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

Bioenergetic Aspects of Mitochondrial Actions of Thyroid Hormones

Much is known, but there is also much more to discover, about the actions that thyroid hormones (TH) exert on metabolism. Indeed, despite the fact that thyroid hormones are recognized as one of the most important regulators of metabolic rate, much remains to be clarified on which mechanisms control/regulate these actions. Given their actions on energy metabolism and that mitochondria are the main cellular site where metabolic transformations take place, these organelles have been the subject of extensive investigations. In relatively recent times, new knowledge concerning both thyroid hormones (such as the mechanisms of action, the existence of metabolically active TH derivatives) and the mechanisms of energy transduction such as (among others) dynamics, respiratory chain organization in supercomplexes and cristes organization, have opened new pathways of investigation in the field of the control of energy metabolism and of the mechanisms of action of TH at cellular level. In this review, we highlight the knowledge and approaches about the complex relationship between TH, including some of their derivatives, and the mitochondrial respiratory chain.

Translational Pharmacology and Physiology of Brown Adipose Tissue in Human Disease and Treatment

Human brown adipose tissue (BAT) is experimentally modeled to better understand the biology of this important metabolic tissue, and also to enable the potential discovery and development of novel therapeutics for obesity and sequelae resulting from the persistent positive energy balance. This chapter focuses on translation into humans of findings and hypotheses generated in nonhuman models of BAT pharmacology. Given the demonstrated challenges of sustainably reducing caloric intake in modern humans, potential solutions to obesity likely lie in increasing energy expenditure. The energy-transforming activities of a single cell in any given tissue can be conceptualized as a flow of chemical energy from energy-rich substrate molecules into energy-expending, endergonic biological work processes through oxidative degradation of organic molecules ingested as nutrients. Despite the relatively tight coupling between metabolic reactions and products, some expended energy is incidentally lost as heat, and in this manner a significant fraction of the energy originally captured from the environment nonproductively transforms into heat rather than into biological work. In human and other mammalian cells, some processes are even completely uncoupled, and therefore purely energy consuming. These molecular and cellular actions sum up at the physiological level to adaptive thermogenesis, the endogenous physiology in which energy is nonproductively released as heat through uncoupling of mitochondria in brown fat and potentially skeletal muscle. Adaptive thermogenesis in mammals occurs in three forms, mostly in skeletal muscle and brown fat: shivering thermogenesis in skeletal muscle, non-shivering thermogenesis in brown fat, and diet-induced thermogenesis in brown fat. At the cellular level, the greatest energy transformations in humans and other eukaryotes occur in the mitochondria, where creating energetic inefficiency by uncoupling the conversion of energy-rich substrate molecules into ATP usable by all three major forms of biological work occurs by two primary means. Basal uncoupling occurs as a passive, general, nonspecific leak down the proton concentration gradient across the membrane in all mitochondria in the human body, a gradient driving a key step in ATP synthesis. Inducible uncoupling, which is the active conduction of protons across gradients through processes catalyzed by proteins, occurs only in select cell types including BAT. Experiments in rodents revealed UCP1 as the primary mammalian molecule accounting for the regulated, inducible uncoupling of BAT, and responsive to both cold and pharmacological stimulation. Cold stimulation of BAT has convincingly translated into humans, and older clinical observations with nonselective 2,4-DNP validate that human BAT's participation in pharmacologically mediated, though nonselective, mitochondrial membrane decoupling can provide increased energy expenditure and corresponding body weight loss. In recent times, however, neither beta-adrenergic antagonism nor unselective sympathomimetic agonism by ephedrine and sibutramine provide convincing evidence that more BAT-selective mechanisms can impact energy balance and subsequently body weight. Although BAT activity correlates with leanness, hypothesis-driven selective β3-adrenergic agonism to activate BAT in humans has only provided robust proof of pharmacologic activation of β-adrenergic receptor signaling, limited proof of the mechanism of increased adaptive thermogenesis, and no convincing evidence that body weight loss through negative energy balance upon BAT activation can be accomplished outside of rodents. None of the five demonstrably β3 selective molecules with sufficient clinical experience to merit review provided significant weight loss in clinical trials (BRL 26830A, TAK 677, L-796568, CL 316,243, and BRL 35135). Broader conclusions regarding the human BAT therapeutic hypothesis are limited by the absence of data from most studies demonstrating specific activation of BAT thermogenesis in most studies. Additionally, more limited data sets with older or less selective β3 agonists also did not provide strong evidence of body weight effects. Encouragingly, β3-adrenergic agonists, catechins, capsinoids, and nutritional extracts, even without robust negative energy balance outcomes, all demonstrated increased total energy expenditure that in some cases could be associated with concomitant activation of BAT, though the absence of body weight loss indicates that in no cases did the magnitude of negative energy balance reach sufficient levels. Glucocorticoid receptor agonists, PPARg agonists, and thyroid hormone receptor agonists all possess defined molecular and cellular pharmacology that preclinical models predicted to be efficacious for negative energy balance and body weight loss, yet their effects on human BAT thermogenesis upon translation were inconsistent with predictions and disappointing. A few new mechanisms are nearing the stage of clinical trials and may yet provide a more quantitatively robust translation from preclinical to human experience with BAT. In conclusion, translation into humans has been demonstrated with BAT molecular pharmacology and cell biology, as well as with physiological response to cold. However, despite pharmacologically mediated, statistically significant elevation in total energy expenditure, translation into biologically meaningful negative energy balance was not achieved, as indicated by the absence of measurable loss of body weight over the duration of a clinical study.

