Phosphorylation potential in the dominant leg is lower, and [ADPfree] is higher in calf muscles at rest in endurance athletes than in sprinters and in untrained subjects.

It has been reported that various types of mammalian muscle fibers differ regarding the content of several metabolites at rest. However, to our knowledge no data have been reported in the literature, concerning the muscle energetic status at rest in high class athletes when considering the dominant and non-dominant leg separately. We have hypothesised that due to higher mechanical loads on the dominant leg in athletes, the metabolic profile in the dominant leg at rest in the calf muscles, characterized by [PCr], [ADP(free)], [AMP(free)] and DeltaG(ATP), will significantly differ among endurance athletes, sprinters and untrained individuals. In this study we determined the DeltaG(ATP) and adenine phosphates concentrations in the dominant and non-dominant legs in untrained subjects (n = 6), sprinters (n = 10) and endurance athletes (n = 7) at rest. The (mean +/- SD) age of the subjects was 23.4 +/- 4.3 years. Muscle metabolites were measured in the calf muscles at rest, by means of (31)P-MRS, using a 4.7 T superconducting magnet (Bruker). When taking into account mean values in the left and right leg, phosphocreatine concentration ([PCr]) and DeltaG(ATP) were significantly lower (p<0.05, Wilcoxon-Mann-Whitney test), and [ADP(free)] was significantly higher (p = 0.04) in endurance athletes than in untrained subjects. When considering the differences between the left and right leg, [PCr] in the dominant leg was significantly lower in endurance athletes than in sprinters (p = 0.01) and untrained subjects (p = 0.02) (25.91 +/- 2.87 mM; 30.02 +/- 3.12 mM and 30.71 +/- 2.88 mM, respectively). The [ADP(free)] was significantly higher (p = 0.02) in endurance athletes than in sprinters and untrained subjects (p = 0.02) (42.19 +/- 13.44 microM; 27.86 +/- 10.19 microM; 25.35 +/- 10.97 microM, respectively). The DeltaG(ATP) in the dominant leg was significantly lower (p = 0.02) in endurance athletes than in sprinters and untrained subjects (p = 0.01) (-60.53 +/- 2.03 kJ.M(-1); -61.82 +/- 1.05 kJ.M(-1), -62.29 +/- 0.73 kJ.M(-1), respectively). No significant differences were found when comparing [PCr], [ADP(free)], [AMP(free)], [Mg(2+)(free)], DeltaG(ATP) in the dominant leg and the mean values for both legs in sprinters and untrained subjects. Moreover, no significant differences were found when comparing the metabolites in non-dominant legs in all groups of subjects. We postulate that higher [ADP(free)] and lower DeltaG(ATP) at rest is a feature of endurance-trained muscle. Moreover,when studying the metabolic profile of the locomotor muscles in athletes one has to consider the metabolic differences between the dominant and non-dominant leg.

How to keep glycolytic metabolite concentrations constant when ATP/ADP and NADH/NAD+ change.

Glycolytic flux may increase over 100 times in skeletal muscle during rest-to-work transition, whereas glycolytic metabolite concentrations remain relatively constant. This constancy cannot be explained by an identical direct activation of all glycolytic enzymes because the concentrations of ATP, ADP, AMP, P(i), NADH and NAD+, modulators of the activity of different glycolytic enzymes, change. It is demonstrated in the present in silico study that a perfect homeostasis of glycolytic metabolite concentrations can be achieved if glycolysis is divided into appropriate blocks of enzymes that are directly activated to a different extent in order to compensate the effect of the modulators.

Training-induced acceleration of oxygen uptake kinetics in skeletal muscle: the underlying mechanisms.

