pH regulation of mitochondrial branch chain alpha-keto acid transport and oxidation in rat heart mitochondria

Metabolomics Profiling of Patients With A-β+ Ketosis-Prone Diabetes During Diabetic Ketoacidosis

When stable and near-normoglycemic, patients with "A-β+" ketosis-prone diabetes (KPD) manifest accelerated leucine catabolism and blunted ketone oxidation, which may underlie their proclivity to develop diabetic ketoacidosis (DKA). To understand metabolic derangements in A-β+ KPD patients during DKA, we compared serum metabolomics profiles of adults during acute hyperglycemic crises, without (n = 21) or with (n = 74) DKA, and healthy control subjects (n = 17). Based on 65 kDa GAD islet autoantibody status, C-peptide, and clinical features, 53 DKA patients were categorized as having KPD and 21 type 1 diabetes (T1D); 21 nonketotic patients were categorized as having type 2 diabetes (T2D). Patients with KPD and patients with T1D had higher counterregulatory hormones and lower insulin-to-glucagon ratio than patients with T2D and control subjects. Compared with patients withT2D and control subjects, patients with KPD and patients with T1D had lower free carnitine and higher long-chain acylcarnitines and acetylcarnitine (C2) but lower palmitoylcarnitine (C16)-to-C2 ratio; a positive relationship between C16 and C2 but negative relationship between carnitine and β-hydroxybutyrate (BOHB); higher branched-chain amino acids (BCAAs) and their ketoacids but lower ketoisocaproate (KIC)-to-Leu, ketomethylvalerate (KMV)-to-Ile, ketoisovalerate (KIV)-to-Val, isovalerylcarnitine-to-KIC+KMV, propionylcarnitine-to-KIV+KMV, KIC+KMV-to-C2, and KIC-to-BOHB ratios; and lower glutamate and 3-methylhistidine. These data suggest that during DKA, patients with KPD resemble patients with T1D in having impaired BCAA catabolism and accelerated fatty acid flux to ketones-a reversal of their distinctive BCAA metabolic defect when stable. The natural history of A-β+ KPD is marked by chronic but varying dysregulation of BCAA metabolism.

Precipitation of Inorganic Salts in Mitochondrial Matrix

In the mitochondrial matrix, there are insoluble, osmotically inactive complexes that maintain a constant pH and calcium concentration. In the present paper, we examine the properties of insoluble calcium and magnesium salts, such as phosphates, carbonates and polyphosphates, which might play this role. We find that non-stoichiometric, magnesium-rich carbonated apatite, with very low crystallinity, precipitates in the matrix under physiological conditions. Precipitated salt acts as pH buffer, and, hence, can contribute in maintaining ATP production in ischemic conditions, which delays irreversible damage to heart and brain cells after stroke.

Cellular stress is a prerequisite for glucose-induced mitochondrial matrix alkalinization in pancreatic β-cells

Changes in mitochondrial and cytosolic pH alter the chemical gradient across the inner mitochondrial membrane. The proton chemical gradient contributes to mitochondrial ATP synthesis as well as the uptake and release of metabolites and ions from the organelle. Here mitochondrial pH and ΔpH were studied for the first time in human pancreatic β-cells. Adenoviruses were used for rat insulin promoter dependent expression of the pH sensor SypHer targeted to either the mitochondrial matrix or the cytosol. The matrix pH in resting human β-cells is low (pH = 7.50 ± SD 0.17) compared to published values in other cell types. Consequently, the ΔpH of β-cells mitochondria is small. Glucose stimulation consistently resulted in acidification of the matrix pH in INS-1E insulinoma cells and β-cells in intact human islets or islet monolayer cultures. We registered acidification with similar kinetics but of slightly smaller amplitude in the cytosol of β-cells, thus glucose stimulation further reduced the ΔpH. Infection of human islets with high levels of adenoviruses caused the mitochondrial pH to increase. The apoptosis inducer and broad-spectrum kinase inhibitor staurosporine had similar effects on pH homeostasis. Although staurosporine alone does not affect the mitochondrial pH, glucose slightly increases the matrix pH of staurosporine treated cells. These two cellular stressors alter the normal mitochondrial pH response to glucose in pancreatic β-cells.

