Effects of thyroid hormone on mitochondrial oxidative phosphorylation

Relationship between Prognosis with Dynamic Changes of Thyroid Hormone and Cortisol Hormone in Patients with Severe Craniocerebral Injury

Objective

To analyze the dynamic changes of thyroid hormone and cortisol hormone (COR) and their relationship with prognosis in patients with severe craniocerebral injury.

Methods

A retrospective analysis of 48 patients with severe craniocerebral injury who were admitted to our hospital from January 2014 to January 2017 was performed. According to the Glasgow Outcome Scale (GOS) after 3 months of treatment, the patients were divided into a favorable prognosis group (GOS score = 4-5) and a poor prognosis group (GOS score = 1–3). Clinical data such as ICU hospitalization time and mechanical ventilation time between the two groups were collected and compared. The GCS score was evaluated and recorded at 24 h and 7 d after injury, respectively. The fasting venous blood was collected from patients at 24 h and 7 d after injury, and the levels of thyrotropin (TSH), total triiodothyronine (TT3), total thyroxine (TT4), free triiodothyronine (FT3), and free thyroxine (FT4) were detected by the time-resolved fluorescence immunoassay, while the cortisol (COR) levels were examined by the chemiluminescence assays. The prognostic risk factors of patients with severe craniocerebral injury were analyzed using logistic regression analysis. A nomogram prediction model was constructed based on the results of the logistic analysis. The value of each factor in predicting the prognosis of patients with severe craniocerebral injury was analyzed using the ROC curve.

Results

Significant differences existed between the poor prognosis group and the favorable prognosis group in age, whether complicated with a cerebral hernia, intracranial hematoma volume, admission time, ICU hospitalization time, GCS score, and mechanical ventilation time (P < 0.05). At 24 h after injury, the levels of TT4, FT3, and FT4 in the poor prognosis group were significantly lower than those in the favorable prognosis group (P < 0.05). On the 7^th day after the injury, the levels of FT3, FT4, TT3, TT4, and TSH in the poor prognosis group were prominently lower than those in the favorable prognosis group (P < 0.05). At 24 h after injury, the COR level in the poor prognosis group was observably higher than that in the favorable prognosis group (P < 0.05). Logistic regression analysis showed that age, complicated with a cerebral hernia, length of stay in ICU, FT3, FT4, TT4, and COR were the risk factors affecting the prognosis of patients with severe craniocerebral injury (P < 0.05), while the GCS score was the protective factor (P < 0.05). ROC curve analysis revealed that the area under the curve (AUC) of ICU length of stay, GCS score, FT3, and FT4 to predict the prognosis of patients with severe craniocerebral injury was better with 0.841, 0.885, 0.881, and 0.850, respectively. The survival curve drawn by the K-M method showed that high levels of serum FT3, FT4, and TT4 and low levels of COR were conducive to improve the overall survival time of patients (P < 0.05).

Conclusion

Abnormal levels of thyroid hormone and cortisol hormone were found in patients with severe craniocerebral injury. Age, combined brain herniation, ICU length of stay, FT3, FT4, TT4, COR, and GCS scores were all prognostic factors in patients with severe traumatic brain injury. These factors have high value in judging the death and survival of patients with severe craniocerebral injury.

Thyroid Hormone Action and Energy Expenditure

Energy metabolism is one of the most recognized targets of thyroid hormone action, which indeed plays a critical role in modulating energy expenditure in all of its components. This is because thyroid hormone receptors are ubiquitous, and thyroid hormones interact and influence most metabolic pathways in virtually all systems throughout the entire life of the organism. The pleiotropic actions of thyroid hormone are the results of interaction between the local availability of T3 and the signal transduction machinery, which confer in physiologic conditions time and tissue specificity of the hormonal signal despite negligible variations in circulating levels. Historically, the measurement of energy expenditure has been used as the gold standard for the clinical assessment of the hormonal action until the advent of the immunoassays for TSH and thyroid hormone, which have since been used as proxy for measurement of thyroid hormone action. Although the clinical correlates between thyroid hormone action and energy expenditure in cases of extreme dysfunction (florid hyperthyroidism or hypothyroidism) are well recognized, there is still controversy on the effects of moderate, subclinical thyroid dysfunction on energy expenditure and, ultimately, on body weight trajectory. Moreover, little information is available on the effects of thyroid hormone replacement therapy on energy expenditure. This mini review is aimed to define the clinical relevance of thyroid hormone action in normal physiology and functional disorders, as well the effects of thyroid hormone therapy on energy expenditure and the effects of changes in energy status on the thyroid hormone axis.

Thyroid Hormone Mediated Modulation of Energy Expenditure

Thyroid hormone (TH) has diverse effects on mitochondria and energy expenditure (EE), generating great interest and research effort into understanding and harnessing these actions for the amelioration and treatment of metabolic disorders, such as obesity and diabetes. Direct effects on ATP utilization are a result of TH's actions on metabolic cycles and increased cell membrane ion permeability. However, the majority of TH induced EE is thought to be a result of indirect effects, which, in turn, increase capacity for EE. This review discusses the direct actions of TH on EE, and places special emphasis on the indirect actions of TH, which include mitochondrial biogenesis and reduced metabolic efficiency through mitochondrial uncoupling mechanisms. TH analogs and the metabolic actions of T2 are also discussed in the context of targeted modulation of EE. Finally, clinical correlates of TH actions on metabolism are briefly presented.

NAD and ADP-ribose metabolism in mitochondria

Mitochondrial metabolism is intimately connected to the universal coenzyme NAD. In addition to its role in redox reactions of energy transduction, NAD serves as substrate in regulatory reactions that lead to its degradation. Importantly, all types of the known NAD-consuming signalling reactions have been reported to take place in mitochondria. These reactions include the generation of second messengers, as well as post-translational protein modifications such as ADP-ribosylation and protein deacetylation. Therefore, the availability and redox state of NAD emerged as important factors in the regulation of mitochondrial metabolism. Molecular mechanisms and targets of mitochondrial NAD-dependent protein deacetylation and mono-ADP-ribosylation have been established, whereas poly-ADP-ribosylation and NAD-derived messenger generation in the organelles await in-depth characterization. In this review, we highlight the major NAD-dependent reactions occurring within mitochondria and describe their metabolic and regulatory functions. We also discuss the metabolic fates of the NAD-degradation products, nicotinamide and ADP-ribose, and how the mitochondrial NAD pool is restored.

