Energy changes during the formation and interconversion of enzyme-substrate complexes

The rate constants and equilibrium constants of the individual steps of several enzyme reactions may be determined by the application of rapid reaction methods and isotope techniques. This makes it possible to complement the formalism of the Haldane relation with details of the reaction mechanism. It has been shown that, in several enzyme reactions, steps involving chemical catalysis are fast and have small free-energy changes compared with those of the substrate binding and product dissociation processes. Data are presented in this paper for three enzyme reactions for which different methods have been used to elucidate the kinetic parameters of the elementary steps. For cardiac lactate dehydrogenase (EC 1.1.1.27), absorption and fluorescence spectroscopy have been used to distinguish the step involved in the chemical process from those involved in the formation of the substrate complex and the release of the product. The rate of interconversion between enzyme-bound substrates and products is fast compared with other steps and the equilibrium constant for the process is near unity. Consequently, the difference of standard free energy changes for the formation of the two ternary complexes correspons approximately to the overall free-energy change of the hydrogen transfer reaction. Isotope kinetic techniques can be used to study the reactions of triosephosphate isomerase (EC 5.3.1.1). With this enzyme, the interconversion of enzyme-bound substrate into product is comparable in rate to product dissociation. The reactions of myosin subfragment 1 with ATP, studied by fluorescence spectroscopy and chemical quenching, follow a similar pattern in that the equilibrium constant of the chemical step in which water reacts with protein-bound ATP is 9. In this case, however, there is a remarkably large free-energy change associated with a first-order process involved in the binding of ATP. The possible significance of these results to energy transduction in muscle contraction as well as in the biosynthesis of ATP is discussed.

Towards time-resolved diffraction studies with glycogen phosphorylase

Laue diffraction with high intensity, broad-spectrum synchrotron radiation sources allows three-dimensional data sets on protein crystals to be recorded in seconds or milliseconds and opens the way for time-resolved studies on dynamic events in crystals. This chapter briefly reviews the field and describes progress towards time-resolved studies with glycogen phosphorylase. Methods for the synchronization of the start of reaction with the start of data collection have been developed for the phosphorolytic reaction of glycogen phosphorylase. The compound 3,5-dinitrophenylphosphate is photolabile, yielding Pi and the by-product, 3,5-dinitrophenol, which is non-reactive with the enzyme. Spectroscopic studies show that the compound has good quantum yield and that photolysis is rapid (greater than 1000 s-1). Release of the dinitrophenylate anion, following a pulse of light from a xenon flash lamp, has been monitored with a diode array spectrophotometer specially adapted for measurements on crystals. In a laboratory X-ray experiment with crystals of glycogen phosphorylase b, release of Pi and formation of the enzyme-product complex have been demonstrated. The way is now open for Laue diffraction studies on the catalytic reaction in the crystal.

Comparative single-molecule and ensemble myosin enzymology: sulfoindocyanine ATP and ADP derivatives

Single-molecule and macroscopic reactions of fluorescent nucleotides with myosin have been compared. The single-molecule studies serve as paradigms for enzyme-catalyzed reactions and ligand-receptor interactions analyzed as individual stochastic processes. Fluorescent nucleotides, called Cy3-EDA-ATP and Cy5-EDA-ATP, were derived by coupling the dyes Cy3.29.OH and Cy5.29.OH (compounds XI and XIV, respectively, in, Bioconjug. Chem. 4:105-111)) with 2'(3')-O-[N-(2-aminoethyl)carbamoyl]ATP (EDA-ATP). The ATP(ADP) analogs were separated into their respective 2'- and 3'-O-isomers, the interconversion rate of which was 30[OH(-)] s(-1) (0.016 h(-1) at pH 7.1) at 22 degrees C. Macroscopic studies showed that 2'(3')-O-substituted nucleotides had properties similar to those of ATP and ADP in their interactions with myosin, actomyosin, and muscle fibers, although the ATP analogs did not relax muscle as well as ATP did. Significant differences in the fluorescence intensity of Cy3-nucleotide 2'- and 3'-O-isomers in free solution and when they interacted with myosin were evident. Single-molecule studies using total internal reflection fluorescence microscopy showed that reciprocal mean lifetimes of the nucleotide analogs interacting with myosin filaments were one- to severalfold greater than predicted from macroscopic data. Kinetic and equilibrium data of nucleotide-(acto)myosin interactions derived from single-molecule microscopy now have a biochemical and physiological framework. This is important for single-molecule mechanical studies of motor proteins.

Response tuning in bacterial chemotaxis

Chemotaxis of enteric bacteria in spatial gradients toward a source of chemoattractant is accomplished by increases in the length of swimming runs up the gradient. Biochemical components of the intracellular signal pathway have been identified, but mechanisms for achieving the high response sensitivity remain unknown. Binding of attractant ligand to its receptor inactivates a receptor-associated histidine kinase, CheA, which phosphorylates the signal protein CheY. The reduction in phospho-CheY, CheY-P, levels prolongs swimming runs. Here, the stimulus-response relation has been determined by measurement of excitation responses mediated by the Tar receptor to defined concentration jumps of the attractant, aspartate, administered within milliseconds by photolysis of a photolabile precursor. The bacteria responded to <1% changes in Tar occupancy when adapted to aspartate over concentrations spanning three orders of magnitude. Response amplitudes increased approximately logarithmically with stimulus strength, extending responsiveness over a greater stimulus range. The extent and form of this relation indicates that, in contrast to mechanisms for adaptive recovery, excitation signal generation involves amplification based on cooperative interactions. These interactions could entail inactivation of multiple receptor-CheA signaling complexes and/or simultaneous activation of CheY-P dephosphorylation.

Dynamic measurement of myosin light-chain-domain tilt and twist in muscle contraction

A new method is described for measuring motions of protein domains in their native environment on the physiological timescale. Pairs of cysteines are introduced into the domain at sites chosen from its static structure and are crosslinked by a bifunctional rhodamine. Domain orientation in a reconstituted macromolecular complex is determined by combining fluorescence polarization data from a small number of such labelled cysteine pairs. This approach bridges the gap between in vitro studies of protein structure and cellular studies of protein function and is used here to measure the tilt and twist of the myosin light-chain domain with respect to actin filaments in single muscle cells. The results reveal the structural basis for the lever-arm action of the light-chain domain of the myosin motor during force generation in muscle.

Chemotactic responses of Escherichia coli to small jumps of photoreleased L-aspartate

Computer-assisted motion analysis coupled to flash photolysis of caged chemoeffectors provides a means for time-resolved analysis of bacterial chemotaxis. Escherichia coli taxis toward the amino acid attractant L-aspartate is mediated by the Tar receptor. The physiology of this response, as well as Tar structure and biochemistry, has been studied extensively. The beta-2, 6-dinitrobenzyl ester of L-aspartic acid and the 1-(2-nitrophenyl)ethyl ether of 8-hydroxypyrene-1,3,6-tris-sulfonic acid were synthesized. These compounds liberated L-aspartate and the fluorophore 8-hydroxypyrene 1,3,6-tris-sulfonic acid (pyranine) upon irradiation with near-UV light. Photorelease of the fluorophore was used to define the amplitude and temporal stability of the aspartate jumps employed in chemotaxis experiments. The dependence of chemotactic adaptation times on aspartate concentration, determined in mixing experiments, was best fit by two Tar aspartate-binding sites. Signal processing (excitation) times, amplitudes, and adaptive recovery of responses elicited by aspartate jumps producing less than 20% change in receptor occupancy were characterized in photorelease assays. Aspartate concentration jumps in the nanomolar range elicited measurable responses. The response threshold and sensitivity of swimming bacteria matched those of bacteria tethered to glass by a single flagellum. Stimuli of similar magnitude, delivered either by rapid mixing or photorelease, evoked responses of similar strength, as assessed by recovery time measurements. These times remained proportional to change in receptor occupancy close to threshold, irrespective of prior occupancy. Motor excitation responses decayed exponentially with time. Rates of excitation responses near threshold ranged from 2 to 7 s-1. These values are consistent with control of excitation signaling by decay of phosphorylated pools of the response regulator protein, CheY. Excitation response rates increased slightly with stimulus size up to values limited by the instrumentation; the most rapid was measured to be 16 +/- 3 (SE) s-1. This increase may reflect simultaneous activation of CheY dephosphorylation, together with inhibition of its phosphorylation.