Adipose tissue branched chain amino acid (BCAA) metabolism modulates circulating BCAA levels

Whereas the role of adipose tissue in glucose and lipid homeostasis is widely recognized, its role in systemic protein and amino acid metabolism is less well-appreciated. In vitro and ex vivo experiments suggest that adipose tissue can metabolize substantial amounts of branched chain amino acids (BCAAs). However, the role of adipose tissue in regulating BCAA metabolism in vivo is controversial. Interest in the contribution of adipose tissue to BCAA metabolism has been renewed with recent observations demonstrating down-regulation of BCAA oxidation enzymes in adipose tissue in obese and insulin-resistant humans. Using gene set enrichment analysis, we observe alterations in adipose-tissue BCAA enzyme expression caused by adipose-selective genetic alterations in the GLUT4 glucose-transporter expression. We show that the rate of adipose tissue BCAA oxidation per mg of tissue from normal mice is higher than in skeletal muscle. In mice overexpressing GLUT4 specifically in adipose tissue, we observe coordinate down-regulation of BCAA metabolizing enzymes selectively in adipose tissue. This decreases BCAA oxidation rates in adipose tissue, but not in muscle, in association with increased circulating BCAA levels. To confirm the capacity of adipose tissue to modulate circulating BCAA levels in vivo, we demonstrate that transplantation of normal adipose tissue into mice that are globally defective in peripheral BCAA metabolism reduces circulating BCAA levels by 30% (fasting)-50% (fed state). These results demonstrate for the first time the capacity of adipose tissue to catabolize circulating BCAAs in vivo and that coordinate regulation of adipose-tissue BCAA enzymes may modulate circulating BCAA levels.

Increased potassium conductance of brain mitochondria induces resistance to permeability transition by enhancing matrix volume

Modulation of K(+) conductance of the inner mitochondrial membrane has been proposed to mediate preconditioning in ischemia-reperfusion injury. The mechanism is not entirely understood, but it has been linked to a decreased activation of mitochondrial permeability transition (mPT). In the present study K(+) channel activity was mimicked by picomolar concentrations of valinomycin. Isolated brain mitochondria were exposed to continuous infusions of calcium. Monitoring of extramitochondrial Ca(2+) and mitochondrial respiration provided a quantitative assay for mPT sensitivity by determining calcium retention capacity (CRC). Valinomycin and cyclophilin D inhibition separately and additively increased CRC. Comparable degrees of respiratory uncoupling induced by increased K(+) or H(+) conductance had opposite effects on mPT sensitivity. Protonophores dose-dependently decreased CRC, demonstrating that so-called mild uncoupling was not beneficial per se. The putative mitoK(ATP) channel opener diazoxide did not mimic the effect of valinomycin. An alkaline matrix pH was required for mitochondria to retain calcium, but increased K(+) conductance did not result in augmented DeltapH. The beneficial effect of valinomycin on CRC was not mediated by H(2)O(2)-induced protein kinase Cepsilon activation. Rather, increased K(+) conductance reduced H(2)O(2) generation during calcium infusion. Lowering the osmolarity of the buffer induced an increase in mitochondrial volume and improved CRC similar to valinomycin without inducing uncoupling or otherwise affecting respiration. We propose that increased potassium conductance in brain mitochondria may cause a direct physiological effect on matrix volume inducing resistance to pathological calcium challenges.

Mitochondrial transport proteins of the brain

In this study, cellular distribution and activity of glutamate and gamma-aminobutyric acid (GABA) transport as well as oxoglutarate transport across brain mitochondrial membranes were investigated. A goal was to establish cell-type-specific expression of key transporters and enzymes involved in neurotransmitter metabolism in order to estimate neurotransmitter and metabolite traffic between neurons and astrocytes. Two methods were used to isolate brain mitochondria. One method excludes synaptosomes and the organelles may therefore be enriched in astrocytic mitochondria. The other method isolates mitochondria derived from all regions of the brain. Immunological and enzymatic methods were used to measure enzymes and carriers in the different preparations, in addition to studying transport kinetics. Immunohistochemistry was also employed using brain slices to confirm cell type specificity of enzymes and carriers. The data suggest that the aspartate/glutamate carriers (AGC) are expressed predominantly in neurons, not astrocytes, and that one of two glutamate/hydroxyl carriers is expressed predominantly in astrocytes. The GABA carrier and the oxoglutarate carrier appear to be equally distributed in astrocytes and neurons. As expected, pyruvate carboxylase and branched-chain aminotransferase were predominantly astrocytic. Insofar as the aspartate/glutamate exchange carriers are required for the malate/aspartate shuttle and for reoxidation of cytosolic NADH, the data suggest a compartmentation of glucose metabolism in which astrocytes catalyze glycolytic conversion of glucose to lactate, whereas neurons are capable of oxidizing both lactate and glucose to CO(2) + H(2)O.