Mitochondrial Actions of Thyroid Hormone

The hypermetabolic effects of thyroid hormones (THs), the major endocrine regulators of metabolic rate, are widely recognized. Although, the cellular mechanisms underlying these effects have been extensively investigated, much has yet to be learned about how TH regulates diverse cellular functions. THs have a profound impact on mitochondria, the organelles responsible for the majority of cellular energy production, and several studies have been devoted to understand the respective importance of the nuclear and mitochondrial pathways for organelle activity. During the last decades, several new aspects of both THs (i.e., metabolism, transport, mechanisms of action, and the existence of metabolically active TH derivatives) and mitochondria (i.e., dynamics, respiratory chain organization in supercomplexes, and the discovery of uncoupling proteins other than uncoupling protein 1) have emerged, thus opening new perspectives to the investigation of the complex relationship between thyroid and the mitochondrial compartment. In this review, in the light of an historical background, we attempt to point out the present findings regarding thyroid physiology and the emerging recognition that mitochondrial dynamics as well as the arrangement of the electron transport chain in mitochondrial cristae contribute to the mitochondrial activity. We unravel the genomic and nongenomic mechanisms so far studied as well as the effects of THs on mitochondrial energetics and, principally, uncoupling of oxidative phosphorylation via various mechanisms involving uncoupling proteins. The emergence of new approaches to the question as to what extent and how the action of TH can affect mitochondria is highlighted. © 2016 American Physiological Society. Compr Physiol 6:1591-1607, 2016.

Effect of residual feed intake on hypothalamic gene expression and meat quality in Angus-sired cattle grown during the hot season

The relationship between heat stress, meat quality, and residual feed intake (RFI) is unknown in growing steers. To address this issue, high RFI (HRFI) and low RFI (LRFI) individuals were compared by assessing RFI in 48 Angus-sired steers during a 70-d feeding trial conducted during July through September to identify steers with calculated RFI at least 2 SD apart. The association of RFI with indices of meat quality and expression of genes within hypothalamic and adipose tissue was then determined in LRFI and HRFI steers. While on test, feed intake was recorded daily with BW and hip heights recorded every 14 d. Ultrasound measurements of rib eye area (REA) and backfat (BF) were recorded initially and before harvest. Carcass and growth data were analyzed using a mixed model with RFI level (LRFI and HRFI) as the independent variable. The least square means for RFI were -1.2 and 0.99 kg DMI/d, respectively, for the LRFI and HRFI cohorts (P < 0.0001). Dry matter intake was higher for the HRFI individuals versus the LRFI steers (P < 0.0001) while on-test gain was not different (P < 0.95). Marbling score was greater in LRFI than HRFI steers (P < 0.05). However, there were no differences in REA (P < 0.53), BF (P < 0.65), yield grade (P < 0.24), or objective Hunter color measures between LRFI and HRFI steers indicating there was no consistent relationship between RFI and indices of meat quality. Hypothalamic neuropeptide Y (NPY), agouti related protein (AGRP), relaxin-3 (RLN3), melanocortin 3 receptor, and relaxin/insulin-like family peptide receptor 1 (RXFP1) mRNA were expressed 280, 185, 202, 183, and 163% greater, respectively (P < 0.01), while proopiomelanocortin (POMC) mRNA was expressed 42% lower in LRFI than HRFI animals (P < 0.05). Hypothalamic GnRH mRNA expression was 67% lower while gonadotropin inhibiting hormone (GnIH) mRNA was 209% higher in LRFI than HRFI animals (P < 0.01). Pituitary expression of FSHβ and LHβ correlated to hypothalamic GnRH levels (P < 0.05) indicating changes in gene expression within the hypothalamus had functional consequences. Leptin mRNA expression levels were not different between adipose tissue of LRFI or HRFI steers (P < 0.84). These data indicate that animals with superior RFI evaluated during warm conditions have higher expression of orexigenic neuropeptide genes independent of the expression of adipose-derived leptin. Furthermore, the gonadotropin axis may also influence feed efficiency under these conditions.

American Thyroid Association Guide to investigating thyroid hormone economy and action in rodent and cell models

BACKGROUND

An in-depth understanding of the fundamental principles that regulate thyroid hormone homeostasis is critical for the development of new diagnostic and treatment approaches for patients with thyroid disease.

SUMMARY

Important clinical practices in use today for the treatment of patients with hypothyroidism, hyperthyroidism, or thyroid cancer are the result of laboratory discoveries made by scientists investigating the most basic aspects of thyroid structure and molecular biology. In this document, a panel of experts commissioned by the American Thyroid Association makes a series of recommendations related to the study of thyroid hormone economy and action. These recommendations are intended to promote standardization of study design, which should in turn increase the comparability and reproducibility of experimental findings.

CONCLUSIONS

It is expected that adherence to these recommendations by investigators in the field will facilitate progress towards a better understanding of the thyroid gland and thyroid hormone dependent processes.

3,5-Diiodo-L-thyronine administration to hypothyroid rats rapidly enhances fatty acid oxidation rate and bioenergetic parameters in liver cells

Growing evidence shows that, among triiodothyronine derivatives, 3,5 diiodo-L-thyronine (T(2)) plays an important role in energy metabolism and fat storage. In the present study, short-term effects of T(2) administration to hypothyroid rats on fatty acid oxidation rate and bioenergetic parameters were investigated. Within 1 h following T(2) injection, state 3 and state 4 respiration rates, which were reduced in hypothyroid mitochondria, were noticeably increased particularly in succinate- with respect to glutamate/malate-energized mitochondria. Maximal respiratory activity, observed when glutamate/malate/succinate were simultaneously present in the respiratory medium, was significantly stimulated by T(2) treatment. A T(2)-induced increase in respiratory rates was also observed when palmitoyl-CoA or L-palmitoylcarnitine were used as substrates. No significant change in respiratory control index and ADP/O ratio was observed. The activities of the mitochondrial respiratory chain complexes, especially Complex II, were increased in T(2)-treated rats. In the latter, Complex V activities, assayed in both ATP synthesis and hydrolysis direction, were enhanced. The rate of fatty acid oxidation, followed by conversion of [(14)C]palmitate to CO(2) and ketone bodies, was higher in hepatocytes isolated from T(2)-treated rats. This increase occurs in parallel with the raise in the activity of carnitine palmitoyltransferase-I, the rate limiting enzyme of fatty acid β-oxidation, assayed in situ in digitonin-permeabilized hepatocytes. Overall, these results indicate that T(2) rapidly increases the ability of mitochondria to import and oxidize fatty acids. An emerging idea in the literature is the ability of T(2) to reduce adiposity and dyslipidemia and to prevent the development in liver steatosis. The results of the present study, showing a rapid T(2)-induced increase in the ability of mitochondria to import and oxidize fatty acids, may contribute to understand the biochemical mechanisms of T(2)-metabolic effects.

