Compartmentation of mitochondrial creatine phosphokinase. I. Direct demonstration of compartmentation with the use of labeled precursors

Energy metabolism design of the striated muscle cell

The design of the energy metabolism system in striated muscle remains a major area of investigation. Here, we review our current understanding and emerging hypotheses regarding the metabolic support of muscle contraction. Maintenance of ATP free energy, so called energy homeostasis, via mitochondrial oxidative phosphorylation is critical to sustained contractile activity, and this major design criterion is the focus of this review. Cell volume invested in mitochondria reduces the space available for generating contractile force, and this spatial balance between mitochondria acontractile elements to meet the varying sustained power demands across muscle types is another important design criterion. This is accomplished with remarkably similar mass-specific mitochondrial protein composition across muscle types, implying that it is the organization of mitochondria within the muscle cell that is critical to supporting sustained muscle function. Beyond the production of ATP, ubiquitous distribution of ATPases throughout the muscle requires rapid distribution of potential energy across these large cells. Distribution of potential energy has long been thought to occur primarily through facilitated metabolite diffusion, but recent analysis has questioned the importance of this process under normal physiological conditions. Recent structural and functional studies have supported the hypothesis that the mitochondrial reticulum provides a rapid energy distribution system via the conduction of the mitochondrial membrane potential to maintain metabolic homeostasis during contractile activity. We extensively review this aspect of the energy metabolism design contrasting it with metabolite diffusion models and how mitochondrial structure can play a role in the delivery of energy in the striated muscle.

Molecular system bioenergics of the heart: experimental studies of metabolic compartmentation and energy fluxes versus computer modeling

In this review we analyze the recent important and remarkable advancements in studies of compartmentation of adenine nucleotides in muscle cells due to their binding to macromolecular complexes and cellular structures, which results in non-equilibrium steady state of the creatine kinase reaction. We discuss the problems of measuring the energy fluxes between different cellular compartments and their simulation by using different computer models. Energy flux determinations by ¹⁸O transfer method have shown that in heart about 80% of energy is carried out of mitochondrial intermembrane space into cytoplasm by phosphocreatine fluxes generated by mitochondrial creatine kinase from adenosine triphosphate (ATP), produced by ATP Synthasome. We have applied the mathematical model of compartmentalized energy transfer for analysis of experimental data on the dependence of oxygen consumption rate on heart workload in isolated working heart reported by Williamson et al. The analysis of these data show that even at the maximal workloads and respiration rates, equal to 174 μmol O₂ per min per g dry weight, phosphocreatine flux, and not ATP, carries about 80–85% percent of energy needed out of mitochondria into the cytosol. We analyze also the reasons of failures of several computer models published in the literature to correctly describe the experimental data.

Regulation of respiration controlled by mitochondrial creatine kinase in permeabilized cardiac cells in situ. Importance of system level properties

The main focus of this investigation is steady state kinetics of regulation of mitochondrial respiration in permeabilized cardiomyocytes in situ. Complete kinetic analysis of the regulation of respiration by mitochondrial creatine kinase was performed in the presence of pyruvate kinase and phosphoenolpyruvate to simulate interaction of mitochondria with glycolytic enzymes. Such a system analysis revealed striking differences in kinetic behaviour of the MtCK-activated mitochondrial respiration in situ and in vitro. Apparent dissociation constants of MgATP from its binary and ternary complexes with MtCK, Kia and Ka (1.94+/-0.86 mM and 2.04+/-0.14 mM, correspondingly) were increased by several orders of magnitude in situ in comparison with same constants in vitro (0.44+/-0.08 mM and 0.016+/-0.01 mM, respectively). Apparent dissociation constants of creatine, Kib and Kb (2.12+/-0.21 mM 2.17+/-0.40 Mm, correspondingly) were significantly decreased in situ in comparison with in vitro mitochondria (28+/-7 mM and 5+/-1.2 mM, respectively). Dissociation constant for phosphocreatine was not changed. These data may indicate selective restriction of metabolites' diffusion at the level of mitochondrial outer membrane. It is concluded that mechanisms of the regulation of respiration and energy fluxes in vivo are system level properties which depend on intracellular interactions of mitochondria with cytoskeleton, intracellular MgATPases and cytoplasmic glycolytic system.