Time-resolved measurements of phosphate release by cycling cross-bridges in portal vein smooth muscle

The rate of release of inorganic phosphate (Pi) from cycling cross-bridges in rabbit portal-anterior mesenteric vein smooth muscle was determined by following the fluorescence of the Pi-reporter, MDCC-PBP (Brune, M., J. L. Hunter, S. A. Howell, S. R. Martin, T. L. Hazlett, J. E. T. Corrie, and M. R. Webb. 1998. Biochemistry. 37:10370-10380). Cross-bridge cycling was initiated by photolytic release of ATP from caged-ATP in Triton-permeabilized smooth muscles in rigor. When the regulatory myosin light chains (MLC20) had been thiophosphorylated, the rate of Pi release was biphasic with an initial rate of 80 microM s-1 and amplitude 108 microM, decreasing to 13.7 microM s-1. These rates correspond to fast and slow turnovers of 1.8 s-1 and 0.3 s-1, assuming 84% thiophosphorylation of 52 microM myosin heads. Activation by Ca2+-dependent phosphorylation subsequent to ATP release resulted in slower Pi release, paralleling the rate of contraction that was also slower than after thiophosphorylation, and was also biphasic: 51 microM s-1 and 13.2 microM s-1. These rates suggest that the activity of myosin light chain kinase and phosphatase ("pseudo-ATPase") contributes <20% of the ATP usage during cross-bridge cycling. The extracellular "ecto-nucleotidase" activity was reduced eightfold by permeabilization, conditions in which the ecto-ADPase was 17% of the ecto-ATPase. Nevertheless, the remaining ecto-ATPase activity reduced the precision of the estimate of cross-bridge ATPase. We conclude that the transition from fast to slow ATPase rates reflects the properties and forces directly acting on cross-bridges, rather than the result of a time-dependent decrease in activation (MLC20 phosphorylation) occurring in intact smooth muscle. The mechanisms of slowing may include the effect of positive strain on cross-bridges, inhibition of the cycling rate by high affinity Mg-ADP binding, and associated state hydrolysis.

Mobilization of intracellular calcium by intracellular flash photolysis of caged dihydrosphingosine in cultured neonatal rat sensory neurones

The ability of dihydrosphingosine to release Ca2+ from intracellular stores in neurones was investigated by combining the whole cell patch clamp technique with intracellular flash photolysis of caged, N-(2-nitrobenzyl)dihydrosphingosine. The caged dihydrosphingosine (100 microM) was applied to the intracellular environment via the CsCl-based patch pipette solution which also contained 0.3% dimethylformamide and 2 mM dithiothreitol. Cultured dorsal root ganglion neurones from neonatal rats were voltage clamped at -90 mV and inward whole cell Ca2+-activated currents were recorded in response to intracellular photorelease of dihydrosphingosine. Intracellular photorelease of dihydrosphingosine (about 5 microM) was achieved using a Xenon flash lamp. Inward Ca2+-activated currents were evoked in 50 out of 57 neurones, the mean delay to current activation following photolysis was 82+/-13 s. The responses were variable with neurones showing transient, oscillating or sustained inward currents. High voltage-activated Ca2+ currents evoked by 100 ms voltage step commands to 0 mV were not attenuated by photorelease of dihydrosphingosine. Controls showed that alone a flash from the Xenon lamp did not activate currents, and that the unphotolysed caged dihydrosphingosine, and intracellular photolysis of 2-(2-nitrobenzylamino) propanediol also did not evoke responses. The dihydrosphingosine current had a reversal potential of -11+/-3 mV (n = 11), and was carried by two distinct Cl- and cation currents which were reduced by 85% and about 20% following replacement of monovalent cations with N-methyl-D-glucamine or application of the Cl- channel blocker niflumic acid (10 microM) respectively. The responses to photoreleased dihydrosphingosine were inhibited by intracellular application of 20 mM EGTA, 10 microM ryanodine or extracellular application of 10 microM dantrolene, but persisted when Ca2+ free saline was applied to the extracellular environment. Intracellular application of uncaged dihydrosphingosine evoked responses which were attenuated by photolysis of the caged Ca2+ chelator Diazo-2. Experiments also suggested that extracellular application of dihydrosphingosine can activate membrane conductances. We conclude that dihydrosphingosine directly or indirectly mobilises Ca2+ from ryanodine-sensitive intracellular stores in cultured sensory neurones.

Mobilization of intracellular calcium by intracellular flash photolysis of caged dihydrosphingosine in cultured neonatal rat sensory neurones

The ability of dihydrosphingosine to release Ca2+ from intracellular stores in neurones was investigated by combining the whole cell patch clamp technique with intracellular flash photolysis of caged, N-(2-nitrobenzyl)dihydrosphingosine. The caged dihydrosphingosine (100 microM) was applied to the intracellular environment via the CsCl-based patch pipette solution which also contained 0.3% dimethylformamide and 2 mM dithiothreitol. Cultured dorsal root ganglion neurones from neonatal rats were voltage clamped at -90 mV and inward whole cell Ca2+-activated currents were recorded in response to intracellular photorelease of dihydrosphingosine. Intracellular photorelease of dihydrosphingosine (about 5 microM) was achieved using a Xenon flash lamp. Inward Ca2+-activated currents were evoked in 50 out of 57 neurones, the mean delay to current activation following photolysis was 82+/-13 s. The responses were variable with neurones showing transient, oscillating or sustained inward currents. High voltage-activated Ca2+ currents evoked by 100 ms voltage step commands to 0 mV were not attenuated by photorelease of dihydrosphingosine. Controls showed that alone a flash from the Xenon lamp did not activate currents, and that the unphotolysed caged dihydrosphingosine, and intracellular photolysis of 2-(2-nitrobenzylamino) propanediol also did not evoke responses. The dihydrosphingosine current had a reversal potential of -11+/-3 mV (n = 11), and was carried by two distinct Cl- and cation currents which were reduced by 85% and about 20% following replacement of monovalent cations with N-methyl-D-glucamine or application of the Cl- channel blocker niflumic acid (10 microM) respectively. The responses to photoreleased dihydrosphingosine were inhibited by intracellular application of 20 mM EGTA, 10 microM ryanodine or extracellular application of 10 microM dantrolene, but persisted when Ca2+ free saline was applied to the extracellular environment. Intracellular application of uncaged dihydrosphingosine evoked responses which were attenuated by photolysis of the caged Ca2+ chelator Diazo-2. Experiments also suggested that extracellular application of dihydrosphingosine can activate membrane conductances. We conclude that dihydrosphingosine directly or indirectly mobilises Ca2+ from ryanodine-sensitive intracellular stores in cultured sensory neurones.

Steady-state fluorescence polarization studies of the orientation of myosin regulatory light chains in single skeletal muscle fibers using pure isomers of iodoacetamidotetramethylrhodamine

The regulatory light chain (RLC) from chicken gizzard myosin was covalently modified on cysteine 108 with either the 5- or 6-isomer of iodoacetamidotetramethylrhodamine (IATR). Labeled RLCs were purified by fast protein liquid chromatography and characterized by reverse-phase high-performance liquid chromatography (HPLC), tryptic digestion, and electrospray mass spectrometry. Labeled RLCs were exchanged into the native myosin heads of single skinned fibers from rabbit psoas muscle, and the ATR dipole orientations were determined by fluorescence polarization. The 5- and 6-ATR dipoles had distinct orientations, and model orientational distributions suggest that they are more than 20 degrees apart in rigor. In the rigor-to-relaxed transition (sarcomere length 2.4 microm, 10 degrees C), the 5-ATR dipole became more perpendicular to the fiber axis, but the 6-ATR dipole became more parallel. This orientation change was absent at sarcomere length 4.0 microm, where overlap between myosin and actin filaments is abolished. When the temperature of relaxed fibers was raised to 30 degrees C, the 6-ATR dipoles became more parallel to the fiber axis and less ordered; when ionic strength was lowered from 160 mM to 20 mM (5 degrees C), the 6-ATR dipoles became more perpendicular to the fiber axis and more ordered. In active contraction (10 degrees C), the orientational distribution of the probe dipoles was similar but not identical to that in relaxation, and was not a linear combination of the orientational distributions in relaxation and rigor.

Fast transient currents in Na,K-ATPase induced by ATP concentration jumps from the P3-[1-(3',5'-dimethoxyphenyl)-2-phenyl-2-oxo]ethyl ester of ATP

Electrogenic ion transport by Na,K-ATPase was investigated by analysis of transient currents in a model system of protein-containing membrane fragments adsorbed to planar lipid bilayers. Sodium transport was triggered by ATP concentration jumps in which ATP was released from an inactive precursor by an intense near-UV light flash. The method has been used previously with the P3-1-(2-nitrophenyl)ethyl ester of ATP (NPE-caged ATP), from which the relatively slow rate of ATP release limits analysis of processes in the pump mechanism controlled by rate constants greater than 100 s(-1) at physiological pH. Here Na,K-ATPase was reinvestigated using the P3-[1-(3,5-dimethoxyphenyl)-2-phenyl-2-oxo]ethyl ester of ATP (DMB-caged ATP), which has an ATP release rate of >10(5) s(-1). Under otherwise identical conditions, photorelease of ATP from DMB-caged ATP showed faster kinetics of the transient current compared to that from NPE-caged ATP. With DMB-caged ATP, transient currents had rate profiles that were relatively insensitive to pH and the concentration of caged compound. Rate constants of ATP binding and of the E1 to E2 conformational change were compatible with earlier studies. Rate constants of enzyme phosphorylation and ADP-dependent dephosphorylation were 600 s(-1) and 1.5 x 10(6) M(-1) s(-1), respectively, at pH 7.2 and 22 degrees C.