It is well known that the oxygen uptake kinetics during rest-to-work transition (V(O2) on-kinetics) in trained subjects is significantly faster than in untrained individuals. It was recently postulated that the main system variable that determines the transition time (t(1/2)) of the V(O2) on-kinetics in skeletal muscle, at a given moderate ATP usage/work intensity, and under the assumption that creatine kinase reaction works near thermodynamic equilibrium, is the absolute (in mM) decrease in [PCr] during rest-to-work transition. Therefore we postulate that the training-induced acceleration of the V(O2) on-kinetics is a marker of an improvement of absolute metabolic stability in skeletal muscles. The most frequently postulated factor responsible for enhancement of muscle metabolic stability is the training-induced increase in mitochondrial proteins. However, the mechanism proposed by Gollnick and Saltin (1982) can improve absolute metabolic stability only if training leads to a decrease in resting [ADP(free)]. This effect is not observed in many examples of training causing an acceleration of the V(O2) on-kinetics, especially in early stages of training. Additionally, this mechanism cannot account for the significant training-induced increase in the relative (expressed in % or as multiples of the resting values) metabolic stability at low work intensities, condition in which oxidative phosphorylation is not saturated with [ADP(free)]. Finally, it was reported that in the early stage of training, acceleration in the V(O2) on-kinetics and enhancement of muscle metabolic stability may precede adaptive responses in mitochondrial enzymes activities or mitochondria content. We postulate that the training-induced acceleration in the V(O2) on-kinetics and the improvement of the metabolite stability during moderate intensity exercise in the early stage of training is mostly caused by an intensification of the "parallel activation" of ATP consumption and ATP supply pathways. A further acceleration in V(O2) on-kinetics, resulting from prolonged periods of training, may be caused by a further and more pronounced improvement in the muscles' absolute metabolic stability, caused by an intensification of the "parallel activation" as well as by an increase in mitochondrial proteins.

MyHC II content in the vastus lateralis m. quadricipitis femoris is positively correlated with the magnitude of the non-linear increase in the VO2 / power output relationship in humans.

In this study we have examined the relationship between the content of different isoforms of MyHC in the vastus lateralis m. quadricipitis femoris and the VO2 / power output relationship during incremental cycling exercise. Twenty-one male subjects: aged 24.0 +/- 2.5 years, body mass 73.0 +/- 7.2 kg, height 179 +/- 5 cm, BMI 22.78 +/- 1.84 kg x m(-2), VO2(max) 3697 +/- 390 ml x min(-1), 50.9 +/- 5.2 ml x kg(-1) x min(-1), participated in this experiment. The subjects performed an incremental exercise test until exhaustion. The exercise test started at power output of 30 W, followed by an increase amounting to 30 W every 3 minutes. The pedalling rate was maintained at 60 rev x min(-1). Gas exchange variables were measured continuously using breath-by-breath system Oxycon Jaeger. At the end of each step blood samples were taken for lactate concentration. Muscle biopsy samples taken from the vastus lateralis m. quadricipitis femoris, using the Bergstrom needle, were analysed for the content of different MyHC (I, IIa, IIx) using SDS-PAGE and Western blotting. The pre-exercise VO2, as a mean value of six-minute measurements, expressed both in ml x min(-1), and in ml x kg(-1) x min(-1), was positively correlated with the content of MyHC II in the vastus lateralis (p < 0.01). We have also found that the pre-exercise values of VO2 in the group of subjects with a high proportion of MyHC II (59.9 +/- 11.2 %) were significantly higher (p < 0.02, when VO2 was expressed in ml x min(-1), and p < 0.01 when VO2 was expressed in ml x kg(-1) x min(-1)) than in the group with low content of MyHC II (27.5 +/- 6.0 %) in the vastus lateralis. Moreover, we have found a significant negative correlation (r = -0.562, p < 0.01) between the slope in the VO2/PO relationship below the lactate threshold (LT) and the content of MyHC IIa in the vastus lateralis. The most interesting finding of our study was that the magnitude of the non-linear increase in the VO2 / power output relationship present above the LT was positively correlated ( r = 0.510, p < 0.02) with the content of MyHC II in the vastus lateralis. Our results show, that there is no simple relationship between the content of different types of MyHC in the vastus lateralis and the oxygen cost of work during incremental exercise test. Individuals with a high content of MyHC II in the vastus lateralis m. quadricipitisfemoris consume more oxygen in the pre-exercise conditions than subjects with a low content of MyHC II in their muscles. Subjects with a high content of MyHC II require a smaller increase in VO2 for maintaining a linear increase in power output up to the lactate threshold (lower slope in this relationship), but after exceeding the LT, they consume more oxygen above that expected from the linear relationship below the LT, than the subjects with a low content of MyHC II in their muscles. Therefore, non-linear increase in the VO2 / power output relationship, present above the LT, is more pronounced in subjects with a higher content of MyHC II in the vastus lateralis m. quadricipitis femoris.