New insights into the bioenergetics of mitochondrial disorders using intracellular ATP reporters

Mutations in mitochondrial DNA (mtDNA) cause impairment of ATP synthesis. It was hypothesized that high-energy compounds, such as ATP, are compartmentalized within cells and that different cell functions are sustained by different pools of ATP, some deriving from mitochondrial oxidative phosphorylation (OXPHOS) and others from glycolysis. Therefore, an OXPHOS dysfunction may affect different cell compartments to different extents. To address this issue, we have used recombinant forms of the ATP reporter luciferase localized in different cell compartments- the cytosol, the subplasma membrane region, the mitochondrial matrix, and the nucleus- of cells containing either wild-type or mutant mtDNA. We found that with glycolytic substrates, both wild-type and mutant cells were able to maintain adequate ATP supplies in all compartments. Conversely, with the OXPHOS substrate pyruvate ATP levels collapsed in all cell compartments of mutant cells. In wild-type cells normal levels of ATP were maintained with pyruvate in the cytosol and in the subplasma membrane region, but, surprisingly, they were reduced in the mitochondria and, to a greater extent, in the nucleus. The severe decrease in nuclear ATP content under "OXPHOS-only" conditions implies that depletion of nuclear ATP plays an important, and hitherto unappreciated, role in patients with mitochondrial dysfunction.

Increased uncoupling protein 3 content does not affect mitochondrial function in human skeletal muscle in vivo

Phosphocreatine (PCr) resynthesis rate following intense anoxic contraction can be used as a sensitive index of in vivo mitochondrial function. We examined the effect of a diet-induced increase in uncoupling protein 3 (UCP3) expression on postexercise PCr resynthesis in skeletal muscle. Nine healthy male volunteers undertook 20 one-legged maximal voluntary contractions with limb blood flow occluded to deplete muscle PCr stores. Exercise was performed following 7 days consumption of low-fat (LF) or high-fat (HF) diets. Immediately following exercise, blood flow was reinstated, and muscle was sampled after 20, 60, and 120 seconds of recovery. Mitochondrial coupling was assessed by determining the rate of PCr resynthesis during recovery. The HF diet increased UCP3 protein content by approximately 44% compared with the LF diet. However, this HF diet-induced increase in UCP3 expression was not associated with any changes in the rate of muscle PCr resynthesis during conditions of maximal flux through oxidative phosphorylation. Muscle acetylcarnitine, free-creatine, and lactate concentrations during recovery were unaffected by the HF diet. Taken together, our findings demonstrate that increasing muscle UCP3 expression does not diminish the rate of PCr resynthesis, allowing us to conclude that the primary role of UCP3 in humans is not uncoupling.

Evolution and physiological roles of phosphagen systems

Phosphagens are phosphorylated guanidino compounds that are linked to energy state and ATP hydrolysis by corresponding phosphagen kinase reactions: phosphagen + MgADP + H(+) <--> guanidine acceptor + MgATP. Eight different phosphagens (and corresponding phosphagen kinases) are found in the animal kingdom distributed along distinct phylogenetic lines. By far, the creatine phosphate/creatine kinase (CP/CK) system, which is found in the vertebrates and is widely distributed throughout the lower chordates and invertebrates, is the most extensively studied phosphagen system. Phosphagen kinase reactions function in temporal ATP buffering, in regulating inorganic phosphate (Pi) levels, which impacts glycogenolysis and proton buffering, and in intracellular energy transport. Phosphagen kinase reactions show differences in thermodynamic poise, and the phosphagens themselves differ in terms of certain physical properties including intrinsic diffusivity. This review evaluates the distribution of phosphagen systems and tissue-specific expression of certain phosphagens in an evolutionary and functional context. The role of phosphagens in regulation of intracellular Pi levels likely evolved early. Thermodynamic poise of the phosphagen kinase reaction profoundly impacts this capacity. Furthermore, it is hypothesized that the capacity for intracellular targeting of CK evolved early as a means of facilitating energy transport in highly polarized cells and was subsequently exploited for temporal ATP buffering and dynamic roles in metabolic regulation in cells displaying high and variable rates of aerobic energy production.

