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PubMed2005

Increased cross-bridge cycling rate in stunned myocardium.

Decreased Ca2+ responsiveness of the myofilaments underlies myocardial stunning. Given that cross-bridge cycling is a major determinant of myofilament behavior, we quantified cross-bridge cycling rate in stunned myocardium. After stabilization, rat hearts were subjected to 20 min of no-flow global ischemia and 30 min of reperfusion at 37 degrees C. Control hearts were perfused continuously at 37 degrees C for 60 min. Trabeculae were dissected and chemically skinned with 1% Triton X-100. The muscles were then activated with solutions of varied Ca2+ concentration ([Ca2+]). Force-[Ca2+] relations, rate of force redevelopment after release (k(tr)), muscle stiffness (k(m)), and myofilament ATP consumption were determined. Maximal Ca2+-activated force (Fmax) was depressed in stunned myocardium (49 +/- 5 vs. 82 +/- 5 mN/mm2, P < 0.01). Western immunoblotting showed degradation of troponin I in stunned myocardium. The k(tr) at Fmax was significantly increased in stunned muscles (19.82 +/- 2.74 vs. 13.19 +/- 0.96 s(-1), 22 degrees C, P < 0.01; 7.49 +/- 0.52 vs. 5.81 +/- 0.54 s(-1), 10 degrees C, P < 0.05). The ratio of k(m) measured at 100 Hz over that at 1 Hz, during Fmax, is lower in stunned muscles (8.22 +/- 1.56 vs. 12.94 +/- 0.71, P < 0.05). In comparison with k(m) at rigor, k(m) at Fmax is significantly lower in the stunned group (78.82 +/- 6.11 vs. 93.27 +/- 3.03%, P < 0.05). Myofilament ATP consumption at Fmax did not change in stunned muscles (5,901 +/- 952 vs. 5,596 +/- 972 pmol x microl(-1) x min(-1), P = 0.49). These results show that cross-bridge cycling is increased in stunned myocardium. Such increases are likely the result of increased transition rate from force-generating states to non-force-generating states. Thus stunned myocardium still maintains ATP consumption in spite of lower force development, rationalizing the long-standing paradox of decreased force but unchanged oxygen consumption in the postischemic heart.

Actin Cytoskeleton↗
PubMed1986

Quantification of ATP-producing and consuming processes of Ehrlich ascites tumour cells.

ATP production of Ehrlich ascites tumour cells was estimated on the basis of their coupled respiration and lactate formation. ATP-consuming processes were assessed from the effects of selective inhibitors of RNA synthesis, protein synthesis and proteolysis, Na+/K+-ATPase on respiration. The extent of protein synthesis and proteolysis were also determined directly. From these values and those of the inhibition of respiration by selective inhibitors, a P/O ratio of 2.2 was calculated. About 75% of the total ATP consumption could be assigned to specific processes. The major ATP-consuming processes of tumour cells in an amino-acid-enriched medium, in which they are in an approximate steady state, are protein synthesis with about 30% of total ATP consumption, and Na+/K+-ATPase with about 20%, while RNA synthesis, ATP-dependent proteolysis and Ca2+-ATPase contribute about 10% each. In an amino-acid-free glucose medium, protein synthesis is reduced to a third, with a corresponding decrease of respiration, whereas the rate of the other ATP-consuming processes is unchanged.

Adenosine Triphosphatases↗
PubMed1981

ATP utilizing reactions of human erythrocyte membranes and the influence of modulator proteins.

