The control of tricarboxylate-cycle oxidations in blowfly flight muscle. The oxidized and reduced nicotinamide-adenine dinucleotide content of flight muscle and isolated mitochondria, the adenosine triphosphate and adenosine diphosphate content of mitochondria, and the energy status of the mitochondria during controlled respiration

The expression of CG9940 affects the adaptation of cardiac function, mobility, and lifespan to exercise in aging Drosophila

The CG9940 gene, which encodes the NAD(+) synthase protein in Drosophila, is conserved in human, zebra fish, and mosquito. NAD(+) synthase is a homodimer, which catalyzes the final step in de novo nicotinamide adenine dinucleotide (NAD(+)) biosynthesis, an amide transfer from either ammonia or glutamine to nicotinic acid adenine dinucleotide (NaAD). Both the CG9940 and exercise are closely relative to NAD(+) level, and NAD(+) plays important roles not only in energy metabolism and mitochondrial functions but also in aging. In our study, the expression of CG9940 was changed by UAS/GAL4 system in Drosophila. Flies were trained by a training device. Cardiac function was analyzed by M-mode traces, climbing index was measured through negative geotaxis assay, and lifespan was measured via lifespan assays. The important new findings from our present study included the following: (1) the expression of the CG9940 could affect cardiac function, mobility, and lifespan in Drosophila. Over-expression of the CG9940 gene had positive effects on Drosophila, such as enhanced aging cardiac output, reduced heart failure, delayed age-related mobility decline, and prolonged lifespan, but lower-expression of the CG9940 had negative effects on them. (2) Different expressions of the CG9940 resulted in different influences on the adaptation of cardiac function, mobility, and lifespan to exercise in aging Drosophila. Both normal-expression and over-expression of the CG9940 resulted in positive influences on the adaptation of cardiac functions, mobility, and lifespan to exercise in aging Drosophila such as exercise slowed age-related decline of cardiac function, mobility and extent of lifespan in these flies, while lower-expression of the CG9940 led to negative impacts on the adaptation of mobility and lifespan to exercise in Drosophila.

Mitochondrial physiology in the major arbovirus vector Aedes aegypti: substrate preferences and sexual differences define respiratory capacity and superoxide production

Adult females of Aedes aegypti are facultative blood sucking insects and vectors of Dengue and yellow fever viruses. Insect dispersal plays a central role in disease transmission and the extremely high energy demand posed by flight is accomplished by a very efficient oxidative phosphorylation process, which take place within flight muscle mitochondria. These organelles play a central role in energy metabolism, interconnecting nutrient oxidation to ATP synthesis, but also represent an important site of cellular superoxide production. Given the importance of mitochondria to cell physiology, and the potential contributions of this organelle for A. aegypti biology and vectorial capacity, here, we conducted a systematic assessment of mitochondrial physiology in flight muscle of young adult A. aegypti fed exclusively with sugar. This was carried out by determining the activities of mitochondrial enzymes, the substrate preferences to sustain respiration, the mitochondrial bioenergetic efficiency and capacity, in both mitochondria-enriched preparations and mechanically permeabilized flight muscle in both sexes. We also determined the substrates preferences to promote mitochondrial superoxide generation and the main sites where it is produced within this organelle. We observed that respiration in A. aegypti mitochondria was essentially driven by complex I and glycerol 3 phosphate dehydrogenase substrates, which promoted distinct mitochondrial bioenergetic capacities, but with preserved efficiencies. Respiration mediated by proline oxidation in female mitochondria was strikingly higher than in males. Mitochondrial superoxide production was essentially mediated through proline and glycerol 3 phosphate oxidation, which took place at sites other than complex I. Finally, differences in mitochondrial superoxide production among sexes were only observed in male oxidizing glycerol 3 phosphate, exhibiting higher rates than in female. Together, these data represent a significant step towards the understanding of fundamental mitochondrial processes in A. aegypti, with potential implications for its physiology and vectorial capacity.

