Mitochondrial metabolism in different thyroid states

Using Steady-State Kinetics to Quantitate Substrate Selectivity and Specificity: A Case Study with Two Human Transaminases

We examined the ability of two human cytosolic transaminases, aspartate aminotransferase (GOT1) and alanine aminotransferase (GPT), to transform their preferred substrates whilst discriminating against similar metabolites. This offers an opportunity to survey our current understanding of enzyme selectivity and specificity in a biological context. Substrate selectivity can be quantitated based on the ratio of the k_cat/K_M values for two alternative substrates (the 'discrimination index'). After assessing the advantages, implications and limits of this index, we analyzed the reactions of GOT1 and GPT with alternative substrates that are metabolically available and show limited structural differences with respect to the preferred substrates. The transaminases' observed selectivities were remarkably high. In particular, GOT1 reacted ~10⁶-fold less efficiently when the side-chain carboxylate of the 'physiological' substrates (aspartate and glutamate) was replaced by an amido group (asparagine and glutamine). This represents a current empirical limit of discrimination associated with this chemical difference. The structural basis of GOT1 selectivity was addressed through substrate docking simulations, which highlighted the importance of electrostatic interactions and proper substrate positioning in the active site. We briefly discuss the biological implications of these results and the possibility of using k_cat/K_M values to derive a global measure of enzyme specificity.

Sites of superoxide and hydrogen peroxide production by muscle mitochondria assessed ex vivo under conditions mimicking rest and exercise

The sites and rates of mitochondrial production of superoxide and H2O2 in vivo are not yet defined. At least 10 different mitochondrial sites can generate these species. Each site has a different maximum capacity (e.g. the outer quinol site in complex III (site IIIQo) has a very high capacity in rat skeletal muscle mitochondria, whereas the flavin site in complex I (site IF) has a very low capacity). The maximum capacities can greatly exceed the actual rates observed in the absence of electron transport chain inhibitors, so maximum capacities are a poor guide to actual rates. Here, we use new approaches to measure the rates at which different mitochondrial sites produce superoxide/H2O2 using isolated muscle mitochondria incubated in media mimicking the cytoplasmic substrate and effector mix of skeletal muscle during rest and exercise. We find that four or five sites dominate during rest in this ex vivo system. Remarkably, the quinol site in complex I (site IQ) and the flavin site in complex II (site IIF) each account for about a quarter of the total measured rate of H2O2 production. Site IF, site IIIQo, and perhaps site EF in the β-oxidation pathway account for most of the remainder. Under conditions mimicking mild and intense aerobic exercise, total production is much less, and the low capacity site IF dominates. These results give novel insights into which mitochondrial sites may produce superoxide/H2O2 in vivo.

L-2-hydroxyglutaric aciduria, a defect of metabolite repair

L-2-hydroxyglutaric aciduria is a metabolic disorder in which L-2-hydroxyglutarate accumulates as a result of a deficiency in FAD-linked L-2-hydroxyglutarate dehydrogenase, a mitochondrial enzyme converting L-2-hydroxyglutarate to alpha-ketoglutarate. The origin of the L-2-hydroxyglutarate, which accumulates in this disorder, is presently unknown. The oxidation-reduction potential of the 2-hydroxyglutarate/alpha-ketoglutarate couple is such that L-2-hydroxyglutarate could potentially be produced through the reduction of alpha-ketoglutarate by a NAD- or NADP-linked oxidoreductase. In fractions of rat liver cytosolic extracts that had been chromatographed on an anion exchanger we detected an enzyme reducing alpha-ketoglutarate in the presence of NADH. This enzyme co-purified with cytosolic L-malate dehydrogenase (cMDH) upon further chromatography on Blue Sepharose. Mitochondrial fractions also contained an NADH-linked, 'alpha-ketoglutarate reductase', which similarly co-purified with mitochondrial L-malate dehydrogenase (mMDH). Purified mMDH catalysed the reduction of alpha-ketoglutarate to L-2-hydroxyglutarate with a catalytic efficiency that was about 10(7)-fold lower than that observed with oxaloacetate. For the cytosolic enzyme, this ratio amounted to 10(8), indicating that this enzyme is more specific. Both cMDH and mMDH are highly active in tissues and alpha-ketoglutarate is much more abundant than oxaloacetate and more concentrated in mitochondria than in the cytosol. As a result of this, the weak activity of mMDH on alpha-ketoglutarate is sufficient to account for the amount of L-2-hydroxyglutarate that is excreted by patients deficient in FAD-linked L-2-hydroxyglutarate dehydrogenase. The latter enzyme appears, therefore, to be responsible for a 'metabolite repair' phenomenon and to belong to the expanding class of 'house-cleaning' enzymes.

