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At least 19 recordsLinked to original sources

PubMed1978

Studies of the chemo-mechanical conversion in artificially produced streamings. I. Reconstruction of a chemo-mechanical system from acto-HMM of rabbit skeletal muscle.

Steady and uniform streamings (SUS) of HMM solutions were set up in the presence of Mg-ATP in a circular slit, on both side-walls of which a Millipore filter was fixed; F-actin filaments from rabbit skeletal muscle were bound onto the Millipore filter by cyanogen bromide in the flow. The direction of the SUS was specificially determined by that of the flow during the fixing of F-actin and was independent of the direction of the initial velocity applied externally to the HMM solutions. The SUS continued for about 90 min with a velocity of about 20 mum/s at 20 degrees C. There was a strong correlation between the acto-HMM ATPase activity and the velocity of SUS when the salt concentration was varied. Moreover, this was also the case when the ATPase activity was controlled by Ca2+, when native tropomyosin was bound to F-actin in the circular slit. Careful examination led to the conclusions that F-actin filaments are fixed on the Millipore filter with a specific polarity and that a chemo-mechanical system had been successfully reconstituted in our "stream cells," in which chemical energy from ATP is converted to the mechanical energy of streaming.

Actins↗

PubMed1975

The amounts of adenosine di- and triphosphates bound to H-meromyosin and the adenosinetriphosphatase activity of the H-meromyosin-F-actin-relaxing protein system in the presence and absence of calcium ions. The physiological functions of the two routes of myosin adenosinetriphosphatase in muscle contraction.

The rates of the ATPase [EC 3.6.1.3] reaction of the H-meromyosin-F-actin-relaxing protein system were measured in 2 mM MgCl2, 50mM KC1, and 10mM Tris-HC1 at pH 7.8 and 20 degrees in the presence and absence of 0.05-0.1 mM Ca2+ ions. The concentrations of H-meromyosin (HMM) and the F-actin-relaxing protein (F-A-PR) complex were 3.4 and 3 mg/ml, respectively, and the ATPase reaction was coupled with 4 mg/ml of pyruvate kinase [EC 2.7.1.40] and 1 or 20 mM phosphoenolpyruvate to regenerate ATP. The amount of ADP bound to HMM during the ATPase reaction was determined by measuring the amount of ADP remaining in the reaction mixture. The amount of ATP bound to HMM was determined by subtracting the amount of bound ADP from the total amount of nucleotides bound to HMM, which was measured by a rapid flow-dialysis method. The following results were obtained. 1. The ATPase activity of the HMM-F-A-RP system increased linearly with increase in the amount of ATP added, and was independent of the presence of 0.05 mM Ca2+, when the amount of ATP added was less than 1 mole/mole of HMM. In the presence of 0.05 mM Ca2+, the ATPase activity reached a maximal level when 1.2-1.5 mole of ATP was added per mole of HMM, and maintained this level even at 3 moles of added ATP/mole of HMM. In the presence of 3mM EGTA, the ATPase activity decreased with increase in the amount of ATP added, from 1.5 to 3 moles of ATP/mole of HMM, and reached the level of the HMM ATPase reaction at 3 moles of added ATP/mole of HMM. Similar results were observed when the concentration of HMM was maintained at 3.4 mg/ml and the concentration of the F-A-RP complex was decreased from 3 to 1 or 0.5 mg/ml.

Adenosine Diphosphate↗

PubMed1976

Structure and function of the two heads of the myosin molecule. III. Cooperativity of the two heads of the myosin molecule, shown by the effect of modification of head A with rho-chloromercuribenzoate on the interaction of head B with F-actin.