Inhibition of calmodulin-activated smooth-muscle myosin light-chain kinase by calmodulin-binding peptides and fluorescent (phosphodiesterase-activating) calmodulin derivatives

Aspects of the biochemistry of calmodulin have been addressed that bear on its cell biological role as a mediator of Ca2+ regulation. Calmodulin-binding peptides derived from the amino acid sequence of smooth-muscle myosin light-chain kinase (MLCK) were characterized as inhibitors of calmodulin activation of MLCK-catalyzed phosphorylation of the smooth-muscle regulatory light chain (MLC). MLCK activity was determined by measuring the rate of formation of one of the reaction products, ADP, in a coupled enzymatic assay by continuous fluorimetric monitoring of NADH removal in 100 microM CaCl2 at ionic strength 0.15 M, pH 7.0 and 21 degreesC. The Km value of calmodulin was 3.5 nM, a value 16-35-fold greater than the Kd value of calmodulin for MLCK [Török, K., and Trentham D. R. (1994) Biochemistry 33, 12807-12820]. The different Km and Kd values are most likely associated with the rate-limiting step in MLC phosphorylation being associated with product release from MLCK. The values of the inhibition constants, Ki, were the following: Ac-R-R-K-W-Q-K-T-G-H-A-V-R-A-I-G-R-L-CONH2 (Trp peptide), 8.6 (+/-1. 4 sd) pM; Y4-analogue of Trp peptide (Tyr peptide), 7.3 (+/-0.1) nM; and A-R-R-K-W-Q-K-T-G-H-A-V-R-A-I-G-R-L-S-S (RS20-like peptide), 0. 11-0.39 nM. The Ki values were consistent with kinetically determined Kd values of the peptides to calmodulin. Kinetic determination of Kd values required the use of a fluorescently labeled calmodulin, 2-chloro-(epsilon-amino-Lys75)-[6-(4-N, N-diethylamino-phenyl)-1,3,5-triazin-4-yl]-calmodulin (TA-calmodulin).1 Since, as here, Lys75 is a convenient labeling site on calmodulin for the introduction of fluorescent probes, the biological activity of the Lys-modified calmodulins was evaluated. TA-calmodulin and calmodulin selectively modified by 1-N, N-dimethylaminonaphthalene-5-sulfonyl chloride (dansyl-C1) at Lys75 (dansyl-calmodulin) were characterized as activators of cyclic AMP phosphodiesterase (PDE) and inhibitors of MLCK. The Km value for dansyl-calmodulin was equal to that of calmodulin, and that of TA-calmodulin was 3.5-fold greater. TA-calmodulin and Lys75-labeled dansyl-calmodulin thus distinguish between PDE and MLCK being agonists to the former and antagonists to the latter.

Activation of NMDA receptors is necessary for the induction of associative long-term potentiation in area CA1 of the rat hippocampal slice

1. It is commonly assumed that the role of the strongly activated heterosynaptic input during the induction of associative long-term potentiation (LTP) is to relieve the magnesium blockade of NMDA receptors located at the weakly stimulated synapses and thereby allow the weak input to undergo potentiation. We tested this assumption by using a caged form of the NMDA receptor antagonist, D-(-)-2-amino-5-phosphonopentanoic acid (D-AP5) to block the activation of NMDA receptors at the weak input in a conditioning protocol for the induction of associative LTP in area CA1 of the rat hippocampal slice. 2. The effect of releasing D-AP5 by flash photolysis of 100 microM caged D-AP5 (N-[1-(2-nitrophenyl)ethoxycarbonyl]-D-AP5) on pharmacologically isolated NMDA receptor-mediated field EPSPs was examined in area CA1. The slope of the EPSP was reduced by 71% within 50 ms of the initiation of the photolytic reaction when the concentration of released D-AP5 had reached 2.0-2.5 microM and was reduced by 95% within 1 min (10 microM D-AP5 released). 3. Associative LTP was induced by pairing a strong tetanus to one input with a weak tetanus (subthreshold for homosynaptic LTP) to a second input. The strong tetanus preceded the weak by 50 ms. Rapid application of D-AP5, by flash photolysis of caged D-AP5, coincident with the last shock of the strong tetanus, resulted in the blockade of NMDA receptor activation during the period of the weak tetanus. Associative LTP was blocked by photolysis of caged D-AP5 but was normally expressed in experiments using caged L-AP5. 4. We conclude that activation of NMDA receptors at the weakly activated input is an essential requirement for synaptically induced associative LTP.

ATPase kinetics on activation of rabbit and frog permeabilized isometric muscle fibres: a real time phosphate assay

1. The rate of appearance of inorganic phosphate (Pi) and hence the ATPase activity of rabbit psoas muscle in single permeabilized muscle fibres initially in rigor was measured following laser flash photolysis of the P3-1-(2-nitrophenyl)ethyl ester of ATP (NPE-caged ATP) in the presence and absence of Ca2+. Pi appearance was monitored from the fluorescence signal of a Pi-sensitive probe, MDCC-PBP, a coumarin-labelled A197C mutant of the phosphate-binding protein from Escherichia coli. Fibres were immersed in oil to optimize the fluorescence signal and to obviate diffusion problems. The ATPase activity was also measured under similar conditions from the rate of NADH disappearance using an NADH-linked coupled enzyme assay. 2. On photolysis of NPE-caged ATP in the presence of Ca2+ at 20 degrees C, the fluorescence increase of MDCC-PBP was non-linear with time. ATPase activity was 41 s-1 in the first turnover based on a myosin subfragment 1 concentration of 150 microM. This was calculated from a linear regression of the fluorescence signal reporting 20-150 microM of Pi release. Tension was at 67% of its isometric level by the time 150 microM Pi was released. ATPase activities were 36 and 31 s-1 for Pi released in the ranges of 150-300 microM and 300-450 microM, respectively. The ATPase activity had a Q10 value of 2.9 based on measurements at 5, 12 and 20 degrees C. 3. An NADH-linked assay showed the ATPase activity had a lower limit of 12.7 s-1 at 20 degrees C. The response to photolytic release of ADP showed that the rate of NADH disappearance was partially limited by the flux through the coupled reactions. Simulations indicated that the linked assay data were consistent with an initial ATPase activity of 40 s-1. 4. On photolysis of NPE-caged ATP in the absence of Ca2+ the ATPase activity was 0.11 s-1 at 20 degrees C with no discernible rapid transient phase of Pi release during the first turnover of the ATPase. 5. To avoid the rigor state, the ATPase rate in the presence of Ca2+ was also measured on activation from the relaxed state by photolytic release of Ca2+ from a caged Ca2+ compound, nitrophenyl-EGTA. At 5 degrees C the ATPase rate was 5.8 and 4.0 s-1 in the first and second turnovers, respectively. These rates are comparable to those when NPE-caged ATP was used. 6. The influence of ADP and Pi on the ATPase activities was measured using the MDCC-PBP and NADH-linked assays, respectively. ADP (0.5 mM) decreased the initial ATPase rate by 23%. Pi (10 mM) had no significant effect. Inhibition by ADP, formed during ATP hydrolysis, contributed to the decrease of ATPase activity with time. 7. The MDCC-PBP assay and NPE-caged ATP were used to measure the ATPase rate in single permeabilized muscle fibres of the semitendinosus muscle of the frog. At 5 degrees C in the presence of Ca2+ the ATPase activity was biphasic being 15.0 s-1 during the first turnover (based on 180 microM myosin subfragment 1). Tension was 74% of its isometric level by the time 180 microM Pi was released. During the third turnover the ATPase rate decreased to about 20% of that during the first turnover. 8. ATPase activity in isometric rabbit muscle fibres during the first few turnovers is about an order of magnitude greater than that when a steady state is reached. Possible reasons and the consequences for understanding the mechanism of muscular contraction are discussed.