Maternal low-protein diet affects epigenetic regulation of hepatic mitochondrial DNA transcription in a sex-specific manner in newborn piglets associated with GR binding to its promoter

Mitochondrial oxidative phosphorylation (OXPHOS) plays an important role in energy homeostasis by controlling electron transfer and ATP generation. Maternal malnutrition during pregnancy affects mitochondrial (mt) DNA-encoded OXPHOS activity in offspring, yet it is unknown whether epigenetic mechanism is involved in the transcriptional regulation of mtDNA-encoded OXPHOS genes. In this study, 14 primiparous purebred Meishan sows were fed either standard- (SP, 12% crude protein) or low-protein (LP; 6% crude protein) diets throughout gestation, and the hepatic expression and transcriptional regulation of mtDNA-encoded OXPHOS genes were analyzed in newborn piglets. Maternal low protein diet decreased hepatic mtDNA copy number in males, but not in females. LP male piglets had significantly higher hepatic AMP concentration and low energy charge, which was accompanied by enhanced mRNA expression of NADH dehydrogenase subunits 6, cytochrome c oxidase subunit 1, 2, 3 and cytochrome b, as well as increased cytochrome c oxidase enzyme activity. In contrast, LP female piglets showed significantly lower hepatic AMP concentrations and higher energy charge with no alterations in OXPHOS gene expression. Moreover, LP males demonstrated higher glucocorticoid receptor (GR) binding to the mtDNA promoter compared with SP males, which was accompanied by lower cytosine methylation and hydroxymethylation on mtDNA promoter. Interestingly, opposite changes were seen in females, which showed diminished GR binding and enriched cytosine methylation and hydroxymethylation on mtDNA promoter. These results suggest that maternal low protein diet during pregnancy causes sex-dependent epigenetic alterations in mtDNA-encoded OXPHOS gene expression, possibly GR is involved in mtDNA transcription regulation.

A novel role for AMP-kinase in the regulation of the Na+/I--symporter and iodide uptake in the rat thyroid gland

The aim of this study was to investigate the role of AMP-kinase (AMPK) in the regulation of iodide uptake by the thyroid gland. Iodide uptake was assessed in PCCL3 follicular thyroid cells exposed to the AMPK agonist 5-aminoimidazole-4-carboxamide-ribonucleoside (AICAR), and also in rat thyroid glands 24 h after a single intraperitoneal injection of AICAR. In PCCL3 cells, AICAR-induced AMPK and acetyl-CoA carboxylase (ACC) phosphorylation decreased iodide uptake in a concentration-dependent manner, while the AMPK inhibitor compound C prevented this effect. In the thyroid gland of rats injected with AICAR, AMPK and ACC phosphorylation was increased and iodide uptake was reduced by ~35%. Under conditions of increased AMPK phosphorylation/activation such as TSH deprivation or AICAR treatment, significant reductions in cellular Na(+)/I(-)-symporter (NIS) protein (~41%) and mRNA content (~65%) were observed. The transcriptional (actinomycin D) and translational (cycloheximide) inhibitors, as well as the AMPK inhibitor compound C prevented AICAR-induced reduction of NIS protein content in PCCL3 cells. The presence of TSH in the culture medium reduced AMPK phosphorylation in PCCL3 cells, while inhibition of protein kinase A (PKA) with H89 prevented this effect. Conversely, the adenylyl cyclase activator forskolin abolished the AMPK phosphorylation response induced by TSH withdrawal in PCCL3 cells. These findings demonstrate that TSH suppresses AMPK phosphorylation/activation in a cAMP-PKA-dependent manner. In summary, we provide novel evidence that AMPK is involved in the physiological regulation of iodide uptake, which is an essential step for the formation of thyroid hormones as well as for the regulation of thyroid function.

Reduction of hepatic steatosis in rats and mice after treatment with a liver-targeted thyroid hormone receptor agonist

Non-alcoholic fatty liver disease (NAFLD) is one of the most common forms of chronic liver disease, with a prevalence ranging from 10% to 30%. The use of thyroid hormone receptor (TR) agonists for the treatment of NAFLD has not been considered viable because thyroid hormones increase free fatty acid (FFA) flux from the periphery to the liver, induce hepatic lipogenesis, and therefore could potentially cause steatosis. MB07811 is an orally active HepDirect prodrug of MB07344, a liver-targeted TR-beta agonist. The purpose of these studies was to assess the effects of MB07811 on whole body and liver lipid metabolism of normal rodents and rodent models of hepatic steatosis. In the current studies, MB07811 markedly reduced hepatic steatosis as well as reduced plasma FFA and triglycerides. In contrast to MB07811, T(3) induced adipocyte lipolysis in vitro and in vivo and had a diminished ability to decrease hepatic steatosis. This suggests the influx of FFA from the periphery to the liver may partially counteract the antisteatotic activity of T(3). Clearance of liver lipids by MB07811 results from accelerated hepatic fatty acid oxidation, a known consequence of hepatic TR activation, as reflected by increased hepatic mitochondrial respiration rates, changes in hepatic gene expression, and increased plasma acyl-carnitine levels. Transaminase levels remained unchanged, or were reduced, and no evidence for liver fibrosis or other histological liver damage was observed after treatment with MB07811 for up to 10 weeks. Additionally, MB07811, unlike T(3), did not increase heart weight or decrease pituitary thyroid-stimulating hormone beta (TSHbeta) expression.

CONCLUSION

MB07811 represents a novel class of liver-targeted TR agonists with beneficial low-density lipoprotein cholesterol-lowering properties that may provide additional therapeutic benefit to hyperlipidemic patients with concomitant NAFLD.

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.

Linking mitochondrial function to diabetes mellitus: an animal's tale

Diabetes mellitus is one of the most common genetic diseases that afflicts humans. It is not a single disease but a collection of diseases having in common an abnormal glucose-insulin relationship and a dysfunctional regulation of glucose homeostasis. Of interest is the diabetic state that results when the mitochondrial genome mutates. Epidemiological studies have shown this to occur in humans. Detailed metabolic studies that are impossible to conduct in humans have been carried out in the BHE/Cdb rat. This rat has a mutated mitochondrial ATPase 6 gene. Strategies to ameliorate the consequences of this mutation have been explored and some of the mechanisms for the transcription and translation of the mitochondrial gene product have been elucidated.