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

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

Mitochondrial genetics and disease

Mitochondrial respiratory chain diseases are a highly diverse group of disorders whose main unifying characteristic is the impairment of mitochondrial function. As befits an organelle containing gene products encoded by both mitochondrial DNA (mtDNA) and nuclear DNA (nDNA), these diseases can be caused by inherited errors in either genome, but a surprising number are sporadic, and a few are even caused by environmental factors.

CK inhibition accelerates transcytosolic energy signaling during rapid workload steps in isolated rabbit hearts

The effect of graded creatine kinase (CK) inhibition on the response time of mitochondrial O2 consumption to dynamic workload jumps (tmito) was studied in isolated rabbit hearts. Tyrode-perfused hearts (n = 7/group) were exposed to 15 min of 0, 0.1, 0.2, or 0.4 mM iodoacetamide (IA) (CK activity = 100, 14, 6, and 3%, respectively). Pretreatment tmito was similar across groups at 6.5 +/- 0.5 s (mean +/- SE). The increase observed over time in control hearts (33 +/- 8%) was progressively reversed to 16 +/- 6, -20 +/- 6 (P < 0.01 vs. control), and -46 +/- 6 (P < 0.01 vs. control) % in the 0.1, 0.2 and 0.4 mM IA groups, respectively. The faster response times occurred without reductions in mitochondrial oxidative capacity (assessed in vitro) or myocardial O2 consumption of the whole heart during workload steps. Isovolumic contractile function assessed as rate-pressure product (RPP) and contractile reserve (increase in RPP during heart rate steps) were significantly reduced by IA. We conclude that CK in the myofibrils and/or cytosol does not speed up transfer of the energy-related signal to the mitochondria but rather acts as an energetic buffer, effectively slowing the stimulus between myofibrils/ion pumps and oxidative phosphorylation. This argues against the existence of an obligatory creatine phosphate energy shuttle, because CK is effectively bypassed.

Function of the outer mitochondrial compartment in regulation of energy metabolism

Electron microscopy showed the organization of several kinases at the mitochondrial surface as complexes between outer membrane (porin), kinase, and inner membrane (presumably adenine nucleotide translocator?). The complexes were enriched in the isolated contact site fraction. Interaction of porin with the kinases in vitro led to formation of tetramers of hexokinase and active creatine kinase. Kinetic analyses of mitochondria with intact outer compartment showed separate ATP/ADP exchange between kinases and oxidative phosphorylation. Considering these results, we postulate that the mitochondrial metabolism in intact cells is not regulated by free ADP, but induced by substrates wf kinases such as glucose or creatine (Fig 1). Increased ATP turnover in muscle during contraction results in only a small change in the free ADP but causes a larger change of creatine because the equilibrium constant of the creatine kinase reaction at pH 7.2 favours ATP formation (ATP creatine/ADP phosphocreatine = 104.7) [38]. In addition, the level of phosphocreatine is roughly 10-times higher compared to ATP. Considering the higher concentration and the equilibrium constant, it can be calculated that a change of ADP between 40 and 70 microM results in creatine increasing from 8 to 12 mM. Thus creatine can be the signal that stimulates the mitochondrial metabolism transmitted by the mitochondrial creatine kinase [39]. Likewise, increased blood glucose in muscle at rest or in the liver stimulates the mitochondrial metabolism transmitted by the activity of bound hexokinase utilizing external ATP. The mitochondrial metabolism provides the UTP for glycogen synthesis through mitochondrial nucleoside-diphosphate kinase activity (Fig 1).