NAD(+)/NADH and skeletal muscle mitochondrial adaptations to exercise

The pyridine nucleotides, NAD(+) and NADH, are coenzymes that provide oxidoreductive power for the generation of ATP by mitochondria. In skeletal muscle, exercise perturbs the levels of NAD(+), NADH, and consequently, the NAD(+)/NADH ratio, and initial research in this area focused on the contribution of redox control to ATP production. More recently, numerous signaling pathways that are sensitive to perturbations in NAD(+)(H) have come to the fore, as has an appreciation for the potential importance of compartmentation of NAD(+)(H) metabolism and its subsequent effects on various signaling pathways. These pathways, which include the sirtuin (SIRT) proteins SIRT1 and SIRT3, the poly(ADP-ribose) polymerase (PARP) proteins PARP1 and PARP2, and COOH-terminal binding protein (CtBP), are of particular interest because they potentially link changes in cellular redox state to both immediate, metabolic-related changes and transcriptional adaptations to exercise. In this review, we discuss what is known, and not known, about the contribution of NAD(+)(H) metabolism and these aforementioned proteins to mitochondrial adaptations to acute and chronic endurance exercise.

NAD(+)/NADH and skeletal muscle mitochondrial adaptations to exercise

The pyridine nucleotides, NAD(+) and NADH, are coenzymes that provide oxidoreductive power for the generation of ATP by mitochondria. In skeletal muscle, exercise perturbs the levels of NAD(+), NADH, and consequently, the NAD(+)/NADH ratio, and initial research in this area focused on the contribution of redox control to ATP production. More recently, numerous signaling pathways that are sensitive to perturbations in NAD(+)(H) have come to the fore, as has an appreciation for the potential importance of compartmentation of NAD(+)(H) metabolism and its subsequent effects on various signaling pathways. These pathways, which include the sirtuin (SIRT) proteins SIRT1 and SIRT3, the poly(ADP-ribose) polymerase (PARP) proteins PARP1 and PARP2, and COOH-terminal binding protein (CtBP), are of particular interest because they potentially link changes in cellular redox state to both immediate, metabolic-related changes and transcriptional adaptations to exercise. In this review, we discuss what is known, and not known, about the contribution of NAD(+)(H) metabolism and these aforementioned proteins to mitochondrial adaptations to acute and chronic endurance exercise.

Pyruvate and acetate metabolism in termite mitochondria

Intact mitochondria have been successfully prepared from body tissues from the termites Nasutitermes walkeri and Coptotermes formosanus. This is the first report of the successful isolation of mitochondria from termites (Isoptera: Termitidae). Using an oxygen electrode, oxygen consumption by the mitochondria during the oxidation of various respiratory substrates was determined and their properties measured in terms of respiratory control index and ADP/O. ADP/O was as expected for substrates such as pyruvate, acetylcarnitine and acetyl-CoA and carnitine. Pyruvate and acetate were the major respiratory substrates in both species. The total activity of the pyruvate dehydrogenase complex (PDHc) in the mitochondria from N. walkeri and C. formosanus was determined to be 72.87+/-8.98 and 8.29+/-0.42 nmol/termite/h, respectively. Mitochondria isolated in the presence of inhibitors of PDHc interconversion were used to determine that about 60% of the PDHc was maintained in the active form in both N. walkeri and C. formosanus. The sufficient PDHc activity and high rate of pyruvate oxidation in mitochondria from N. walkeri suggest that pyruvate is rapidly metabolised, whereas the low mitochondrial PDHc activity of C. formosanus suggests that in this species more pyruvate is produced than can be oxidised in the termite tissues.

Calcium activation of heart mitochondrial oxidative phosphorylation: rapid kinetics of mVO2, NADH, AND light scattering

