Rate Of ATP Regeneration Research Articles

Quantifying ATP turnover in anoxic coleoptiles of rice (Oryza sativa) demonstrates preferential allocation of energy to protein synthesis.

Oxygen deprivation limits the energy available for cellular processes and yet no comprehensive ATP budget has been reported for any plant species under O2 deprivation, including Oryza sativa. Using 3-d-old coleoptiles of a cultivar of O. sativa tolerant to flooding at germination, (i) rates of ATP regeneration in coleoptiles grown under normoxia (aerated solution), hypoxia (3% O2), and anoxia (N2) and (ii) rates of synthesis of proteins, lipids, nucleic acids, and cell walls, as well as K+ transport, were determined. Based on published bioenergetics data, the cost of synthesizing each class of polymer and the proportion of available ATP allocated to each process were then compared. Protein synthesis consumed the largest proportion of ATP synthesized under all three oxygen regimes, with the proportion of ATP allocated to protein synthesis in anoxia (52%) more than double that in normoxic coleoptiles (19%). Energy allocation to cell wall synthesis was undiminished in hypoxia, consistent with preferential elongation typical of submerged coleoptiles. Lipid synthesis was also conserved strongly in O2 deficits, suggesting that membrane integrity was maintained under anoxia, thus allowing K+ to be retained within coleoptile cells. Rates of protein synthesis in coleoptiles from rice cultivars with contrasting tolerance to oxygen deficits (including mutants deficient in fermentative enzymes) confirmed that synthesis and turnover of proteins always accounted for most of the ATP consumed under anoxia. It is concluded that successful establishment of rice seedlings under water is largely due to the capacity of coleoptiles to allocate energy to vital processes, particularly protein synthesis.

Interaction among Skeletal Muscle Metabolic Energy Systems during Intense Exercise.

High-intensity exercise can result in up to a 1,000-fold increase in the rate of ATP demand compared to that at rest (Newsholme et al., 1983). To sustain muscle contraction, ATP needs to be regenerated at a rate complementary to ATP demand. Three energy systems function to replenish ATP in muscle: (1) Phosphagen, (2) Glycolytic, and (3) Mitochondrial Respiration. The three systems differ in the substrates used, products, maximal rate of ATP regeneration, capacity of ATP regeneration, and their associated contributions to fatigue. In this exercise context, fatigue is best defined as a decreasing force production during muscle contraction despite constant or increasing effort. The replenishment of ATP during intense exercise is the result of a coordinated metabolic response in which all energy systems contribute to different degrees based on an interaction between the intensity and duration of the exercise, and consequently the proportional contribution of the different skeletal muscle motor units. Such relative contributions also determine to a large extent the involvement of specific metabolic and central nervous system events that contribute to fatigue. The purpose of this paper is to provide a contemporary explanation of the muscle metabolic response to different exercise intensities and durations, with emphasis given to recent improvements in understanding and research methodology.

Nitrogenase activity and regeneration of the cellular ATP pool in Azotobacter vinelandii adapted to different oxygen concentrations.

The in vivo activity of nitrogenase under aerobiosis was studied with diazotrophic chemostat cultures of Azotobacter vinelandii grown under glucose- or phosphate-limited conditions at different dilution rates (Ds, representing the growth rate mu) and different dissolved oxygen concentrations. Under steady-state conditions, the concentration as well as the cellular level of ATP increased in glucose-limited cultures when D was increased. Irrespective of the type of growth limitation or the dissolved oxygen concentration, the steady-state concentrations of ATP and of dinitrogen fixed by nitrogenase increased in direct proportion to each other. Specific rates of dinitrogen fixation as well as of the regeneration of the cellular ATP pool were compared with specific rates of cellular respiration. With glucose-limited cultures, the rate of regeneration of the ATP pool and the rate of respiration varied in direct proportion to each other. This relationship, however, was dependent on the dissolved oxygen concentration. As compared to the phosphate-sufficient control, phosphate-limited cultures exhibited the same nitrogenase activity but significantly increased respiratory activities. Rates of ATP regeneration and of cellular respiration of phosphate-limited cultures did not fit into the relationship characteristic of glucose-limited cultures. However, a linear relationship between the rates of dinitrogen fixation and ATP regeneration was identified irrespective of the type of growth limitation and the dissolved oxygen concentration. The results suggest that the ATP supply rather than cellular oxygen consumption is of primary importance in keeping nitrogenase activity in aerobic cultures of A. vinelandii.

Microtubule stabilization and potentiation of taxol activity by the creatine analog cyclocreatine.

Creatine kinase (CK), a key enzyme of cellular energetics, has been implicated in tumorigenesis. Cyclocreatine (CCr), which forms a stable phosphagen with a reduced rate of ATP regeneration through CK, inhibits the growth of many solid tumors. We report that CCr induces the formation of unusually stable microtubules that resist depolymerization by nocodazole. By reducing ATP availability, CCr may modulate the activity of kinases that regulate microtubule dynamics. Further, combinations of CCr and taxol resulted in the synergistic killing of breast tumor cells indicating that CCr may be a useful addition to chemotherapy's that include taxanes.

