The effect of magnesium ions on the two-stage kinetics of superprecipitation and ATPase activity of natural actomyosin

The effect of magnesium ions on the two-stage kinetics of superprecipitation (SPP) and ATP activity of natural skeletal muscle actomyosin was studied. It was found that the changes in the ratios of two independent steps of SPP and ATPase activity are mainly induced by the Mg-ATP2- complex, but not by free Mg2+. These changes in the kinetics of SPP and ATPase are regarded as being due to the shift in the dynamic equilibrium between the two types of the actomyosin complexes in solution, each of which is characterized by different reaction mechanisms. The role of the Mg-ATP2(-)-induced alteration of at least two structural-and-functional states of actomyosin in muscle contractibility is discussed.     

The character of actin-myosin interaction at two stages of the superprecipitation reaction]

It has been shown that two-stage kinetics of superprecipitation (SPP) and ATPase of natural and synthetic actomyosin can be modulated by changing Mg-ATp2- concentration. The I stage is activated at low substrate concentrations, and the II stage--at high concentrations. Resynthesis of ATP completely inhibited the II stage of SPP (and ATPase) and produced no effect in the clearing phase, as well as in the I stage of these reactions. We conclude that active myosin bridges function during the I stage of SPP. However, the II stage ends with the formation of rigorous bridges. It is suggested that division of two different types of actomyosin complexes which participated in the alternative kinetic mechanisms of both, SPP and ATPase reactions, takes place at the moment when ATP is bound in active sites of myosin and dependent on substrate concentration.     

A possible role of myosin conformers in provision of dual-stage superprecipitation kinetics of natural actomyosin

It has been shown that synchronous starting and successive accomplishment of superprecipitation on the two types of actomyosin complexes lead to the two-stage kinetics of this reaction. By means of a temperature change different balance of two types of actomyosin macromolecules can be achieved. We conclude that two different structural forms (conformers) of myosin cause two non-equivalent functional states of the whole actomyosin complex.     

pH-dependence of kinetic parameters in the superprecipitation reaction and ATPase activity of myometrial actomyosin

The investigation of pH-dependence of superprecipitation reaction and ATPase activity of myometrium actomyosin in the interval of pH 5.5-8.0 has detected cupola-shaped curves with maximal activity of both processes by pH 6.5. On the basis of calculating the constants of ionization it was supposed that in the case of actomyosin ATPase imidazole groups of two histidins had an essential role in reaction of ATP hydrolysis and in superprecipitation process--imidazol group of histidine and carboxyl group of asparagin acid. The investigation of [ATP]- and [Mg2+]-dependence of superprecipitation reaction by pH 6.0, 6.5 and 7.0 has demonstrated different pH-sensitiveness of Michaelis constants and maximal speeds relatively Mg2+ and ATP for both processes. It was shown that pH-optimum of ATPase activity of myometrium actomyosin coincided with maximal affinity of actomyosin with ATP and Mg2+ while as for superprecipitation reaction the correlation between value of process by certain pH and affinity with ATP and Mg2+ was not detected.     

Structural changes in the contractile proteins of muscle fiber studied by polarization ultraviolet fluorescence microscopy. IX. The effect of the pH and ionic strength of the solution on the conformational restructurings of F-actin induced by the binding of heavy meromyosin

The dependence of F-actin conformational changes induced by the F-actin-HMM complex on pH and ionic strength was found by polarized ultraviolet fluorescence microscopy. It is discovered that pH affects sufficiently the cooperativity of F-actin structural changes, while the ionic strength affects their depth. The actomyosin complex was supposed to be at least in two structural states, differing in their orientation as well as in flexibility of F-actin monomers.     

Estimation of the amount of energy transformed in the actomyosin--ATP system according to kinetics findings]

A kinetic scheme for actomyosin ATPase reaction, including two different states of the energy-rich actomyosin intermediate and different ways of its formation and decomposition, is considered. The energy, liberated in this reaction, amounts to 9 kcal. mol-1. Energy liberation is coupled with myosin head rotation. The energy-rich intermediate is stabilized by hindrances to head motion. By this mechanism the energy transduction in a filamentous actomyosin system can occur only after a dissociation act, and an ordered cyclical reaction can take place.     

Steady-state studies of the actin-activated adenosine triphosphatase activity of myosin.