Mitochondrial Bioenergetics, Diabetes, and Aging: Top-Down Analysis Using the Diabetic Goto-Kakizaki (GK) Rat as a Model

In the present study we investigated the changes in the oxidative phosphorylation system of liver mitochondria, isolated from diabetic Goto-Kakizaki (GK) and Wistar (control) rats with different ages (6, 12, 26, and 52 weeks). We used a kinetic approach known as "top-down" analysis, which conceptually divides the oxidative phosphorylation system into two subsystems: one producing the protonmotive force (Deltap) and another that consumes Deltap. The overall response of the Deltap generators to Deltap was obtained from an uncoupler titration of respiration rate versus Deltap, while the overall response of Deltap consumers to Deltap was obtained from an inhibitor titration of respiration rate versus Deltap. Our results showed that GK liver mitochondrial preparations presented an increase in Deltap production and phosphorylative subsystems (using succinate as respiratory substrate). The alterations observed may suggest the existence of biochemical compensatory mechanisms to type 2 diabetes mellitus in GK rats during their first year of life, in order to reduce the injury associated with the disease. Furthermore, we observed that liver metabolic efficiency of mitochondrial respiration declined with age, this decrease in respiratory activity being visible both in control and diabetic rats.

Differential Expression ESTs Associated with Fluorosis in Rats Liver

The fluoride has volcanic activity and abundantly exists in environment combining with other elements as fluoride compounds. Recent researches indicated that the molecular mechanisms of intracellular fluoride toxicity were very complex. However, the molecular mechanisms underlying the effects on gene expression of chronic fluoride-induced damage is unknown, especially the detailed regulatory process of mitochondria. In the present study, we screened the differential expression ESTs associated with fluorosis by DDRT-PCR in rat liver. We gained 8 genes, 3 new ESTs, and 1 unknown function sequence and firstly demonstrated that microsomal glutathione S-transferase 1 (MGST1), ATP synthase H⁺ transporting mitochondrial F₀ complex subunit C1, selenoprotein S, mitochondrial IF1 protein, and mitochondrial succinyl-CoA synthetase alpha subunit were participated in mitochondria metabolism, functional and structural damage process caused by chronic fluorosis. This information will be very helpful for understanding the molecular mechanisms of fluorosis.

Mitochondria as pharmacological targets: the discovery of novel anti-obesity mitochondrial uncouplers from Africa's medicinal plants

Obesity results from prolonged positive imbalance between energy in take and expenditure. When food intake chronically exceeds the body's energy need, an efficient metabolism results in the storage of the excess energy as fat. Mitochondria are the main centre for energy production in eukaryotic cells. Mitochondrial proton cycling is responsible for a significant proportion of basal or standard metabolic rate, therefore, further uncoupling of mitochondria may be a good way to increase energy expenditure and hence represent a good pharmacological target for the treatment of obesity. This implies that, any chemical agent or photochemical compound that further uncouples the mitochondria in vivo without having any effect on mitochondria activity could be a potential target in finding treatment for obesity. In the past, uncoupling by 2, 4-dinitrophenol has been used this way with notable success. This paper discusses the mitochondria as targets in the discovery of potential plant natural anti-obesity products from Africa's rich rainforests.

Phenomenological kinetic and control analysis of oxidative phosphorylation in isolated mitochondria

Metabolic control analysis provides a quantitative framework for analyzing regulatory properties of enzymes in various metabolic pathways. It has been used for estimation of control parameters of the enzymatic pathways at isolated enzyme, cellular, or whole organism levels. This chapter describes how control and elasticities analysis can be experimentally applied to measure control properties of the components of the oxidative phosphorylation system and how a variant of such analysis - phenomenological kinetic analysis - can be used to investigate the effects of various factors (physiological or pathological) on the system of oxidative phosphorylation in isolated mitochondria.

3,5-diiodo-L-thyronine increases FoF1-ATP synthase activity and cardiolipin level in liver mitochondria of hypothyroid rats

Short-term effects of 3,5-L-diiodothyronine (T(2)) administration to hypothyroid rats on F(o)F(1)-ATP synthase activity were investigated in liver mitochondria. One hour after T(2) injection, state 4 and state 3 respiration rates were noticeably stimulated in mitochondria subsequently isolated. F(o)F(1)-ATP synthase activity, which was reduced in mitochondria from hypothyroid rats as compared to mitochondria from euthyroid rats, was significantly increased by T(2) administration in both the ATP-synthesis and hydrolysis direction. No change in β-subunit mRNA accumulation and protein amount of the α-β subunit of F(o)F(1)-ATP synthase was found, ruling out a T(2) genomic effect. In T(2)-treated rats, changes in the composition of mitochondrial phospholipids were observed, cardiolipin (CL) showing the greatest alteration. In mitochondria isolated from hypothyroid rats the decrease in the amount of CL was accompanied by an increase in the level of peroxidised CL. T(2) administration to hypothyroid rats enhanced the level of CL and decreased the amount of peroxidised CL in subsequently isolated mitochondria, tending to restore the CL value to the euthyroid level. Minor T(2)-induced changes in mitochondrial fatty acid composition were detected. Overall, the enhanced F(o)F(1)-ATP synthase activity observed following injection of T(2) to hypothyroid rats may be ascribed, at least in part, to an increased level of mitochondrial CL associated with decreased peroxidation of CL.

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.

The emerging functions of UCP2 in health, disease, and therapeutics

The uncoupling proteins (UCPs) are attracting an increased interest as potential therapeutic targets in a number of important diseases. UCP2 is expressed in several tissues, but its physiological functions as well as potential therapeutic applications are still unclear. Unlike UCP1, UCP2 does not seem to be important to thermogenesis or weight control, but appears to have an important role in the regulation of production of reactive oxygen species, inhibition of inflammation, and inhibition of cell death. These are central features in, for example, neurodegenerative and cardiovascular disease, and experimental evidence suggests that an increased expression and activity of UCP2 in models of these diseases has a beneficial effect on disease progression, implicating a potential therapeutic role for UCP2. UCP2 has an important role in the pathogenesis of type 2 diabetes by inhibiting insulin secretion in islet beta cells. At the same time, type 2 diabetes is associated with increased risk of cardiovascular disease and atherosclerosis where an increased expression of UCP2 appears to be beneficial. This illustrates that therapeutic applications involving UCP2 likely will have to regulate expression and activity in a tissue-specific manner.