Subfragment-1 of HMM was prepared by tryptic [EC 3.4.21.4] digestion of HMM, which had been modified with 1 mole of CMB per mole of HMM at a specific SH group, SHr. S-1(T) obtained from CMB-HMM retained almost all the CMB, and the amount of bound CMB was about 0.8-0.9 mole per 2 moles of S-1(T). S-2 of CMB-HMM contained no bound CMB. The ATPase [EC 3.6.1.3] activity of HMM increased gradually with increase in the concentration of FA, and the acto-HMM ATPase was inhibited by excess substrate or removal of Ca2+ ions in the presence of RP. The ATPase activity of CMB-HMM increased to a maximum level on adding a small amount of FA, and the acto-CMB-HMM ATPase showed neither substrate inhibition nor Ca2+ sensitivity in the presence of RP. On the other hand, the dependence on the concentration of FA of the ATPase activity of acto-S-1(T) was unaffected by modification of S-1 with CMB. The Ca2+ sensitivity of the ATPase activity of acto-S-1(T) in the presence of RP was also unaffected by the modification. Acto-S-1(T) dissociated almost completely, while acto-CMB-S-1(T) was only 50% dissociated on adding ATP. More than 80% of the bound CMB was contained in S-1(T) undissociated from FA. Furthermore, superprecipitation of actomyosin induced by ATP was completely inhibited by adding about 2 moles of CMB-S-1(T) per mole of actin monomer. On the other hand, about 90% of the burst size of Pi liberation was retained in S-1(T) dissociated from FA. It was concluded that the two heads of the myosin molecule are different: one shows the initial burst of Pi liberation, and does not contain the SHr group which binds CMB (head B), and the other does not show the initial burst and contains the SHr group (head A). It was also concluded that modification of head A of HMM or myosin with CMB increases its binding strength to FA, and consequently the substrate inhibition and Ca2+ sensitivity of acto-HMM or actomyosin ATPase at head B are lost on modification of head A with CMB. CMB-S-1(CT) was prepared by chymotryptic [EC 3.4.21.1] digestion of CMB-myosin, and separated into two fractions by ultracentrifugation of acto-CMB-S-1(CT) in the presence of ATP. Three components of CMB-S-1(CT) with molecular weights of 9, 2.4, and 1.2 X 10(4) were separated by SDS-polyacrylamide gel electrophoresis. The ratios of the peak areas of the three components in electrophoretograms were the same in CMB-S-1(CT) and in the two fractions (1 : 0.18 : 0.09), indicating that heads A and B have the same subunit structure.

Actins↗

PubMed1979

Heavy meromyosin from skipjack tuna, Euthynus pelamis. Preparation and enzymic properties.

A method was developed to obtain heavy meromyosin (HMM) from the tryptic digest of skipjack tuna dorsal myosin. The tuna HMM thus obtained was shown to be homogeneous on gel filtration-gel electrophoresis, and on ultracentrifugation. The sedimentation constant (S20,w) was estimated to be 6.1S for tuna HMM. The ATPase activity of tuna dorsal HMM was found to be very similar to that of rabbit skeletal HMM in many respects: KCl concentration dependence, pH dependence, effect of pCMB, kinetic parameters (Vmax and Ka) in actin activation, and Arrhenius activation energy. The only difference found between tuna HMM and rabbit HMM was in heat denaturation behavior: the ATPase activities of tuna HMM were approximately four times as sensitive to heat inactivation as those of rabbit HMM. Thus, tuna HMM should represent a good experimental material for investigations of the molecular basis of susceptibility to denaturation, and of the characteristics of fish myosins in general. A new type of heat denaturation of myosin was observed. It occurred in a very early stage of heat treatment of either tuna dorsal myosin or rabbit skeletal myosin; however, it did not occur upon heat treatment of HMM of either tuna or rabbit, and it was detectable in terms of the Mg-ATPase activity only when the activity was measured in the presence of untreated actin.

Actins↗

PubMed1979

Chicken gizzard heavy meromyosin that retains the two light-chain components, including a phosphorylatable one.