Hypothyroidism down-regulates mitochondrial citrate carrier activity and expression in rat liver

The effect of hypothyroidism on citrate carrier (CiC) activity has been investigated in rat-liver mitochondria. The rate of citrate transport was reduced by approximately 50% in mitochondria from hypothyroid as compared with euthyroid rats. In parallel, a decrease in the rate of de novo fatty acid synthesis was observed in the cytosol of the former animals. Kinetic analysis of citrate transport revealed that only the Vmax was reduced by hypothyroidism, while Km was almost unaffected. Hypothyroidism increased the mitochondrial percentage of phosphatidylcholine while decreased that of phosphatidylethanolamine; an altered fatty acid pattern but no significant difference in the sum of saturated and unsaturated fatty acids as well as in the unsaturation index was observed. The CiC Arrhenius plot did not show appreciable difference between the two groups of rats. However, Western blot analysis associated with mRNA quantitation indicated that both protein level and mRNA accumulation of hepatic CiC were noticeably decreased in hypothyroid state. Therefore, a reduced content of the carrier protein can represent a plausible mechanism to explain the decline in the CiC activity observed in rat liver mitochondria of hypothyroid rats.

Differing mechanisms of hepatic glucose overproduction in triiodothyronine-treated rats vs. Zucker diabetic fatty rats by NMR analysis of plasma glucose

The metabolic mechanism of hepatic glucose overproduction was investigated in 3,3'-5-triiodo-l-thyronine (T3)-treated rats and Zucker diabetic fatty (ZDF) rats (fa/fa) after a 24-h fast. 2H2O and [U-13C3]propionate were administered intraperitoneally, and [3,4-13C2]glucose was administered as a primed infusion for 90 min under ketamine-xylazine anesthesia. 13C NMR analysis of monoacetone glucose derived from plasma glucose indicated that hepatic glucose production was twofold higher in both T3-treated rats and ZDF rats compared with controls, yet the sources of glucose overproduction differed significantly in the two models by 2H NMR analysis. In T3-treated rats, the hepatic glycogen content and hence the contribution of glycogenolysis to glucose production was essentially zero; in this case, excess glucose production was due to a dramatic increase in gluconeogenesis from TCA cycle intermediates. 13C NMR analysis also revealed increased phosphoenolpyruvate carboxykinase flux (4x), increased pyruvate cycling flux (4x), and increased TCA flux (5x) in T3-treated animals. ZDF rats had substantial glycogen stores after a 24-h fast, and consequently nearly 50% of plasma glucose originated from glycogenolysis; other fluxes related to the TCA cycle were not different from controls. The differing mechanisms of excess glucose production in these models were easily distinguished by integrated 2H and 13C NMR analysis of plasma glucose.

Thyroid hormone influences antioxidant defense system in adult rat brain

The objective of the current study was to find out whether thyroid hormone influences antioxidant defense parameters of rat brain. Several oxidative stress and antioxidant defense parameters of mitochondrial (MF) and post-mitochondrial (PMF) fractions of cerebral cortex (CC) of adult rats were compared among euthyroid (control), hypothyroid [6-n-propylthiouracil (PTU)-challenged], and hyperthyroid (T3-treatment to PTU-challenged rats) states. Oxidative stress parameters, such as thiobarbituric acid-reactive substances (TBA-RS) and protein carbonyl content (PC), in MF declined following PTU challenge in comparison to euthyroid rats. On the other hand, when PTU-challenged rats were treated with T3, a significant increase in the level of oxidative stress parameters in MF was recorded. Hydrogen peroxide content of MF as well as PMF of CC was elevated by PTU-challenge and brought to normal level by subsequent treatment of T3. Although mitochondrial glutathione (reduced or oxidized) status did not change following PTU challenge, a significant reduction in oxidized glutathione (GSSG) level was noticed in PMF following the treatment. T3 administration to PTU-challenged rats had no effect on mitochondrial glutathione status. Total and CN-resistant superoxide dismutase (SOD) activities in MF of CC augmented following PTU challenge. CN-resistant SOD activity did not change when PTU-challenged rats were treated with T3. Although CN-sensitive SOD activity of PMF remained unaltered in response to PTU challenge, its activity increased when PTU-challenged rats were treated with T3. Catalase activity in PMF of CC of PTU-challenged rats increased, whereas the activity was decreased when hypothyroid rats were treated with T3. Similarly, total and Se-dependent glutathione peroxidase (GPx) activities of MF increased following PTU challenge and reduced following administration of T3. Se-independent GPx activity of MF and PMF and glutathione reductase activity of PMF decreased following PTU challenge and did not change further when rats were treated with T3. On the other hand, glutathione S-transferase activity of MF and PMF of CC did not change following PTU challenge but decreased below detectable level following T3 treatment. Results of the current investigation suggest that antioxidant defense parameters of adult rat brain are considerably influenced by thyroid states of the body.

Experimentally induced hypo- and hyper-thyroidism influence on the antioxidant defence system in adult rat testis

The objective of the present experiment was to study the effect of thyroid hormone on the antioxidant defence system of rat testis. Hypothyroidism induced in rats by 6-n propyl 2-thiouracil (PTU) treatment resulted in a reduction in body weight, seminal vesicle and ventral prostate gland. A further decrease in the weight of seminal vesicle was recorded following administration of T3 to hypothyroid rats. The oxidative stress parameters such as hydrogen peroxide and protein carbonyl content increased in the crude homogenate of testis of hypothyroid rats. T3 administration to hypothyroid rats resulted in no further change in the hydrogen peroxide level but the protein carbonyl content further elevated in the crude homogenate of testis. No significant change was observed in the endogenous lipid peroxidation level of the crude homogenate of testis whereas the FeSO4/ascorbic acid induced lipid peroxidation level decreased in hypothyroid rats and did not change further by T3 administration. Although the reduced glutathione level in the crude homogenate of testis did not change following hypothyroidism, oxidized glutathione level increased. The reduced and oxidized glutathione level decreased and increased, respectively following T3 administration to hypothyroid rats in comparison with PTU-treated rats. Activities of superoxide dismutase and catalase decreased in the post-mitochondrial fraction (PMF) of testis of hypothyroid rats. T3 injection to PTU-treated rats resulted in an elevation in the level of catalase activity only. The activity of glutathione peroxidase in the PMF of testis elevated in the hypothyroid rats and reduced following T3 treatment to hypothyroid rats. The results of the present study suggest that any alteration in the thyroid hormone level in the body affects the antioxidant defence system of testis of adult rats and, thereby, may affect the physiology of testis through oxidative stress.