The importance of the outer mitochondrial compartment in regulation of energy metabolism

Substitution of physiologically present macromolecules during isolation of mitochondria and investigation of their functions led to a significant change in regulation of oxidative phosphorylation. The differences compared to conventionally isolated mitochondria were that stimulation of oxidative phosphorylation appeared to rather depend on the activity of peripheral kinases than on the addition of free ADP. The localisation of peripheral kinases such as hexokinase and mitochondrial creatine kinase are described as well as the effects of macromolecules on the regulation of bound hexokinase and of oxidative phosphorylation via this enzyme.

Extracellular phosphate requirement for insulin action on isolated rat hepatocytes

Isolated rat hepatocytes were prepared in KHB buffer, pH 7.4; were centrifuged and washed twice in KHB buffer containing various amounts of phosphate and calcium; and were incubated at 30 degrees in the presence of tracer [2,3-14C]succinate and a 0.5 mM concentration of each of the 20 natural amino acids. Hepatocytes washed and incubated in KHB buffer containing less than 0.1 mM phosphate failed to show any insulin stimulation of [2,3-14C]succinate oxidation or protein incorporation of tracer carbons. The absence or presence of extracellular phosphate did not alter the specific activity of 32P-adenine nucleotides; they remained the same in the presence or absence of insulin. The maximal insulin stimulatory effect on succinate oxidation and tracer incorporation into protein was observed in the presence of 1.18 mM phosphate and 1.9 mM calcium ion. The lack of external phosphate did not prevent the stimulation of succinate oxidation by either glucagon on epinephrine, whereas removal of calcium from the medium abolished their hormonal effects. The lack of medium calcium also prevented the insulin stimulation of succinate oxidation and protein synthesis. Our data indicate that a diminished insulin responsiveness in hypophosphatemic patients may be due to the insensitivity of mitochondria to insulin in the hypophosphatemic state.

The role of contact sites between inner and outer mitochondrial membrane in energy transfer

Three functions have been suggested to be localized in contact sites between the inner and the outer membrane of mitochondria from mammalian cells: (i) transfer of energy from matrix to cytosol through the action of peripheral kinases; (ii) import of mitochondrial precursor proteins; and (iii) transfer of lipids between outer and inner membrane. In the contact site-related energy transfer a number of kinases localized in the periphery of the mitochondrion play a crucial role. Two examples of such kinases are relevant here: (i) hexokinase isoenzyme I which is capable of binding to the outer aspect of the outer membrane; and (ii) the mitochondrial isoenzyme of creatine kinase which is localized in the intermembrane space. Recently, evidence was presented that both hexokinase and creatine kinase are preferentially localized in contact sites (Adams, V. et al. (1989) Biochim. Biophys. Acta 981, 213-225). The aim of the present experiments was two-fold. First, to establish methods which enable the bioenergetic aspects of energy transfer mediated by kinases in contact sites to be measured. In these experiments emphasis was on hexokinase, while 31P-NMR was the major experimental technique. Second, we wanted to develop methods which can give insight into factors playing a role in the formation of contact sites involved in energy transfer. In the latter approach, mitochondrial creatine kinase was studied using monolayer techniques.

A spin-label electron spin resonance study of the binding of mitochondrial creatine kinase to cardiolipin

The binding of the mitochondrial creatine kinase to aqueous dispersions of beef heart cardiolipin has been studied via the perturbation of the mobility of spin-labelled cardiolipin, using electron spin resonance (ESR) spectroscopy. In the presence of creatine kinase (1:1 protein/lipid ratio, by mass), the ESR spectra of cardiolipin labelled in a single acyl chain [n-(4,4-dimethyl-oxazolidinyl-N- oxy)stearoylcardiolipin] indicate a restriction of motion both at the C-5 and C-14 positions (n = 5, 14) of the lipid chains. The restriction in mobility was reversed by addition of phosphate or adriamycin, which are thought to inhibit the binding of creatine kinase to the mitochondrial membrane or to displace it from its binding site on the membrane. The effect of the protein on the chain mobility is consistent with surface binding of the protein; no positive evidence was obtained for penetration of the protein into the hydrophobic region of the membrane.