The ATP production of human erythrocytes in the steady state (approximately 2 mmoles . 1 cells-1 . h-1, 37 degrees C, pHi 7.2) is maintained by glycolysis and the ATP consumption is essentially limited to the cell membrane. About 25% of the ATP consumption is used for ion transport ATPases. The bulk of the ATP consuming processes in intact erythrocytes remains poorly understood. "Isotonic" erythrocyte membranes prepared under approximate intracellular conditions after freeze-thaw hemolysis have high (Ca2+, Mg2+)-ATPase activities (80% of the total membrane ATPase activity). There is a great discrepancy between the high capacity of the (Ca2+, Mg2+)-ATPase in isotonic membranes and the actual activity in the intact cell. The (Ca2+, Mg2+)-ATPase of isotonic membranes has a "high" Ca2+-affinity (Ka less than 0.5 microM) and a "low" Mg-ATP affinity (Km approximately 760 microM). This state of (Ca2+, Mg2+)-ATPase is caused by the association of calmodulin and 30000 Dalton polypeptides (ATP affinity modulator protein). Hypotonic washings of isotonic membranes result in a loss of the 30 kD polypeptides. EGTA (0.5 mM) extracts derived from isotonic membranes contain the 30 kD modulator protein and restore the properties of the (Ca2+, Mg2+)-ATPase of hypotonic membrane preparations to the isotonic characteristics. The Mg-ATP affinity modulator protein is assumed to form a complex with calmodulin and (Ca2+, Mg2+)-ATPase.

Adenosine Triphosphate↗
PubMed1998

Energy requirements for two aspects of phospholipid metabolism in mammalian brain.

Previous estimates have placed the energy requirements of total phospholipid metabolism in mammalian brain at 2% or less of total ATP consumption. This low estimate was consistent with the very long half-lives (up to days) reported for fatty acids esterified within phospholipids. However, using an approach featuring analysis of brain acyl-CoA, which takes into account dilution of the precursor acyl-CoA pool by recycling of fatty acids, we reported that half-lives of fatty acids in phospholipids are some 100 times shorter (min-h) than previously thought. Based on these new estimates of short half-lives, palmitic acid and arachidonic acid were used as prototype fatty acids to calculate energy consumption by fatty acid recycling at the sn-1 and sn-2 positions of brain phospholipids. We calculated that the energy requirements for reacylation of fatty acids into lysophospholipids are 5% of net brain ATP consumption. We also calculated ATP requirements for maintaining asymmetry of the aminophospholipids, phosphatidylserine and phosphatidylethanolamine across brain membrane bilayers. This asymmetry is maintained by a translocase at a stoichiometry of 1 mol of ATP per mol of phospholipid transferred in either direction across the membrane. The energy cost of maintaining membrane bilayer asymmetry of aminophospholipids at steady-state was calculated to be 8% of total ATP consumed. Taken together, deacylation-reacylation and maintenance of membrane asymmetry of phosphatidylserine and phosphatidylethanolamine require about 13% of ATP consumed by brain as a whole. This is a lower limit for energy consumption by processes involving phospholipids, as other processes, including phosphorylation of polyphosphoinositides and de novo phospholipid biosynthesis, were not considered.

Acyl Coenzyme A↗
PubMed1991

Mitochondrial effects of the guanidino group-containing cytostatic drugs, m-iodobenzylguanidine and methylglyoxal bis (guanylhydrazone).

The involvement of mitochondrial damage in the antiproliferative effects of m-iodobenzylguanidine [MIBG] and methylglyoxal bis (guanylhydrazone) [methylGAG] was studied in human neuroblastoma SK-N-SH, mouse neuroblastoma N1E115 and mouse lymphosarcoma S49 cells. Proliferation of SK-N-SH cells was insensitive to MIBG (100 microM gave 15% inhibition), but sensitive to methylGAG (IC50 = 50 microM). MIBG and methylGAG were approximately equitoxic to N1E115 cells (IC50 of 92 and 87 microM, respectively). S49 cells were most sensitive to both MIBG (IC50 = 11 microM) and methylGAG (IC50 = 5 microM). In isolated sonicated mitochondria, MIBG inhibited respiration a complex I of the respiratory chain (EC50 = 0.5 mM), whereas methylGAG was much less effective (EC50 greater than 15 mM). In intact cells, MIBG at 31 microM impaired mitochondrial respiration and stimulated the glycolytic flux. In contrast, equimolar concentrations of methylGAG had no effect on oxygen consumption, ATP content, glucose consumption and lactate production. MethylGAG significantly increased putrescine levels in N1E115 and S49 cells within 12 hr via inhibition of S-adenosylmethionine decarboxylase. No such effects were seen in SK-N-SH cells for up to 48 hr. Equimolar concentrations of MIBG had no effect on the putrescine levels in the various cell lines, suggesting that MIBG did not inhibit S-adenosylmethionine decarboxylase. It is concluded that the antiproliferative mechanisms of the guanidino compounds are essentially different. MIBG inhibited mitochondrial respiration at complex I with concomitant stimulation of the glycolytic flux but was essentially without effect on polyamine levels. On the other hand, cytotoxicity of methylGAG was not associated with mitochondrial dysfunction.