Kinetics and Mechanism of Acetate Kinase From Bacillus Stearothermophilus.

The initial rates of ATP regeneration catalyzed by acetate kinase (ATP: acetate phosphotransferase; EC 2.7.2.1) from Bacillus stearothermophilus were measured under a wide range of MgADP–, acetyl phosphate, MgATP2– and acetate concentrations. The experimental results showed that the enzyme exhibited positive co-operativity for MgATP2– and no co-operativity for MgADP–, acetyl phosphate and acetate. Furthermore, the initial rates of the forward reaction, in which MgATP2– and acetate were produced from MgADP– and acetyl phosphate, were inhibited by MgATP2– and acetate, and those of the reverse reaction were inhibited by MgADP– and acetyl phosphate. The initial rate data were correlated well by using the rate equation derived based on the assumption that the reaction obeys the general reaction scheme based on the Random Bi Bi mechanism, and on the previous experimental result that the enzyme behaves like a dimeric enzyme, even though it is a tetrameric enzyme.

Evidence for Mitochondrial Regulation of Photosynthesis by a Starchless Mutant of Nicotiana sylvestris

Mutant NS458 of Nicotiana sylvestris (Speg. et Comes) contains a defective plastid phosphoglucomutase and accumulates only trace amounts of starch. Determinations of carbon partitioning using tracer d-[3-(14)C]glyceric acid showed that the maximal CO(2) assimilation by mature leaves of the mutant at saturating [CO(2)] and light and low [O(2)] was close to the flux for sucrose formation in the wild type. The mutant is characterized by exceptionally slow oscillations in maximal CO(2) assimilation. The postulate that these slow oscillations follow changes in the cytosolic rate of sucrose phosphate synthesis has been investigated. Studies with wild-type and mutant leaf discs subjected to various treatments failed to indicate that any significant activation-inactivation cycle in sucrose-P synthase activity can occur. The rate of sucrose phosphate synthesis, however, might be altered by variations in the supply of uridine UDP-glucose which is controlled by the rate of ATP regeneration (via UTP regeneration). Treating mutant leaf protoplasts and young leaves with oligomycin, an inhibitor of mitochondrial ATP regeneration, reduced photosynthesis by as much as 25 and 40%, respectively. The wild type failed to show inhibition by oligomycin, i.e. its effect is masked when starch and sucrose synthesis can interact. It is concluded that maximal CO(2) assimilation in the mutant is fine tuned by mitochondrial metabolism such that interactions between sucrose synthesis and mitochondrial processes may generate the observed oscillations.

How important is endogenous muscle glycogen to fatigue in prolonged exercise?

Endogenous muscle glycogen represents a primary fuel source during large muscle group activity in the human. The depletion of this fuel source during submaximal exercise at intensities ranging between 60 and 85% of maximal aerobic power (Vo2max) is widely believed to be the cause of an inability to sustain exercise. Alterations of preexercise muscle glycogen reserves by dietary and exercise manipulations and changing the degree of dependency on endogenous glycogen during exercise by modifying the availability of other fuel sources have in general served to establish a close relationship between muscle glycogen and fatigue resistance. However, in spite of the evidence implicating glycogen depletion to fatigue, the mechanism remains elusive. The most popular theory is that glycogen is an essential substrate, the depletion of which results in a reduction in the rate of ATP regeneration and an inability to maintain energy supply to one or more of the processes involved in excitation and contraction in the muscle. As a consequence, the muscle is unable to translate the motor drive into an expected force and fatigue develops. However, there is little experimental evidence to support this theory. Most studies report no or only minimal changes in ATP concentration at fatigue with low glycogen and no further change in the by-products of ATP hydrolysis. These findings suggest that fatigue might be caused by other nonmetabolic factors. This review examines these other nonmetabolic factors and analyzes their potential role in fatigue during prolonged exercise with depletion of muscle glycogen reserves.

Changes in Plasma Concentration of Hypoxanthine and Uric Acid in Man with Short-Distance Running at Various Intensities

The relationship between running intensity and the accumulation of hypoxanthine and uric acid in plasma was studied in four well-trained runners. The runners each performed several 800-m runs at different velocities, each run being performed on separate days. Venous blood samples were collected before and at regular intervals after the runs. The concentration of hypoxanthine and uric acid was determined with high performance liquid chromatography (HPLC) in plasma extracts. A marked increase in the plasma concentration of both hypoxanthine and uric acid occurred simultaneously at intensities corresponding to 110, 108, 115 and 107% of VO2max for subject a, b, c and d, respectively. The sudden sharp increase in plasma concentration of hypoxanthine and uric acid may indicate that at a certain level of running intensity ATP catabolism exceeds the rate of ATP regeneration from the normal metabolic pathways.