Further characterization of contact sites from mitochondria of different tissues: topology of peripheral kinases

A membrane fraction of intermediate density between inner and outer membrane was isolated by density gradient centrifugation from osmotically disrupted mitochondria of rat liver, brain, and kidney. The fraction was hexokinase rich and could therefore be further purified using specific antibodies against hexokinase and immunogold labelling techniques. In agreement with recent findings the gradient fraction which cosedimented with hexokinase contained the boundary membrane contact sites because it was composed of outer and inner membrane components and beside hexokinase, was enriched also by activity of creatine kinase and nucleoside diphosphate kinase. In contrast the activity of adenylate kinase appeared to be concentrated beyond the contact sites in the outer membrane fraction. By employing surface proteolysis analysis and specific blockers of the outer membrane pore we observed that the location of the kinases relative to the membrane components in the contact fraction resembled that of intact mitochondria. This specific organization of some peripheral kinases in the contact sites suggested an important role of the voltage dependence of the outer membrane pore, in that the pore may become limiting in anion exchange because of influence of the inner membrane potential on the closely attached outer membrane. Such control of anion exchange would lead to a dynamic compartmentation at the mitochondrial surface by the formation of contact sites, which may explain the preferential utilization of cytosolic creatine by the mitochondrial creatine kinase, as postulated in the phosphocreatine shuttle.

Ultrastructural localisation of creatine kinase activity in the contact sites between inner and outer mitochondrial membranes of rat myocardium

The mitochondrial isoenzyme of creatine kinase, together with the ADP/ATP translocase, most probably belongs to a functional multi-enzyme complex located on the inner mitochondrial membrane. The outer membrane is a necessary constituent of this microcompartment. On the other hand, electron microscopic visualisation demonstrated the formation of contact sites between inner and outer mitochondrial membranes as a reaction to variations of the energy metabolism. In search for a possible correlation between these biochemical and morphological phenomena, rat myocardia were brought into the required energy state by stimulation through catecholaminergic mechanisms or adjusted perfusion with amytal. Subsequently, creatine kinase was cytochemically localised. Creatine kinase activity is demonstrated in membrane contacts between inner and outer mitochondrial membranes. The extent of contact sites and creatine kinase activity depends on the metabolic state as shown by morphometric analysis of the surface density of cytochemical reaction product. This surface density diminishes drastically after inhibiting the metabolic activity with amytal. It is concluded that these contact sites are dynamic micro-environments in which the active site of creatine kinase, oxidative phosphorylation and ADP/ATP transport interact during basal and stimulated metabolism.

Heart mitochondrial creatine kinase revisited: the outer mitochondrial membrane is not important for coupling of phosphocreatine production to oxidative phosphorylation

The state of mitochondrial creatine kinase (CKmi-mi) in intact dog heart mitochondria and mitoplasts and the mechanism of its functional coupling with the oxidative phosphorylation system have been reinvestigated under different osmotic conditions and ionic compositions of the medium. It has been established that in a medium which mimics the cardiac cell cytoplasma, dissociation of CKmi-mi from the membrane of mitoplasts increases when the mitoplasts are swollen due to hypoosmotic treatment. It was shown by EPR that hypoosmotic treatment results in the enhancement of the mobility of phospholipids in the membrane bilayer. It has been also shown that when CKmi-mi is detached from the inner membrane in intact mitochondria in isotonic KCl solution, the effects of the coupling between CKmi-mi and oxidative phosphorylation via ATP/ADP translocase disappear in spite of the presence of CKmi-mi in the intermembrane space and intactness of the outer mitochondrial membrane. Therefore, this coupling cannot be explained by the "compartmented coupling" mechanism or "dynamic adenine nucleotide compartmentation" in the intermembrane space due to diffusion limitation for adenine nucleotides through the outer mitochondrial membrane, as has been supposed by several authors (F.N. Gellerich et al. (1987) Biochim. Biophys. Acta 890, 117-126; S.P.J. Brooks and C.H. Suelter (1987) Arch. Biochem. Biophys. 253, 122-132). The data obtained show that the displacement of the enzyme from the membrane results in significantly increased sensitivity of the coupled processes of aerobic phosphocreatine synthesis to inhibition by the product, phosphocreatine. Thus, all results show that under physiological osmotic and ionic conditions CKmi-mi remains firmly attached to the inner mitochondrial membrane and effectively coupled with ATP/ADP translocase due to intimate dynamic interaction between those proteins.