Reconstituted actomyosin (ATP phosphohydrolase, EC 3.6.1.3) (0.400 mg F-actin/mg myosin) in 10.0 muM ATP loses 96% of its specific ATPase activity when its reaction concentration is decreased from 42.0 mug/ml down to 0.700 mug/ml. The loss of specific activity at the very low enzyme concentrations is prevented by the addition of more F-actin to 17.6 mug/ml. It is concluded that at low actomyosin concentrations the complex dissociates into free myosin with a very low specific ATPase activity and free F-actin with no ATPase. The dissociation of the essential low molecular weight subunits of myosin from the heavy chains at very low actomyosin concentrations may be a contributing factor. Actomyosin has its maximum specific activity at pH 7.8-8.2. The Km for ATP is 9.4 muM, which is at least 20-fold greater than myosin's Km for ATP. The actin-activated ATPase of myosin follows hyperbolic kinetics with varying F-actin concentrations. The Km values for F-actin are 0.110 muM (4.95 mug/ml) at pH 7.4 and 0.241 muM (10.8 mug/ml) at pH 7.8. The actin-activated maximum turnover numbers for myosin are 9.3 s-1 at pH 7.4 and 11.6 s-1 at pH 7.8. The actomyosin ATPase is inhibited by KCl. This KCl inhibition is not competitive with respect to F-actin, and it is not a simple form of non-competitive inhibition.     

Comparison of kinetic properties of the ATPase reaction of arterial smooth muscle myosin with skeletal muscle myosin

Myosin was prepared from arterial smooth muscle, and a hybrid actomyosin was formed from arterial myosin and rabbit skeletal muscle F-actin. We performed kinetics on the ATPase reaction [EC 3.6.1.3] of arterial myosin and the hybrid actomyosin at high ionic strength, and compared the kinetic properties of arterial myosin ATPase with those of skeletal muscle myosin ATPase. No significant difference was found between these two myosins in the size of the initial Pi burst, the amount of bound nucleotides, and the rates of various elementary steps in the ATPase reaction. On the other hand, two important differences were observed between the hybrid actomyosin and skeletal muscle actomyosin: (i) The amounts of ATP necessary for complete dissociation of the hybrid and skeletal muscle actomyosins were 2 and 1 mol/mol of myosin, respectively. (ii) The rate of dissociation of the hybrid actomyosin induced by ATP was much lower than that of skeletal muscle actomyosin and also was lower than that of fluorescence enhancement.     

Two distinct rhodopsin molecules within the disc membrane of vertebrate rod outer segments

The kinetics of the metarhodopsin I–II reaction have been measured over a wide range of temperatures (1–37°C) and pH values (4.5–8) with suspensions containing fragments of bovine rod outer segments. It was found that for all conditions the occurrence of metarhodopsin II could be described by two independent first-order processes. The fast component: slow component amplitude ratio depends upon pH and temperature.The kinetics of the lumi-metarhodopsin I reaction show the same pH dependence for the fast component: slow component amplitude ratio as the one observed for the metarhodopsin II signals.All the results observed could be described with the assumption that rhodopsin itself exists in two conformational states before bleaching which are in a pH and temperature-dependent equilibrium. This hypothesis is confirmed by its ability to explain some apparently anomalous observations in the literature.     

Two distinct rhodopsin molecules within the disc membrane of vertebrate rod outer segments.

The kinetics of the metarhodopsin I-II reaction have been measured over a wide range of temperatures (1-37C ) and pH values (4.5-8) with suspensions containing fragments of bovine rod outer segments. It was found that for all conditions the occurrence of metarhodopsin II could be described by two independent first-order processes. The fast component: slow component amplitude ratio depends upon pH and temperature. The kinetics of the lumi-metarhodopsin I reaction show the same pH dependence for the fast component: slow component amplitude ratio as the one observed for the metarhodopsin II signals. All the results observed could be described with the assumption that rhodopsin itself exists in two conformational states before bleaching which are in a pH and temperature-dependent equilibrium. This hypothesis is confirmed by its ability to explain some apparently anomalous observations in the literature.     

The causes of the nonuniform thermostability of Mg-ATPase and of the contractility of muscle models

With longer periods of preliminary heat-treatment of actomyosin suspension the decrease in the rate of superprecipitation (SPP) is followed by that in the extent of SPP, and, finally, in the Mg-ATPase activity. A similar uncoupling of mechanical and enzymatic activities is observed when the ratio between the native and the inactivated myosin in reconstructed actomyosin varied. This uncoupling is supposed to result from the formation during heat-treatment of myosin bridges incapable of dissociating in the presence of Mg-ATP. The bridges affect largely the mechanical properties of actomyosin, and in a lesser degree, its enzymatic properties.     

Kinetic manifestations of slow conformation rearrangements of lactate dehydrogenase. An experiment and a mathematical model

Non-steady-state kinetics of lactate dehydrogenase (LDH) catalyzed reaction was investigated for a wide time interval (from 100 msec to 1-3 min) by using stopped-flow methods. A two-stage character of LDH reaction, slow changes like a lag-period on kinetic curves at pH 8.0, flexions on kinetic curves after pre-mixing LDH with NAD+ and pyruvate have been revealed. The graph theory for mathematical analysis of experimental data was applied, which has been developed for the non-steady-state kinetics. An enzyme model of the two-conformer LDH structure was used. The reaction scheme with a preferential inhibition of one of the conformers (pH 8.0) is suggested. The obtained values of kinetic constants prove that transitions between LDH conformers must be slow.     

