. We suggest that the lowering of the apparent ADP/O ratio in mitochondria from hypothyroid rats at low concentrations of mitochondrial protein is an experimental artefact resulting from the breakdown of ADP to AMP.

Thyroid hormone uptake into the cell and its subsequent localisation to the mitochondria

The nature of thyroid hormone uptake into the cell and the possible involvement of the serum carrier proteins and receptor-mediated endocytosis in this process are reviewed. The evidence that there is a specific thyroid hormone-binding receptor in the inner mitochondrial membrane and the relation of this to the adenine nucleotide translocator is discussed. Direct effects of thyroid hormone on mitochondrial function that might be mediated by such a receptor are also considered.

Evidence for ADP-ribosylation in the mechanism of rapid thyroid hormone control of mitochondria

Triiodothyronine in vitro at concentrations between 10(-13) and 10(-11) M very rapidly activates oxidative phosphorylation in hypothyroid rat liver mitochondria. Comparing the concentrations of hormone with estimates of the amounts of respiratory chain components present suggests that this activation may involve an amplification mechanism. Here we present evidence that while no changes in phosphorylation were detected following hormone administration, nicotinamide, an inhibitor of mono ADP-ribosylation reported to occur rapidly and reversibly in mitochondria, prevented activation by hormone. Moreover incubation with nicotinamide of euthyroid mitochondria and derived intact inner membrane vesicles revealed lowered ADP/O ratios under the same conditions as shown by hypothyroid preparations. While this lesion could be reversed simply by washing the intact mitochondria, the membrane vesicles required triiodothyronine addition.

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.

Thyroxine-induced changes in rat liver mitochondrial ubiquinone

Ubiquinone was extracted from liver mitochondria isolated from euthyroid and hyperthyroid rats. The redox state of ubiquinone was determined during States III and IV respiration with succinate or glutamate-malate substrates. Ubiquinone was more reduced during State III or IV in the hyperthyroid mitochondria with either substrate. Furthermore, the concentration of ubiquinone increased in the hyperthyroid rats.

Alpha-adrenergic stimulation of glutamine metabolism in isolated rat hepatocytes

The mechanisms by means of which phenylephrine stimulates glutamine metabolism were studied in isolated rat hepatocytes. In the first 2 min after phenylephrine addition there was a rapid fall in the concentrations of intracellular 2-oxoglutarate and glutamate, presumably owing to activation of 2-oxoglutarate dehydrogenase. This was followed 2-3 min later by activation of glutaminase and by increases in glutamate and 2-oxoglutarate. Activation of glutaminase by phenylephrine was due to direct stimulation of the enzyme rather than to reversal of inhibition by the decrease in 2-oxoglutarate and glutamate. The stimulation of glutaminase by phenylephrine is partly due to an increase in the affinity of the enzyme for ammonia, its essential activator. It is concluded that stimulation of steady-state flux through the pathway from glutamine to glucose and urea can only be achieved by stimulation of glutaminase, the first enzyme in the pathway.