In vivo functioning of creatine phosphokinase in human forearm muscle, studied by 31P NMR saturation transfer

31P nuclear magnetic resonance (NMR) saturation transfer has been used to measure enzymatic flux through the creatine phosphokinase reaction in the direction of ATP synthesis in the human forearm muscle flexor digitorum superficialis. Modification of the ratio method for measurement of spin-lattice relaxation (R. Freeman, H.D.W. Hill, and R. Kaptein, J. Magn. Reson. 7, 82(1972]was tested and used to appreciably shorten the duration of the measurement. Under conditions of steady state work intracellular pH decreased slightly by 0.06 units and the spin-lattice relaxation time of phosphocreatine in muscle was unchanged, while flux from phosphocreatine to ATP was 64 +/- 10% of the resting value. This is contrary to the increase in flux of 155% predicted from previous saturation transfer studies carried out in vitro on rabbit skeletal muscle creatine phosphokinase using metabolite concentrations to mimic those in vivo (E.A. Shoubridge, J.L. Bland, and G.K. Radda, Biochim. Biophys. Acta 805, 72 (1984]. This discrepancy could be accounted for by an underestimation of the ADP concentrations to which the enzyme is exposed due to inaccurate assumptions about the total metabolite concentrations, or possibly by compartmentation of creatine phosphokinase and its reactants.

Effects of magnesium depletion on myocardial high-energy phosphates and contractility

The effect of prolonged magnesium depletion on contractility, phosphorylating activity, and organic phosphates of spontaneously beating isolated rat atria was studied. Rats were fed a Mg-deficient diet for 8 weeks, during which serum Mg fell from 1.85 +/- 0.02 to 0.52 +/- 0.10 mg/dl. Atrial contractile activity was measured for 1 hr and at the end of this period tissue samples were taken for the determination of the phosphorylated intermediates. Mg depletion was associated with (a) reduced intracellular inorganic phosphorus and adenine nucleotides; (b) elevated creatine phosphate; (c) reduction in contractile force (CF) with no change in atrial beat rate (BR). There were no significant differences in the activities of creatine phosphokinase and adenylate kinase in control and Mg-depleted rat atrial homogenates determined in the presence of 5 mM MgCl2. Addition of various concentrations of MgCl2 to the medium resulted in an immediate reduction in both CF and BR of normal and Mg-depleted rat atria. Intraperitoneal administration of MgCl2 to Mg-depleted rats resulted in complete recovery of CF of isolated atria. This improvement in CF occurred without changes in the levels of inorganic phosphate and adenine nucleotides. The reduced intracellular level of high-energy phosphate or inorganic phosphate cannot therefore be responsible for the impaired contractility seen in Mg-depleted heart muscle. On the other hand, the fact that the creatine phosphate levels were higher in magnesium depletion suggests that myofibrillar utilization of creatine phosphate is more impaired than production, analogous to phenomena seen in postanoxic recovery.

A synthetic functional metabolic compartment. The role of propinquity in a linked pair of immobilized enzymes

A system was created to model the influence of microcompartments on linked enzymatic reactions. Creatine kinase and hexokinase were covalently attached to Sepharose beads. The gel could be perfused in a specially constructed chamber inside a 360-MHz NMR spectrometer at different flow rates with solutions containing various concentrations of substrates. 31P NMR studies were carried out on the linked enzymatic reaction, creatine phosphate + glucose----creatine + glucose 6-phosphate in two enzyme gels differing in only one aspect, the average distance between hexokinase and creatine kinase. At a distance on the order of 0.1 mm between the enzymes, the average bulk concentrations of substrates and products in the perfusate determined the overall function of the linked system. At an average distance of the order of 10 nm, flux through the linked pair was much higher and much less dependent on the concentration of the intermediate substrate/product ADP/ATP. Even at adenine nucleotide concentrations far below the Km of hexokinase, substantial amounts of glucose 6-phosphate were produced when the enzymes were near but not when they were distant. From saturation transfer measurements and turnover calculations, the lifetime of ATP in the system is estimated to be 0.14-0.5 s when the enzymes are near. This compares to 6 s for distant enzymes. From this it appears that the pair of linked enzymes comprise a functional compartment supported by propinquity in which hexokinase has preferential access to ATP produced by creatine kinase, and creatine kinase to ADP from the hexokinase reaction.