A method was developed to obtain a preparation of chicken gizzard heavy meromyosin (HMM) that retains the two light-chain components of parent myosin: the 20,000-dalton and 17,000-dalton light-chains. The HMM preparation was also shown to retain two characteristics of the ATPase activity of the parent myosin: the characteristic effect of phosphorylation of the 20,000-dalton light-chain component on the ATPase activity, and the characteristic dependence of the ATPase activity on the KCl concentration. 1. Two distinct stages were observed in the Mg-ATPase reaction catalyzed by gizzard HMM and rabbit skeletal actin in the presence of gizzard "native" tropomyosin (NTM) and Ca2+ ions: an early lag phase, in which the reaction rate gradually increased, and a subsequent steady state, in which the reaction proceeded at a high, constant rate. Urea-gel electrophoresis revealed that the 20,000-dalton light-chain component was gradually phosphorylated in the lag phase, and was fully phosphorylated in the steady state. It was also observed that addition of EGTA (to remove Ca2+ ions) at various times in the lag phase caused neither a further increase nor a decrease in the reaction rate, and that addition of EGTA in the steady state caused no change in the reaction rate. These observations imply that the ATPase activity increased as the amount of phosphorylated 20,000-dalton light-chain component increased, and also that Mg-ATPase of acto-phosphorylated HMM was no longer calcium-sensitive. 2. The Mg-ATPase activity of HMM in the presence of gizzard NTM and Ca2+ ions or EGTA was studied as a function of the concentration of rabbit skeletal actin. The maximal activity (Vmax) and the apparent affinity constant of acto-HMM (KA) were thus estimated from the double-reciprocal plot of Eisenberg-Moos: the Vmax and KA values for phosphorylated HMM (in the presence of Ca2+ ions) were 5 S(-1) and 5.5 mg/ml actin, respectively, and the Vmax value for unphosphorylated HMM (in the presence of EGTA) was 0.3 S(-1), assuming that the KA value with unphosphorylated HMM is equal to that with phosphorylated HMM.

Actins↗

PubMedDate not supplied

ATPase activity and light scattering of acto-heavy meromyosin: dependence on ATP concentration and on ionic strength.

1. The dependence on ATP concentration of ATPase activity and light scattering decrease of acto-HMM could be described at very low ionic strength by one hyperbolic adsorption isotherm with a dissociation constant of 3 X 10(-6)M. Hence the increase of ATP ase activity was paralleled by a decrease in light scattering. At higher values of ionic strength ATPase activity stopped rising before HMM was completely saturated with ATP. Higher ionic strength prevented ATPase activity from further increasing when the rigor links (links between actin and nucleotide-free myosin), which have formerly protected the ATPase against the suppressing action of higher ionic strength have fallen below a certain amount. This protecting influence of rigor links did not require tropomyosin-troponin. 2. For complete activation of ATPase activity by actin less actin was needed when HMM was incompletely saturated with ATP than when it was completely saturated with ATP. 3. The apparent affinity of ATP to regulated acto-HMM (which contained tropomyosin-troponin) was lower than to unregulated acto-HMM (which was devoid of tropomyosin-troponin). In the presence of rigor complexes (indicated by an incomplete decrease of light scattering) the ATPase activity of regulated acto-HMM was higher than that of unregulated acto-HMM. At increasing ATP concentrations the ATPase activity of regulated acto-HMM stopped rising at a similar degree of saturation with ATP as the ATPase activity of unregulated acto-HMM at the same ionic strength.

Actins↗

PubMed1976

Evidence for myosin-linked regulation in guinea pig taenia coli muscle.

The ATPase activity of actomyosin prepared from taenia coli muscle of guinea pig was found to increase upon adding rabbit skeletal heavy meromyosin (HMM) in the absence of Ca2+. SDS-gel electrophoresis of muscle homogenates did not reveal the presence of troponin. Ca2+-regulation in taenia coli muscle thus appears to be myosin-linked. The glycerinated muscles which did not develop any tension in the presence of EGTA contracted after irrigation with rabbit skeletal myosin. Skeletal HMM could also cause tension generation in strips of glycerinated taenia coli in the presence of EGTA. The tension developed by the muscles in the presence of Ca2+ was increased if HMM was added. The HMM-induced tension was associated with a marked increase in ATPase activity both in the presence and in the absence of Ca2+. No HMM-associated tension could be detected when inactivated HMM was employed or when MgATP was substituted with Mg-pyrophosphate or Mg-AMP-PNP. The mechanical effect of HMM probably results from a mechanochemical interaction between the added HMM and muscle actin.