3-Iodobenzylguanidine↗
PubMed1985

Effects of a met-enkephalin analogue on motility, O2 consumption, and ATP content of human spermatozoa.

Opioid narcotics are present in seminal plasma, although their physiological effect on spermatozoa is still unknown. This study reports data on metabolic parameters of human spermatozoa in the presence of a met-enkephalin analogue: D-Ala2-Mephe4-Met-(o)-ol-Enkephalin, FK 33824, Sandoz, Basel, Switzerland (DAMME), and its receptor antagonist naloxone hydrochloride, Endo Laboratories, Garden City, New York. Our findings indicate that the metenkephalin analogue reduces sperm motility and cellular O2 consumption without affecting cellular ATP content and viability. The hypothesis that DAMME acts on adenylate-cyclase is briefly discussed.

Adenosine Triphosphate↗
PubMed1993

On the coupling between ATP usage and DNA transport by yeast DNA topoisomerase II.

The initial rates of ATP hydrolysis and relaxation of negatively supercoiled DNA by highly purified wild-type and mutant yeast DNA topoisomerase II were measured under identical conditions to study the coupling between the ATPase activity of a type II DNA topoisomerase and its catalysis of the transport of one DNA segment through another. The results indicate that the binding of the enzyme to DNA stimulates its intrinsic ATPase activity by about 20-fold, and ATP binding to the pair of ATPase sites in a DNA-bound dimeric enzyme appears to be cooperative. The cooperativity in ATP binding may be significant in the coordination of the two halves of a DNA-bound enzyme dimer. At low ATP concentrations, the rate-limiting step in ATP usage appears to be slower than that in DNA transport, and DNA transport is relatively efficient in terms of ATP consumption: 1.9 +/- 0.5 ATP molecules are hydrolyzed/DNA transport event. At a saturating ATP concentration, however, there appears to be a reversal of these rate-limiting steps, and DNA transport is less efficient: 7.4 +/- 1.0 ATP molecules are hydrolyzed/DNA transport event. These data are interpreted in terms of a model in which a DNA-bound enzyme acts as an ATP-operated clamp for the capture and transport of a second DNA segment.

Adenosine Diphosphate↗
PubMed1991

Ischemic preconditioning preserves creatine phosphate and intracellular pH.

BACKGROUND: Ischemic preconditioning slows ATP depletion and ultrastructural damage during the final episode of ischemia. To define the influence of creatine phosphate (CP) and intracellular pH (pHi) on this effect, CP and pHi were serially measured in porcine hearts without collateral circulation by using 31P-NMR spectroscopy and ultrastructural examination. METHODS AND RESULTS: Farm pigs weighing 12-15 kg were anesthetized with Fluothane. The control group underwent a single occlusion (20 minutes or 60 minutes); the preconditioned group underwent four episodes of 5-minute occlusion and 5-minute reperfusion followed by a sustained occlusion (20 minutes or 60 minutes). After ischemic preconditioning, CP increased to 115 +/- 11% (p less than 0.05) of preischemic value and ATP decreased to 84 +/- 8% (p less than 0.05) of preischemic value, but pHi returned to preischemic value. At 5 and 10 minutes of sustained ischemia, CP was significantly preserved in the preconditioned group (control group, 19 +/- 3% versus preconditioned group, 29 +/- 4% at 5 minutes; control group, 5 +/- 3% versus preconditioned group, 11 +/- 3% at 10 minutes; p less than 0.05). At 15 and 20 minutes of sustained ischemia, ATP was significantly preserved in the preconditioned group (control group, 64 +/- 3% versus preconditioned group, 73 +/- 3% at 15 minutes; control group, 51 +/- 7% versus preconditioned group, 62 +/- 2% at 20 minutes; p less than 0.05). At 10, 15, 20, and 25 minutes of sustained ischemia, pHi was significantly higher in the preconditioned group (control group, 6.5 +/- 0.05 versus preconditioned group, 6.7 +/- 0.1 at 10 minutes; control group, 6.3 +/- 0.05 versus preconditioned group, 6.6 +/- 0.06 at 15 minutes; control group, 6.1 +/- 0.1 versus preconditioned group, 6.4 +/- 0.1 at 20 minutes; control group, 6.0 +/- 0.2 versus preconditioned group, 6.3 +/- 0.1 at 25 minutes; p less than 0.05). Ultrastructural changes were milder in the preconditioned group at 20 minutes of sustained ischemia. CONCLUSIONS: In addition to ATP and ultrastructure, preconditioning preserved CP and pHi during sustained ischemia. These protective effects might be due to overshoot phenomenon of CP and/or reduced ATP consumption. The relatively longer period of preservation of pHi, which probably is the result of reduced ATP consumption, indicates its greater contribution to reducing infarct size than that of CP and ATP.