Diphenylhydantoin suppresses glucose-induced insulin release by decreasing cytoplasmic H+ concentration in pancreatic islets

Diphenylhydantoin (DPH), which is clinically used in the treatment of epilepsy, inhibits glucose-induced insulin release from pancreatic islets by a mechanism that remains unknown. In the present study, DPH is shown to suppress glucose-induced insulin release concentration-dependently. In dynamic experiments, 20 microm DPH suppressed 16.7 mm glucose-induced biphasic insulin release. DPH also suppressed insulin release in the presence of 16.7 mm glucose, 200 microm diazoxide, and 30 mm K+ without affecting the intracellular Ca2+ concentration. DPH suppressed ATP content and mitochondrial membrane hyperpolarization in the presence of 16.7 mm glucose without affecting glucose utilization, glucose oxidation, and reduced nicotinamide adenine dinucleotide phosphate fluorescence. DPH increased cytoplasmic pH in the presence of high glucose, but the increase was abolished under Na+ -deprived conditions and HCO3- -deprived conditions, suggesting that Na+ and HCO3- transport across the plasma membrane are involved in the increase in cytoplasmic pH by DPH. Alkalization by adding NH4+ to the extracellular medium also suppressed insulin release, ATP content, and mitochondrial membrane hyperpolarization. Because ATP production from the mitochondrial fraction in the presence of substrates was decreased by increased pH in the medium, DPH suppresses mitochondrial ATP production by reducing the H+ gradient across mitochondrial membrane. Using permeabilized islets, the increase in pH was shown to decrease Ca2+ efficacy at a clamped concentration of ATP in the exocytotic system. Taken together, DPH inhibits glucose-induced insulin secretion not only by inhibiting mitochondrial ATP production, but also by reducing Ca2+ efficacy in the exocytotic system through its alkalizing effect on cytoplasm.

Use of sulfhydryl reagents to investigate branched chain alpha-keto acid transport in mitochondria

The goal of this paper was to determine the contribution of the mitochondrial branched chain aminotransferase (BCATm) to branched chain alpha-keto acid transport within rat heart mitochondria. Isolated heart mitochondria were treated with sulfhydryl reagents of varying permeability, and the data suggest that essential cysteine residues in BCATm are accessible from the cytosolic face of the inner membrane. Treatment with 15 nmol/mg N-ethylmaleimide (NEM) inhibited initial rates of alpha-ketoisocaproate (KIC) uptake in reconstituted mitochondrial detergent extracts by 70% and in the intact organelle by 50%. KIC protected against inhibition suggesting that NEM labeled a cysteine residue that is inaccessible when substrate is bound to the enzyme. Additionally, the apparent mitochondrial equilibrium KIC concentration was decreased 50-60% after NEM labeling, and this difference could not be attributed to effects of NEM on matrix pH or KIC oxidation. In fact, NEM was a better inhibitor of KIC oxidation than rotenone. Measuring matrix aspartate and glutamate levels revealed that the effects of NEM on the steady-state KIC concentration resulted from inhibition of BCATm catalyzed transamination of KIC with matrix glutamate to form leucine. Furthermore, circular dichroism spectra of recombinant human BCATm with liposomes showed that the commercial lipids used in the reconstituted transport assay contain BCAT amino acid substrates. Thus BCATm is distinct from the branched chain alpha-keto acid carrier but may interact with the inner mitochondrial membrane, and it is necessary to inhibit or remove transaminase activity in both intact and reconstituted systems prior to quantifying transport of alpha-keto acids which are transaminase substrates.