Fluorescence polarization of skeletal muscle fibers labeled with rhodamine isomers on the myosin heavy chain

Fluorescence polarization was used to examine orientational changes of Rhodamine probes in single, skinned muscle fibers from rabbit psoas muscle following either photolysis of caged nucleotides or rapid length changes. Fibers were extensively and predominantly labeled at SH1 (Cys-707) of the myosin heavy chain with either the 5- or the 6-isomer of iodoacetamidotetramethylrhodamine. Results from spectroscopic experiments utilizing the two Rhodamine isomers were quite similar. Following photolysis of either caged ATP or caged ADP, probes promptly reoriented toward the muscle fiber axis. Changes in the fluorescence polarization signals with transients elicited by the photolysis of caged ATP in the presence of saturating Ca2+ greatly preceded active force generation. Photolysis of caged ADP caused only a small, rapid decrease in force but elicited changes in the fluorescence polarization signals with time course and amplitude similar to those following photolysis of caged ATP. Fluorescence polarization signals were virtually unchanged by rapid length steps in both rigor and active muscle fibers. These results indicate that structural changes monitored by Rhodamine probes at SH1 are not associated directly with the force-generating event of muscle contraction. However, the fluorescence polarization transients were slightly faster than the estimated rate of cross-bridge detachment following photolysis of caged ATP, suggesting that the observed structural changes at SH1 may be involved in the communication pathway between the nucleotide- and actin-binding sites of myosin.

Kinetics of the phosphorylation of Na,K-ATPase by inorganic phosphate detected by a fluorescence method

Phosphorylation by Pi of the Na,K-ATPase from rabbit kidney in the absence of Na+ ions but in the presence of Mg2+ ions has been studied. In the absence of K+ ions, unphosphorylated and phosphorylated states induce different fluorescence levels in the membrane-bound styryl dye RH421, and hence transitions between the two states were monitored. Transient kinetic studies of phosphorylation were initiated by manual addition of Pi or by photochemical release of Pi from 1-(2-nitrophenyl)ethyl phosphate (caged Pi) using laser flash photolysis at 308 nm. Equilibrium studies of phosphorylation showed that the apparent Km for Pi was 23.0 +/- 0.3 microM (mean +/- sem) at pH 7.1 and 21 degrees C. The dye fluorescence increased in a biphasic manner on addition of 500 microM Pi to the enzyme: a rapid phase (t 1/2 < 1 s) and a slower exponential phase at 0.059 +/- 0.003 s-1. The rate of the rapid phase was studied by fast concentration-jump experiments and exhibited first-order kinetics in Pi up to 60 microM. Fluorescence records vs time were exponential, and a plot of the rate constant versus [Pi] had a slope of 1.47 x 10(5) M-1 s-1 and ordinate [Pi] = 0) intercept of 3.1 s-1. Addition of 50 mM NaCl to the phosphorylated enzyme induced an exponential decay in the dye fluorescence from which a rate constant of 0.10 +/- 0.005 s-1 was determined. These data were interpreted in terms of transformations between conformational states E1 and E2, and the phosphorylated state P-E2 defined in the Post-Albers mechanism of the Na,K-ATPase [Läuger, P., (1991) Electrogenic Ion Pumps, Sinauer Associates Inc., Sunderland, MA] as follows: [formula: see text] The RH421 fluorescence of state P-E2 was studied over the pH range 6-8.5. Fluorescence was greatest at pH 8.5 and lowest at pH 6.0 in a simple binding isotherm with pK 7.5. The apparent Km for Pi rose cooperatively with increasing pH (pKa 8.6 and a Hill coefficient of 2). Therefore in the absence of monovalent metal ions, occupation of the cation (K+) binding sites by protons promotes phosphorylation by Pi.

The role of MgADP in force maintenance by dephosphorylated cross-bridges in smooth muscle: a flash photolysis study

The effect of [MgADP] on relaxation from isometric tension, initiated by reducing free [Ca2+] through photolysis of the caged photolabile Ca2+ chelator diazo-2, was determined at 20 degrees C in alpha-toxin permeabilized tonic (rabbit femoral artery, Rf) and phasic (rabbit bladder, Rb) smooth muscle. In Rf, the shape of the relaxation curve was clearly biphasic, consisting of a slow "plateau" phase followed by a monotonic exponential decline with rate constant k. The duration of the plateau (d = 44 +/- 4 s, mean +/- SEM, n = 28) was well correlated (R = 0.92) with the total t1/2 of relaxation that was 66 +/- 3 s (n = 28) in the presence of 20 mM creatine phosphate (CP), and was prolonged in the absence of CP (t1/2 = 83 +/- 3 s, n = 7); addition of 100 microM MgADP further slowed relaxation (t1/2 = 132 +/- 7 s, n = 14). In Rb, a plateau was not detectable and t1/2 (= 15 +/- 2 s, n = 6) was not affected by 100 microM MgADP. In Rf the Q10 between 20 degrees C and 30 degrees C was 4.3 +/- 0.4 for d-1 and 2.8 +/- 0.3 for k (n = 8; p = 0.006). The regulatory myosin light chain (MLC20) in Rf was dephosphorylated at 0.07 +/- 0.02 s-1, from 42 +/- 3% before to 20 +/- 2% after photolysis of diazo-2, reaching basal values at a time when force had fallen by only 40%. We conclude that, in the presence of ATP, as during rigor, the affinity of dephosphorylated cross-bridges for MgADP is significantly higher in tonic than in phasic smooth muscle and contributes to the maintenance of force at low levels of phosphorylation. The MgADP dependence of the post-dephosphorylation phase of relaxation is consistent with its being rate-limited by the slow off-rate of ADP from cross-bridges that were dephosphorylated while in force-generating ADP-bound (AM*D) cross-bridge states. The fourfold faster off-rate of ADP from AM*D in the phasic, Rb, compared to tonic, Rf, smooth muscle is a major determinant of the different kinetics of relaxation in the two types of smooth muscle.

Chemotactic signal integration in bacteria

Chemotactic signaling in Escherichia coli involves transmission of both negative and positive signals. In order to examine mechanisms of signal processing, behavioral responses to dual inputs have been measured by using photoactivable "caged" compounds, computer video analysis, and chemoreceptor deletion mutants. Signaling from Tar and Tsr, two receptors that sense amino acids and pH, was studied. In a Tar deletion mutant the photoactivated release of protons, a Tsr repellent, and of serine, a Tsr attractant, in separate experiments at pH 7.0 resulted in tumbling (negative) or smooth-swimming (positive) responses in ca. 50 and 140 ms, respectively. Simultaneous photorelease of protons and serine resulted in a single tumbling or smooth-swimming response, depending on the relative amounts of the two effectors. In contrast, in wild-type E. coli, proton release at pH 7.0 resulted in a biphasic response that was attributed to Tsr-mediated tumbling followed by Tar-mediated smooth-swimming. In wild-type E. coli at more alkaline pH values the Tar-mediated signal was stronger than the Tsr signal, resulting in a strong smooth-swimming response preceded by a diminished tumbling response. These observations imply that (i) a single receptor time-averages the binding of different chemotactic ligands generating a single response; (ii) ligand binding to different receptors can result in a nonintegrated response with the tumbling response preceding the smooth-swimming response; (iii) however, chemotactic signals of different intensities derived from different receptors can also result in an apparently integrated response; and (iv) the different chemotactic responses to protons at neutral and alkaline pH may contribute to E. coli migration toward neutrality.

Tilting of the light-chain region of myosin during step length changes and active force generation in skeletal muscle

Force generation and relative sliding between the myosin and actin filaments in muscle are thought to be caused by tilting of the head region of the myosin crossbridges between the filaments. Structural and spectroscopic experiments have demonstrated segmental flexibility of myosin in muscle, but have not shown a direct linkage between tilting of the myosin heads and either force generation or filament sliding. Here we use fluorescence polarization to detect changes in the orientation of the light-chain region of the head, the part most likely to tilt, and synchronized head movements by imposing rapid length steps. We found that the light-chain region of the myosin head tilts both during the imposed filament sliding and during the subsequent quick force recovery that is thought to signal the elementary force-generating event.