Thyroid status is a key regulator of both flux and efficiency of oxidative phosphorylation in rat hepatocytes

Thyroid status is crucial in energy homeostasis, but despite extensive studies the actual mechanism by which it regulates mitochondrial respiration and ATP synthesis is still unclear. We studied oxidative phosphorylation in both intact liver cells and isolated mitochondria from in vivo models of severe not life threatening hyper- and hypothyroidism. Thyroid status correlated with cellular and mitochondrial oxygen consumption rates as well as with maximal mitochondrial ATP production. Addition of a protonophoric uncoupler, 2,4-dinitrophenol, to hepatocytes did not mimic the cellular energetic change linked to hyperthyroidism. Mitochondrial content of cytochrome oxidase, ATP synthase, phosphate and adenine nucleotide carriers were increased in hyperthyroidism and decreased in hypothyroidism as compared to controls. As a result of these complex changes, the maximal rate of ATP synthesis increased in hyperthyroidism despite a decrease in ATP/O ratio, while in hypothyroidism ATP/O ratio increased but did not compensate for the flux limitation of oxidative phosphorylation. We conclude that energy homeostasis depends on a compromise between rate and efficiency, which is mainly regulated by thyroid hormones.

Mitochondrial respiratory chain adjustment to cellular energy demand

Because adaptation to physiological changes in cellular energy demand is a crucial imperative for life, mitochondrial oxidative phosphorylation is tightly controlled by ATP consumption. Nevertheless, the mechanisms permitting such large variations in ATP synthesis capacity, as well as the consequence on the overall efficiency of oxidative phosphorylation, are not known. By investigating several physiological models in vivo in rats (hyper- and hypothyroidism, polyunsaturated fatty acid deficiency, and chronic ethanol intoxication) we found that the increase in hepatocyte respiration (from 9.8 to 22.7 nmol of O(2)/min/mg dry cells) was tightly correlated with total mitochondrial cytochrome content, expressed both per mg dry cells or per mg mitochondrial protein. Moreover, this increase in total cytochrome content was accompanied by an increase in the respective proportion of cytochrome oxidase; while total cytochrome content increased 2-fold (from 0.341 +/- 0.021 to 0.821 +/- 0.024 nmol/mg protein), cytochrome oxidase increased 10-fold (from 0.020 +/- 0.002 to 0.224 +/- 0.006 nmol/mg protein). This modification was associated with a decrease in the overall efficiency of the respiratory chain. Since cytochrome oxidase is well recognized for slippage between redox reactions and proton pumping, we suggest that this dramatic increase in cytochrome oxidase is responsible for the decrease in the overall efficiency of respiratory chain and, in turn, of ATP synthesis yield, linked to the adaptive increase in oxidative phosphorylation capacity.

Modulation of rat liver mitochondrial antioxidant defence system by thyroid hormone

In the present study the effect of thyroid hormone (T(3)) on oxidative stress parameters of mitochondria of rat liver is reported. Hypothyroidism is induced in male adult rats by giving 0.05% propylthiouracil (PTU) in drinking water for 30 days and in order to know the effect of thyroid hormone, PTU-treated rats were injected with 20 microg T(3)/100 g body weight/day for 3 days. The results of the present study indicate that administration of T(3) to hypothyroid (PTU-treated) rats resulted in significant augmentation of oxidative stress parameters such as thiobarbituric acid reactive substances and protein carbonyl content of mitochondria in comparison to its control and euthyroid rats. The hydrogen peroxide content of the mitochondria of liver increased in hypothyroid rats and was brought to a normal level by T(3) treatment. Induction of hypothyroidism by PTU treatment to rats also resulted in the augmentation of total and CN-sensitive superoxide dismutase (SOD) activities of the mitochondria, which was reduced when hypothyroid rats were challenged with T(3). Although CN-resistant SOD activity of the mitochondria remained unaltered in response to hypothyroidism induced by PTU treatment, its activity decreased when hypothyroid rats were injected with T(3). The catalase activity of the mitochondria decreased significantly by PTU treatment and was restored to normal when PTU-treated rats were given T(3). Total, Se-independent and Se-dependent glutathione peroxidase activities of the mitochondria were increased following PTU treatment and reduced when T(3) was administered to PTU-treated rats. The reduced and oxidised glutathione contents of the mitochondria of liver increased significantly in hypothyroid rats and their level was restored to normal when hypothyroid rats were injected with T(3). The results of the present study suggest that the mitochondrial antioxidant defence system is considerably influenced by the thyroid states of the body.

Correlation between decreased expression of mitochondrial F0F1-ATP synthase and low regenerating capability of the liver after partial hepatectomy in hypothyroid rats

In hypothyroid rats, partial hepatectomy does not induce liver regeneration until 120 h after surgical operation. when, instead, in normal rats a complete recovery of the liver mass, in this interval, is observed. In normal rats, a good efficiency of mitochondrial oxidative phosphorylation is needed as an energy source for liver regeneration (Guerrieri, F. et al., 1995); in hypothyroid rats the efficiency of mitochondrial oxidative phosphorylation is low in the 0-120 h interval after partial hepatectomy. This low efficiency of oxidative phosphorylation appears to be related to a low mitochondrial content of F0F1-ATP synthase, in liver of hypothyroid rats, which does not recover after partial hepatectomy. In the liver of hypothyroid rats, low levels of the nuclear-encoded mitochondrial catalytic betaF1 subunit and of its transcript are observed and they do not increase, as occurs in normal rats, after partial hepatectomy.

A convenient diagnostic function test of peripheral blood neutrophils in glycogen storage disease type Ib

Neutrophils from patients suffering from glycogen storage disease type Ib (GSD-Ib) show several defects. one of which is a decreased rate of glucose utilization. In this study, we established experimental conditions to show the stimulation of the neutrophil respiratory burst by extracellular glucose. With phorbol-myristate-acetate as stimulus of the burst, the activity of the NADPH oxidase in GSD-Ib neutrophils hardly increased on addition of glucose. In control and GSD-type Ia neutrophils, a clear increase was observed. The lack of response to extracellular glucose in GSD-Ib neutrophils is correlated with the inability to raise intracellular glucose-6-P levels on glucose addition, thereby limiting the activity of the generation of NADPH in the hexose-monophosphate shunt. Our study shows the usefulness of this test for the diagnosis of neutrophil function abnormality in GSD-Ib patients.