A model of oxidative phosphorylation in mammalian skeletal muscle.

A dynamic computer model of oxidative phosphorylation in oxidative mammalian skeletal muscle was developed. The previously published model of oxidative phosphorylation in isolated skeletal muscle mitochondria was extended by incorporation of the creatine kinase system (creatine kinase plus phosphocreatine/creatine pair), cytosolic proton production/consumption system (proton production/consumption by the creatine kinase-catalysed reaction, efflux/influx of protons), physiological size of the adenine nucleotide pool and some additional minor changes. Theoretical studies performed by means of the extended model demonstrated that the CK system, which allows for large changes in P(i) in relation to isolated mitochondria system, has no significant influence on the kinetic properties of oxidative phosphorylation, as inorganic phosphate only slightly modifies the relationship between the respiration rate and [ADP]. Computer simulations also suggested that the second-order dependence of oxidative phosphorylation on [ADP] proposed in the literature refers only to the ATP synthesis flux, but not to the oxygen consumption flux (the difference between these two fluxes being due to the proton leak). Next, time courses of changes in fluxes and metabolite concentrations during transition between different steady-states were simulated. The model suggests, in accordance with previous theoretical predictions, that activation of oxidative phosphorylation by an increase in [ADP] can (roughly) explain the behaviour of the system only at low work intensities, while at higher work intensities parallel activation of different steps of oxidative phosphorylation is involved.

Physiological background of the change point in VO2 and the slow component of oxygen uptake kinetics.

It is generally believed that oxygen uptake during incremental exercise--until VO2max, increases linearly with power output (see eg. Astrand & Rodahl, 1986). On the other hand, it is well established that the oxygen uptake reaches a steady state only during a low power output exercise, but during a high power output exercise, performed above the lactate threshold (LT), the oxygen uptake shows a continuous increase until the end of the exercise. This effect has been called the slow component of VO2 kinetics (Whipp & Wasserman, 1972). The presence of a slow component in VO2 kinetics implies that during an incremental exercise test, after the LT has been exceeded, the VO2 to power output relationship has to become curvilinear. Indeed, it has recently been shown that during the incremental exercise, the exceeding of the power output, at which blood lactate begins to accumulate (LT), causes a non-proportional increase in VO2 (Zoladz et al. 1995) which indicates a drop in muscle mechanical efficiency. The power output at which VO2 starts to rise non-proportionally to the power output has been called "the change point in VO2" (Zoladz et al. 1998). In this paper, the significance of the factors most likely involved in the physiological mechanism responsible for the change point in oxygen uptake (CP-VO2) and for the slow component of VO2 kinetics, including: increase of activation of additional muscle groups, intensification of the respiratory muscle activity, recruitment of type II muscle fibres, increase of muscle temperature, increase of the basal metabolic rate, lactate and hydrogen ion accumulation, proton leak through the inner mitochondrial membrane, slipping of the ATP synthase and a decrease in the cytosolic phosphorylation potential, are discussed. Finally, an original own model describing the sequence of events leading to the non-proportional increase of oxygen cost of work at a high exercise intensity is presented.

Effect of 'binary mitochondrial heteroplasmy' on respiration and ATP synthesis: implications for mitochondrial diseases.

Respiratory-chain-complex subunits in mitochondria are encoded by nuclear or mitochondrial DNA. This property might have profound implications for the phenotypic expression of mutations affecting oxidative phosphorylation complexes. The aim of this paper is to study the importance of the origin of the mutation (nuclear or mitochondrial) on the expression of mitochondrial defects. We have therefore developed theoretical models illustrating three mechanisms of nuclear or mitochondrial DNA mutation giving rise to a deficiency in the respiratory-chain complex: (1) a partial deficiency, homogeneously distributed in all of the mitochondria; (2) a complete deficiency, only affecting some of the mitochondria ('binary mitochondrial heteroplasmy'); and (3) a partial deficiency, affecting only some of the mitochondria. We show that mutations affecting oxidative phosphorylation complexes will be expressed in different ways depending on their origins. Although the expression of nuclear or mitochondrial mutations is evidence of a biochemical threshold, we demonstrate that the threshold value depends on the origin and distribution of the mutation (homogeneous or not) and also on the energy demand of the tissue. This last prediction has been confirmed in an experimental model using hexokinase for the simulation of the energy demand and a variation in mitochondrial concentration. We also emphasize the possible role of 'binary mitochondrial heteroplasmy' in the expression of mitochondrial DNA mutations and thus the importance of the origin of the deficit (mutation) for the diagnosis or therapy of mitochondrial diseases.