Compartmented coupling of chicken heart mitochondrial creatine kinase to the nucleotide translocase requires the outer mitochondrial membrane

The kinetic coupling of mitochondrial creatine kinase (MiMi-CK) to ADP/ATP translocase in chicken heart mitochondrial preparations is demonstrated. Measuring the MiMi-CK apparent Km value for MgATP2- (at saturating creatine) gives a value of 36 microM when MiMi-CK is coupled to oxidative phosphorylation. This Km value is threefold lower than the Km for enzyme bound to mitoplasts or free in solution. The nucleotide translocase Km value for ADP decreases from 20 to 10 microM in the presence of 50 mM creatine only with intact mitochondria. Similar experiments with mitoplasts do not give decreased Km values. The observed Km differences can be used to calculate the concentration of ATP and ADP under steady-state conditions showing that the observed differences in the kinetic constants accurately reflect the enzyme activities of MiMi-CK under the different conditions. The behavior of the Km values provides evidence for what we term compartmented coupling. Therefore, like the rabbit heart system (S. Erickson-Viitanen, P. Viitanen, P. J. Geiger, W. C. T. Yang, and S. P. Bessman (1982) J. Biol. Chem. 257, 14395-14404) compartmented coupling requires an intact outer mitochondrial membrane. The apparent Km values for normal or compartmentally coupled systems can be used to calculate steady-state values of ATP and ADP by coupling enzyme theory. Hence, the overall kinetic parameters accurately reflect the behavior of the enzymes whether free in solution or in the intermembrane space.

The creatine phosphate energy shuttle--the molecular asymmetry of a "pool"

The creatine phosphate shuttle energy transfer mechanism was postulated on the basis of the hexokinase acceptor theory of insulin action. It proposes that the movement of chemical energy from the mitochondrion to the myofibril is in the form of creatine phosphate. This occurs because there are isozymes of creatine phosphokinase bound to the inner membrane of the sarcosome and to the A band of the myofibril. These isozymes have been shown to act as transducers of energy from ATP to creatine phosphate at the translocase site and from creatine phosphate back to ATP at the myofibrillar compartment. Calculations show that there is no significant amount of transformation of creatine phosphate to ATP in the intervening space between the mitochondrion and the myofibril so that, essentially, transport between the oxidative sites and the contractile apparatus is through the creatine phosphate shuttle. There is also evidence that another terminus for this shuttle is the microsome so that muscle activity tends to increase energy supply for protein synthesis.

Association of chicken mitochondrial creatine kinase with the inner mitochondrial membrane

The stoichiometry and dissociation constant for the binding of homogeneous chicken heart mitochondrial creatine kinase (MiMi-CK) to mitoplasts was examined under a variety of conditions. Salts and substrates release MiMi-CK from mitoplasts in a manner that suggests an ionic interaction. The binding of MiMi-CK to mitoplasts is competitively inhibited by Adriamycin, suggesting that they compete for the same binding site. Fluorescence measurements also show that Adriamycin binds to MiMi-CK so that the effect of Adriamycin on the binding of MiMi-CK to mitoplasts is not simple. Titrating mitoplasts with homogeneous MiMi-CK at different pH values shows a pH-dependent equilibrium involving a group(s) on either the membrane or the enzyme with a pKa = 6. Extrapolating these titrations to infinite MiMi-CK concentration gives 14.6 IU bound/nmol cytochrome aa3 corresponding to 1.12 mol MiMi-CK/mol cytochrome aa3. Chicken heart mitochondria contain, after isolation, 2.86 +/- 0.42 IU/nmol cytochrome aa3. Titrating respiring mitoplasts with carboxyatractyloside gives at saturation 3.3 mol ADP/ATP translocase/mol cytochrome aa3. Therefore, chicken heart mitoplasts can maximally bind about 1 mol of MiMi-CK per 3 mol translocase; in normal chicken heart mitochondria about 1 mol of MiMi-CK is present per 13 mol translocase.