Adenosine Triphosphate↗
PubMed2000

Regulation of ATP supply in mammalian skeletal muscle during resting state-->intensive work transition.

In the present debating paper, the problem how the rate of ATP supply by oxidative phosphorylation in mitochondria is adjusted to meet a greatly increased demand for ATP during intensive exercise of skeletal muscle is discussed. Different experimental results are collected from different positions of the literature and confronted with five conceptual models of the regulation of the oxidative phosphorylation system. The previously performed computer simulations using a dynamic model of oxidative phosphorylation are also discussed in this context. The possible regulatory mechanisms considered in the present article are: (A) output activation: an external effector activates directly only the output of the system (ATP turnover); (B) input/output activation: an external effector activates directly the output (ATP usage) and input (substrate dehydrogenation) of the system; (C) removal of substrate shortage: only ATP consumption and substrate supply by blood are directly activated; (D) removal of oxygen shortage: only ATP consumption and oxygen supply by blood are directly activated; (E) each step activation: an external effector activates both the ATP-consuming subsystem and all the steps in the ATP-producing subsystem (particular enzymes/carriers/blocks of oxidative phosphorylation, substrate supply, oxygen supply). The performed confrontation of the considered mechanisms with the presented results leads to the conclusion that only the each step activation model is quantitatively consistent with the whole set of experimental data discussed. It is therefore postulated that a universal effector/regulatory mechanism of a still unknown nature which activates all steps of oxidative phosphorylation should exist and be discovered. A possible nature of such an effector is shortly discussed.

Adenosine Triphosphate↗
PubMed1994

Energetics of crossbridge phosphorylation and contraction in vascular smooth muscle.

Ca(2+)-dependent crossbridge phosphorylation is the primary mechanism governing crossbridge cycling in smooth muscle. A four-state crossbridge model in which phosphorylation is the only proposed regulatory mechanism was successful in predicting the mechanical properties of the swine carotid media including latch (sustained force with reduced crossbridge cycling). This model also predicts that the ATP consumption of crossbridge phosphorylation is approximately equal to that of crossbridge cycling and that ATP consumption will rise hyperbolically with increases in steady-state force. This review shows these predictions to be consistent with the available energetics data for the carotid media. The absolute energetic cost of covalent regulation is modest and less than the energy savings associated with latch. However, covalent regulation should reduce the total mechanical efficiency of smooth muscle relative to striated muscle.

Animals↗
PubMed1987

Regulation of arginine-ornithine exchange and the arginine deiminase pathway in Streptococcus lactis.