Adenosine Triphosphatases↗

PubMed1976

Structure and function of the two heads of the myosin molecule. IV. Physiological functions of various reaction intermediates in myosin adenosinetriphosphatase, studied by the interaction between actomyosin and 8-bromoadenosine triphosphate.

The kinetic properties of the hydrolyses of 8-Br ATP and 8-SCH3 ATP by myosin [EC 3.6.1.3] and actomyosin were compared with those of ATP, and the following results were obtained. The Ca-NTPase activities of myosin using these two ATP analogs as substrates were smaller than that of ATPase, and the NTPase activities toward these analogs were strongly suppressed by EDTA. The Mg-NTPase activities toward these analogs were higher in a medium of high ionic strength than in a medium of low ionic strength, in contrast to the activity of Mg-ATPase. These analogs did not produce any initial burst of Pi liberation, activation of myosin NTPase by F-actin, or superprecipitation of actomyosin. The interactions between 8-Br ATP and HMM, acto-HMM, actomyosin, and myofibrils were studied in detail in the presence of Mg2+ in medium of low ionic strength. The Michaelis constant, Km, and the maximum rate, Vm, of 8-Br ATPase of HMM were 27 muM and 21 min-1, respectively. The fluorescence change of HMM induced by 8-Br ATP also followed the Michaelis-Menten equation, and the Michaelis constant, Kf1, was as low as 4 muM. Acto-HMM and acto-S-1 were fully dissociated by the addition of 8-Br ATP. The relation between the extent of dissociation of acto-HMM and the concentration of 8-Br ATP followed the Michaelis-Menten equation, and the apparent dissociation constant, Kd, was 22 muM. This Kd value is almost equal to the Km value of 8-Br ATPase of HMM described above. Myofibrillar contraction was not supported by 8-Br ATP. It was concluded that in the myosin NTPase reaction with 8-Br ATP as a substrate, M2NTP but not MNDPP is formed in route (1), while MNTP is formed in route (2). It was also concluded that the key intermediate for the actomyosin NTPase reaction is MNDPP, and that dissociation of acto-HMM is induced by the formation of M2NTP and MNTP in routes (1) and (2), respectively.

Actins↗

PubMed1978

Elementary steps in the acto-H-meromyosin ATPase reaction to arterial smooth muscle.

Transient and steady state kinetics were studied in the interactions of ATP with acto-H-meromyosin reconstituted from bovine arterial heavy-meromyosin (HMM) and rabbit skeletal muscle F-actin. The results showed that the rate of dissociation of the hybrid acto-HMM induced by ATP was slower than the rate of the fluorescence enhancement of HMM, and that the rate of the P1 burst of HMM was unaffected by addition of skeletal muscle F-actin. The ATPase [EC 3.6.1.3] activity of arterial HMM was activated only slightly even with addition of high concentrations of skeletal muscle F-actin. Furthermore, the rates of dissociation of the hybrid acto-HMM induced by ATP and reassociation of dissociated arterial HMM with skeletal muscle F-actin after decomposition of ATP were much lower than those of skeletal muscle acto-HMM.

Actins↗

PubMed2024

Networks of human milk microbiota are associated with host genomics, childhood asthma, and allergic sensitization.

