The control of tricarboxylate-cycle of oxidations in blowfly flight muscle. The steady-state concentrations of coenzyme A, acetyl-coenzyme A and succinyl-coenzyme A in flight muscle and isolated mitochondria

A method for assessing mitochondrial physiology using mechanically permeabilized flight muscle of Aedes aegypti mosquitoes

Aedes aegypti is the most important and widespread vector of arboviruses, including dengue and zika. Insect dispersal through the flight activity is a key parameter that determines vector competence, and is energetically driven by oxidative phosphorylation in flight muscle mitochondria. Analysis of mitochondrial function is central for a better understanding of cellular metabolism, and is mostly studied using isolated organelles. However, this approach has several challenges and methods for assessment of mitochondrial function in chemically-permeabilized tissues were designed. Here, we described a reliable protocol to assess mitochondrial physiology using mechanically permeabilized flight muscle of single A. aegypti mosquitoes in combination with high-resolution respirometry. By avoiding the use of detergents, high respiratory rates were obtained indicating that substrate access to mitochondria was not limited. This was confirmed by using selective inhibitors for specific mitochondrial substrates. Additionally, mitochondria revealed highly coupled, as ATP synthase or adenine nucleotide translocator inhibition strongly impacted respiration. Finally, we determined that pyruvate and proline induced the highest respiratory rates compared to other substrates tested. This method allows the assessment of mitochondrial physiology in mosquito flight muscle at individual level, and can be used for the identification of novel targets aiming rational insect vector control.

Site-specific and kinetic characterization of enzymatic and nonenzymatic protein acetylation in bacteria

Reversible N^ε-lysine acetylation has emerging as an important metabolic regulatory mechanism in microorganisms. Herein, we systematically investigated the site-specific and kinetic characterization of enzymatic (lysine acetyltransferase) and nonenzymatic acetylation (AcP-dependent or Acyl-CoA-dependent), as well as their different effect on activity of metabolic enzyme (AMP-forming acetyl-CoA synthetase, Acs). It was found that Bacillus subtilis acetyl-CoA synthetase (BsAcsA) can be acetylated in vitro either catalytically by lysine acetyltransferase BsAcuA and Ac-CoA (at low concentration), or nonenzymatically by Ac-CoA or AcP (at high concentration). Two distinct mechanisms show preference for different lysine acetylation site (enzymatic acetylation for K549 and nonenzymatic acetylation for K524), and reveal different dynamics of relative acetylation changes at these lysine sites. The results demonstrated that lysine residues on the same protein exhibit different acetylation reactivity with acetyl-phosphate and acetyl-CoA, which was determined by surface accessibility, three-dimensional microenvironment, and pKa value of lysine. Acetyl-CoA synthetase is inactivated by AcuA-catalyzed acetylation, but not by nonenzymatic acetylation.

Metabolic control of methylation and acetylation

Methylation and acetylation of DNA and histone proteins are the chemical basis for epigenetics. From bacteria to humans, methylation and acetylation are sensitive to cellular metabolic status. Modification rates depend on the availability of one-carbon and two-carbon substrates (S-adenosylmethionine, acetyl-CoA, and in bacteria also acetyl-phosphate). In addition, they are sensitive to demodification enzyme cofactors (α-ketoglutarate, NAD(+)) and structural analog metabolites that function as epigenetic enzyme inhibitors (e.g., S-adenosylhomocysteine, 2-hydroxyglutarate). Methylation and acetylation likely initially evolved to tailor protein activities in microbes to their metabolic milieu. While the extracellular environment of mammals is more tightly controlled, the combined impact of nutrient abundance and metabolic enzyme expression impacts epigenetics in mammals sufficiently to drive important biological outcomes such as stem cell fate and cancer.

Physiologic mechanisms of sequential products synthesis in 1,3-propanediol fed-batch fermentation by Klebsiella pneumoniae

The glycerol fed-batch fermentation by Klebsiella pneumoniae CGMCC 1.6366 exhibited the sequential synthesis of products, including acetate, 1,3-propanediol (1,3-PD), 2,3-butanediol, ethanol, succinate, and lactate. The dominant flux distribution was shifted from acetate formation to 1,3-PD formation in early- exponential growth phase and then to lactate synthesis in late-exponential growth phase. The underlying physiological mechanism of the above observations has been investigated via the related enzymes, nucleotide, and intermediary metabolites analysis. The carbon flow shift is dictated by the intrinsic physiological state and enzymatic activity regulation. Especially, the internal redox state could serve as a rate-controlling factor for 1,3-PD production. The q(1,3-PD) formation was the combined outcomes of regulations of glycerol dehydratase activity and internal redox balancing. The q(ethanol)/q(acetate) ratios demonstrated the flexible adaptation mechanism of K. pneumoniae preferring ATP generation in early-exponential growth phase. A low PEP to pyruvate ratio corresponded LDH activity increase, leading to lactate accumulation in stationary phase.