Streptococcus lactis metabolizes arginine by the arginine deiminase (ADI) pathway. Resting cells of S. lactis grown in the presence of galactose and arginine maintain a high intracellular ornithine pool in the absence of arginine and other exogenous energy sources. Addition of arginine results in a rapid release of ornithine concomitant with the uptake of arginine. Subsequent arginine metabolism results intracellularly in high citrulline and low ornithine pools. Arginine-ornithine exchange was shown to occur in a 1-to-1 ratio and to be independent of a proton motive force. The driving force for arginine uptake in intact cells is supplied by the ornithine and arginine concentration gradients formed during arginine metabolism. These results confirm studies of arginine and ornithine transport in membrane vesicles of S. lactis (A. J. M. Driessen, B. Poolman, R. Kiewiet, and W. N. Konings, Proc. Natl. Acad. Sci. USA, 84:6093-6097). The activity of the ADI pathway appears to be affected by the internal concentration of (adenine) nucleotides. Conditions which lower ATP consumption (dicyclohexylcarbodiimide, high pH) decrease the ADI pathway activity, whereas uncouplers and ionophores which stimulate ATP consumption increase the activity. The arginine-ornithine exchange activity matches the ADI pathway most probably by adjusting the intracellular levels of ornithine and arginine. Regulation of the ADI pathway and the arginine-ornithine exchanger at the level of enzyme synthesis is exerted by glucose (repressor, antagonized by cyclic AMP) and arginine (inducer). An arginine/ornithine antiport was also found in Streptococcus faecalis DS5, Streptococcus sanguis 12, and Streptococcus milleri RH1 type 2.

Adenine Nucleotides↗
PubMed1986

ATP production and consumption of rabbit reticulocytes increase in an amino-acid-enriched medium.

The ATP production and the main processes of ATP consumption, globin synthesis, Na+, K+-ATPase, and proteolysis of rabbit reticulocytes in two different media (1. complete medium with amino acids and glucose and 2. glucose containing electrolyte medium) were measured. The ATP formation in the complete medium amounts to 157 mmol/l X h which is 28% higher than in the incomplete medium. The increased ATP formation rate is due to increased ATP consumption by the more than doubled protein synthesis in the complete medium, whereas proteolysis and Na+,K+-ATPase are nearly unchanged. The increase in the glucose consumption in the complete medium is not fully accounted for by the increased oxidative degradation of glucose in the citric acid cycle and by lactate accumulation.

Adenosine Triphosphate↗
PubMed1993

The regulation of bidirectional mitochondrial transport is coordinated with axonal outgrowth.

Although small molecules such as ATP diffuse freely in the cytosol, many types of cells nonetheless position their mitochondria in regions of intense ATP consumption. We reasoned that in the highly elongated axonal processes of growing neurons in culture, the active growth cone would form a focus of ATP consumption so distant from the cell body as to require the positioning of mitochondria nearby via regulated axonal transport. To test this hypothesis, we quantified the distribution and transport behavior of mitochondria in live, aerobically respiring chick sympathetic neurons. We found that in the distal region of actively growing axons, the distribution of mitochondria was highly skewed toward the growth cone, with a sevenfold higher density in the region immediately adjacent to the growth cone than in the region 100 microns away. When axonal outgrowth was blocked by substratum-associated barriers or mild cytochalasin E treatment, the gradient of mitochondrial distribution collapsed as mitochondria exited retrogradely from the distal region, becoming uniformly distributed along the axon within one hour. Analysis of individual mitochondrial behaviors revealed that mitochondrial movement everywhere was bidirectional but balanced so that net transport was anterograde in growing axons and retrograde in blocked axons. This reversal in net transport derived from two separate modulations of mitochondrial movement. First, moving mitochondria underwent a transition to a persistently stationary state in the region of active growth cones that was reversed when growth cone activity was halted. Second, the fraction of time that mitochondria spent moving anterogradely was sharply reduced in non-growing axons. Together, these could account for the formation of gradients of mitochondria in growing axons and their dissipation when outgrowth was blocked. This regulated transport behavior was not dependent upon the ability of mitochondria to produce ATP. Our data indicate that mitochondria possess distinct motor activities for both directions of movement and that mitochondrial transport in axons is regulated by both recruitment between stationary and moving states, and direct regulation of the anterograde motor.

Anaerobiosis↗
PubMed1984

Energy consumption in a cyclic phosphorylation/dephosphorylation cascade.