Cybernetic formulation of the definition of life.

A definition of life (a living individual) in cybernetic terms is proposed. In this formulation, life (a living individual) is defined as a network of inferior negative feedbacks (regulatory mechanisms) subordinated to (being at service of) a superior positive feedback (potential of expansion). It is suggested that this definition is the minimal definition, necessary and sufficient, for life to be distinguished from inanimate phenomena and, as such, it describes the essence of life. Subsequently, a quantitative expression for the amount of the biologically relevant ("purposeful") information (as opposed to the amount of information in the thermodynamic sense) is proposed. This is followed by the application of the formulated approach to different phenomena of a dubious status existing presently on the Earth as well as to the process of origination of life on our planet.

Theoretical studies on the regulation of oxidative phosphorylation in intact tissues.

The theoretical studies on the regulation of oxidative phosphorylation that were performed with the aid of kinetic models of this process are overviewed. A definition of the regulation of the flux through a metabolic pathway is proposed and opposed to the control exerted by particular enzymes over this flux. Different kinetic models of oxidative phosphorylation proposed in the literature are presented, of which only the model proposed by myself and co-workers was extensively used in theoretical studies on the regulation and compensation in the oxidative phosphorylation system. These theoretical studies have led to the following conclusions: (1) in isolated mitochondria, an increase in the activity of an artificial ATP-using system stimulates mitochondria mainly via changes in [ADP], while changes in [ATP] and [P(i)] play only a minor role; (2) in non-excitable tissues (e.g. liver), hormones (acting via some cytosolic factor(s)) activate directly both ATP usage and at least some enzymes of the ATP-producing block; (3) in excitable tissues (e.g. skeletal muscle), neural signals stimulate (via some cytosolic factor(s)) in parallel all the steps of oxidative phosphorylation together with ATP usage and substrate dehydrogenation; (4) the decrease in the flux through cytochrome oxidase caused by a decrease in oxygen concentration is, at least partially, compensated by a decrease in Delta p and increase in the reduction level of cytochrome c. A theoretical prediction is formulated that there should exist and be observable a universal cytosolic factor/regulatory mechanism which directly activates (at least in excitable tissues) all complexes of oxidative phosphorylation during an increased energy demand.

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.

Quantification of the relative contribution of parallel pathways to signal transfer: application to cellular energy transduction.

A simple mathematical formalism designed to quantify the relative contribution of parallel pathways to signal transduction is presented and applied to the regulation of the respiration rate by ATP, ADP and Pi concentrations in response to an increase of energy demand in isolated mitochondria. Theoretical studies were performed by means of the computer model of oxidative phosphorylation developed previously. Many earlier experimental studies have shown that externally-manipulated concentrations of all three metabolites can influence the respiration rate significantly. However, the effect of changes in [ATP], [ADP] and [Pi] that actually take place during an increased energy demand have not been determined in a quantitative way. It was shown in the present paper that [ADP] is the main regulatory factor which stimulates respiration during transition from state 4 to state 3 imposed by an addition of increasing amounts of an artificial ADP-regenerating system. Changes in [ATP] and [Pi] contribute to the respiration rate increase very weakly, and only in the nearest neighbourhood of state 3. Generally, changes in [ADP] are responsible for approx. 90% of the respiration rate increase during the state 4-->state 3 transition, while the remaining approx. 10% is due to changes in [Pi] and [ATP].

Is it possible to predict any properties of oxidative phosphorylation in a theoretical way?