Effect of primary congenital hypothyroidism upon expression of genes mediating murine brain glucose uptake

Using hyt/hyt mice that exhibit naturally occurring primary hypothyroidism (n = 72) and Balb/c controls (n = 66), we examined the mRNA, protein, and activity of brain glucose transporters (Glut 1 and Glut 3) and hexokinase I enzyme at various postnatal ages (d 1, 7, 14, 21, 35, and 60). The hyt/hyt mice showed an age-dependent decline in body weight (p < 0.04) and an increase in serum TSH levels (p < 0.001) at all ages. An age-dependent translational/posttranslational 40% decline in Glut 1 (p = 0.02) with no change in Glut 3 levels was observed. These changes were predominant during the immediate neonatal period (d 1). A posttranslational 70% increase in hexokinase enzyme activity was noted at d 1 alone (p < 0.05) with no concomitant change in brain 2-deoxy-glucose uptake. This was despite a decline in the hyt/hyt glucose production rate. We conclude that primary hypothyroidism causes a decline in brain Glut 1 associated with no change in Glut 3 levels and a compensatory increase in hexokinase enzyme activity. These changes are pronounced only during the immediate neonatal period and disappear in the postweaned stages of development. These hypothyroid-induced compensatory changes in gene products mediating glucose transport and phosphorylation ensure an adequate supply of glucose to the developing brain during transition from fetal to neonatal life.

Hypothyroidism leads to a decreased expression of mitochondrial F0F1-ATP synthase in rat liver

In liver mitochondria isolated from hypothyroid rats, the rate of ATP synthesis is lower than in mitochondria from normal rats. Oligomycin-sensitive ATP hydrolase activity and passive proton permeability were significantly lower in submitochondrial particles from hypothyroid rats compared to those isolated from normal rats. In mitochondria from hypothyroid rats, the changes in catalytic activities of F0F1-ATP synthase are accompanied by a decrease in the amount of immunodetected beta-F1, F0 1-PVP, and OSCP subunits of the complex. Northern blot hybridization shows a decrease in the relative cytosolic content of mRNA for beta-F1 subunit in liver of hypothyroid rats. Administration of 3,5,3'-triodo-L-thyronine to the hypothyroid rats tends to remedy the functional and structural defects of F0F1-ATP synthase observed in the hypothyroid rats. The results obtained indicate that hypothyroidism leads to a decreased expression of F0F1-ATP synthase complex in liver mitochondria and this contributes to the decrease of the efficiency of oxidative phosphorylation.

3,5,3'-triiodothyronine induces mitochondrial permeability transition mediated by reactive oxygen species and membrane protein thiol oxidation

Ca(2+)-loaded rat liver mitochondria treated with 3,5,3'-triiodothyronine (T3) undergo nonspecific inner membrane permeabilization, as evidenced by mitochondrial swelling, a decrease in membrane potential (delta psi), and an increase in the rate of oxygen uptake. T3 analogues thyroxine (T4), 3',5'-diiodothyronine (T2), and 3,5',3'-triiodothyronine (reverse T3), in decreasing order of potency, resulted in a similar but less extensive effect. Permeabilization induced by T3 is dependent on Ca2+ (1 microM) and T3 (0.5-25 microM) concentrations and is inhibited by cyclosporin A, a known inhibitor of mitochondrial permeability transition. Catalase or dithiothreitol also prevents membrane permeabilization, suggesting the participation of membrane protein thiol group oxidation induced by reactive oxygen species. The determination of the mitochondrial membrane protein thiol group content after treatment with Ca2+ and T3 shows a significant decrease, due to thiol oxidation. When mitochondria are incubated in the presence of inorganic phosphate and the protonophore carbonyl cyanide p-trifluoromethoxyphenylhydrazone, mitochondrial swelling still occurs after treatment with T3 and high Ca2+ concentrations, suggesting that mitochondrial permeabilization is not dependent on T3-induced delta psi or matrix pH alterations. Under these experimental conditions, when no oxygen is present in the incubation medium, no permeabilization occurs, suggesting that the permeabilization is dependent on mitochondrial-generated reactive oxygen species. Confirming this hypothesis, superoxide generation in a suspension of submitochondrial particles is increased when T3 is present. Our results lead to the conclusion that T3 induces a situation of oxidative stress in isolated liver mitochondria, with Ca(2+)-mediated membrane protein thiol oxidation and nonspecific inner membrane permeabilization.

Thyroid hormone-induced expression of the ADP/ATP carrier and its effect on fatty acid-induced uncoupling of oxidative phosphorylation

Liver mitochondria from rats made hypothyroid by administration of 2-mercapto-1-methylimidazole were less sensitive to the uncoupling effect of myristic acid, as measured by the increase of resting state respiration, than mitochondria from euthyroid animals, whereas subsequent administration to the animals of triiodothyronine ('hyperthyroidism') resulted in an increased uncoupling action of myristate. 'Hyperthyroidism' also resulted in doubling of the carboxyatractyloside-sensitive portion of the myristate-stimulated respiration. Parallel to this was a twofold increase of the mitochondrial content of the ADP/ATP carrier protein and an over threefold increase of its activity. The uncoupling effect of phytanic acid was less sensitive to carboxyatractyloside and was increased in the hyperthyroid state to a smaller extent than in the case of myristate. These results provide further support to the thesis [Skulachev, V.P., FEBS Lett. 294 (1991) 158-162] that the ADP/ATP carrier is involved in the mechanism of the uncoupling effect of long-chain fatty acids.

Mobilization of Mg2+ from rat heart and liver mitochondria following the interaction of thyroid hormone with the adenine nucleotide translocase

The in vitro addition of thyroid hormone to isolated rat heart or liver mitochondria induces the extrusion of approximately 2-4 nmol Mg2+/mg protein from both mitochondria preparations. The mobilization of Mg2+ is not accompanied by extrusion of matrix ATP or K+, or by mitochondria swelling, thus excluding that the phenomenon occurs through the nonspecific opening of the mitochondrial permeability transition pore. Moreover, the Mg2+ extrusion is completely prevented by bongkrekic acid, a membrane-permeant inhibitor of the adenine nucleotide translocase (AdNT), and by cyclosporine, which has also been reported to inhibit AdNT in a bongkrekate-like manner, operating at the matrix site of the translocase. By contrast, atractyloside, another specific inhibitor of AdNT that operates at the cytosolic site of the AdNT, only partially affects the Mg2+ mobilization (< 30% inhibition). These findings and the binding of 125I-labeled thyroid hormone to both the dimeric and monomeric moiety of AdNT support the hypothesis that AdNT can operate as a specific receptor for thyroid hormone in the mitochondria, and suggest that thyroid hormone operates at the matrix site of the translocase. In addition, these observations may imply that some of the so called "nongenomic effects" exerted by thyroid hormone on mitochondrial metabolism could occur through changes in the matrix content of Mg2+.