Acute depression of mitochondrial protein synthesis during anoxia: contributions of oxygen sensing, matrix acidification, and redox state

Mitochondrial protein synthesis is acutely depressed during anoxia-induced quiescence in embryos of Artemia franciscana. Oxygen deprivation is accompanied in vivo by a dramatic drop in extramitochondrial pH, and both of these alterations strongly inhibit protein synthesis in isolated mitochondria. Here we show that the oxygen dependence is not explained simply by blockage of the electron transport chain or by the increased redox state. Whereas oxygen deprivation substantially depressed protein synthesis within 5 min and resulted in a 77% reduction after 1 h, aerobic incubations with saturating concentrations of cyanide or antimycin A had little effect during the first 20 min and only a modest effect after 1 h (36 and 20% reductions, respectively). Yet the mitochondrial NAD(P)H pools were fully reduced after 2-3 min with all three treatments. This cyanide- and antimycin-insensitive but hypoxia-sensitive pattern of protein synthesis depression suggests the presence of a molecular oxygen sensor within the mitochondrion. Second, we show for the first time that acidification of extramitochondrial pH exerts inhibition on protein synthesis specifically through changes in matrix pH. Matrix pH was 8.2 during protein synthesis assays performed at the extramitochondrial pH optimum of 7.5. When this proton gradient was abolished with nigericin, the extramitochondrial pH optimum for protein synthesis displayed an alkaline shift of approximately 0.7 pH unit. These data suggest the presence of proton-sensitive translational components within the mitochondrion.

Branched-chain alpha-keto acid dehydrogenase complex in rat skeletal muscle: regulation of the activity and gene expression by nutrition and physical exercise

Branched-chain alpha-keto acid dehydrogenase complex is the rate-limiting enzyme in the catabolism of branched-chain amino acids in skeletal muscle. It is suggested that activation of this enzyme in the muscle during exercise plays an important role in the increased oxidation of branched-chain amino acids in the muscle. Evidence suggests that branched-chain alpha-keto acids, the substrates for the enzyme, regulate the activity state of the enzyme in the muscle during exercise through phosphorylation/dephosphorylation cycle of the enzyme protein. We propose a model for the mechanism of enzyme activation by exercise. In addition to this acute effect of exercise, we present evidence suggesting that exercise training modulates the enzyme activity and gene expression for the enzyme. Increases in the total activity as well as enzyme proteins by exercise training are suggested to be associated with mitochondrial biogenesis in the muscle.

Modulation of branched-chain 2-oxo acid dehydrogenase complex activity in rat skeletal muscle by endurance training

Effects of endurance training and high-fat diet intake on the branched-chain 2-oxo acid dehydrogenase complex in skeletal muscle were examined in rats. The basal activities of the enzyme complex (approximately 4% in active form of the total enzyme) in the muscle of rats under the fed conditions were not different between trained and untrained rats. The basal activity in the muscle was elevated by 24 h starvation in both groups of rats, but the level of the elevation was significantly greater in the trained rats than in the untrained rats. On the other hand, high-fat diet intake did not alter the basal activity of the enzyme complex in the muscle or the profile of activation of the enzyme complex by muscle contractions elicited by the electrical stimulation, suggesting that the fat content in the diet does not affect the enzyme activity in the muscle. Neither training nor diet affected the total enzyme activity or the amount of enzyme protein. Activation by leucine administration of the enzyme complex in the muscle was greater in the trained rats than in the untrained rats, suggesting that the activity state of the enzyme complex is more responsive to regulation by the 2-oxo acid derived from leucine in the muscles of endurance-trained rats.

Methionine transamination--metabolic function and subcellular compartmentation

Enzymatic activities catalysing the inter-conversion of L-methionine and its oxy analogue 4-methylthio-2-oxobutyric acid (2,4-KMB) were detected in the liver, skeletal muscle and heart of the laboratory rat and of sheep. In both species the highest activity of methionine transamination was found in the liver and was located in the cytoplasm and mitochondria. We propose that physiological and nutritional role of the cytoplasmic methionine transamination is amination of 2,4 KMB and formation of L-methionine while in mitochondria the activity is responsible for disposal of excess methionine is oxidised through oxidative decarboxylation of 2,4 KMB.