Parallel activation of heart mitochondria NADH and ATP production by Ca(2+) has been shown to involve the Ca(2+)-sensitive dehydrogenases and the F(0)F(1)-ATPase. In the current study we hypothesize that the response time of Ca(2+)-activated ATP production is rapid enough to support step changes in myocardial workload ( approximately 100 ms). To test this hypothesis, the rapid kinetics of Ca(2+) activation of mV(O(2)), [NADH], and light scattering were evaluated in isolated porcine heart mitochondria at 37 degrees C using a variety of optical techniques. The addition of Ca(2+) was associated with an initial response time (IRT) of mV(O(2)) that was dose-dependent with a minimum IRT of 0.27 +/- 0.02 s (n = 41) at 535 nm Ca(2+). The IRTs for NADH fluorescence and light scattering in response to Ca(2+) additions were similar to mV(O(2)). The Ca(2+) IRT for mV(O(2)) was significantly shorter than 1.6 mm ADP (2.36 +/- 0.47 s; p < or = 0.001, n = 13), 2.2 mm P(i) (2.32 +/- 0.29, p < or = 0.001, n = 13), or 10 mm creatine (15.6.+/-1.18 s, p < or = 0.001, n = 18) under similar experimental conditions. Calcium effects were inhibited with 8 microm ruthenium red (2.4 +/- 0.31 s; p < or = 0.001, n = 16) and reversed with EGTA (1.6 +/- 0.44; p < or = 0.01, n = 6). Estimates of Ca(2+) uptake into mitochondria using optical Ca(2+) indicators trapped in the matrix revealed a sufficiently rapid uptake to cause the metabolic effects observed. These data are consistent with the notion that extramitochondrial Ca(2+) can modify ATP production, via an increase in matrix Ca(2+) content, rapidly enough to support cardiac work transitions in vivo.

NAD(+)-linked isocitrate dehydrogenase in fish tissues

NAD(+)-linked isocitrate dehydrogenase was found in the brain, heart, gills, kidney, liver and muscle of trout, and in the liver and muscle of eel. A complex homogenization buffer containing 1 mM ADP, 5 mM MgSO4, 5 mM citrate and 40% glycerol is required for retrieval of significant amounts of stable enzyme. The highest activities were found in brain of trout and the lowest in white muscle of trout and eel. The enzyme was partially purified from frozen trout heart to a final activity of 0.04 μM/min/mg protein, and the kinetic properties of this partially purified enzyme were studied. The enzyme requires either Mn(2+) or Mg(2+) for activity, higher activities being observed with Mn(2+). Saturation kinetics for DL-isocitrate were sigmoidal, apparent S0·5=8.2±0.6 mM and nH=1.8±0.2, in the absence of ADP, changing to hyperbolic, apparent S0·5=1.4±0.3 mM and nH=1.0, with 1 mM ADP added. Citrate and Ca(2+) were found to activate the enzyme to a small extent. NADH strongly inhibited the enzyme, I50=3.7±0.5 μM. ATP was also found to be an inhibitor, I50=7.2±1.4 mM. These properties are consistent with the role of the enzyme as a major control site of the tricarboxylic acid cycle.

Mitochondrial respiratory control in the myocardium

The heart muscle has proved to be a practical model for studying respiratory control in intact tissues. It also demonstrates that control at the level of the respiratory chain is augmented by metabolic control at the substrate level as exemplified by the very narrow range of changes in the redox state of the mitochondrial NADH/NAD couple even during extensive changes in ATP and oxygen consumption. The behaviour of mitochondria when isolated can largely be duplicated in the intact myocardium. Moreover, the high intracellular concentrations of enzymes, coenzymes and adenine nucleotides create conditions of high reaction rates, enabling the formation of a near equilibrium network of certain main pathways. This equilibrium network in connection with metabolic regulation of the hydrogen pressure upon the matrix NADH/NAD pool is a prerequisite for the regulation of cellular respiration at a high efficiency of energy transfer. Experimentation on the intact myocardium also seems to be capable of resolving some of the uncertainties about prevailing mechanisms for the regulation of cellular respiration.

Regulation of pyruvate oxidation in blowfly flight muscle mitochondria: requirement for ADP