Kinetics of cytosolic Ca2+ concentration after photolytic release of 1-D-myo-inositol 1,4-bisphosphate 5-phosphorothioate from a caged derivative in guinea pig hepatocytes

The influence of 1-D-myo-inositol 1,4,5-trisphosphate (InsP3) breakdown by InsP3 5-phosphatase in determining the time course of Ca2+ release from intracellular stores was investigated with flash photolytic release of a stable InsP3 derivative, 5-thio-InsP3, from a photolabile caged precursor. The potency and Ca(2+)-releasing properties of the biologically active D isomers of 5-thio-InsP3 and InsP3 itself were compared by photolytic release in guinea pig hepatocytes. After a light flash, cytosolic free calcium concentration ([Ca2+]i) showed an initial delay before rising quickly to a peak and declining more slowly to resting levels, with time course and amplitude generally similar to those seen with photolytic release of InsP3. Differences were a three- to eightfold lower potency of 5-thio-InsP3 in producing Ca2+ release, much longer delays between photolytic release and Ca2+ efflux with low concentrations of 5-thio-InsP3 than with InsP3, and persistent reactivation of Ca2+ release, producing periodic fluctuations of cytosolic [Ca2+]i with high concentrations of 5-thio-InsP3 but not InsP3 itself. The lower potency of 5-thio-InsP3 may be a result of a lower affinity for closed receptor/channels or a lower open probability of liganded receptor/channels. The longer delays with 5-thio-InsP3 at low concentration suggest that metabolism of InsP3 by 5-phosphatase may reduce the concentration sufficiently to prevent receptor activation and may have a similar effect on InsP3 concentration during hormonal activation. The maximal rate of rise of [Ca2+]i, the duration of the period of high Ca2+ efflux, and the initial decline of [Ca2+]i are similar with5-thio-lnsP3 and lnsP3, indicating that lnsP3 breakdown is not important in terminating Ca2+ release. The second activation ofInsP3 receptors with 5-thio-lnsP3 and particularly the sustained periodic fluctuations of [Ca2+]i indicate persistence of 5-thio-lnsP3,suggesting that InsP3 breakdown prevents reactivation of InsP3 receptors. The photochemical properties of 1-(2-nitrophenyl)-ethyl caged 5-thio-lnsP3 are photolytic quantum yield = 0.57 (cf. 0.65 for caged InsP3) and rate of photolysis = 87 s-I (half-life approximately 8 ms; cf. 3 ms for caged lnsP3; pH7.1; ionic strength, 0.2 M; 21 OC). Caged 5-thio-lnsP3 at concentrations up to 360 pM did not activate lnsP3 receptors to produce Ca2+ release or block Ca2+ release by free 5-thio-lnsP3.

A new method for the time-resolved measurement of phosphate release in permeabilized muscle fibers

A new method for the measurement of phosphate release in contracting and relaxed permeabilized muscle fibers is described. The assay is based on a genetically engineered phosphate-binding protein labeled with a coumarin fluorescent probe, which binds inorganic phosphate tightly and shows a fourfold increase in fluorescence upon binding. Measurements of Pi release on the millisecond time scale with sensitivity in the 10 microM range are obtained that provide new information about the relationship between ATP hydrolysis and force production.

Fluorescence polarization from isomers of tetramethylrhodamine at SH-1 in rabbit psoas muscle fibers

We have used fluorescence polarization to examine orientational changes of the 5- and 6-isomers of acetamidotetramethylrhodamine (ATR) covalently bound to SH-1 (Cys-707 of the myosin heavy chain) in single, skinned fibers from rabbit psoas muscle after rapid length steps or photolysis of caged nucleotides. Similar results were obtained with both the 5- and 6-isomers of ATR. After the photolysis of caged ATP, large and rapid changes in the fluorescence polarization signals were observed and were complete well before appreciable force had been generated. Changes in the fluorescence polarization signals after the photolysis of caged ADP were similar to those after the photolysis of caged ATP, despite an almost negligible change in force. The fluorescence polarization signals remained almost constant after rapid length steps in both rigor and active muscle fibers. These results suggest that structural changes at SH-1 monitored by 5- or 6-ATR are not associated directly with the force-generating event of muscle contraction, but may be involved in the communication pathway between the nucleotide and actin-binding sites of myosin.

Kinetics of relaxation from rigor of permeabilized fast-twitch skeletal fibers from the rabbit using a novel caged ATP and apyrase

The complex time course of tension decay was investigated in fast-twitch permeabilized rabbit muscle fibers when they were relaxed from the rigor state using photochemical generation of ATP. A novel caged ATP compound, the P3-3',5'-dimethoxybenzoin ester of ATP (DMB-caged ATP), as well as the P3-1-(2-nitrophenyl)ethyl ester of ATP (NPE-caged ATP), have been used. DMB-caged ATP photolyzes at least three orders of magnitude more rapidly than NPE-caged ATP. The role of ADP on relaxation kinetics from rigor was examined by using apyrase to remove ADP from the rigor muscle solutions. The presence of Pi-sensitive states was investigated from the effect of Pi on relaxation. Rigor tension was varied enabling the influence of tension on the relaxation to be examined. The time course of relaxation was faster with DMB-caged ATP compared with NPE-caged ATP for concentrations of ATP released by photolysis greater than 0.7 mM. Most of the complexity in the relaxation tension records was caused by ADP. In the absence of ADP, tension decayed monotonically after photochemical release of ATP in a process whose rate was unaffected by Pi. In the presence of ADP, relaxation was more complex and tension passed through a maximum. A model invoking cooperative interactions involving ADP-containing myosin heads provides a reasonable description of the data.

Mechanism of 2-chloro-(epsilon-amino-Lys75)-[6-[4-(N,N- diethylamino)phenyl]-1,3,5-triazin-4-yl]calmodulin interactions with smooth muscle myosin light chain kinase and derived peptides

The mechanism of the interactions of 2-chloro-(epsilon-amino-Lys75)-[6-[4-(N,N-diethylamino)phenyl]- 1,3,5-triazin-4-yl]calmodulin (TA-calmodulin) with smooth muscle myosin light-chain kinase (MLCK) and two 17-residue peptides, Ac-R-R-K-W-Q-K-T-G-H-A-V-R-A-I-G-R-L-CONH2 (Trp peptide) and Tyr peptide, in which W is replaced by Y, were studied by measurements of equilibrium and transient fluorescence changes in the nanomolar range. Most reactions were carried out in 100 microM CaCl2 at ionic strength 0.15 M, pH 7.0, and 21 degrees C. In each case association of MLCK or peptide to TA-calmodulin could be described by a two-step process, a bimolecular step and an isomerization. In the case of the interaction between TA-calmodulin and Tyr peptide it was shown that the isomerization involved the binary complex of TA-calmodulin and Tyr peptide as opposed to an isomerization of either TA-calmodulin or Tyr peptide in isolation. These distinctions depended in part on development for transient kinetic experiments of a general theory to quantify relative phase amplitudes in two-step mechanisms. The kinetics for all three association reactions were then interpreted in terms of a bimolecular association (rate constants k+1 and k-1) followed by an isomerization of the binary complex (rate constants k+2 and k-2). For the interaction of TA-calmodulin and Tyr peptide, values of the rate constants are k+1, 8.8 x 10(8) M-1 s-1; k-1, 5.7 s-1; k+2, 0.38 s-1; and k-2, 0.65 s-1. The fluorescence intensities (lambda ex 365 nm, lambda ex 365 nm, lambda em > 400 nm) of TA-calmodulin, the initial binary complex of TA-calmodulin and Tyr peptide, and the isomerized binary complex are in the ratio 1:2.8:1.3. Analogous mechanisms were found for TA-calmodulin binding to Trp peptide and to MLCK, but values for the rate constants and relative fluorescence intensities of the binary complexes were generally not so completely defined. Values for the Trp peptide and MLCK, respectively, are k+1, 8.8 x 10(8) M-1 s-1 and 1.1 x 10(8) M-1 s-1; (k+2 + k-2), 0.97 s-1 and 1.3 s-1; and k-1k-2/(k+2 + k-2), 0.0079 s-1 and 0.025-0.056 s-1. Equilibrium dissociation constants (Kd) for interactions of TA-calmodulin and targets determined from these data are Tyr peptide, 4.1 nM; Trp peptide, 0.011 nM; and MLCK, 0.23-0.51 nM.(ABSTRACT TRUNCATED AT 400 WORDS)

Excitatory signaling in bacterial probed by caged chemoeffectors

Chemotactic excitation responses to caged ligand photorelease of rapidly swimming bacteria that reverse (Vibrio alginolyticus) or tumble (Escherichia coli and Salmonella typhimurium) have been measured by computer. Mutants were used to assess the effects of abnormal motility behavior upon signal processing times and test feasibility of kinetic analyses of the signaling pathway in intact bacteria. N-1-(2-Nitrophenyl)ethoxycarbonyl-L-serine and 2-hydroxyphenyl 1-(2-nitrophenyl) ethyl phosphate were synthesized. These compounds are a 'caged' serine and a 'caged' proton and on flash photolysis release serine and protons and attractant and repellent ligands, respectively, for Tsr, the serine receptor. The product quantum yield for serine was 0.65 (+/- 0.05) and the rate of serine release was proportional to [H+] near-neutrality with a rate constant of 17 s-1 at pH 7.0 and 21 degrees C. The product quantum yield for protons was calculated to be 0.095 on 308-nm irradiation but 0.29 (+/- 0.02) on 300-350-nm irradiation, with proton release occurring at > 10(5) s-1. The pH jumps produced were estimated using pH indicators, the pH-dependent decay of the chromophoric aci-nitro intermediate and bioassays. Receptor deletion mutants did not respond to photorelease of the caged ligands. Population responses occurred without measurable latency. Response times increased with decreased stimulus strength. Physiological or genetic perturbation of motor rotation bias leading to increased tumbling reduced response sensitivity but did not affect response times. Exceptions were found. A CheR-CheB mutant strain had normal motility, but reduced response. A CheZ mutant had tumbly motility, reduced sensitivity, and increased response time to attractant, but a normal repellent response. These observations are consistent with current ideas that motor interactions with a single parameter, namely phosphorylated CheY protein, dictate motor response to both attractant and repellent stimuli. Inverse motility motor mutants with extreme rotation bias exhibited the greatest reduction in response sensitivity but, nevertheless, had normal attractant response times. This implies that control of CheY phosphate concentration rather than motor reactions limits responses to attractants.