Branched-chain 2-oxo acid dehydrogenase complex activation by tetanic contractions in rat skeletal muscle

Branched-chain 2-oxo acid dehydrogenase complex in rat skeletal muscle was activated by muscle contractions elicited by electrical stimulation. This activation was attributed to dephosphorylation of the phosphorylated enzyme complex, and the total enzyme activity was not altered by muscle contractions. The activation of the enzyme complex occurred in the muscle of the electrically stimulated leg, but not in the muscle of the non-stimulated (control) leg, indicating that blood components are not involved in the mechanism of the enzyme activation in the muscle. Adenine nucleotides, branched-chain amino and 2-oxo acids and lactate in the muscle were determined as possible factors modulating the enzyme complex activity through inhibition of branched-chain 2-oxo acid dehydrogenase kinase activity. The profile of enzyme activation induced by muscle contractions was different from the alteration of the adenine nucleotide concentrations but was similar to the alteration of the concentrations of branched-chain amino and 2-oxo acids in the muscle. The lactate concentration in the stimulated muscle was elevated 3-5-fold during the contractions, indicating intracellular acidification. Previous studies have shown that the 2-oxo acid derived from leucine is a potent inhibitor of the kinase. These results suggest that intracellular branched-chain 2-oxo acids increased by muscle contractions accumulate in the mitochondria due to exercise-induced acidification of the muscle cell, resulting in activation of branched-chain 2-oxo acid dehydrogenase complex by inhibition of the kinase.

Ethanol and oleate inhibition of alpha-ketoisovalerate and 3-hydroxyisobutyrate metabolism by isolated hepatocytes

Ethanol inhibited glucose synthesis from alpha-ketoisovalerate by isolated rat hepatocytes without significant inhibition of flux through the branched-chain alpha-ketoacid dehydrogenase complex. Accumulation of 3-hydroxyisobutyrate, an intermediate in the catabolism of alpha-ketoisovalerate, was increased by ethanol, indicating inhibition of flux at the level of 3-hydroxyisobutyrate dehydrogenase. 3-Hydroxybutyrate caused the same effects as ethanol, suggesting inhibition was a consequence of an increase in the mitochondrial NADH/NAD+ ratio. Flux through the 3-hydroxyisobutyrate dehydrogenase was more sensitive to regulation by the mitochondrial NADH/NAD+ ratio than flux through the branched-chain alpha-ketoacid dehydrogenase. Oleate also inhibited glucose synthesis from alpha-ketoisovalerate, but marked inhibition of flux through the branched-chain alpha-ketoacid dehydrogenase complex was caused by this substrate.

Protein sparing during treatment of obesity: ketogenic versus nonketogenic very low calorie diet

Although it is generally agreed that both ketogenic and nonketogenic very low calorie diets promote weight reduction, there is no consensus on a preference of one diet over the other in regard to protein sparing. In the present study, we compared the effects of isocaloric (600 kcal/d) and isonitrogenous (8 g nitrogen/d) ketogenic (low carbohydrate) and nonketogenic diets on parameters of protein and amino acid metabolism, in 16 morbidly obese women maintained on these diets for 4 weeks while confined to a metabolic ward. Cumulative urinary nitrogen excretion (g/4 wk) was significantly (P less than .01) greater (248 +/- 6 v 207 +/- 12, mean +/- SEM, n = 8), and cumulative nitrogen balance significantly (P less than .02) more negative (-50.4 +/- 4.4 v -18.8 +/- 5.7), during treatment with the ketogenic than with the nonketogenic diet. Plasma leucine concentration (mumol/L) was significantly higher (P less than .05) during treatment with the ketogenic than with the nonketogenic diet at day 14 (210 +/- 17 v 150 +/- 8), but not at day 28 (174 +/- 9 v 148 +/- 8). Whole-body rates of leucine oxidation (mmol/h) were significantly higher (P less than .05) during treatment with the ketogenic than with the nonketogenic diet at day 14 (1.29 +/- 0.20 v 0.92 +/- 0.10) and at day 28 (1.00 +/- 0.16 v 0.75 +/- 0.10). Conversely, proteolysis, as measured by leucine turnover rate and urinary excretion of 3-methylhistidine, was not significantly different between the diets.(ABSTRACT TRUNCATED AT 250 WORDS)

A 31P NMR study of mitochondrial inorganic phosphate visibility: effects of Ca2+, Mn2+, and the pH gradient