Effect of perinatal hypothyroidism on expression of cytochrome c oxidase subunit I gene, which is cloned by differential plaque screening from the cerebellum of newborn rat

Early development of the central nervous system is influenced by several hormones including thyroid hormone. This study was designed to clone the gene whose expression is changed in association with perinatal hypothyroidism in the rat cerebellum. Rats were sacrificed at 15 day-old postnatal age (P15) and their cerebella were removed. Poly (A)+ RNA was extracted to construct a cDNA library using lambda gt 10 cloning vector. Differential plaque screening was then performed using 32P-labeled antisense cDNA synthesized from poly (A)+ RNA of the methimazole-treated (hypothyroid) P15 rat cerebellum (hypothyroid probe), and of the euthyroid P15 rat cerebellum (euthyroid probe). The clones, which hybridized strongly to the euthyroid probe and weakly or not at all to the hypothyroid probe, were isolated. Sequence analysis of these clones revealed that all isolated clones encode cytochrome c oxidase subunit I (COX I), which is located in the mitochondrial DNA. The decrease in COX I gene expression was not seen in the animals, which received methimazole treatment and daily replacement of thyroid hormone. In situ hybridization detection showed not only overall decrease in COX I gene expression but also change in distribution of hybridization signal in the cerebellar cortex of hypothyroid rat. Such change was not observed in the T4-replaced animals. Based on the evidence that thyroid hormone greatly influences brain development, the results of the present study indicate that the terminal enzyme of mitochondrial respiratory chain, COX I is one of the important target molecules regulated by thyroid hormone in the newborn rat cerebellum.

EPR study of annexin V-cardiolipin Ca-mediated interaction in phospholipid vesicles and isolated mitochondria

The properties of the binding of annexin V to variously composed phospholipid vesicles have been studied by applying a recently developed EPR method, using an annexin V spin label. By this approach, this protein is seen to bind to acidic phospholipid-containing vesicles, as reported, thus confirming the reliability of the method. In addition, binding of this annexin to cardiolipin-containing vesicles has been studied in more depth, and the protein has been shown to have a distinct affinity for this phospholipid. As a cardiolipin-rich natural membrane system, mitochondrial membranes and mitoplasts from rat liver were considered, and a strong binding of AV to these membranes was observed. Having compared this binding with that to phospholipid vesicles, cardiolipin-rich microdomains in the mitochondrial membranes are proposed as the putative mitochondrial binding sites for annexin V.

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.

Selective labelling and inactivation of creatine kinase isoenzymes by the thyroid hormone derivative N-bromoacetyl-3,3',5-tri-iodo-L-thyronine

Besides their well-known regulation of transcription by binding to nuclear receptors, thyroid hormones have been suggested to have direct effects on mitochondria. In a previous study, incubation of rat heart mitochondria with 125I-labelled N-bromoacetyl-3,3',5-tri-iodo-L-thyronine (BrAcT3), a thyroid hormone derivative with an alkylating side chain, resulted in the selective labelling of a protein doublet around M(r) 45,000 on SDS/polyacrylamide gels [Rasmussen, Köhrle, Rokos and Hesch (1989) FEBS Lett. 255, 385-390]. Now, this protein doublet has been identified as mitochondrial creatine kinase (Mi-CK). Immunoblotting experiments with the cytoplasmic and mitochondrial fractions of rat heart, brain and liver, as well as inactivation studies with the purified chicken CK isoenzymes have further demonstrated that all four CK isoenzymes (Mia-, Mib-, M- and B-CK) are indeed selectively labelled by BrAcT3. However, in contrast with their bromoalkyl derivatives, thyroid hormones themselves did not compete for CK labelling, suggesting that not the thyroid hormone moiety but rather the bromoacetyl-driven alkylation of the highly reactive 'essential' thiol group of CK accounts for this selective labelling. Therefore the assumption that CK isoenzymes are thyroid-hormone-binding proteins has to be dismissed. Instead, bromoacetyl-based reagents may allow a very specific covalent modification and inactivation of CK isoenzymes in vitro and in vivo.

Respiratory activity of isolated liver Mitochondria following Trypanosoma congolense infection in rabbits: the role of thyroxine

1. The effect of trypanosome infection on rabbit liver mitochondrial oxidative phosphorylation was investigated, with and without thyroxine replacement. 2. State 3 respiration, respiratory control ratio (RCR) and ADP/O ratio were significantly reduced in mitochondria from trypanosome-infected animals whereas there was no change in state 4 respiration. 3. State 3 respiration, RCR and ADP/O ratio were not significantly altered in trypanosome-infected animals given thyroxine replacement therapy. 4. Trypanosome infection leads to impairment of mitochondrial integrity, apparently through lowered thyroxine levels. Replacement of thyroxine therefore sustains optimal mitochondrial respiratory activity.

Control of respiration and ATP synthesis in mammalian mitochondria and cells

We have seen that there is no simple answer to the question 'what controls respiration?' The answer varies with (a) the size of the system examined (mitochondria, cell or organ), (b) the conditions (rate of ATP use, level of hormonal stimulation), and (c) the particular organ examined. Of the various theories of control of respiration outlined in the introduction the ideas of Chance & Williams (1955, 1956) give the basic mechanism of how respiration is regulated. Increased ATP usage can cause increased respiration and ATP synthesis by mass action in all the main tissues. Superimposed on this basic mechanism is calcium control of matrix dehydrogenases (at least in heart and liver), and possibly also of the respiratory chain (at least in liver) and ATP synthase (at least in heart). In many tissues calcium also stimulates ATP usage directly; thus calcium may stimulate energy metabolism at (at least) four possible sites, the importance of each regulation varying with tissue. Regulation of multiple sites may occur (from a teleological point of view) because: (a) energy metabolism is branched and thus proportionate regulation of branches is required in order to maintain constant fluxes to branches (e.g. to proton leak or different ATP uses); and/or (b) control over fluxes is shared by a number of reactions, so that large increases in flux requires stimulation at multiple sites because each site has relatively little control. Control may be distributed throughout energy metabolism, possibly due to the necessity of minimizing cell protein levels (see Brown, 1991). The idea that energy metabolism is regulated by energy charge (as proposed by Atkinson, 1968, 1977) is misleading in mammals. Neither mitochondrial ATP synthesis nor cellular ATP usage is a unique function of energy charge as AMP is not a significant regulator (see for example Erecinska et al., 1977). The near-equilibrium hypothesis of Klingenberg (1961) and Erecinska & Wilson (1982) is partially correct in that oxidative phosphorylation is often close to equilibrium (apart from cytochrome oxidase) and as a consequence respiration and ATP synthesis are mainly regulated by (a) the phosphorylation potential, and (b) the NADH/NAD+ ratio. However, oxidative phosphorylation is not always close to equilibrium, at least in isolated mitochondria, and relative proximity to equilibrium does not prevent the respiratory chain, the proton leak, the ATP synthase and ANC having significant control over the fluxes. Thus in some conditions respiration rate correlates better with [ADP] than with phosphorylation potential, and may be relatively insensitive to mitochondrial NADH/NAD+ ratio.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Cardiolipins and biomembrane function