Blowfly (Phormia regina) flight muscle mitochondria oxidized pyruvate ( + proline) in the presence of either ADP (coupled respiration) or carbonylcyanide-p-trifluoromethoxyphenylhydrazone (FCCP-uncoupled respiration). There was an absolute requirement for ADP (Km = 8.0 microM) when pyruvate oxidation was stimulated by FCCP in the presence of oligomycin. This requirement for ADP was limited to the oxidation of pyruvate; uncoupled alpha-glycerolphosphate oxidation proceeded maximally even in the absence of added ADP. Atractylate inhibited uncoupled pyruvate oxidation whether added before (greater than 99%) or after (95%) initiation of respiration with FCCP. In the presence of FCCP, oligomycin, and limiting concentrations of ADP (less than 110 microM), there was a shutoff in the uptake of oxygen. This inhibition of respiration was completely reversed by the addition of more ADP. Plots of net oxygen uptake as a function of the limiting ADP concentration were linear; the observed ADP/O ratio was 0.22 +/- 0.025. An ADP/O ratio of 0.2 was predicted if phosphorylation occurred only at the succinyl-CoA synthetase step of the tricarboxylate cycle. Experiments performed in the presence of limiting concentrations of ADP, and designed to monitor changes in the mitochondrial content of ADP and ATP, demonstrated that the shutoff in oxygen uptake was not due to the presence of a high intramitochondrial concentration of ATP. Indeed, ATP, added to the medium prior to the addition of FCCP, inhibited uncoupled pyruvate oxidation; the apparent KI was 0.8 mM. These results are consistent with the hypothesis that it is the intramitochondrial ATP/ADP ratio that is one of the controlling factors in determining the rate of flux through the tricarboxylate cycle. Changes in the mitochondrial content of citrate, isocitrate, alpha-ketoglutarate, and malate during uncoupled pyruvate oxidation in the presence of a limiting concentration of ADP were consistent with the hypothesis that the mitochondrial NAD + -linked isocitric dehydrogenase is a major site for such control through the tricarboxylate cycle.

Intramitochondrial and extramitochondrial free calcium ion concentrations of suspensions of heart mitochondria with very low, plausibly physiological, contents of total calcium

The 2-oxoglutarate dehydrogenase of intact rat heart mitochondria is activated by Ca2+, with 50% activation at approximately 0.5 nmol of total Ca/mg of mitochondrial protein, in the presence of Pi and Mg2+. Mitochondrial Ca contents in excess of 2 nmol/mg of protein result in 100% activation of the enzyme. Investigation of Ca2+ release from the mitochondria using the metallochromic indicator Arsenazo III defines a S0.5 of 5.4 +/- 0.4 nmol of Ca/mg of protein, when the endogenous Ca content of the mitochondria is progressively depleted with EGTA, prior to the initiation of the release process being studied. The subsequent determination of matrix free Ca2+ concentration by the "null-point" technique has allowed expression of these results in terms of free concentration rather than Ca content, with an activity coefficient of approximately 0.001 for matrix Ca2+. From the above, Ca2+ efflux from heart mitochondria is not saturated at the mitochondrial Ca contents or Ca2+ concentrations which give effective regulation of dehydrogenase activity. A consequence is that heart mitochondria do not buffer the pCa of the extramitochondrial medium at these Ca contents (less than 2 nmol/mg of protein), and this is shown in direct measurements of extramitochondrial pCa. This is taken to question the physiological significance of mitochondrial buffering of cytosolic free Ca2+ in normal heart.

Changes in the contents of adenine nucleotides and intermediates of glycolysis and the citric acid cycle in flight muscle of the locust upon flight and their relationship to the control of the cycle