Characterization of murine sphingosine-1-phosphate phosphohydrolase.

In the present study we have characterized mammalian sphingosine-1-phosphate phosphohydrolase (SPP1), an enzyme that specifically dephosphorylates sphingosine 1-phosphate (S1P) and which differs from previously described lipid phosphate phosphohydrolases. Based on sequence homology to murine SPP1, we cloned the human homolog. Transfection of human embryonic kidney 293 and Chinese hamster ovary cells with murine or human SPP1 resulted in marked increases in SPP1 activity in membrane fractions that were used to examine its enzymological properties. Unlike other known type 2 lipid phosphate phosphohydrolases (LPPs), but similar to the yeast orthologs, mammalian SPP1s are highly specific toward long chain sphingoid base phosphates and degrade S1P, dihydro-S1P, and phyto-S1P. SPP1 exhibited apparent Michaelis-Menten kinetics with S1P as substrate with an apparent K(m) of 38.5 microm and optimum activity at pH 7.5. Similar to other LPPs, SPP1 activity was also independent of any cation requirements, including Mg(2+), and was not inhibited by EDTA but was markedly inhibited by NaF and Zn(2+). However, SPP1 has some significantly different enzymological properties than the LPPs: the aliphatic cation propanolol, which is an effective inhibitor of type 1 phosphatidate phosphohydrolase activities and is only modestly effective as an inhibitor of LPPs, is a potent inhibitor of SPP1; the activity was partially sensitive to N-ethylmaleimide but not to the thioreactive compound iodoacetamide; and importantly, low concentrations of Triton X-100 and other non-ionic detergents were strongly inhibitory. Thus, in agreement with Cluster analysis which shows that outside of the consensus motif there is very little homology between SPP1s and the other type 2 lipid phosphohydrolases, SPP1s are significantly different and divergent from the mammalian LPPs.     

Direct inhibition of the actomyosin motility by local anesthetics in vitro.

Using a recently developed in vitro motility assay, we have demonstrated that local anesthetics directly inhibit myosin-based movement of single actin filaments in a reversible dose-dependent manner. This is the first reported account of the actions of local anesthetics on purified proteins at the molecular level. In this study, two tertiary amine local anesthetics, lidocaine and tetracaine, were used. The inhibitory action of the local anesthetics on actomyosin sliding movement was pH dependent; the anesthetics were more potent at higher pH values, and this reaction was accompanied by an increased proportion of the uncharged form of the anesthetics. QX-314, a permanently charged derivative of lidocaine, had no effect on actomyosin sliding movement. These results indicate that the uncharged form of local anesthetics is predominantly responsible for the inhibition of actomyosin sliding movement. The local anesthetics inhibited sliding movement but hardly interfered with the binding of actin filaments to myosin on the surface or with actomyosin ATPase activity at low ionic strength. To characterize the actomyosin interaction in the presence of anesthetics, we measured the binding and breaking force of the actomyosin complex. The binding of actin filaments to myosin on the surface was not affected by lidocaine at low ionic strength. The breaking force, measured using optical tweezers, was approximately 1.5 pN per micron of an actin filament, which was much smaller than in rigor and isometric force. The binding and breaking force greatly decreased with increasing ionic strength, indicating that the remaining interaction is ionic in nature. The result suggests that the binding and ATPase of actomyosin are governed predominantly by ionic interaction, which is hardly affected by anesthetics; whereas the force generation requires hydrophobic interaction, which plays a major part of the strong binding and is blocked by anesthetics, in addition to the ionic interaction.     

Participation of two types of actomyosin complexes in the process of superprecipitation

It has been shown that addition of turbidity changes accompanying two different types of actomyosin complexes synchronous function in solution allows observation of biphasic kinetics of superprecipitation.     

Ordering of the myofilament lattice in muscle fibers

The effect of pH on the muscle filament lattice in skinned rabbit psoas fibers was studied by X-ray diffraction. In relaxed fibers, the intensity of the 11 equatorial reflection, I11, remained constant between pH 7.0 and pH 6.0 and fell markedly when the pH was decreased to 5.5. The intensity of the 10 reflection was almost constant over this pH range. These results indicate that the thick-filament lattice is more stable than that of the thin filaments, and that the thin filaments are positioned within the thick-filament lattice by a charge-dependent force. In rigor fibers, the decrease in I11 over this pH range was much smaller, which shows that the thin filament lattice can also be stabilized by the presence of actomyosin crossbridges. These conclusions were confirmed by electron microscopy. Thus, the thin filaments can be positioned in the trigonal positions of the thick-filament lattice by two different mechanisms, one electrostatic and the other steric.     