31P-NMR spectroscopic investigations and mitochondrial studies on the cardioprotective efficiency of 2-mercaptopropionylglycine

Contents of high energy phosphates in the isolated perfused rat heart were followed during ischemia and reperfusion using 31P NMR spectroscopy. Application of 2-mercaptopropionylglycine resulted in significantly higher content of ATP in the reperfusion phase whereas during ischemia no differences between control and therapy hearts were found. Analysis of postischemic mitochondrial function reveals that improved ATP level is paralleled by an increased respiratory control index and a reduced ATPase activity. It is suggested that 2-mercaptopropionylglycine may cause increase of high energy phosphates during reperfusion by improving mitochondrial oxidative phosphorylation.

Mitochondrial respiratory control in the myocardium

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

Metabolite channeling: a phosphorylcreatine shuttle to mediate high energy phosphate transport between sperm mitochondrion and tail

Energy utilization by the flagellum of motile sea urchin sperm is tightly coupled to the rate of energy production by the mitochondrion. This tight coupling depends upon the transport of high energy phosphate (P) from mitochondrion to axoneme, which we propose to be mediated by a phosphorylcreatine shuttle. The shuttle employs distinct mitochondrial and axonemal creatine kinase isozymes, the latter being a novel creatine kinase of 145 kd. To examine whether P is directed to the tail by such a shuttle, we inactivated creatine kinase specifically with fluorodinitrobenzene. Creatine kinase inactivation led to an inhibition of coupled, but not uncoupled, respiration and affected the pattern of sperm motility as predicted for the disruption of an obligatory link in P transport.

Is the "soluble" phase of cells structured?

Evidence suggesting that small molecular-weight precursors as well as polymers such as proteins and RNA, are not homogeneously distributed in the "soluble" phase of the cell, and that there may be, on one hand, hitherto unidentified barriers to normal diffusion within the cell and, on the other, channels through which certain molecular species may move in the cell sap much faster than their diffusion coefficients will permit, is discussed. Some implications of such barriers and channels, if they exist, are stated.

Regulation of energy flux through the creatine kinase reaction in vitro and in perfused rat heart. 31P-NMR studies

Fluxes catalyzed by soluble creatine kinase (MM) in equilibrium in vitro and by the creatine kinase system in perfused rat hearts were studied by 31P-NMR saturation transfer method. It was found that in vitro both forward and reverse fluxes through creatine kinase at equilibrium were almost equal and very stable to changes in phosphocreatine/creatine ratio (from 0.2 to 3.0) as well as to changes in pH (from 7.4 to 6.5 or 8.1), free Mg2+ concentration and 2-fold decrease of total adenine nucleotides and creatine pools (from 8.0 to 4.0 mM and from 30 to 14 mM, respectively). In the rat hearts perfused by the Langendorff method the creatine kinase-catalyzed flux from phosphocreatine to ATP was increased by 50% when oxygen consumption grew from 8 to 55 mumol/min per g of dry wt. due to transition from rest to high workload. These changes could not be exclusively explained on the basis of the equilibrium model by activation of heart creatine kinase due to some decrease in [phosphocreatine]/[creatine] ratio (from 1.8 to 0.8) observed during transition from rest to high workload. Analysis of our data showed that an increase in the flux via creatine kinase is correlated with an increase in the rate of ATP synthesis with a linearity coefficient higher than 1.0. These data are more consistent with the concept of energy channeling by phosphocreatine shuttle than with that of the creatine kinase equilibrium in the heart.