The effects of external pH, temperature, and Ca2+ and Mn2+ concentrations on the compartmentation and NMR visibility of inorganic phosphate (Pi) were studied in isolated rat liver mitochondria respiring on succinate and glutamate. Mitochondrial matrix Pi is totally visible by NMR at 8 degrees C and at low external concentrations of Pi. However, when the external Pi concentration is increased above 7 mM, the pH gradient decreases, the amount of matrix Pi increases, and the fraction not observed by NMR increases. Raising the temperature to 25 degrees C also decreases the pH gradient and the Pi fraction observed by NMR. At physiologically relevant concentrations, Ca2+ and Mn2+ do not seem to play a major role in matrix Pi NMR invisibility. For Ca2+ concentrations above 30 nmol/mg of protein, formation of insoluble complexes will cause loss of Pi signal intensity. For Mn2+ concentrations above 2 nmol/mg of protein, the Pi peak can be broadened sufficiently to preclude detection of a high-resolution signal. The results indicate that mitochondrial matrix Pi should be mostly observable up to 25 degrees C by high-resolution NMR. While the exact nature of the NMR-invisible phosphate in perfused or in vivo liver is yet to be determined, better success at detecting and resolving both Pi pools by NMR is indicated at high field, low temperature, and optimized pulsing conditions.

2-Ketoisocaproate transport in insulin-secreting cells

The transport of the nutrient secretagogue 2-ketoisocaproate (KIC) was studied in isolated rat pancreatic islets and in the HIT-T15 insulinoma cell line using an oil-filtration technique. In both islets and HIT-T15 cells, KIC uptake was a slow process, not reaching equilibrium within 10 min KIC transport was not dependent upon Na+ in the medium, was not inhibited by alpha-cyano-4-hydroxycinnamate nor by 2-amino-2-norborane carboxylic acid (BCH) and did not appear to be electrogenic. Evidence was obtained to suggest that KIC uptake occurred via passive diffusion into the cell of the undissociated acid species. This possibility was supported by the apparent unsaturability of KIC uptake in HIT-T15 cells. Addition of 10-30 mM KIC to dispersed islets cells or HIT-T15 cells produced a rapid intracellular acidification. In islets, the rate of transport of 10 mM KIC was comparable with oxidation rate of the keto-acid suggesting that uptake could be rate-limiting factor for KIC oxidation and thus stimulated insulin release. However, in HIT-T15 cells, the rate of uptake of KIC greatly exceeded the oxidation rate. The low rate of KIC oxidation could explain the poor secretory response of HIT-T15 cells to KIC.

Effect of acute acidosis and alkalosis on leucine kinetics in man

The effects of acute pH changes on whole body leucine kinetics (1-13C-leucine infusion technique) were determined in normal subjects. Plasma insulin, glucagon, and growth hormone concentrations were kept constant by somatostatin and replacement infusions of the three hormones. When acidosis was produced by ingestion of NH4Cl (4 mmol kg-1 p.os; n = 8) arterialized pH decreased within 3 h from 7.39 +/- 0.01 to 7.31 +/- 0.01 (P less than 0.001) and leucine plasma appearance increased by 0.13 +/- 0.04 mumol kg-1 min-1 (P less than 0.02); in contrast, when alkalosis was produced by intravenous infusion of 4 mmol kg-1 NaHCO3 (n = 7, pH 7.47 +/- 0.01), leucine plasma appearance decreased by -0.09 +/- 0.04 mumol kg-1 min-1 (P less than 0.01 vs. acidosis). Whole body leucine flux also increased during acidosis compared to alkalosis (P less than 0.05), suggesting an increase in whole body protein breakdown during acidosis. Apparent leucine oxidation increased during acidosis compared to alkalosis (P = 0.05). Net forearm leucine exchange remained unaffected by acute pH changes. Plasma FFA concentrations decreased during acidosis by -107 +/- 67 mumol l-1 (P less than 0.05) and plasma glucose increased by 1.90 +/- 0.25 mmol l-1 (P less than 0.02); in contrast, alkalosis resulted in an increase in plasma FFA by 83 +/- 40 mumol l-1 (P less than 0.02; P less than 0.01 vs. acidosis), suggesting an increase in lipolysis; plasma glucose decreased compared to acidosis (P less than 0.01). The data demonstrate that acute metabolic acidosis and alkalosis, as they occur in clinical conditions, influence protein breakdown, and in the opposite direction, lipolysis.