Diabetes and mitochondrial bioenergetics: alterations with age

Several studies have been carried out to evaluate the alterations in mitochondrial functions of diabetic rats. However, some of the results reported are controversial, since experimental conditions, such as aging, and/or strain of animals used were different. The purpose of this study was to evaluate the metabolic changes in liver mitochondria, both in the presence of severe hyperglycaemia (STZ-treated rats) and mild hyperglycaemia (Goto-Kakizaki (GK) rats). Moreover, metabolic alterations were evaluated both at initial and at advanced states of the disease. We observed that both models of type 1 and type 2 diabetes presented alterations on respiratory chain activity. Because of continual severe hyperglycaemia, 9 weeks after the induction of diabetes, the respiratory function declined in STZ-treated rats, as observed by membrane potential and respiratory ratios (RCR, P/O, and FCCP-stimulated respiration) assessment. In contrast, GK rats of 6 months age presented increased respiratory ratios. To localize which respiratory complexes are affected by diabetes, enzymatic respiratory chain activities were evaluated. We observed that succinate dehydrogenase and cytochrome c oxidase activities were significantly augmented both in STZ-treated rats and GK rats of 6 months age. Moreover, H(+)-ATPase activity was also significantly increased in STZ-treated rats with 3 weeks of diabetes and in GK rats of 6 months age as compared to controls. Therefore, these results clearly suggest that both animal models of diabetes present some metabolic adjustments in order to circumvent the deleterious effects promoted by the high glucose levels typical of the disease.

Superoxide activates uncoupling proteins by generating carbon-centered radicals and initiating lipid peroxidation: studies using a mitochondria-targeted spin trap derived from alpha-phenyl-N-tert-butylnitrone

Although the physiological role of uncoupling proteins (UCPs) 2 and 3 is uncertain, their activation by superoxide and by lipid peroxidation products suggest that UCPs are central to the mitochondrial response to reactive oxygen species. We examined whether superoxide and lipid peroxidation products such as 4-hydroxy-2-trans-nonenal act independently to activate UCPs, or if they share a common pathway, perhaps by superoxide exposure leading to the formation of lipid peroxidation products. This possibility can be tested by blocking the putative reactive oxygen species cascade with selective antioxidants and then reactivating UCPs with distal cascade components. We synthesized a mitochondria-targeted derivative of the spin trap alpha-phenyl-N-tert-butylnitrone, which reacts rapidly with carbon-centered radicals but is unreactive with superoxide and lipid peroxidation products. [4-[4-[[(1,1-Dimethylethyl)-oxidoimino]methyl]phenoxy]butyl]triphenylphosphonium bromide (MitoPBN) prevented the activation of UCPs by superoxide but did not block activation by hydroxynonenal. This was not due to MitoPBN reacting with superoxide or the hydroxyl radical or by acting as a chain-breaking antioxidant. MitoPBN did react with carbon-centered radicals and also prevented lipid peroxidation by the carbon-centered radical generator 2,2'-azobis(2-methyl propionamidine) dihydrochloride (AAPH). Furthermore, AAPH activated UCPs, and this was blocked by MitoPBN. These data suggest that superoxide and lipid peroxidation products share a common pathway for the activation of UCPs. Superoxide releases iron from iron-sulfur center proteins, which then generates carbon-centered radicals that initiate lipid peroxidation, yielding breakdown products that activate UCPs.

Thyroid hormone administration to hypothyroid rats restores the mitochondrial membrane permeability properties

We have investigated the effect of thyroid hormone on the mitochondrial membrane permeability properties in a hypothyroid rat model. The role played by calcium in affecting these properties has been also examined. Cyclosporin A-sensitive mitochondrial calcium efflux, swelling, and external release of matrix proteins are events that occur normally during the permeability transition process induced by calcium loading of mitochondria. We demonstrate that these events are impaired in mitochondria isolated from the liver of hypothyroid rats, even in the presence of high calcium content. However, after thyroid hormone administration to hypothyroid rats, the mitochondrial permeability transition process in response to calcium loading is restored. Consequently, mitochondrial calcium efflux, swelling, and release of matrix proteins, like glutamate dehydrogenase, malate dehydrogenase, and aspartate aminotransferase occur. These effects are abrogated by the concomitant administration of cyclosporin A. The results of the present study suggest that hypothyroidism may be a potential source of adverse effects in patients receiving cyclosporin A.

Specific targeting of a DNA-alkylating reagent to mitochondria. Synthesis and characterization of [4-((11aS)-7-methoxy-1,2,3,11a-tetrahydro-5H-pyrrolo[2,1-c][1,4]benzodiazepin-5-on-8-oxy)butyl]-triphenylphosphonium iodide

The selective manipulation of the expression and replication of mitochondrial DNA (mtDNA) within mammalian cells has proven difficult. In progressing towards this goal we synthesized a novel mitochondria-targeted DNA-alkylating reagent. The active alkylating moiety [(11aS)-8-hydroxy-7-methoxy-1,2,3,11a-tetrahydro-5H-pyrrolo[2,1-c][1,4]benzodiazepin-5-one (DC-81)], irreversibly alkylates guanine bases in DNA (with a preference for AGA triplets), preventing its expression and replication. To target this compound to mitochondria it was covalently coupled to the lipophilic triphenylphosphonium (TPP) cation to form a derivative referred to as mitoDC-81. Incorporation of this lipophilic cation led to the rapid uptake of mitoDC-81 by mitochondria, driven by the large membrane potential across the inner membrane. This compound efficiently alkylated isolated supercoiled, relaxed-circular or linear plasmid DNA and isolated mtDNA. However mitoDC-81 did not alkylate mtDNA within isolated mitochondria or cells, even though it accessed the mitochondrial matrix at concentrations up to 100-fold higher than those required to alkylate isolated DNA. This surprising finding suggests that mtDNA within intact mitochondria may not be accessible to this class of alkylating reagent. This inability to alkylate mtDNA in situ has significant implications for the design of therapies for mtDNA diseases and for studies on the packaging, expression and turnover of mtDNA in general.