Postsynaptic activation at the squid giant synapse by photolytic release of L-glutamate from a 'caged' L-glutamate

1. Pharmacological evidence suggests L-glutamate is a strong candidate as a transmitter at the giant synapse of the squid. Postsynaptic activation at the giant synapse cannot be effected by conventional application of putative neurotransmitters by iontophoresis or perfusion, apparently because the complex structure of the synapse prevents a sufficiently rapid change in concentration at the postsynaptic membrane. Flash photolytic release of L-glutamate from a pharmacologically inert 'caged' L-glutamate pre-equilibrated in the stellate ganglion of Alloteuthis or Loligo was used to determine whether L-glutamate can produce postsynaptic activation when released rapidly in the synaptic clefts. 2. The preparation, reaction mechanism and properties of the caged L-glutamate, N-1-(2-nitrophenyl)ethoxycarbonyl-L-glutamate, are described. The product quantum yield on photolysis was 0.65 (+/- 0.05). On flash photolysis glutamate release followed a single exponential time-course in the pH range 5.5-7.8. The rate constant was proportional to [H+] and was 93 s-1 at pH 5.5 and 16 degrees C in artificial sea water (ionic strength, I = 0.68 M). 3. At pH 7.8 flash photolysis of caged glutamate pre-equilibrated in the synapse caused only a slow depolarization. A second photolytic release of L-glutamate or transsynaptic activation produced no further depolarization, suggesting desensitization and inactivation of postsynaptic mechanisms by the initial pulse of L-glutamate. 4. Synaptic transmission in the giant synapse was normal at pH 5.5. Flash photolysis at pH 5.5 caused rapid production of L-glutamate within the synaptic cleft and a fast postsynaptic depolarization which generated postsynaptic action potentials.(ABSTRACT TRUNCATED AT 250 WORDS)

Kinetics of Ca2+ release and contraction induced by photolysis of caged D-myo-inositol 1,4,5-trisphosphate in smooth muscle. The effects of heparin, procaine, and adenine nucleotides

The kinetics of Ca2+ release and contraction induced by photolytic release of inositol 1,4,5-trisphosphate (InsP3) were determined in permeabilized smooth muscle. The rate of Ca2+ release was half-maximal at 1 microM InsP3. The concentration-dependent delay of Ca2+ release at saturating InsP3 concentration was approximately 10 ms and within the uncertainty of the measurements. The relationship between the delay and InsP3 concentration showed no evidence of a high level (n = 4 or higher) of cooperativity but could not distinguish between no cooperativity (n = 1) or a low level (n = 2) of cooperativity. Submaximal [InsP3] caused only partial Ca2+ release from the InsP3-sensitive stores. InsP3-induced Ca2+ release was markedly potentiated by ATP or by adenosine 5'-(beta,gamma-methylene-triphosphate), but neither the rate nor the amplitude of release was significantly affected by procaine (2-5 mM). Heparin increased the delay between photolysis and Ca2+ release, indicating that the off rate of inert ligand(s) bound to InsP3 receptors may contribute to the physiological delay in Ca2+ release. There was a much longer (370 ms +/- 45 S.E.) delay between the rise of Ca2+ and force development, presumably reflecting events preceding and associated with myosin light chain phosphorylation.

Synthesis and properties of a conformationally restricted spin-labeled analog of ATP and its interaction with myosin and skeletal muscle

The synthesis is described of a spin-labeled analog of ATP, 2',3'-O-(1-oxy-2,2,6,6-tetramethyl-4-piperidylidene)adenosine 5'-triphosphate (SL-ATP). The spin-label moiety is attached by two bonds to the ribose ring as a spiroketal and hence has restricted conformational mobility relative to the ribose moiety of ATP. The synthesis proceeds via an acid-catalyzed addition of adenosine 5'-monophosphate to 1-acetoxy-4-methoxy-2,2,6,6-tetramethyl-1,2,5,6-tetrahydropyridine in acetonitrile. The spiroketal product is pyrophosphorylated, and alkaline hydrolysis with concomitant aerial oxidation gives the required product. The spin-labeled moiety probably takes up two rapidly interconverting conformations with respect to the ribose ring on the basis of the 1H NMR spectra of its precursors and related uridine derivatives [Alessi et al. (1991) J. Chem. Soc., Perkin Trans.1,2243-2247]. SL-ATP is a substrate for myosin and actomyosin with similar kinetic parameters to ATP during triphosphatase activity. SL-ATP supports muscle contraction and permits relaxation of permeabilized rabbit skeletal muscle fibers. SL-ADP is a substrate for yeast 3-phosphoglycerate kinase, thus permitting regeneration of SL-ATP from SL-ADP within muscle fibers. Electron paramagnetic resonance (EPR) studies of SL-ADP bound to myosin filaments and to myofibrils show a degree of nanosecond motion independent of that of the protein, which may be due to conformational flexibility of the ribose moiety of ATP bound to myosin's active site. This nanosecond motion is more restricted in myofibrils than in myosin filaments, suggesting that the binding of actin affects the ribose binding site in myosin. EPR studies on SL-ADP bound to rigor cross-bridges in muscle fiber bundles showed the nucleotide to be highly oriented with respect to the fiber axis.

Bacterial chemoreceptor signaling probed by flash photorelease of a caged serine

A caged serine, a photolabile compound that liberates serine upon photolysis, has been synthesized. Smooth-swimming responses of the bacterium Escherichia coli to caged serine photorelease were videotaped. The mean latency was measured from the videorecords using computerized motion analysis. This time was approximately 0.2 s. Caged photorelease of a photolabile but nonchemotactic serine analogue had no effect on the swimming behavior of the bacteria. A tumbly mutant strain lacking tsr, the serine chemoreceptor, did not respond to caged serine photorelease.

GTP gamma S causes contraction of skinned frog skeletal muscle via the DHP-sensitive Ca2+ channels of sealed T-tubules

We have investigated the involvement of G-proteins in excitation-contraction coupling of fast-twitch skeletal muscle, using a fibre preparation designed to retain intact T-tubules and sarcoplasmic reticulum. The nonhydrolysable analogue of guanosine triphosphate, GTP gamma S (50-500 microM) caused a strong, transient isometric contraction in this preparation. Reduction of ethylene-bis(oxonitrilo)tetraacete (EGTA) in the sealed T-tubules from 5 mM to 0.1 mM lowered the threshold to GTP gamma S and removal of sodium reversibly raised it. The dihydropyridine (DHP) calcium channel antagonists nicardipine and nifedipine allowed a first contraction and then blocked subsequent GTP gamma S action. The phenylalkylamine methoxyverapamil (D-600) did likewise, reversibly, at 10 degrees C. The guanosine diphosphate analogue, GDP beta S, and procaine reversibly blocked the action of GTP gamma S; pertussis toxin also blocked it. Photolytic release of 40-100 microM GTP gamma S within 0.1 s from S-caged GTP gamma S caused contraction after a latent period of 0.3-20 s. We conclude that GTP gamma S can activate contraction in frog skeletal muscle via a route requiring both the integrity of the T-tubular DHP-sensitive calcium channel (DHPr) and the presence of sodium in the sealed T-tubules. We propose that in this preparation GTP gamma S activates a G-protein, which in turn activates the DHPr as a calcium channel and releases stored calcium from within the sealed T-tubule. Implications of these results for the excitation-contraction coupling mechanism in skeletal muscle are discussed.