Evidence is discussed for roles of cardiolipins in oxidative phosphorylation mechanisms that regulate State 4 respiration by returning ejected protons across and over bacterial and mitochondrial membrane phospholipids, and that regulate State 3 respiration through the relative contributions of proteins that transport protons, electrons and/or metabolites. The barrier properties of phospholipid bilayers support and regulate the slow proton leak that is the basis for State 4 respiration. Proton permeability is in the range 10(-3)-10(-4) cm s-1 in mitochondria and in protein-free membranes formed from extracted mitochondrial phospholipids or from stable synthetic phosphatidylcholines or phosphatidylethanolamines. The roles of cardiolipins in proton conductance in model phospholipid membrane systems need to be assessed in view of new findings by Hübner et al. [313]: saturated cardiolipins form bilayers whilst natural highly unsaturated cardiolipins form nonlamellar phases. Mitochondrial cardiolipins apparently participate in bilayers formed by phosphatidylcholines and phosphatidylethanolamines. It is not yet clear if cardiolipins themselves conduct protons back across the membrane according to their degree of fatty acyl saturation, and/or modulate proton conductance by phosphatidylcholines and phosphatidylethanolamines. Mitochondrial cardiolipins, especially those with high 18:2 acyl contents, strongly bind many carrier and enzyme proteins that are involved in oxidative phosphorylation, some of which contribute to regulation of State 3 respiration. The role of cardiolipins in biomembrane protein function has been examined by measuring retained phospholipids and phospholipid binding in purified proteins, and by reconstituting delipidated proteins. The reconstitution criterion for the significance of cardiolipin-protein interactions has been catalytical activity; proton-pumping and multiprotein interactions have yet to be correlated. Some proteins, e.g., cytochrome c oxidase are catalytically active when dimyristoylphosphatidylcholine replaces retained cardiolipins. Cardiolipin-protein interactions orient membrane proteins, matrix proteins, and on the outerface receptors, enzymes, and some leader peptides for import; activate enzymes or keep them inactive unless the inner membrane is disrupted; and modulate formation of nonbilayer HII-phases. The capacity of the proton-exchanging uncoupling protein to accelerate thermogenic respiration in brown adipose tissue mitochondria of cold-adapted animals is not apparently affected by the increased cardiolipin unsaturation; this protein seems to take over the protonophoric role of cardiolipins in other mitochondria. Many in vivo influences that affect proton leakage and carrier rates selectively alter cardiolipins in amount per mitochondrial phospholipids, in fatty acyl composition and perhaps in sidedness; other mitochondrial membrane phospholipids respond less or not at all.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

The influence of hypothyroidism on the transport of phosphate and on the lipid composition in rat-liver mitochondria

The influence of hypothyroidism on the transport of phosphate and on the lipid composition in rat-liver mitochondria was examined. It was found that the rate of phosphate transport is reduced (around 40%) in mitochondria from hypothyroid rats compared to that obtained in mitochondria from normal rats. Treatment of hypothyroid rats with thyroid hormone reverses this effect completely. Kinetic analysis of the phosphate transport indicates that only the Vmax of this process is affected, while there is no change in the Km values. The lower rate of phosphate transport in mitochondria from hypothyroid rats is also demonstrated by swelling experiments. There is no significant difference either in the respiratory control ratios or in the ADP/O ratios between these two types of mitochondria. The hepatic mitochondrial lipid composition is altered significantly in hypothyroid rats. The total cholesterol increases, the phospholipids decrease and the cholesterol/phospholipid molar ratio increases (around 40%). Among the phospholipids, cardiolipin shows the greatest alteration (30% decrease in the hypothyroid rats). The phosphatidylethanolamine/phosphatidylcholine ratio also decreases. Alterations were also found in the pattern of fatty acids. These changes in lipid composition may be responsible, at least in part, for the depression of the phosphate carrier activity in mitochondria from hypothyroid rats.

Differential action of thyroid hormones on the activity of certain enzymes in rat kidney and brain

In rat kidney several mitochondrial and soluble enzyme activities are stimulated by thyroid hormones and the mitochondrial membrane fluidity is also increased. However, the ketone metabolism enzyme activities of D-3-hydroxybutyrate dehydrogenase and of 3-oxoacid CoA-transferase are not significantly affected by the hyperthyroid state and the ketone body concentration is not greatly changed. Therefore, in hyperthyroid rats the response of the kidney, as far as the ketone bodies and their metabolizing enzymes are concerned, is at variance with that of the liver and the heart. In the brain of young rats, age 8-9 weeks, the activities of the enzymes of ketone body metabolism and those responsible for other metabolic pathways are not influenced by the hyperthyroid state. In these animals, however, the activities of two enzymes, NAD-isocitrate dehydrogenase and pyruvate kinase, are still stimulated by 28 and 41%, respectively. This can be probably related to the higher energy requirement for definitive brain maturation in young hyperthyroid rats.

Effects of thyroid hormones on the bypasses of the antimycin A block in the bc1 complex of rat liver mitochondria

The effect of thyroid hormones on the electron flow through the bc1 complex of rat liver mitochondria was studied using two dye bypasses of the Antimycin A block of the bc1 complex by the method of Alexandre and Lehninger (Biochim. Biophys. Acta 767:120; 1984). Bypass respiration rates with both DCIP (2,6-dichlorophenolindophenol) and TMPD (N,N,N',N'-tetramethyl-p-phenylenediamine dihydrochloride) were elevated in the hyperthyroid rats and depressed in the hypothyroid groups compared to the euthyroid controls. T3 treatment of hypothyroid rats returned the bypass rates to control levels in 24 hours with the TMPD dye but not for the DCIP. This further demonstrates that different portions of the bc1 complex respond individually to the thyroid state.