Regulation of the mitochondrial Na+/Ca2+ antiport by matrix pH

The effect of matrix pH (pHi) on the activity of the mitochondrial Na+/Ca2+ antiport has been studied using the fluorescence of SNARF-1 to monitor pHi and Na(+)-dependent efflux of accumulated Ca2+ to follow antiport activity. Heart mitochondria respiring in a KCl medium maintain a large delta pH (interior alkaline) and show optimal Na+/Ca2+ antiport only when the pH of the medium (pH0) is acid. Addition of nigericin to these mitochondria decreases delta pH and increases the membrane potential (delta psi). Nigericin strongly activates Na+/Ca2+ antiport at values of pH0 near 7.4 but inhibits antiport activity at acid pH0. When pHi is evaluated in these protocols, a sharp optimum in Na+/Ca2+ antiport activity is seen near pHi 7.6 in the presence or absence of nigericin. Activity falls off rapidly at more alkaline values of pHi. The effects of nigericin on Na+/Ca2+ antiport are duplicated by 20 mM acetate and by 3 mM phosphate. In each case the optimum rate of Na+/Ca2+ antiport is obtained at pHi 7.5 to 7.6 and changes in antiport activity do not correlate with changes in components of the driving force of the reaction (i.e., delta psi, delta pH, or the steady-state Na+ gradient). It is concluded that the Na+/Ca2+ antiport of heart mitochondria is very sensitive to matrix [H+] and that changes in pHi may contribute to the regulation of matrix Ca2+ levels.

Regulation of the mitochondrial multienzyme complexes in complex metabolic systems

During the past two decades, I and the members of my laboratory have enjoyed very much contributing to the body of knowledge concerning the alpha-keto acid dehydrogenase complexes. We would hope that some of our work has allowed those interested in this area a perception as to how these interesting enzyme complexes are regulated in intact metabolic systems. And last, but certainly not least, I have treasured both my professional relationship and my personal friendship with Lester Reed. His work and his presence in this field of inquiry serve as a gold standard for all of us.

Regulation of substrate availability for the branched-chain alpha-keto acid dehydrogenase enzyme complex

The tissue distribution of BCAT and BCKDH is largely responsible for the unique metabolism of branched-chain amino acids in rat tissues. Because BCKDH is a mitochondrial enzyme, tissue capacity for branched-chain amino acid oxidation will be a function of mitochondrial specific activity and tissue mitochondrial content, as well as the activity state of the BCKDH complex. In muscle tissues, the activity of the BCKDH appears to restrict branched-chain amino acid oxidation. Therefore, in muscle, transamination exceeds oxidation. Depending on muscle fiber type, the branched-chain alpha-keto acid transporter operates primarily as either an efflux or an exchange pathway and keto acids are released from the tissue. The liver contains very low cytosolic BCAT activity and no mitochondrial BCAT. Since the BCKDH is largely in the active state in hepatic tissue, the liver is a major site of branched-chain amino acid oxidation. Thus, control of the metabolism of these essential amino acids in vivo is achieved through distribution and regulation of the activity of the first two enzymes in the catabolic pathway.

Estimation of matrix pH in isolated heart mitochondria using a fluorescent probe

Isolated heart mitochondria hydrolyze the acetoxymethyl esters of the Ca2+-sensitive fluorescent probe fura-2 and the pH-sensitive 2',7'-biscarboxyethyl-5(6)-carboxyfluorescein (BCECF). The resulting charged forms of the probes are retained in the mitochondrial matrix and appear well-suited for the estimation of pCa and pH in this compartment. The mitochondria esterase activity is stimulated by Ca2+, inhibited by butacaine and quinine, and shows an alkaline pH optimum. The esterase has a similar affinity for the two probes (Km about 1.5 microM) and a somewhat higher Vmax for BCECF. Intramitochondrial pH can be determined by recording the ratio of the fluorescence of matrix BCECF at its excitation maximum of 509 nm to that at 450 nm, an excitation wavelength that is unresponsive to pH. A calibration plot relating the fluorescence ratio to pH is constructed using detergent-lysed mitochondria and the excitation maximum of 500 nm for BCECF in aqueous solution. Estimates of matrix pH by BCECF fluorescence in its useful range (pH 6 to 8) agree well with values obtained using the distribution of 5,5-dimethyl-2,4-oxazolidenedione. In protocols in which the fluorescence with excitation at 450 nm does not vary, a direct recording of BCECF fluorescence with excitation at 509 nm can be used to follow the kinetics of matrix pH changes.