Control of energy metabolism by iodothyronines

One of the most widely recognized effects of thyroid hormones (TH) in adult mammals is their influence over energy metabolism. In the past, this has received much attention but, possibly because of the complex mode of action of thyroid hormones, no universally accepted mechanism to explain this effect has been put forward so far. Significant advances in our understanding of the biochemical processes involved in the actions of TH have been made in the last three decades and now it seems clear that TH can act through both nuclear-mediated and extranuclear-mediated pathways. TH increase energy expenditure, partly by reducing metabolic efficiency, with control of specific genes at the transcriptional level, being is thought to be the major molecular mechanism. However, both the number and the identity of the thyroid-hormone-controlled genes remain unknown, as do their relative contributions. The recent discovery of uncoupling proteins (UCPs) (in addition to UCP1 in brown adipose tissue) in almost all tissues in animals, including humans, has opened new perspectives on the understanding of the mechanisms involved in the regulation of energy metabolism by thyroid hormones. Other approaches have included the various attempts made to attribute changes in respiratory activity to a direct influence of thyroid hormones over the mitochondrial energy-transduction apparatus. In addition, an increasing number of studies has revealed that TH active in the regulation of energy metabolism include not only T3, but also other iodothyronines present in the biological fluids, such as 3,5-diiodothyronine (3,5-T2). This, in turn, may make it possible to explain some of the effects exerted by TH on energy metabolism that cannot easily be attributed to T3.

Mechanisms of the deleterious effects of tamoxifen on mitochondrial respiration rate and phosphorylation efficiency

Tamoxifen (TAM), the widely prescribed drug in the prevention and therapy of breast cancer, is a well-known modulator of estrogen receptor (ER) that also inhibits the proliferation of different cell types that lack the ER. However, the ER-independent action mechanisms of TAM and its side effects have not been yet clarified. Mitochondria are essential in supporting the energy-dependent regulation of cell functions. Changes in mitochondria result in bioenergetic deficits leading to the loss of vital functions to cell survival. Therefore, this study describes the effects of TAM on mitochondrial bioenergetics, contributing to a better understanding of the biochemical mechanisms underlying the multiple antiproliferative and toxic effects of this drug. TAM at concentrations above 20 nmol/mg protein, preincubated with isolated rat liver mitochondria at 25 degrees C for 3 min, significantly depresses, in a dose-dependent manner, the phosphorylation efficiency of mitochondria as inferred from the decrease in the respiratory control and ADP/O ratios, the perturbations in mitochondrial transmembrane potential (DeltaPsi), the fluctuations associated with mitochondrial energization, and the phosphorylative cycle induced by ADP. Furthermore, TAM at up to 40 nmol/mg protein stimulates the rate of state 4 respiration and at higher concentrations it strongly inhibits state 3 and uncouples the mitochondrial respiration. The stimulation of state 4 respiration parallels the decrease of DeltaPsi as a consequence of proton permeability. The TAM-stimulatory action of ATPase is also observed in intact mitochondria, suggesting that TAM promotes extensive permeability to protons due to destructive effects in the structural integrity of the mitochondrial inner membrane. These multiple effects of TAM on mitochondrial bioenergetic functions, causing changes in the respiration, phosphorylation efficiency, and membrane structure, may explain the cell death induced by this drug in different cell types, its anticancer activity in ER-negative cells, and its side effects.

Mitochondrial uncoupling as a target for drug development for the treatment of obesity

Mitochondrial proton cycling is responsible for a significant proportion of basal or standard metabolic rate, so further uncoupling of mitochondria may be a good way to increase energy expenditure and represents a good pharmacological target for the treatment of obesity. Uncoupling by 2,4-dinitrophenol has been used in this way in the past with notable success, and some of the effects of thyroid hormone treatment to induce weight loss may also be due to uncoupling. Diet can alter the pattern of phospholipid fatty acyl groups in the mitochondrial membrane, and this may be a route to uncoupling in vivo. Energy expenditure can be increased by stimulating the activity of uncoupling protein 1 (UCP1) in brown adipocytes either directly or through beta 3-adrenoceptor agonists. UCP2 in a number of tissues, UCP3 in skeletal muscle and the adenine nucleotide translocase have also been proposed as possible drug targets. Specific uncoupling of muscle or brown adipocyte mitochondria remains an attractive target for the development of antiobesity drugs.

Physiological and molecular basis of thyroid hormone action

Thyroid hormones (THs) play critical roles in the differentiation, growth, metabolism, and physiological function of virtually all tissues. TH binds to receptors that are ligand-regulatable transcription factors belonging to the nuclear hormone receptor superfamily. Tremendous progress has been made recently in our understanding of the molecular mechanisms that underlie TH action. In this review, we present the major advances in our knowledge of the molecular mechanisms of TH action and their implications for TH action in specific tissues, resistance to thyroid hormone syndrome, and genetically engineered mouse models.

How understanding the control of energy metabolism can help investigation of mitochondrial dysfunction, regulation and pharmacology

Understanding the control of mitochondrial energy metabolism is central to knowing how mitochondria function within cells. Metabolic control analysis is the best approach available for studying the control of mitochondrial energy metabolism. Here I outline how metabolic control analysis has been used to help understand mitochondrial regulation, damage and pharmacology.

Cardiolipins and mitochondrial proton-selective leakage

The proton-selective leak (State 4 respiratory rate) but not delta psi, in mitochondria from thyroid-sensitive tissues, responds to in vivo stimuli in unique correlation with changes in cardiolipins, saturated and mono-unsaturated (extended) fatty acyl contents, cardiolipins/phospholipids ratios, and/or membrane outer-sidedness. Liver mitochondrial State 4 respiration, basal in fasted rats, contributes little to resting metabolic rate in fed rats, where State 3 depresses delta psi. In a proposed model, an essential inner-membrane outer-surface proton antenna collects protons and donates them, via a water-shuttle, to transmembrane porters: transient water-molecule-chains between extended phospholipid acyls; protonophores, and uncoupling proteins. Only cardiolipin microdomains can donate, from an anomalously-dissociating phosphate group in each headgroup; unadapted cardiolipins have few conducting water chains. Thyroid states regulate each cardiolipin property, and are permissive, via the proton antenna, for proton leaks, including those through adapted and possibly constitutive BAT and ectopic uncoupling proteins. Slow leakage in liposomes may reflect insufficient cardiolipin proton antennas.