Cyclic phosphorylation/dephosphorylation cascade systems are responsible for regulating numerous metabolic pathways. The capacity of a cyclic cascade system to maintain a steady-state level of phosphorylation and, hence, a specific biological activity of a phosphorylatable protein is dependent upon a constant supply of metabolic energy (ATP). Quantification of the extent of ATP consumption in a cyclic cascade was examined experimentally with the model in vitro phosphorylation/dephosphorylation system described in detail in the previous paper (Shacter, E., Chock, P. B., and Stadtman, E. R. (1984) J. Biol. Chem. 259, 12252-12259). The results indicate that (a) when the concentrations of converter enzymes and interconvertible substrate are held constant and the fractional phosphorylation of the substrate is varied by changing the allosteric effector concentrations, the rate of ATP consumption in the monocyclic cascade is directly proportional to the steady-state level of phosphorylation being maintained. (b) Attainment of a particular steady-state level of phosphorylation is determined by the net ratio of the protein kinase and phosphatase activities and is independent of the absolute concentrations of these enzymes. (c) Whereas the time required to reach a given steady state is inversely proportional to the converter enzyme concentrations, the amount of ATP consumed in maintaining that steady state is directly proportional to the kinase and phosphatase concentrations. In addition, a theoretical analysis based upon experimentally determined parameters for two in vivo cyclic cascade systems (pyruvate kinase and glycogen phosphorylase) revealed that under normal conditions, cyclic phosphorylation/dephosphorylation cascades consume only a small proportion (less than 0.02%) of the total cellular energy flux.

Adenosine Triphosphate↗
PubMed1977

The effects of calcium on glycolysis and ATP concentration in complete and membrane-poor hemolyzates of human erythrocytes.

Hemolyzates prepared from packed human red cells with 30 micrometer total calcium were employed as a means to examine the relationship between ATP consumption and lactate formation. Hemolyzates exposed to ultracentrifugation accumulate membrane fragments in the top layer yielding membrane-poor fractions in the buttom layers of the centrifuge tube. Lactate formation accompanied by ATP depletion amounts to 12 mumoles per ml and hour in complete hemolyzates fortified with NAD. Complexation of calcium results in about 50% inhibition of the lactate formation with a concomitant increase of ATP. Lactate formation is reduced in membrane-poor hemolyzates approximately concurrently to the extent of membrance removal which produces no discernible change in the glyceral-dehydephosphate dehydrogenase activity. 50--200 micrometer total calcium has no effect on the membrane-independent lactate formation which amounts to 1--2 mumoles per ml and hour. Triton X-100 seems to solubilise also the membrane components responsible for the high calcium-dependent ATP consumption which governs the lactate formation.

Adenosine Triphosphate↗
PubMed1992

Adenosine 5'-triphosphate consumption by smooth muscle as predicted by the coupled four-state crossbridge model.

We have proposed a four-state crossbridge model to explain contraction and the latch state in arterial smooth muscle. Ca(2+)-dependent crossbridge phosphorylation was the only postulated regulatory mechanism and the latchbridge (a dephosphorylated, attached crossbridge) was the only novel element in the model. In this study, we used the model to predict rates of ATP consumption by crossbridge phosphorylation (JPhos) and cycling (JCycle) during isometric and isotonic contractions in arterial smooth muscle; then we compared model predictions with experimental data. The model predicted that JPhos and JCycle were similar in magnitude in isometric contractions, and both increased almost linearly with myosin phosphorylation. The predicted relationship between isometric stress and ATP consumption was quasihyperbolic, but approximately linear when myosin phosphorylation was below 35%, in agreement with most of the available data. Muscle shortening increased the predicted values of JCycle up to 3.7-fold depending on shortening velocity and the level of myosin phosphorylation. The predicted maximum work output per ATP was 7.4-7.8 kJ/mol ATP and was relatively insensitive to changes in myosin phosphorylation. The predicted increase in JCycle with shortening was in agreement with available data, but the model prediction that work output per ATP was insensitive to changes in myosin phosphorylation was unexpected and remains to be tested in future experiments.

Adenosine Triphosphate↗