The human milk microbiota (HMM) is thought to influence the long-term health of offspring. However, its role in asthma and atopy and the impact of host genomics on HMM composition remain unclear. Through the CHILD Cohort Study, we followed 885 pregnant mothers and their offspring from birth to 5 years and determined that HMM was associated with maternal genomics and prevalence of childhood asthma and allergic sensitization (atopy) among human milk-fed infants. Network analysis identified modules of correlated microbes in human milk that were associated with subsequent asthma and atopy in preschool-aged children. Moreover, reduced alpha-diversity and increased Lawsonella abundance in HMM were associated with increased prevalence of childhood atopy. Genome-wide association studies (GWASs) identified maternal genetic loci (e.g., ADAMTS8, NPR1, and COTL1) associated with HMM implicated with asthma and atopy, notably Lawsonella and alpha-diversity. Thus, our study elucidates the role of host genomics on the HMM and its potential impact on childhood asthma and atopy.

Humans↗

PubMed1979

Crystalloids of actin-like filaments in the Sertoli cell of the swine testis.

Normal swine testes, congenital cryptorchid swine testes, and normal human tests were exposed to HMM (heavy meromyosin) after either glycerination or saponin treatment in order to determine whether the fine filaments composing the crystalloids in the Sertoli cells of the cryptorchid swine testes bind HMM to form arrowhead complexes. Short bundles of microfilaments observed in the basal part of the Sertoli cells in both normal and cryptorchid testes also bind HMM. Similar bundles of HMM-bound filaments are observed in the vicinity of spermatocytes. The periodicity of the arrowhead complexes is about 35 nm, and all arrowheads on a given filament point in the same direction. In addition, the polarity of the HMM-bound filaments in a given crystalloid or bundle is uni-directional. A mechanism for the formation of the swine crystalloids has been strongly support this hypothesis. Fine filaments of Charcot-Boettcher's crystalloid in human Sertoli cells did not bind HMM. Therefore the fine filaments of the human crystalloid are not actin-like in nature.

Actins↗

PubMed1976

Tension development in skinned glycerinated rabbit psoas fiber segments irrigated with soluble myosin fragments.

Single glycerinated rabbit psoas muscle fibers were skinned by splitting them lengthwise. The fiber segments thus obtained were more easily accessible to solutes in the surrounding medium than the intact fibers. Using such segments, active tension could be fully abolished by adding N-ethylmaleimide under conditions which lead to inhibition of actin activation of the ATPase activity of myosin. Such muscles could, however, develop tension after irrigation with myosin or with the water-soluble active myosin fragments heavy meromyosin (HMM) or its subfragment 1 (HMM-S1). The induced tensions increased with increasing protein concentration in the irrigating solution. At any given protein concentration, the tension generated by myosin was larger than that produced by HMM which was, in turn, greater than that induced by HMM-S1 e.g. at 15 mg/ml protein the tensions produced by these three myosin moieties were 44.0, 14.0 and 2.8 g/cm2, respectively. The tension was found to be intimately associated with ATP splitting; thus, HMM and HMM-S1 which have been treated with reagents abolishing actin-activated ATPase failed to induce tension development. A contractile force may thus be generated through the interaction with actin of the water-soluble, enzymatically active, myosin subfragments involving the splitting of ATP.

Actins↗

PubMed1979

N-ethylmaleimide-modified heavy meromyosin. A probe for actomyosin interactions.

Treatment of rabbit skeletal muscle heavy meromyosin (HMM) with the sulfhydryl reagent N-ethylmaleimide (NEM) produces a species of HMM which remains tightly bound to actin in the presence of MgATP. NEM-HMM forms characteristic "arrowhead" complexes with actin which persist despite rinses with MgATP. NEM-HMM inhibits the actin activation of native HMM-ATPase activity, the superprecipitation of actomyosin, the contraction of glycerinated muscle myofibrils, and the contraction of cytoplasmic strands of the soil amoeba Chaos carolinensis. However, NEM-HMM does not interfere with in vitro microtubule polymerization or beating of demembranated cilia.

Actins↗

PubMedDate not supplied

Phase II study of hexamethylmelamine alone and in combination with mitomycin C and vincristine in advanced breast carcinoma.