Age-related increase in mitochondrial proton leak and decrease in ATP turnover reactions in mouse hepatocytes

Age-related changes in mitochondria, including decreased respiratory control ratios and altered mitochondrial inner membrane lipid composition, led us to study oxidative phosphorylation in hepatocytes from old (30 mo) and young (3 mo) male C57BL/J mice. Top-down metabolic control analysis and its extension, elasticity analysis, were used to identify changes in the control and regulation of the three blocks of reactions constituting the oxidative phosphorylation system: substrate oxidation, mitochondrial proton leak, and the ATP turnover reactions. Resting oxygen consumption of cells from old mice was 15% lower (P < 0.05) than in young cells. This is explained entirely by a decrease in oxygen consumption supporting ATP turnover reactions. At all values of mitochondrial membrane potential assessed, the proportion of total oxygen consumption used to balance the leak was greater in the old cells than in the young cells. Metabolic control coefficients indicate a shift in control over respiration and phosphorylation away from substrate oxidation toward increased control by leak and by ATP turnover reactions. Control of the actual number of ATP molecules synthesized by mitochondria for each oxygen atom consumed by the ATP turnover and leak reactions was greater in old than in young cells, showing that efficiency in older cells is more sensitive to changes in these two blocks of reactions than in young cells.

Top-down elasticity analysis and its application to energy metabolism in isolated mitochondria and intact cells

This paper reviews top-down elasticity analysis, which is a subset of metabolic control analysis. Top-down elasticity analysis provides a systematic yet simple experimental method to identify all the primary sites of action of an effector in complex systems and to distinguish them from all the secondary, indirect, sites of action. In the top-down approach, the complex system (for example, a mitochondrion, cell, organ or organism) is first conceptually divided into a small number of blocks of reactions interconnected by one or more metabolic intermediates. By changing the concentration of one intermediate when all others are held constant and measuring the fluxes through each block of reactions, the overall kinetic response of each block to each intermediate can be established. The concentrations of intermediates can be changed by adding new branches to the system or by manipulating the activities of blocks of reactions whose kinetics are not under investigation. To determine how much an effector alters the overall kinetics of a block of reactions, the overall kinetic response of the block to the intermediate is remeasured in the presence of the effector. Blocks that contain significant primary sites of action will display altered kinetics; blocks that change rate only because of secondary alterations in the concentrations of other metabolites will not. If desired, this elasticity analysis can be repeated with the primary target blocks subdivided into simpler blocks so that the primary sites of action can be defined with more and more precision until, with sufficient subdivision, they are mapped onto individual kinetic steps. Top-down elasticity analysis has been used to identify the targets of effectors of oxygen consumption in mitochondria, hepatocytes and thymocytes. Effectors include poisons such as cadmium and hormones such as triiodothyronine. However, the method is more general than this; in principle it can be applied to any metabolic or other steady-state system.

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.

Changes in the hepatic mitochondrial respiratory system in the transition from weaning to adulthood in rats

In the present study we investigated the changes in the hepatic mitochondrial respiratory system in the transition from weaning to adulthood in the rat. We conceptually divided the system into blocks of reactions that produced or consumed mitochondrial membrane potential and then measured the kinetic responses of these blocks of reactions to changes in this potential in isolated liver mitochondria from 25- and 60-day-old rats using succinate as substrate. Moreover, we considered the mitochondrial membrane potential producers to be divided into blocks of reactions that reduced or oxidized ubiquinone-2 (Q-2) and then measured the kinetic responses of these two blocks to changes in Q-2 redox state as well as the flux control coefficients and the cytochrome content. We found that adult rats exhibited significantly higher state 3 respiratory rates with increased kinetic response of the substrate oxidation pathway to the mitochondrial membrane potential, slightly decreased activity of the phosphorylating system, increased kinetic responses of both Q-2 reducers and oxidizers to Q-2 redox state, and increased cytochrome content. Our results indicate that important changes in the hepatic mitochondrial respiratory system occur in the transition from weaning to adulthood in rats.

Effect of 3,5-di-iodo-L-thyronine on the mitochondrial energy-transduction apparatus

We examined the effect of a single injection of 3,5-di-iodo-L-thyronine (3,5-T2) (150 microg/100 g body weight) on the rat liver mitochondrial energy-transduction apparatus. We applied 'top-down' elasticity analysis, which allows identification of the site of action of an effector within a metabolic pathway. This kinetic approach considers oxidative phosphorylation as two blocks of reactions: those generating the mitochondrial inner-membrane potential (DeltaPsi; 'substrate oxidation') and those 'consuming' it ('proton leak' and 'phosphorylating system'). The results show that 1 h after the injection of 3,5-T2, state 4 (respiratory state in which there is no ATP synthesis and the exogenous ADP added has been exhausted) and state 3 (respiratory state in which ATP synthesis is at maximal rate) of mitochondrial respiration were significantly increased (by approx. 30%). 'Top-down' elasticity analysis revealed that these increases were due to the stimulation of reactions involved in substrate oxidation; neither 'proton leak' nor the 'phosphorylating system' was influenced by 3,5-T2. Using the same approach we divided the respiratory chain into two blocks of reactions: cytochrome c reducers and cytochrome c oxidizers. We found that both cytochrome c reducers and cytochrome c oxidizers are targets for 3,5-T2. The rapidity with which 3,5-T2 acts in stimulating the mitochondrial respiration rate suggests to us that di-iodo-L-thyronine may play an important role in the physiological conditions in which rapid energy utilization is required, such as cold exposure or overfeeding.