Allosteric behavior of platelet myosin.

The allosteric properties of platelet actomyosin and myosin have been further studied. At pH 7.2, both exhibit sigmoid kinetics with at least two interacting ATP binding sites. At pH 8.9, the velocity versus substrate curve is shifted to the right and becomes more sigmoidal. In contrast, at pH 5.5, the enzyme appears to follow hyperbolic kinetics and the K_m is reduced. In the presence of 1.4 m urea, the sigmoidicity is lost and the enzyme obeys Michaelis-Menten kinetics. The effect of ADP on the ATPase activity was also investigated. ADP shows characteristics of a competitive inhibitor; it increases K_m (shifts sigmoid curve to the right) without affecting V. When the enzyme is desensitized by low pH (5.5) or urea (1.4 m), the allosteric interaction is abolished without impairing the catalytic activity and ADP is no longer inhibitory. These findings suggest that platelet myosin possesses two interacting sites and that ADP binds to the allosteric site which appears to be different from the catalytic site.     

Influence of C-protein on mechano-chemical properties and structure of reconstituted actomyosin

C-protein on the mechano-chemical properties (ATPase activity and superprecipitation) of actomyosin systems has been investigated. The presence of C-protein in AM-complexes has been shown to decrease the rate of superprecipitation (SPP) and simultaneously increase the ATPase activity. Both effects of C-protein are dependent on its quantity in the system. Tropomyosin decreased considerably but does not eliminate completely the inhibitory influence of C-protein on the SPP. Electron microscopy does not reveal considerable structural differences in the initial AM-complexes depending on the presence or absence of C-protein. It is supposed that the discovered effects of C-protein on the behaviour of AM-systems are determined by the fine local structural and (or) charge changes produced by C-protein in the region of myosin cross-bridges, which in its turn results in a modification of the actin-myosin interaction. Possible participation of C-protein in the regulation of the interaction of thin and thick filaments in the muscle is discussed.     

Thermodynamics of nucleotide binding to actomyosin V and VI: a positive heat capacity change accompanies strong ADP binding

We have measured the energetics of ATP and ADP binding to single-headed actomyosin V and VI from the temperature dependence of the rate and equilibrium binding constants. Nucleotide binding to actomyosin V and VI can be modeled as two-step binding mechanisms involving the formation of collision complexes followed by isomerization to states with high nucleotide affinity. Formation of the actomyosin VI-ATP collision complex is much weaker and slower than for actomyosin V. A three-step binding mechanism where actomyosin VI isomerizes between two conformations, one competent to bind ATP and one not, followed by rapid ATP binding best accounts for the data. ADP binds to actomyosin V more tightly than actomyosin VI. At 25 degrees C, the strong ADP-binding equilibria are comparable for actomyosin V and VI, and the different overall ADP affinities arise from differences in the ADP collision complex affinity. The actomyosin-ADP isomerization leading to strong ADP binding is entropy driven at >15 degrees C and occurs with a large, positive change in heat capacity (DeltaC(P) degrees ) for both actomyosin V and VI. Sucrose slows ADP binding and dissociation from actomyosin V and VI but not the overall equilibrium constants for strong ADP binding, indicating that solvent viscosity dampens ADP-dependent kinetic transitions, presumably a tail swing that occurs with ADP binding and release. We favor a mechanism where strong ADP binding increases the dynamics and flexibility of the actomyosin complex. The heat capacity (DeltaC(P) degrees ) and entropy (DeltaS degrees ) changes are greater for actomyosin VI than actomyosin V, suggesting different extents of ADP-induced structural rearrangement.     

Schematic methods for probabilistic enzyme kinetics.

The theory of steady-state enzyme processes which avoids using the mass action law of chemical kinetics and consistently describes catalytic mechanisms by probabilistic concepts has recently been proposed (Mazur, 1991, J. theor. Biol. 148, 229-242). To facilitate the analysis of complex reaction graphs by this theory the possibility of constructing schematic rules similar to those used in classical kinetics is studied. It is found that due to the similarity of algebraic procedures the popular method of King & Altman can be applied in probabilistic kinetics in addition to the earlier proposed rule based on enumeration of cycles of the reaction graph. This similarity also allows one to adapt many other shortcut methods of classical kinetics for probabilistic reaction graphs. The paper considers separately the possibility of transforming reaction mechanisms so that the initial graph is replaced by a simpler but equivalent one. It is shown that there are few cases when a group of states can be replaced by one united state, with earlier known rules such as the rule of Cha for equilibrium stages being particular cases of a more general procedure. In addition a novel method is proposed which performs step-by-step reduction of any reaction graph. All the new methods can be adapted for traditional kinetics as well. The results obtained demonstrate that many schematic rules of classical kinetics are of probabilistic origin.