Two theoretical approaches applied to oxidative phosphorylation, namely Metabolic Control Analysis (MCA) [ 1-7] and Non-Equilibrium Thermodynamics (NET) [8-11], turned out to be very useful tools for quantitative description and understanding of control and regulation of this process. However, they were not able to predict any new properties of the considered system. On the other hand, the previously developed dynamic model of oxidative phosphorylation [12-17], representing a kinetic approach, allowed to formulate several interesting predictions which can be tested experimentally. The most important of these predictions are: (1) Different steps of ATP-production must be directly activated to a similar extent as ATP-consumption during stimulation of ATP turnover by calcium-acting hormones as well as by neural signals during muscle contraction; (2) A universal activator/regulatory mechanism responsible for such a precise balance of activation should be identified; (3) The flux-force relationship for cytochrome oxidase can be inverse during the transition towards hypoxia and anoxia, when oxygen concentration falls below 30 microM; (4) The flux-force relationship can depend on the way in which the thermodynamic force is changed; (5) The pattern of metabolic control is completely different in normoxic and hypoxic conditions; in the latter case cytochrome oxidase has the flux control coefficient close to unity. Thus, the kinetic model of oxidative phosphorylation seems to be a useful scientific tool, offering some novel theoretical predictions, which then can be tested in the experimental way.

Dextran strongly increases the Michaelis constants of oxidative phosphorylation and of mitochondrial creatine kinase in heart mitochondria.

Macromolecules restore the morphological changes which occur upon isolation of mitochondria in normally used isolation media. It was shown that in the presence of dextrans the permeability of mitochondrial outer membrane for adenine nucleotides decreases which may have considerable implications for the transport of ADP into the mitochondria. In this study the effect of dextran on the apparent Michaelis constants of oxidative phosphorylation and mitochondrial creatine kinase (mi-CK) of rat heart mitochondria was investigated. Mitochondria were isolated either in normally used isolation media or in the additional presence of 15% dextran 20 in order to avoid changes in the oncotic conditions on the mitochondria during preparation and investigation. Except for an increased contamination with extramitochondrial ATPases the basic functional properties of these mitochondria were normal. With oxygraphic measurements it was found that Km(ADP) of oxidative phosphorylation increased from 16 +/- 4 microM ADP (without dextran) to 50 +/- 15 microM (15% dextran 20) and to 122 +/- 62 microM (25% dextran 20) irrespective of the mode of preparation of the mitochondria. Using spectrophotometric measurements the effect of dextran on the Km(ATP) of mi-CK was investigated in three systems (a) as soluble enzyme, (b) bound to mitoplasts, (c) and in intact rat heart mitochondria. The addition of 10% dextran had no effect on kinetic properties of solubilized mi-CK. In intact heart mitochondria, however, the addition of dextran caused an augmentation of Km(ATP) from 332 +/- 91 microM (control) to 525 +/- 150 microM ATP (10% dextran) and 641 +/- 160 microM ATP (30% dextran). In mitoplasts the effect of dextran disappeared (control, 230 +/- 19 microM ATP; 10% dextran, 238 +/- 28 microM ATP) indicating that the outer mitochondrial membrane is a prerequisite for the modulation of the transport of adenine nucleotides into the intermembrane space by macromolecules. To investigate the effects of viscosity of dextran solutions on the diffusion of adenine nucleotides across the outer membrane, dextrans with different molecular size (20, 40 70 and 500 kDa) were used. The viscosity of the 10% solutions drastically increased with the molecular size of the dextrans used, but the effects of different dextran solutions on the kinetic constants were the same. From these results it was concluded that neither the viscosity nor the molar concentration but the content of macromolecules (mass/vol.) correlates with restrictions of diffusion into the intermembrane space of mitochondria with intact outer membranes. Assuming that a dextran concentration of 15% mimicks the intracellular oncotic pressure on mitochondria in vivo, the apparent Km(ATP) of oxidative phosphorylation within the intact cell seems to be about 50 microM ADP which is somewhat higher than the cytoplasmic free ADP concentration as reported for the intact heart.

Regulation of ATP supply during muscle contraction: theoretical studies.