Nutrient-gene interactions determine mitochondrial function: effect of dietary fat

The effect on mitochondrial respiration of feeding hydrogenated coconut oil, corn oil, or menhaden oil (MO) to diabetes-prone BHE/cdb rats and normal Sprague Dawley (SD) rats was studied. Both fat source and strain affected the temperature dependence of succinate-supported respiration. The transition temperature was greater in BHE/cdb rats than in the SD rats. The efficiency of ATP synthesis as reflected by the ADP:O ratio was decreased in the BHE/cdb rats compared to SD rats, with the exception of the comparison made at 37 degrees C with the MO-fed rats; at this temperature, the ADP:O ratios were similar. The diet and strain differences suggest a dietary lipid-gene interaction with respect to the mobility of subunit 6 of the F1F0ATPase. This subunit has two errors in its gene: one that affects the proton channel and another that likely affects its mobility within the inner mitochondrial membrane.

Minimal model of beta-cell mitochondrial Ca2+ handling

We develop a simplified, but useful, mathematical model to describe Ca2+ handling by mitochondria in the pancreatic beta-cell. The model includes the following six transport mechanisms in the inner mitochondrial membrane: proton pumping via respiration and proton uptake by way of the F1Fzero-ATPase (adapted from D. Pietrobon and S. Caplan. Biochemistry 24: 5764-5778, 1985), a proton leak, adenine nucleotide exchange, the Ca2+ uniporter, and Na+/Ca2+ exchange. Each mechanism is developed separately into a kinetic model for the rate of transport, with parameters taken from experiments on isolated mitochondrial preparations. These mechanisms are combined in a modular fashion first to describe state 4 (nonphosphorylating) and state 3 (phosphorylating) mitochondria with mitochondrial NADH and Ca2+ concentrations as fixed parameters and then to describe Ca2+ handling with variable mitochondrial Ca2+ concentration. Simulations are compared to experimental measurements and agree well with the threshold for Ca2+ uptake, measured mitochondrial Ca2+ levels, and the influence of Ca2+ on oxygen uptake. In the absence of Ca2+ activation of mitochondrial dehydrogenases, the simulations predict a significant reduction in the rate of production of ATP that involves a "short circuit" via Ca2+ uptake through the uniporter. This effect suggests a potential role for mitochondrial Ca2+ handling in determining the ATP-ADP ratio in the pancreatic beta-cell.

F1F0-ATPase, early target of the radical initiator 2,2'-azobis-(2-amidinopropane) dihydrochloride in rat liver mitochondria in vitro

This study was designed to determine which enzyme activities were first impaired in mitochondria exposed to 2,2'-azobis-(2-amidinopropane) dihydrochloride (AAPH), a known radical initiator. EPR spin-trapping revealed generation of reactive oxygen species although malondialdehyde formation remained very low. With increasing AAPH concentrations, State-3 respiration was progressively depressed with unaltered ADP/O ratios. A top-down approach demonstrated that alterations were located at the phosphorylation level. As shown by inhibitor titrations, ATP/ADP translocase activity was unaffected in the range of AAPH concentrations used. In contrast, AAPH appeared to exert a deleterious effect at the level of F1F0-ATPase, comparable with dicyclohexylcarbodi-imide, which alters Fo proton channel. A comparison of ATP hydrolase activity in uncoupled and broken mitochondria reinforced this finding. In spite of its pro-oxidant properties, AAPH was shown to act as a dose-dependent inhibitor of cyclosporin-sensitive permeability transition initiated by Ca2+, probably as a consequence of its effect on F1F0-ATPase. Resveratrol, a potent antiperoxidant, completely failed to prevent the decrease in State-3 respiration caused by AAPH. The data suggest that AAPH, when used under mild conditions, acted as a radical initiator and was capable of damaging F1F0-ATPase, thereby slowing respiratory chain activity and reducing mitochondrial antioxidant defences.

Nongenomic actions of thyroid hormone

Nongenomic actions of thyroid hormone are by definition independent of nuclear receptors for the hormone and have been described at the plasma membrane, various cell organelles, the cytoskeleton, and in cytoplasm. The actions include alterations in solute transport (Ca2+, Na+, glucose), changes in activities of several kinases, including protein kinase C, cAMP-dependent protein kinase and pyruvate kinase M2 (PKM2), effects on efficiency of specific mRNA translation and mRNA t1/2, modulation of mitochondrial respiration, and regulation of actin polymerization (promotion of formation of F-actin). Iodothyronines also can regulate nongenomically the state of contractile elements in vascular smooth muscle cells (VSMC). The physiologic significance at the cellular level of certain of these actions has been demonstrated, for example, in the cases of myocardiocyte Na+ current, red cell Ca2+ content, and the control by hormone-induced alterations in actin solubility of cell surface activity of iodothyronine 5'-monodeiodinase activity and the intracellular distribution of protein disulfide isomerase activity. The physiologic significance of these actions at the organ or system level is less clear, but extranuclear effects of thyroid hormone on myocardial Na+ channel, sarcoplasmic reticulum Ca(2+)-ATPase activity, and contractile state of VSMC may each contribute to acute effects of thyroid hormone on cardiac output that have recently been described clinically. The molecular mechanisms for nongenomic actions are incompletely understood; relevant binding sites and signal transduction pathways have been described for hormone actions on plasma membrane Ca(2+)-ATPase activity, and PKM2 monomer is known to bind T3 and, as a result, prevent activation of the kinase via tetramer formation. Nongenomic actions of thyroid hormone may have different structure-activity relationships of iodothyronines from those effects that depend upon nuclear receptors; they may have different time courses and may invoke complex signal transduction pathways before the action is detected.

Relationship between membrane potential and respiration rate in isolated liver mitochondria from rats fed an energy dense diet

We studied the relationship between membrane potential and respiration rate in isolated liver mitochondria from rats fed an energy dense diet. We conceptually divided the system into blocks of reactions that produced or consumed mitochondrial membrane potential and then measured the kinetic response of these blocks of reactions to this potential using NAD-linked and FAD-linked substrates. We show that decreased respiration rate with an NAD-linked substrate is accounted for by decreased kinetic response of the substrate oxidation pathway to the potential. No variation in the kinetic response of the above blocks of reactions to the potential was found using an FAD-linked substrate. These results indicate that FAD-linked and NAD-linked pathways are differently affected in rats fed an energy dense diet.