Fifty-three patients with metastatic breast carcinoma were randomized to treatment with hexamethylmelamine (HMM) as a single agent versus a three-drug reimen of HMM, vincristine, and mitomycin C (HOM). All patients had received prior treatment with 5-fluorouracil, Adriamycin, and cyclophosphamide with or without methotrexate. HMM alone was used in a dose of 300 mg/m2/day x 14 days every 21 days. In the HOM regimen, the HMM dose was 200 mg/m2/day x 21 days, the vincristine dose was 1.5 mg on Days 1, 8, and 15, and the mitomycin C dose was 12 mg/m2 once every 6 weeks. No objective responses were observed with HMM in 15 evaluable patients. The HOM regimen resulted in five partial responses among the 27 evaluable patients. Gastrointestinal toxicity was the limiting toxicity of HMM, and thrombocytopenia was the major toxicity of the HOM regimen.

Adult↗

PubMed1977

18O-exchange catalyzed by myosin, heavy meromyosin, heavy meromyosin subfragment 1 and their complexes with actin.

Myosin, HMM and HMM S1 catalyze 18O-exchange between P1 and H218O of the medium at an intermediate stage of ATP hydrolysis ("intermediate 18O-exchange") in the presence of Mg2+. Natural complexes of actomyosin and acto-HMM S1 do not catalyze intermediate 18O-exchange but facilitate "direct" or "medium" 18O-exchange (KH2P18O4 in equilibrium H2O) even without ATP. Reconstituted complexes of actomyosin, acto-HMM, acto-HMM S1, PABC-HMM S1, congo-myosin and TNP-myosin do not catalyze direct 18O-exchange in the presence of Mg2+ and absence of ATP. From the data obtained a hypothetical sequence of phosphorylation and 18O-exchange reactions in myofibril action has been suggested.

Actins↗

PubMed1975

An ultrastructural study of the microfilaments in rat brain by means of heavy meromyosin labeling. I. The perikaryon, the dendrites and the axon.

Nervous tissue pieces from the caudate nucleus and the substantia nigra of the rat were incubated in cold glycerol solutions of decreasing concentrations and then transferred into standard phosphate buffer (pH 7.0) or into tris-K+-Mg++-Ca++ buffer (pH 7.9) containing HMM, prepared from rabbit skeletal muscle by tryptic digestion. As controls, pieces were immersed for an identical period in the same buffers (1) without HMM or (2) with HMM to which had been added 2.5 mM Na+ pyrophosphate or 5 mM ATP. In control neurons smooth-surfaced microfilaments, about 50 A in diameter, were observed. After reaction with HMM, the microfilaments were increased in number and density and in width to 180-200 A. A meshwork was formed. Arrowheads pointing in the same direction were spaced at regular intervals (300-350 A) among short segments of the surfaces of the microfilaments, depending upon the plane of section. More often, however, typical arrowheads were not observed, and the surfaces of the microfilaments were seen coated with polarized side-arms cross-bridging the spaces between adjacent elements at more or less regular intervals. When cross-sectioned, the microfilaments appeared as dense dots from which a material of lesser electron density radiated. Following incubation in HMM solutions containing Na+ pyrophosphate or ATP, no arrowhead structures were seen. Of particular interest was the structural relation of the actin-like filaments with occasional, tapered myosin-like filaments, and with the plasma membrane, which served as anchor points. Mitochondria and smooth ER membranes were observed to be attached to the actin-like filaments or enmeshed in the network. The microtubules, as well as most of the neurofilaments, were disrupted by the glycerination procedure at 4 degrees, and thus no precision about the structural relationship of the actin-like filaments with the latter elements could be added. The role of the actin-like filaments in the transport of material, by a mechanism of chemomechanical transduction, throughout the neuron from sites of synthesis to functional locations, and between several functional locations, is discussed.

Actins↗

PubMed1975

An ultrastructural study of the microfilaments in rat brain by means of E-PTA staining and heavy meromyosin labeling. II. The synapses.