Efficiency factors and ATP/ADP ratios in nitrogen-fixing Bacillus polymyxa and Bacillus azotofixans

The efficiency factor, the number of moles of ATP generated per mole of glucose fermented, was determined in anaerobic, non-carbon-limited N2-fixing cultures of Bacillus polymyxa, Bacillus macerans, Bacillus azotofixans, and Clostridium butyricum through identification and quantitation of the fermentation products by 13C nuclear magnetic resonance spectroscopy and measurement of acetate kinase activities. All three Bacillus species had acetate kinase activities and produced acetate and ethanol as the major fermentation products. The maximum amounts of ATP generated per mole of glucose fermented were 2.70, 2.64, and 2.88 mol in B. polymyxa, B. macerans, and B. azotofixans, respectively, compared with 3.25 mol in C. butyricum. Thus, in the N2-fixing Bacillus species, the efficiency factors are lower than that in C. butyricum. Steady-state ATP/ADP concentration ratios were measured in non-carbon-limited N2-fixing cultures of B. polymyxa and B. azotofixans through separation and quantitation of the adenylates in cell extracts by ion-pair reversed-phase high-performance liquid chromatography. The observed ATP/ADP ratios were 4.5 and 3.8, and estimated energy charges were 0.81 to 0.86 and 0.81 to 0.83, respectively, for B. polymyxa and B. azotofixans. The results suggest that under these growth conditions, the rate of ATP regeneration is adequate to meet the energy requirement for N2 fixation in the Bacillus species, in contrast to N2-fixing Clostridium pasteurianum and Klebsiella pneumoniae, for which substantially lower steady-state ATP/ADP ratios and energy charges have been reported. Implications of the results are discussed in relation to possible differences between Bacillus and Clostridium species in energy requirements for N2 fixation and concomitant ammonia assimilation.

Effect of Oxygen and Malate on NO3− Inhibition of Nitrogenase in Soybean Nodules

Soybean (Glycine max cv Hodgson) nitrogenase activity (C(2)H(2) reduction) in the presence or absence of nitrate was studied at various external O(2) tensions. Nitrogenase activity increased with oxygen partial pressure up to 30 kilopascals, which appeared to be the optimum. A parallel increase in ATP/ADP ratios indicated a limitation of respiration rate by low O(2) tensions in the nodule, and the values found for adenine nucleotide ratios suggested that the nitrogenase activity was limited by the rate of ATP regeneration. In the presence of nitrate, the nitrogenase activity was low and less stimulated by increased pO(2), although the nitrite content per gram of nodules decreased from 0.05 to 0.02 micromole when pO(2) increased from 10 to 30 kilopascals. Therefore, the accumulation of nitrite inside the nodule was probably not the major cause of the inhibition. Instead, inhibition by nitrate could be due to competition for reducing power between nitrate reduction and bacteroid or mitochondrial respiration inside the nodule. This is supported by the observation of decrease in ATP/ADP ratios from 1.65, in absence of nitrate, to 0.93 in the presence of this anion at 30 kilopascals O(2). Furthermore, the inhibition was suppressed by the addition, to the plant nutrient solution, of 15 millimolar l-malate, a carbon substrate that is considered to be the major source of reductant for the bacteroids in the symbiosis.

A study of the impaired growth of roots of Zea mays seedlings at low oxygen concentrations

Abstract. Seedlings of Zea mays L. were grown in the dark at 27°C. Four‐day‐old seedlings were then exposed for 3 days to solutions equilibrated with gas mixtures to give O2 concentrations between 0.02 and 0.25 mol m−3. Root growth was impaired just as severely at 0.06 as 0.02 mol O2 m−3 while growth at 0.16 mol O2 m−3 was about the same as in solutions in equilibrium with air (0.25 mol O2 m−3).Growth of young seedlings at low O2 concentrations was inhibited to the same extent in nutrient solution and 0.5 ml m−3 CaCl2, showing that the adverse effect of O2 deficits on growth was not due to less uptake of inorganic nutrients. Furthermore, at low O2 concentrations neither exposure of the shoots to a relative humidity of 100% (26.0 g H2O m−3) nor excision of the entire shoot enhanced root growth relative to that in plants with shoots at a relative humidity of 50% (13.0 g H2O m−3). Therefore, for these seedlings growing in the dark, impairment of root growth at low O2 concentrations was not a consequence of water deficits in the shoot or of other shoot‐root interactions.Total soluble sugars and amino acid concentrations were generally greater at low (0.02–0.06 mol O2m−3) than at high O2 concentrations (0.16–0.25 mol O2 m −3). This applied specifically to the root apices (0–2 mm) and expanding (2–15 mm) tissue except in some experiments where sugar concentrations in expanding tissue were slightly greater at high than at low O2 concentrations.Critical O2 pressures for respiration of excised root segments were approximately 0.117 and 0.065 mol O2 m−3 in the expanding and expanded zones of the roots, respectively. In contrast, the critical O2 pressure exceeded 0.20 mol O2 m−3 in the apex, suggesting that O2 supply for metabolic processes is most likely to be sub‐optimal in this zone. Our results show clearly that the adverse effects of low O2 concentrations are unlikely to be a consequence of substrate shortage for either respiration or synthesis of macromolecules; low rates of ATP regeneration in growing root tissues are the logical cause for impaired growth in young seedlings while they are being sustained by seed reserves.