Changes in cerebral oxygen consumption are independent of changes in body oxygen consumption after severe head injury in childhood

This study examines the relation between cerebral O2 consumption (CMRO2) and the O2 consumption of the rest of the body (BVO2) after severe head injury. Seventy nine serial measurements of whole body O2 consumption, CMRO2, plasma adrenaline, T3, and glucagon concentrations were made in 15 children with severe head injuries receiving neurointensive care. Body O2 consumption was measured with indirect calorimetry and CMRO2 with the Kety-Schmidt technique. There was no evidence of a significant relation between CMRO2 and BVO2. Within each child there were statistically significant positive relations between BVO2 and adrenaline, T3, and glucagon. By contrast, there was only a weak significant positive relation between CMRO2 and T3. In conclusion, CMRO2 and BVO2 seem to be determined independently after severe head injury. Thus therapeutic measures aiming to reduce CMRO2 need to be specific to the brain and it should not be assumed that measures which decrease whole body energy expenditure will necessarily have the same effect on CMRO2.

A 43-kDa protein related to c-Erb A alpha 1 is located in the mitochondrial matrix of rat liver

In order to characterize Sterling's triiodothyronine (T3) mitochondrial receptor using photoaffinity labeling, we observed two specific T3-binding proteins in the inner membrane (28 kDa) and in the matrix (43 kDa) of rat liver mitochondria. Western blots and immunoprecipitation using antibodies raised against the T3-binding domain of the T3 nuclear receptor c-Erb A alpha 1 indicated that at least the 43-kDa protein was c-Erb A alpha 1-related. In addition, gel mobility shift assays demonstrated the occurrence of a c-Erb A alpha 1-related mitochondrial protein that specifically binds to a natural or a palindromic thyroid-responsive element. Moreover, this protein specifically binds to a direct repeat 2 sequence located in the D-loop of the mitochondrial genome. Furthermore, electron microscopy studies allowed the direct observation of a c-Erb A-related protein in mitochondria. Lastly, the relative amounts of the 43-kDa protein related to c-Erb A alpha 1 were in good correlation with the known mitochondrial mass in three typical tissues. Interestingly, expression of a truncated form of the c-Erb A alpha 1 nuclear receptor in CV1 cells was associated with a mitochondrial localization and a stimulation of mitochondrial activity. These results supply evidence of the localization of a member of the nuclear receptor superfamily in the mitochondrial matrix involved in the regulation of mitochondrial activity that could act as a mitochondrial T3-dependent transcription factor.

Thyroid hormone action: effect of triiodothyronine on mitochondrial adenine nucleotide translocase in vivo and in vitro

Adenine nucleotide translocase (AdNT) levels were measured as the exchange of extramitochondrial against intramitochondrial adenosine diphosphate (ADP) in liver, spleen, and testes mitochondria isolated from normal and hypothyroid rats using the "back-exchange" and atractyloside-stop method of Pfaff and Klingenberg. The results provide confirmation of previous reports that mitochondria from hypothyroid rats show a markedly diminished AdNT activity, which is restored to normal levels within 72 hours by intraperitoneal injection of 10 to 20 micrograms triiodothyronine (T3)/100 g body weight. The latter dose was found in dose-response studies to result in maximal stimulation of AdNT in liver mitochondria. Qualitatively similar results on AdNT activity were obtained in liver mitochondria within 30 to 60 minutes following intravenous injection into hypothyroid rats of a more physiological dose of T3 (40 ng/100 g body weight). AdNT in mitochondria isolated from spleen and testes (organs that do not exhibit a calorigenic response after administration of thyroid hormone to the whole animal) failed to respond to thyroidectomy and to administration of T3. More recently, we have observed that in vitro replacement of T3 also stimulates AdNT activity in hypothyroid liver mitochondria. The enzyme adenosine triphosphate (ATP) synthase was examined as another possible candidate for direct hormonal stimulation of mitochondria. Simultaneous determinations on the same rats after intraperitoneal injection of T3 (20 micrograms/100 g body weight) showed little or no effect on ATP synthase until after 37 to 85 hours, whereas enhanced activity of the translocator was regularly observed at 17 hours.(ABSTRACT TRUNCATED AT 250 WORDS)

Inorganic phosphate is the major component of the thermostable cytoplasmic fraction which stimulates mitochondrial anion uniport

A low molecular weight thermostable cytoplasmic fraction isolated from rat liver homogenate when pre-incubated with mitochondria increases the rate at which anions enter mitochondria via the pH-dependent anion-conducting channel in the inner membrane. The crude fraction obtained by centrifuging and heating the liver homogenate was purified by gel filtration and chromatography on DEAE-cellulose. The resulting factor is stable to heating at 100 degrees C, freeze-drying and extremes of pH. Inorganic phosphate co-purified with activity and activity was lost when the phosphate was removed by barium salt precipitation. A pure sample of KH2PO4 produced stimulation of anion conductivity. These results show that the major portion of the activity which stimulates anion uniport can be accounted for by the presence of phosphate in the crude and purified fractions. Mersalyl blocks stimulation when added before, but not when added after, incubation with phosphate which shows that the stimulation is produced by phosphate in the mitochondrial matrix. The proposed role of this factor in thyroid hormone action is discussed in the light of its identification as inorganic phosphate.

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

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

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)

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)

Interaction of butylated hydroxyanisole with mitochondrial oxidative phosphorylation

The antioxidant, butylated hydroxyanisole (BHA), has a number of effects on mitochondrial oxidative phosphorylation. In this study we apply the novel approach developed by Brand (Brand MD, Biochim Biophys Acta 1018: 128-133, 1990) to investigate the site of action of BHA on oxidative phosphorylation in rat liver mitochondria. Using this approach we show that BHA increases the proton leak through the mitochondrial inner membrane and that it also inhibits the delta p (proton motive force across the mitochondrial inner membrane) generating system, but has no effect on the phosphorylation system. This demonstrates that compounds having pleiotypic effects on mitochondrial oxidative phosphorylation in vitro can be analysed and their many effects distinguished. This approach is of general use in analysing many other compounds of pharmacological interest which interact with mitochondria. The implications of these results for the mechanism of interaction of BHA with mitochondrial oxidative phosphorylation are discussed.

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.