To identify structures involved in the translocation of the synaptic vesicles towards the presynaptic membrane, an ultrastructural study has been undertaken by means of (1) the E-PTA stain and (2) the HMM-labeling procedure. Using serial sections of E-PTA stained nervous tissue, especially those made in transversal and tangential planes, the geometric order of the presynaptic grid and of its constituents has been described in detail. It consisted of dense projections having the shape of small truncated pyramids cut parallel to their hexagonal bases which rested on the electron-lucent presynaptic membrane. The dense projections were arranged at the points of equilateral triangles. Around each dense projection, six asymmetric hexagonal holes were seen to be arrayed in an hexagonal pattern, forming thus the presynaptic sieve. From the spiny tops of the dense projections, which appeared as specialized structures of the dense material coating the inner surface of the plasma membrane at the level of the synaptic cleft, fine filaments, 40--60 A in diameter, radiated and formed a three-dimensional meshwork pervading the presynaptic bag. The dense cytoplasmic coating delineating the plasma membrane served as anchor points for these microfilaments. Upon incubation with rabbit skeletal muscle HMM the microfilaments underwent specific structural changes, consisting of: (1) a striking increase in diameter; (2) the association of periodic and polarized substructures with their surfaces. The synaptic vesicles and mitochondria were seen to be attached to the numerous HMM-decorated filaments or enmeshed in the network formed by these filaments. The actin-like filaments were anchored to the plasma membrane at many points and to the presynaptic dense projections. Following incubation in the buffer alone or in buffer HMM solutions containing Na+ pyrophosphate or ATP, no arrowheaded structures were observed. Thus, a network consisting of actin-like filaments was demonstrated in the presynaptic bag. Of particular interest was the structural relation of the actin-like filaments with the occasional, tapered myosin-like filaments. The role of the presynaptic actin-like network in the transport of synaptic vesicles towards the presynaptic membrane by a mechanism of chemomechanical transduction is discussed. In the postsynaptic dendrite or dendritic spine, a filamentous network was observed to be attached to the subsynaptic web by means of the E-PTA stain and of the HMM-labeling procedure. The occurrence of an actin-like meshwork in the postsynaptic region is suggested to produce changes in the macromolecular configuration of the postsynaptic membrane by a "mechanoenzyme" system similar to that described in the mitochondrial membrane.

Actins↗

PubMed1975

Standard free energy changes for formation of various intermediates in the reaction of H-meromyosin ATPase.

Two reaction intermediates of H-meromyosin (HMM) ATPase [EC 3.6.1.3], E2AT32P, and (see article), were formed by mixing excess HMM with AT32P. Then a large excess of unlabelled ATP was added, and the amount of AT32P liberated from E2AT32P was measured as the difference between the total amount of AT32P in the reaction mixture and the amount of AT32P bound to HMM, obtained by filtering the mixture after adding charcoal to adsorb nucleotides (charcoal-filtration method). The amount of free AT32P was also measured as the amount of glucose-6-32P formed within 15 sec after adding large excesses of hexokinase [EC 2.7.1.1] and glucose to the reaction mixture. The rate constant, k-2, for the step E2ATP yields E plus ATP was calculated at various KCl concentrations from the time-course of liberation of AT32P. The intermediate, (see article), was formed by mixing HMM with AT32P in a molar ratio of 1:2, and the rate constant, k-6, for the step (see article) was also determined by the same procedures used for k-2. In 0.5 M KCl and 2 mM MgCl2 at pH 7.8 and 0 degrees, k-2 and k-6 were 0.002 sec-1 and 0.1 sec-1 or more, respectively. From the rate constants determined in this work and the rate and equilibrium constants which we reported previously, the standard free energy changes (kcal/mole) for formation of various reaction intermediates in the reaction of HMM ATPase in 0.5 M KCl and 2 mM MgCl2 at pH 7.8 and 0 degrees were calculated to be as follows: (see article).

Adenosine Triphosphatases↗