Cross-bridge kinetics in the presence of MgADP investigated by photolysis of caged ATP in rabbit psoas muscle fibres

1. The interaction between MgADP and rigor cross-bridges in glycerol-extracted single fibres from rabbit psoas muscle has been investigated using laser pulse photolysis of caged ATP (P3-1(2-nitrophenyl)ethyladenosine 5'-triphosphate) in the presence of MgADP and following small length changes applied to the rigor fibre. 2. Addition of 465 microM-MgADP to a rigor fibre caused rigor tension to decrease by 15.3 +/- 0.7% (S.E.M., n = 24 trials in thirteen fibres). The half-saturation value for this tension reduction was 18 +/- 4 microM (n = 23, thirteen fibres). 3. Relaxation from rigor by photolysis of caged ATP in the absence of Ca2+ was markedly slowed by inclusion of 20 microM-2 mM-MgADP in the photolysis medium. 4. Four phases of tension relaxation occurred with MgADP in the medium: at, a quick partial relaxation (in pre-stretch fibres); bt, a slowing of relaxation or a rise in tension for 50-100 ms; ct, a sudden acceleration of relaxation; and dt, a final, nearly exponential relaxation. 5. Experiments at varied MgATP and MgADP concentrations suggested that phase at is due to MgATP binding to nucleotide-free cross-bridges. 6. Phase bt was abbreviated by including 1-20 mM-orthophosphate (Pi) in the photolysis medium, or by applying quick stretches before photolysis or during phase bt. These results suggest that phases bt and ct are complex processes involving ADP dissociation, cross-bridge reattachment and co-operative detachment involving filament sliding and the Ca(2+)-regulatory system. 7. Stretching relaxed muscle fibres to 3.2-3.4 microns striation spacing followed by ATP removal and release of the rigor fibre until tension fell below the relaxed level allowed investigation of the strain dependence of relaxation in the regions of negative cross-bridge strain. In the presence of 50 microM-2 mM-MgADP and either 10 mM-Pi or 20 mM-2,3-butanedione monoxime, relaxation following photolysis of caged ATP was 6- to 8-fold faster for negatively strained cross-bridges than for positively strained ones. This marked strain dependence of cross-bridge detachment is predicted from the model of A. F. Huxley (1957). 8. In the presence of Ca2+, activation of contraction following photolysis of caged ATP was slowed by inclusion of 20-500 microM-MgADP in the medium. An initial decrease in tension related to cross-bridge detachment by MgATP was markedly suppressed in the presence of MgADP. 9. Ten millimolar Pi partly suppressed active tension generation in the presence of MgADP.(ABSTRACT TRUNCATED AT 400 WORDS)

Reversible inactivation of K+ channels of Vicia stomatal guard cells following the photolysis of caged inositol 1,4,5-trisphosphate

RECENT investigations suggest that cytoplasmic D-myo-inositol 1,4,5-trisphosphate (InsP3) functions as a second messenger in plants, as in animals, coupling environmental and other stimuli to intracellular Ca2+ release. Cytoplasmic levels of InsP3 and the turnover of several probable precursors in plants are affected by physiological stimuli--including light, osmotic stress and the phytohormone indoleacetic acid--and InsP3 activates Ca2+ channels and Ca2+ flux across plant vacuolar and microsomal membranes. Complementary data also link changes in cytoplasmic free Ca2+ to several physiological responses, notably in guard cells which regulate gas exchange through the stomatal pores of higher plant leaves. Recent evidence indicates that guard cell K+ channels and, hence, K+ flux for stomatal movements may be controlled by cytoplasmic Ca2+. So far, however, direct evidence of a role for InsP3 in signalling in plants has remained elusive. Here we report that InsP3 released from an inactive, photolabile precursor, the P5-1-(2-nitrophenyl)ethyl ester of InsP3 (caged InsP3) reversibly inactivates K+ channels thought to mediate K+ uptake by guard cells from Vicia faba L. while simultaneously activating an apparently time-independent, inward current to depolarize the membrane potential and promote K+ efflux through a second class of K+ channels. The data are consistent with a transient rise in cytoplasmic free Ca2+ and demonstrate that intact guard cells are competent to use InsP3 in signal cascades controlling ion flux through K+ channels.

Kinetics and mechanism of DNA repair. Preparation, purification and some properties of caged dideoxynucleoside triphosphates

Caged dideoxyribosylthymine triphosphate, dideoxyadenosine triphosphate and arabinosylcytosine triphosphate were prepared in high yield by reaction with 1-(2-nitrophenyl)diazoethane at pH 4 and room temperature for 24 h. Synthesis of caged alpha-32P-labelled dideoxyadenosine triphosphate (approx. 5000 Ci/mmol) in 85% yield was achieved by a modification of the method used for the synthesis of the unlabelled compounds. ATP was shown to be an excellent buffer in the synthesis of alpha-32P-labelled material, and in caged form to be an effective carrier in h.p.l.c. purification. Preparative h.p.l.c. was used to achieve purification of unlabelled caged compounds to greater than 98% purity and 32P-labelled material to 97% purity. Photolysis of unlabelled and 32P-labelled caged compounds by using XeF-excimer laser irradiation at 351 nm was characterized by using difference spectrophotometry and h.p.l.c. analysis. The stability of caged dideoxyadenosine [a-32P]triphosphate in the presence of cultured mammalian cells was evaluated; the adenosine derivative is essentially stable for 1 h.

Kinetics of the conductance evoked by noradrenaline, inositol trisphosphate or Ca2+ in guinea-pig isolated hepatocytes

1. Guinea-pig hepatocytes respond to noradrenaline (NA, 5-10 microM) with a large membrane conductance increase to K+ and Cl-. The response has a long initial delay (range 2-30 s). Following the delay, the K+ conductance (studied in Cl(-)-free solutions) rises quickly to a peak in 1-2 s and is maintained in the continued presence of NA, though often with superimposed oscillations of conductance. The roles of intracellular Ca2+ and D-myo-inositol 1,4,5-trisphosphate (InsP3) in this complex response have been investigated by rapid photolytic release of intracellular Ca2+ (from Nitr5-Ca2+ buffers) or InsP3 from 'caged' InsP3. 2. A rapid increase of intracellular [Ca2+] produced an immediate membrane conductance increase which rose approximately exponentially to a new steady level, consistent with a direct activation of Ca2(+)-dependent ion channels. 3. Following a pulse of InsP3, conductance rose after a brief delay (range 70-1500 ms) which was shortest at high [InsP3] or if the initial cytosolic [Ca2+] had been raised above normal levels. The maximum conductance produced by InsP3 was similar in each cell to the peak recorded with NA and could be evoked by InsP3 concentrations of 0.5-1 microM. 4. The rates of rise of conductance increased with InsP3 concentration in the range 0.25-12.5 microM (range 10-90%, rise times 90-1000 ms), indicating that InsP3-evoked Ca2(+)-efflux from stores increases with InsP3 concentration in this range. 5. Photochemically released InsP3 and Ca2+ activate at physiological concentrations the same membrane conductances as NA. The results indicate that the long initial delay in NA action occurs prior to or during generation of InsP3. The mechanism of the delay and the subsequent apparently all-or-none conductance increase during NA action are discussed in terms of the high co-operativity in InsP3 and Ca2+ actions and an additional positive feedback step. 6. Evidence was found of a negative interaction between [Ca2+] and InsP3-evoked Ca2+ release. The time course of the recovery of InsP3-evoked Ca2+ release following a rise of cytosolic [Ca2+] suggests that this interaction may be important in regulating oscillatory responses of [Ca2+] during hormonal stimulation of guinea-pig hepatocytes.

Photolabile precursors of inositol phosphates. Preparation and properties of 1-(2-nitrophenyl)ethyl esters of myo-inositol 1,4,5-trisphosphate

1-(2-Nitrophenyl)ethyl esters of D-myo-inositol 1,4,5-trisphosphate (InsP3) have been synthesized and shown to have suitable properties for use as photolabile precursors of InsP3. Synthesis was accomplished by treatment of InsP3 with 1-(2-nitrophenyl)diazoethane in a CHCl3/water mixture. This resulted in esterification of each of the three phosphate residues in InsP3, the 1-phosphate being more reactive than the 4- or 5-phosphate. Singly esterified P-1, P-4, and P-5 esters, termed P-1, P-4, and P-5 caged InsP3, were isolated from the reaction mixture by anion-exchange HPLC and characterized by 500-MHz 1H NMR spectroscopy. Each of these caged InsP3 esters exists as a pair of diastereoisomers and was identified by examining the effects of pH and nitrophenyl ring current shielding on the chemical shifts of nonexchangeable inositol protons. 1H NMR spectra of InsP3 were analyzed for comparison. On photolysis the compounds released InsP3 with rate constants of 175 (P-1), 225 (P-4), and 280 s-1 (P-5) as determined by monitoring the aci-nitro decay reaction at pH 7.1, 0.2 M ionic strength, 21 degrees C. Quantum yields determined by steady-state near-UV photolysis were 0.65 +/- 0.08 for each compound. P-4 and P-5 caged InsP3 were the most promising biologically inactive InsP3 precursors since at concentrations up to 50 microM they did not release Ca2+ from smooth muscle sarcoplasmic reticulum (SR) and were not metabolized by vascular smooth muscle InsP3 5-phosphatase or bovine brain InsP3 3-kinase.(ABSTRACT TRUNCATED AT 250 WORDS)