Mitochondrial function in flying honeybees (Apis mellifera): respiratory chain enzymes and electron flow from complex III to oxygen

The biochemical bases for the high mass-specific metabolic rates of flying insects remain poorly understood. To gain insights into mitochondrial function during flight, metabolic rates of individual flying honeybees were measured using respirometry, and their thoracic muscles were fixed for electron microscopy. Mitochondrial volume densities and cristae surface densities, combined with biochemical data concerning cytochrome content per unit mass, were used to estimate respiratory chain enzyme densities per unit cristae surface area. Despite the high content of respiratory enzymes per unit muscle mass, these are accommodated by abundant mitochondria and high cristae surface densities such that enzyme densities per unit cristae surface area are similar to those found in mammalian muscle and liver. These results support the idea that a unit area of mitochondrial inner membrane constitutes an invariant structural unit. Rates of O(2) consumption per unit cristae surface area are much higher than those estimated in mammals as a consequence of higher enzyme turnover rates (electron transfer rates per enzyme molecule) during flight. Cytochrome c oxidase, in particular, operates close to its maximum catalytic capacity (k(cat)). Thus, high flux rates are achieved via (i) high respiratory enzyme content per unit muscle mass and (ii) the operation of these enzymes at high fractional velocities.

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.

Regulation of coenzyme A biosynthesis by glucagon and glucocorticoid in adult rat liver parenchymal cells

We studied the effects of glucagon, dibutyryl cyclic AMP and dexamethasone on the rate of [(14)C]pantothenate conversion to CoA in adult rat liver parenchymal cells in primary culture. The presence of 30nm-glucagon increased the rate by about 1.5-fold relative to control cultures (range 1.4-2.3) and 2.4-fold relative to cultures containing 1-3m-i.u. of insulin/ml. The half-maximal effect was obtained at 3nm-glucagon. Dibutyryl cyclic AMP plus theophylline also enhanced the rate by about 1.5-fold. Dexamethasone acted synergistically with glucagon; glucagon at 0.3nm had no effect when added alone, but resulted in a 1.7-fold enhancement when added in the presence of dexamethasone (maximum effect at 50nm). The 1.4-fold enhancement caused by the addition of saturating glucagon concentrations was increased to a 3-fold overall enhancement by the addition of dexamethasone. However, dexamethasone added alone over the range 5nm to 5mum had no effect on the rate of [(14)C]pantothenate conversion to CoA. The stimulatory effect of dibutyryl cyclic AMP plus theophylline was also enhanced by the addition of dexamethasone. Changes in intracellular pantothenate concentration or radioactivity could not account for the stimulatory effects of glucagon, dibutyryl cyclic AMP or dexamethasone. Addition of 18mum-cycloheximide, an inhibitor of protein synthesis, decreased the rate of incorporation of [(14)C]pantothenate into CoA and the enhancement of this rate by glucagon and dibutyryl cyclic AMP plus theophylline in a reversible manner. These results demonstrate an influence of glucagon, dibutyryl cyclic AMP and glucocorticoids on the intracellular mechanism regulating total CoA concentrations in the liver.

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.

Cytochrome c-cytochrome oxidase interaction at subzero temperatures

Cytochrome oxidase forms two distinctive compounds with oxygen at --105 and --90 degrees C, one appears to be oxycytochrome oxidase (Compound A) and the other peroxycytochrome oxidase (Compound B). The functional role of compound B in the oxidation of cytochrome c has been examined in a variety of mitochondrial preparations. The rate and the extent of the reaction have been found to be dependent upon the presence of a fluid phase in the vicinity of the site of the reaction of cytochrome c and cytochrome oxidase. The kinetics of cytochrome c oxidation and of the slowly reacting component of cytochrome oxidase are found to be linked to one another even in cytochrome c depleted preparations, but under appropriate conditions, especially low temperatures, the oxidation of cytochrome c precedes that of this component of cytochrome oxidase. Based upon the identification of the slowly reacting components of cytochrome oxidase with cytochrome c, various mechanisms are considered which allow cytochrome c to be oxidized without the intervention of cytochrome a at very low temperatures, and tunneling seems an appropriate mechanism.

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).

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

1. A study is presented of the mitochondrial NADH content during controlled (state 4) and active (state 3) pyruvate oxidation by blowfly flight-muscle mitochondria. The results confirm and extend those of an earlier study (Hansford, 1972), which indicated an increased reduction in state 3. Nicotinamide nucleotide is normally highly oxidized during state 4; however, there can be substantial reduction in the presence of carnitine or high concentrations of proline, or on lengthy incubation in the presence of either of the systems used to generate intramitochondrial tricarboxylate-cycle intermediate. 2. Omission of phosphate leads to substantial reduction and this can be reversed by adding phosphate or acetate. 3. Estimations of NAD-+ and NADH in fly thoraces show a marked increase in NADH on flight, tending to corroborate the results of mitochondrial experiments and testifying to the importance of dehydrogenase activation in this tissue. 4. Determination of intramitochondrial adenine nucleotides reveals a total of 4-5 nmol/mg of protein, and an ADP content of less than 0.1 nmol/mg during state 4 oxidation of pyruvate and proline. ATP content is found to increase slowly during state 4 and this is attributed to the net phosphorylation of AMP. 5. The uncoupling agent carbonyl cyanide p=trifluoromethoxyphenylhydrazone leads to hydrolysis of some, but not all, of the mitochondrial ATP. Studies of mitochondrial ATPase (adenosine triphosphatase), measured by external pH change, show that it is inactive unless the mitochondria are allowed to respire for several minutes in state 4 in the presence of phosphate before the addition of carbonyl cyanide p-trifluoromethoxyphenylhydrazone. It is suggested that phosphate uptake is essential for maximal ATPase activity. 6. Studies of the fluorescence of the fluorochrome 8-anilino-1-naphthalensulphonic acid suggest that the energy status of the mitochondrion is high during state 4-pyruvate oxidattion, and decrease slightly in state 3. The implications of these findings are discussed.