1. The contents of some intermediates of glycolysis, the citric acid cycle and adenine nucleotides have been measured in the freeze-clamped locust flight muscle at rest and after 10s and 3min flight. The contents of glucose 6-phosphate, pyruvate, alanine and especially fructose bisphosphate and triose phosphates increased markedly upon flight. The content of acetyl-CoA is decreased after 3min flight whereas that of acetylcarnitine is decreased markedly after 10s flight, but returns towards the resting value after 3min flight. The content of citrate is markedly decreased after both 10s and 3min flight, whereas that of isocitrate is changed very little after 10s and is increased by 50% after 3min. The content of oxaloacetate is very low in insect flight muscle and hence it was measured by a sensitive radiochemical assay. The content of oxaloacetate increased about 2-fold after 3min flight. A similar change was observed in the content of malate. The content of ATP decreased about 15%, whereas those of ADP and AMP increased about 2-fold after 3min flight. 2. Calculations based on O(2) uptake of the intact insect indicate that the rate of the citric acid cycle must be increased >100-fold during flight. Consequently, if citrate synthase catalyses a non-equilibrium reaction, the activity of the enzyme must increase >100-fold during flight. However, changes in the concentrations of possible regulators of citrate synthase, oxaloacetate, acetyl-CoA and citrate (which is an allosteric inhibitor), are not sufficient to account for this change in activity. It is concluded that there may be much larger changes in the free concentration of oxaloacetate than are indicated by the changes in the total content of this metabolite or that other unknown factors must play an additional role in the regulation of citrate synthase activity. 3. The increased content of oxaloacetate could be produced via pyruvate carboxylase, which may be stimulated during the early stages of flight by the increased concentration of pyruvate. 4. The decreases in the concentrations of citrate and alpha-oxoglutarate indicate that isocitrate dehydrogenase and oxoglutarate dehydrogenase may be stimulated by factors other than their pathway substrates during the early stages of flight. 5. Calculated mitochondrial and cytosolic NAD(+)/NADH ratios are both increased upon flight. The change in the mitochondrial ratio indicates the importance of the intramitochondrial ATP/ADP concentration ratio in the regulation of the rate of electron transfer in this muscle.

NAD in muscle of man at rest and during exercise

NAD can be used to assess the adequacy of oxygen availability to the respiratory chain. An enzymatic assay was established for NAD in human muscle biopsy samples. It gave reliable, reproducible results. The variation within and between subjects was less than 12%. Muscle NAD and lactate were determined at rest, and after bicycle ergometry work requiring approximately 75 and approximately 100% VO2 max (six subjects, four tests each). A positive (P less than 0.01) linear relationship between resting muscle NAD and percent slow twitch fibers was found, suggesting that fiber types may have different NAD content. Muscle NAD decreased during submaximal and maximal work (P less than 0.05). A large portion (73%) of the NAD reduction could be accounted for by increased muscle water. No relationship could be established between NAD and lactate. The negative linear relationship (P less than 0.01) between the muscle/blood ratio and percent slow twitch fibers is another indication of the fiber having different metabolic responses to the activity.

The nature of controlled respiration and its relationship to protonmotive force and proton conductance in blowfly flight-muscle mitochondria

1. To determine whether controlled (State 4) pyruvate oxidation can support a high energy state, measurements of the redox span NAD-cytochrome c, phosphorylation potential and protonmotive force (the gradient in electrochemical activity of protons across the mitochondrial inner membrane) were made as indices of energy status. For comparison, these three measurements were also made with glycerol 3-phosphate, an alternative substrate. The two substrates gave essentially identical values for the redox span NAD-cytochrome c in State 4, and the phosphorylation potential was of sufficient magnitude to be considered in equilibrium with the redox span over the first two phosphorylation sites. The magnitude of the protonmotive force in State 4 was much less and the implications of this finding are discussed. 2. Measurements made during the controlled (State 4) to active (State 3) transition indicated that with glycerol 3-phosphate as substrate, both the redox span NAD-cytochrome c and the protonmotive force were diminished; the State 4 --> State 3 transition with pyruvate as substrate was accompanied by an increase in the redox span but a decrease in protonmotive force. The contrary behaviour of these two energetic parameters in the presence of pyruvate was ascribed to a transient excess in the flux of protons through the adenosine triphosphatase relative to the protonpumping respiratory chain, in spite of the increased dehydrogenase activity. 3. The lower protonmotive force seen in State 3 relative to State 4 with pyruvate as substrate was due to a diminution of both the electrical (DeltaPsi) and the chemical (DeltapH) components; with glycerol 3-phosphate, the magnitude of the decrease in protonmotive force during the State 4 --> State 3 transition was similar to that seen with pyruvate, but was due to a large decrease in the electrical component (DeltaPsi) and a small rise in the chemical component (DeltapH). The reason for the difference seen in the behaviour of the components of the protonmotive force was investigated but not established. 4. In the presence of oligomycin and ADP, oxidation of pyruvate, but not of glycerol 3-phosphate, supported a greater protonmotive force than in State 4, in keeping with the dehydrogenase activation and increased redox span NAD-cytochrome c found under these conditions. 5. Experiments involving the use of uncoupling agent to stimulate respiration are compared with those in which limiting concentrations of ADP were used. Estimates of the proton conductance of the inner membrane indicate a similar non-linear dependence on uncoupler concentration with the two substrates. 6. A model is proposed as an explanation of the high rates of controlled glycerol 3-phosphate oxidation. The model relies on a high permeability of the inner membrane to protons and other ions being induced by glycerol 3-phosphate oxidation in State 4.