Thyroid hormone effects on the State IV proton motive force in rat liver mitochondria

In order to further investigate the mechanisms regulating the control of mitochondrial respiration by thyroid hormone, the proton motive force was measured during State IV respiration in liver mitochondria isolated from euthyroid, hyperthyroid, hypothyroid and T3-treated hypothyroid rats. The proton motive force was significantly higher in the hyperthyroid group due to an increased delta pH. The proton motive force of hypothyroid mitochondria was lower than controls due to a decreased membrane potential. The proton motive force for the T3-treated hypothyroid group did not differ from the euthyroid group due to negating changes in the pH gradient and the membrane potential. The intramitochondrial volume was decreased in the hyperthyroid group and unchanged in the other groups. The results indicate that the thyroid status alters the proton motive force in State IV through individual changes in the pH and membrane potential components of the force. The component that changes in hyperthyroid mitochondria is different from that changing in hypothyroid mitochondria.

The effect of thyroid state and cold exposure on rat liver oxidative phosphorylation

In this paper we have examined the effect of cold exposure on hepatic mitochondrial state 3 respiration and ATP synthesis, using succinate as the substrate, in euthyroid, hypothyroid and hyperthyroid rats. The results show that cold exposure does not elicit any variation in the above parameters in euthyroid and hyperthyroid rats, whereas when hypothyroid rats are exposed to cold, a significant increase (about +45%) occurs in state 3 respiration and ATP synthesis. We have also measured succinic dehydrogenase specific activity and uncoupled respiration during cold exposure in various thyroid states. The finding that cold exposure elicits no variation in the above parameters indicates that there is some control on ATP synthase and/or adenine nucleotide translocator. The above findings, as a whole, suggest that cold exposure acts on oxidative phosphorylation only if triiodothyronine is lacking, by controlling ATP synthase and/or adenine nucleotide translocator.

Enhanced activity of the tricarboxylate carrier and modification of lipids in hepatic mitochondria from hyperthyroid rats

The effect of hyperthyroidism on the activity of the mitochondrial tricarboxylate carrier has been studied. The activity of this transporting system in liver mitochondria was quantitatively determined by the rate of malate-[14C]citrate exchange using the 1,2,3-benzene-tricarboxylate inhibitor stop technique. It has been found that the rate of citrate uptake is significantly enhanced in liver mitochondria from hyperthyroid rats as compared to that obtained in mitochondria from control rats. Kinetic analysis of the malate-citrate exchange reaction indicates that only the Vmax of this transporting process is enhanced, while there is practically no change in the Km values. Inhibitor titrations with the inhibitor palmitoyl-CoA show that mitochondria from hyperthyroid rats require the same concentrations of inhibitor to produce 100% inhibition of citrate uptake as control mitochondria, suggesting that the amount of functional translocase enzyme present is unaffected. The Arrhenius plot characteristics differ for tricarboxylate carrier activity in mitochondria from hyperthyroid rats as compared with control rats in that the break point of the biphasic plot decreases from 18.1 +/- 1.4 degrees C in controls to 12.9 +/- 1.2 degrees C in hyperthyroid animals. The hepatic mitochondrial lipid composition is altered significantly in hyperthyroid rats; the total cholesterol decreases and the phospholipids increase. The liver mitochondrial phospholipid composition is altered significantly in hyperthyroid rats. In particular negatively charged phospholipid cardiolipin increases by more than 50%. Minor alterations were found in the pattern of fatty acids. The thyroid hormone induced change in the activity of the tricarboxylate carrier can be ascribed either to a general modification of membrane lipid composition which increases the membrane fluidity and in turn the mobility of the carrier or to a more localized change of lipid domain (cardiolipin content) surrounding the carrier molecule in the mitochondrial membrane.

Effect of aging on the oxidative phosphorylation pathway

The proposed study was undertaken to investigate the effect of aging on control of the oxidative phosphorylation pathway. Flux control coefficients for adenine nucleotide translocase and cytochrome c oxidase were determined using the procedure of Groen et al. [J. Biol. Chem., 257 (1982) 137-144]. Hepatic mitochondrial fractions from Fischer 344 rats were isolated from control (average age 6.5 months), and aged (average age 27.3 months) groups. No aging-related changes in the extent of control of respiration by the oxidase were obtained, however, differences were observed for the translocase. For the control group of animals, the greatest regulation occurred at 80-85% maximal respiratory rates, and declined at higher rates. For the aged group, a similar flux control coefficient was obtained at 80-85% respiration, but was maintained as respiration increased to maximal rates. It is proposed that changes in the flux control coefficients at maximal respiratory rates are associated with an aging-related decrease in translocase activity. Evaluation of translocase content revealed no significant differences between the two groups supporting the concept that the decreased activity was not due to decreased content. During the course of these experiments, it also became apparent that there was a significant aging-related decrease in the rate of succinate oxidation providing an adequate supply of ADP was present. No significant changes in respiratory rates, or RCR, were evident at suboptimal concentrations of ADP as reported previously from this laboratory [Vorbeck, M.L. et al., Arch. Biochem. Biophys., 214 (1982) 67-79]. Since similar decreases in respiration were obtained upon addition of an uncoupler, the aging-related changes in respiration are attributed to differences at the level of the electron transport system, including its associated reactions. The aging-related differences in respiratory rates, and extent of control of respiration, were both observed under conditions of maximal stimulation of respiration. This suggests an inability of mitochondria from aged animals to respond to the increased demands of oxidation. Basic to these differences may be the lipid-membrane associated changes seen during aging.

Concerning the decreased D-3-hydroxybutyrate dehydrogenase activity in the liver and heart of hyperthyroid rats

Whereas in rat liver mitochondria the hyperthyroid state causes an increase both in fatty acid unsaturation and in the Ea of D-3-hydroxybutyrate dehydrogenase and a decrease in phase transition temperature, in hyperthyroid rat heart mitochondria these changes are negligible. D-3-hydroxybutyrate dehydrogenase in both the liver and the heart mitochondria of hyperthyroid rats is reduced by about 35% [12] but this reduction is not due to changes in membrane fluidity in either tissue. Hypothyroidism, on the other hand, affects BDH activity in neither heart nor liver.

Thyroid-hormone control of state-3 respiration in isolated rat liver mitochondria

Oxidative phosphorylation can be treated as two groups of reactions; those that generate protonmotive force (dicarboxylate carrier, succinate dehydrogenase and the respiratory chain) and those that consume protonmotive force (adenine nucleotide and phosphate carriers. ATP synthase and proton leak). Mitochondria from hypothyroid rats have lower rates of respiration in the presence of ADP (state 3) than euthyroid controls. We show that the kinetics of the protonmotive-force generators are unchanged in mitochondria from hypothyroid animals, but the kinetics of the protonmotive-force consumers are altered, supporting proposals that the important effects of thyroid hormone on state 3 are on the ATP synthase or the adenine nucleotide translocator.