Mechanisms of thyroid hormone receptor-specific nuclear and extra nuclear actions

Triiodothyronine (T3) classically regulates gene expression by binding to high-affinity thyroid hormone receptors (TR) that recognize specific response elements in the promoters of T3-target genes and activate or repress transcription in response to hormone. However, a number of thyroid hormone effects occur rapidly and are unaffected by inhibitors of transcription and translation, suggesting that thyroid hormones may also mediate non-genomic actions. Such actions have been described in many tissues and cell types, including brown adipose tissue, the heart and pituitary. The site of non-genomic hormone action has been localized to the plasma membrane, cytoplasm and cellular organelles. These non-genomic actions include the regulation of ion channels, oxidative phosphorylation and mitochondrial gene transcription and involve the generation of intracellular secondary messengers and induction of [Ca(2+)](I), cyclic AMP or protein kinase signalling cascades. These observations have been interpreted to imply the presence of a specific, membrane associated, TR isoform or an unrelated high affinity membrane receptor for thyroid hormone. The recent identification of a progestin membrane receptor and the sub cellular targeted nuclear receptor isoforms ER46, mtRXR, mtPPAR, p28 and p46, has highlighted the potential importance of non-genomic actions of steroid hormones. Here we compare these recently identified receptors with the genomic, non-genomic and mitochondrial actions of thyroid hormones and consider their implications.

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.

A variant form of the nuclear triiodothyronine receptor c-ErbAalpha1 plays a direct role in regulation of mitochondrial RNA synthesis

In earlier research, we identified a 43-kDa c-ErbAalpha1 protein (p43) in the mitochondrial matrix of rat liver. In the present work, binding experiments indicate that p43 displays an affinity for triiodothyronine (T3) similar to that of the T3 nuclear receptor. Using in organello import experiments, we found that p43 is targeted to the organelle by an unusual process similar to that previously reported for MTF1, a yeast mitochondrial transcription factor. DNA-binding experiments demonstrated that p43 specifically binds to four mitochondrial DNA sequences with a high similarity to nuclear T3 response elements (mt-T3REs). Using in organello transcription experiments, we observed that p43 increases the levels of both precursor and mature mitochondrial transcripts and the ratio of mRNA to rRNA in a T3-dependent manner. These events lead to stimulation of mitochondrial protein synthesis. In transient-transfection assays with reporter genes driven by the mitochondrial D loop or two mt-T3REs located in the D loop, p43 stimulated reporter gene activity only in the presence of T3. All these effects were abolished by deletion of the DNA-binding domain of p43. Finally, p43 overexpression in QM7 cells increased the levels of mitochondrial mRNAs, thus indicating that the in organello influence of p43 was physiologically relevant. These data reveal a novel hormonal pathway functioning within the mitochondrion, involving a truncated form of a nuclear receptor acting as a potent mitochondrial T3-dependent transcription factor.

Oligomycin sensitivity of mitochondrial F(1)F(0)-ATPase in diabetes-prone BHE/Cdb rats

BHE/Cdb and Sprague-Dawley rats differ in their mitochondrial DNA sequence for the ATPase 6 ("subunit a") gene. Base substitutions in this sequence result in the substitution of asparagine for aspartate at position 101 and the substitution of serine for leucine at position 129. Differences in sensitivity to oligomycin were observed. When the isolated F(1)F(0)-ATPase complex was studied and ATPase activity was assessed, that which was isolated from the BHE/Cdb rats was less sensitive to oligomycin inhibition than that which was isolated from the Sprague-Dawley rats. In contrast, when oxygen consumption was measured [oxygen phosphorylation (OXPHOS)] and a dose-response curve was generated with isolated mitochondria from these two strains, there was a shift to the left for the BHE/Cdb rat mitochondria. These mitochondria were more sensitive to oligomycin inhibition of OXPHOS than were mitochondria isolated from Sprague-Dawley rats. The OXPHOS results are consistent with those from human fibroblasts having either a normal or mutated ATPase 6 gene.

Integrating cytosolic calcium signals into mitochondrial metabolic responses

Stimulation of hepatocytes with vasopressin evokes increases in cytosolic free Ca2+ ([Ca2+]c) that are relayed into the mitochondria, where the resulting mitochondrial Ca2+ ([Ca2+]m) increase regulates intramitochondrial Ca2+-sensitive targets. To understand how mitochondria integrate the [Ca2+]c signals into a final metabolic response, we stimulated hepatocytes with high vasopressin doses that generate a sustained increase in [Ca2+]c. This elicited a synchronous, single spike of [Ca2+]m and consequent NAD(P)H formation, which could be related to changes in the activity state of pyruvate dehydrogenase (PDH) measured in parallel. The vasopressin-induced [Ca2+]m spike evoked a transient increase in NAD(P)H that persisted longer than the [Ca2+]m increase. In contrast, PDH activity increased biphasically, with an initial rapid phase accompanying the rise in [Ca2+]m, followed by a sustained secondary activation phase associated with a decline in cellular ATP. The decline of NAD(P)H in the face of elevated PDH activity occurred as a result of respiratory chain activation, which was also manifest in a calcium-dependent increase in the membrane potential and pH gradient components of the proton motive force (PMF). This is the first direct demonstration that Ca2+-mobilizing hormones increase the PMF in intact cells. Thus, Ca2+ plays an important role in signal transduction from cytosol to mitochondria, with a single [Ca2+]m spike evoking a complex series of changes to activate mitochondrial oxidative metabolism.

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.