The nature and control of the tricarboxylate cycle in beetle flight muscle

The only exogenous substrates oxidized by mitochondria isolated from the flight muscle of the Japanese beetle (Popillia japonica) are proline, pyruvate and glycerol 3-phosphate. The highest rate of oxygen consumption is obtained with proline. The oxidation of proline leads to the production of more NH3 than alanine, indicating a functioning glutamate dehydrogenase (EC 1.4.1.2). Studies of mitochondrial extracts confirm the presence of a very active glutamate dehydrogenase, and this enzyme is found to be activated by ADP and inhibited by ATP. These extracts also show high alanine aminotransferase activity (EC 2.6.1.2) and a uniquely active "malic' enzyme (EC 1.1.1.39). The "malic' enzyme is activated by succinate and inhibited by ATP and by pyruvate. It is suggested that the input of tricarboxylate-cycle intermediate from proline oxidation is balanced by the formation of pyruvate from malate, and the complete oxidation of the majority of the pyruvate. Studies of the steady-state concentrations of mitochondrial CoASH and CoA thioesters during proline oxidation show a high succinyl (3-carboxypropionyl)-CoA content which falls on activating respiration with ADP. There is a concomitant rise in CoASH. However, the reverse transition, from state-3 to state-4 respiration, causes only very slight changes in acylation. The reasons for this are discussed. Studies of the mitochondrial content of glutamate, 2-oxoglutarate, malate, pyruvate, citrate and isocitrate during the same phases of proline oxidation give results consistent with control at the level of glutamate dehydrogenase and isocitrate dehydrogenase during proline oxidation, with the possibility of further control at "malic' enzyme. During the oxidation of pyruvate all of the tricarboxylate-cycle intermediates and NAD(P)H follow the pattern of changes described in the blowfly (Johnson & Hansford, 1975; Hansford, 1974) and isocitrate dehydrogenase is identified as the primary site of control.?2OAuthor

The control of tricarboxylate-cycle oxidations in blowfly flight muscle. The steady-state concentrations of citrate, isocitrate 2-oxoglutarate and malate in flight muscle and isolated mitochondria

1. Blowfly (Phormia regina) flight-muscle mitochondria were allowed to oxidize pyruvate under a variety of experimental conditions, and determinations of the citrate, isocitrate, 2-oxoglutarate and malate contents of both the mitochondria and the incubation medium were made. For each intermediate a substantial portion of the total was present within the mitochondria. 2. Activation of respiration by either ADP or uncoupling agent resulted in a decreased content of citrate and isocitrate and an increased content of 2-oxoglutarate and malate when the substrate was pyruvate, APT and HCO3 minus. Such a decrease in citrate content was obscured when the substrate was pyruvate and proline owing to a large rise in the total content of tricarboxylate-cycle intermediates in the presence of proline and ADP. 3. An experiment involving oligomycin and uncoupling agent demonstrated that the ATP/ADP ratio is the main determinant of flux through the tricarboxylate cycle, with the redox state of nicotinamide nucleotide being of lesser importance. 4. Addition of ADP and Ca-2+ to activate the oxidation of both glycerol 3-phosphate and pyruvate, simulating conditions on initiation of flight, gave a decrease in citrate and isocitrate and an increase in 2-oxoglutarate and malate content. 5. There was a good correlation between these results with isolated flight-muscle mitochondria and the changes found in fly thoraces after 30s and 2 mihorax. 6. It is concluded that NAD-isocitrate dehydrogenase (EC 1.1.1.41) controls the rate of pyruvate oxidation in both resting fly flight muscle in vivo and isolated mitochondria in state 4 (nomenclature of Change & Williams, 1955).