The dynamic computer model of oxidative phosphorylation developed previously and successfully tested for large-scale changes in fluxes and metabolite concentrations was used to study the question of how the rate of ATP production by oxidative phosphorylation is adjusted to meet the energy demand during muscle contraction, which causes a great increase in ATP consumption in relation to the resting state. The changes in the respiration rate and ATP/ADP ratio after the onset of maximal work measured experimentally were compared with simulated changes in the respiration rate and ATP/ADP in several different cases, assuming direct activation of different steps by an external effector. On the basis of the computer simulations performed, it was possible to conclude which enzymes/metabolic blocks should be directly activated to cause the experimentally observable changes in fluxes and metabolite concentrations. The theoretical results obtained suggest that the parallel direct activation of actinomyosin-ATP-ase and oxidative phosphorylation by an external effector (for example calcium ions) is the main mechanism responsible for fitting of ATP production to ATP consumption, while the negative feedback via an increase in ADP concentration (decrease in ATP/ADP), which indirectly activates the ATP supply, plays only a minor role. Additionally, the conclusion is drawn that most of the oxidative phosphorylation steps should be directly activated in order to explain the observed changes in the respiration rate and ATP/ADP ratio (and also in other parameters) during muscle contraction. It is suggested that there should exist a universal external activator/regulatory mechanism which causes a parallel stimulation of different enzymes/processes. A possible nature of such an activator is shortly discussed.

Thermodynamic regulation of cytochrome oxidase.

It was concluded that cytochrome oxidase was a strange enzyme for three reasons. (1) The thermodynamic flux-force relationship of this enzyme was inverse in some conditions: flux decreased when force increased. (2) The flux-force relationship was not unique and depended on the way in which the thermodynamic span of cytochrome oxidase was changed. (3) The regulation of cytochrome oxidase was different in the same conditions when different external parameters (energy demand, oxygen concentration) were changed. It was also shown that the flux control coefficient of cytochrome oxidase, small at saturating oxygen concentration, increases when oxygen pressure diminishes, approaching unity at very low oxygen concentrations.

Metabolic control analysis and threshold effect in oxidative phosphorylation: implications for mitochondrial pathologies.

We have shown that the Metabolic Control Analysis (MCA) can explain the threshold effect observed in the expression of mitochondrial diseases. As a matter of fact, the effect of a specific inhibitor on the flux of O2 consumption mimics a defect in a step of oxidative phosphorylation. The observed threshold is correlated to the value of the control coefficient of the inhibited step. For this reason, we have studied the repartition of the control coefficients of different steps in oxidative phosphorylation on various tissues (liver, kidney, brain, skeletal muscle and heart). We discuss the results in terms of metabolic control theory and provide a possible explanation for the heterogeneous phenotype of those pathologies. We present the double threshold hypothesis of both a threshold in the energy demand of a tissue and in the energy supply by oxidative phosphorylation.

A simple mechanism decreasing free metabolite pool size in static spatial channelling.

We propose a simple mechanism which enables decrease of the free pool of channelled metabolite in static spatial channelling, when the concentration of the enzyme consuming the channelled metabolite is greater than the concentration of the enzyme producing this metabolite. Spatial channelling occurs between two enzymes when the common metabolite is released to a small space between these enzymes and does not from a ternary covalent complex with them, as is the case in covalent (dynamic or static) channelling. The mechanism proposed is qualitatively independent of rate constants, metabolite concentrations as well as other kinetic properties and is quantitatively significant for all physiologically relevant conditions. Calculations show that the free metabolite pool must decrease, when the concentration of the enzyme consuming the channelled metabolite is greater than the enzyme producing it. This mechanism is much more effective than increase in the concentration (or rate constant) of the enzyme consuming the metabolite in the absence of spatial channelling.

Theoretical studies on control of oxidative phosphorylation in muscle mitochondria at different energy demands and oxygen concentrations.

The mathematical dynamic model of oxidative phosphorylation in muscle mitochondria developed previously was used to calculate the flux control coefficients of particular steps of this process in isolated mitochondria at different amounts of hexokinase and oxygen concentrations. The pattern of control was completely different under different conditions. For normoxic concentration, the main controlling steps in state 4, state 3.5 and state 3 were proton leak, ATP usage (hexokinase) and complex III, respectively. The pattern of control in state 4 was not changed at hypoxic oxygen concentration, while in state 3.5 and state 3 much of the control was shifted from other steps to cytochrome oxidase. The implications of the theoretical results obtained for the regulation of oxidative phosphorylation in intact muscle are discussed.