Relaxation of muscle fibers with adenosine 5'-[gamma-thio]triphosphate (ATP[gamma S]) and by laser photolysis of caged ATP[gamma S]: evidence for Ca2+-dependent affinity of rapidly detaching zero-force cross-bridges

The relationship between the mechanical and biochemical states of the muscle cross-bridge cycle and the control of contraction were investigated by using the nucleotide analogs adenosine 5'-[gamma-thio]triphosphate (ATP[gamma S]) and caged ATP[gamma S] [the O-1(2-nitrophenyl)ethyl P3-ester of ATP[gamma S]]. ATP[gamma S] interacts with actomyosin in a manner similar to ATP but is hydrolyzed (by a factor of 500) more slowly. Generation of ATP[gamma S] by photolysis of caged ATP[gamma S] within a permeabilized fiber in rigor in the absence of Ca2+ relaxed tension and stiffness as occurs with ATP. The transient rise in tension prior to final relaxation observed with photolysis of caged ATP was absent with caged ATP[gamma S]. This result suggests that following detachment of a cross-bridge, ATP is normally hydrolyzed before force generation. In the presence of Ca2+, photolysis of caged ATP[gamma S] within rigor fibers caused tension to relax fully but significant stiffness remained. Stiffness also developed without concomitant tension when Ca2+ concentration was raised from less than 1 nM to 30 microM in the presence of ATP[gamma S]. The amplitude of the tension response to ramp stretches in the presence of Ca2+ and ATP[gamma S] increased with ramp stretch velocity, suggesting that the cross-bridges have detachment rate constants extending into the 10(3) s-1 range. The results provide evidence that the Ca2+-regulatory system can directly control attachment of cross-bridges into states before the power stroke.

Inositol trisphosphate, calcium and muscle contraction

The identity of organelles storing intracellular calcium and the role of Ins(1,4,5)P3 in muscle have been explored with, respectively, electron probe X-ray microanalysis (EPMA) and laser photolysis of 'caged' compounds. The participation of G-protein(s) in the release of intracellular Ca2+ was determined in saponin-permeabilized smooth muscle. The sarcoplasmic reticulum (SR) is identified as the major source of activator Ca2+ in both smooth and striated muscle; similar (EPMA) studies suggest that the endoplasmic reticulum is the major Ca2+ storage site in non-muscle cells. In none of the cell types did mitochondria play a significant, physiological role in the regulation of cytoplasmic Ca2+. The latency of guinea pig portal vein smooth muscle contraction following photolytic release of phenylephrine, an alpha 1-agonist, is 1.5 +/- 0.26 s at 20 degrees C and 0.6 +/- 0.18 s at 30 degrees C; the latency of contraction after photolytic release of Ins(1,4,5)P3 from caged Ins(1,4,5)P3 is 0.5 +/- 0.12 s at 20 degrees C. The long latency of alpha 1-adrenergic Ca2+ release and its temperature dependence are consistent with a process mediated by G-protein-coupled activation of phosphatidylinositol 4,5 bisphosphate (PtdIns(4,5)P2) hydrolysis. GTP gamma S, a non-hydrolysable analogue of GTP, causes Ca2+ release and contraction in permeabilized smooth muscle. Ins(1,4,5)P3 has an additive effect during the late, but not the early, phase of GTP gamma S action, and GTP gamma S can cause Ca2+ release and contraction of permeabilized smooth muscles refractory to Ins(1,4,5)P3. These results suggest that activation of G protein(s) can release Ca2+ by, at least, two G-protein-regulated mechanisms: one mediated by Ins(1,4,5)P3 and the other Ins(1,4,5)P3-independent. The low Ins(1,4,5)P3 5-phosphatase activity and the slow time-course (seconds) of the contractile response to Ins(1,4,5)P3 released with laser flash photolysis from caged Ins(1,4,5)P3 in frog skeletal muscle suggest that Ins(1,4,5)P3 is unlikely to be the physiological messenger of excitation-contraction coupling of striated muscle. In contrast, in smooth muscle the high Ins(1,4,5)P3-5-phosphatase activity and the rate of force development after photolytic release of Ins(1,4,5)P3 are compatible with a physiological role of Ins(1,4,5)P3 as a messenger of pharmacomechanical coupling.

Photoactivation of intracellular guanosine triphosphate analogues reduces the amplitude and slows the kinetics of voltage-activated calcium channel currents in sensory neurones

The influence of guanine nucleotide analogues on calcium channel currents in cultured rat dorsal root ganglion neurones has been studied using a technique in which the rate of diffusion of the analogues to their site of action is by-passed by photochemical release of the analogues within the neurones. The 1(2-nitrophenyl)ethyl P3-ester derivatives of guanosine 5'-0(3-thio)triphosphate (caged GTP-gamma-S) and 5'-guanylylimidodiphosphate (caged GMP-PNP) were synthesised and found to be completely photolysable by light, yielding free GTP-gamma-S and GMP-PNP. Calcium channel currents were recorded using the whole cell patch technique and either caged GTP-gamma-S or caged GMP-PNP (2 mM) were included in the patch pipette. Stable currents were recorded for 5-10 min, and a single pulse of 300-350 nm irradiation was directed using a liquid light guide onto the recording dish. Calcium channel currents were then recorded every 30-120 s following photochemical release of approximately 20 microM GTP-gamma-S. The peak calcium channel current was reduced by about 70% with a slow time course [t1/2 1.5 +/- 0.2 min (mean +/- SEM); n = 5]. The transient component of the peak current was usually completely abolished, whereas the sustained current measured at the end of the 100 ms depolarising pulse was less affected. Qualitatively similar effects were observed on photolysis of caged GMP-PNP.(ABSTRACT TRUNCATED AT 250 WORDS)

Cross-bridge kinetics, cooperativity, and negatively strained cross-bridges in vertebrate smooth muscle. A laser-flash photolysis study

The effects of laser-flash photolytic release of ATP from caged ATP [P3-1(2-nitrophenyl)ethyladenosine-5'-triphosphate] on stiffness and tension transients were studied in permeabilized guinea pig protal vein smooth muscle. During rigor, induced by removing ATP from the relaxed or contracting muscles, stiffness was greater than in relaxed muscle, and electron microscopy showed cross-bridges attached to actin filaments at an approximately 45 degree angle. In the absence of Ca2+, liberation of ATP (0.1-1 mM) into muscles in rigor caused relaxation, with kinetics indicating cooperative reattachment of some cross-bridges. Inorganic phosphate (Pi; 20 mM) accelerated relaxation. A rapid phase of force development, accompanied by a decline in stiffness and unaffected by 20 mM Pi, was observed upon liberation of ATP in muscles that were released by 0.5-1.0% just before the laser pulse. This force increment observed upon detachment suggests that the cross-bridges can bear a negative tension. The second-order rate constant for detachment of rigor cross-bridges by ATP, in the absence of Ca2+, was estimated to be 0.1-2.5 X 10(5) M-1s-1, which indicates that this reaction is too fast to limit the rate of ATP hydrolysis during physiological contractions. In the presence of Ca2+, force development occurred at a rate (0.4 s-1) similar to that of intact, electrically stimulated tissue. The rate of force development was an order of magnitude faster in muscles that had been thiophosphorylated with ATP gamma S before the photochemical liberation of ATP, which indicates that under physiological conditions, in non-thiophosphorylated muscles, light-chain phosphorylation, rather than intrinsic properties of the actomyosin cross-bridges, limits the rate of force development. The release of micromolar ATP or CTP from caged ATP or caged CTP caused force development of up to 40% of maximal active tension in the absence of Ca2+, consistent with cooperative attachment of cross-bridges. Cooperative reattachment of dephosphorylated cross-bridges may contribute to force maintenance at low energy cost and low cross-bridge cycling rates in smooth muscle.

Kinetics of smooth and skeletal muscle activation by laser pulse photolysis of caged inositol 1,4,5-trisphosphate

Inositol 1,4,5-trisphosphate (InsP3) can stimulate skinned smooth and skeletal muscle to contract by initiating Ca2+ release from the sarcoplasmic reticulum. Whether this process is an integral component of the in vivo muscle activation mechanism was tested by releasing InsP3 rapidly within skinned muscle fibers of rabbit main pulmonary artery and frog semitendinosus. InsP3 was liberated on laser pulse photolysis of a photolabile but biologically inactive precursor of InsP3 termed caged InsP3. Caged InsP3 is a mixture of compounds in which InsP3 is esterified with 1(2-nitrophenyl)diazoethane (probably at the P4- or P5-position). Photochemical release of InsP3 induced a full contraction in both muscles at physiological free Mg2+ concentrations, but only in the smooth muscle were the InsP3 concentration (0.5 microM) and the activation rate compatible with the in vivo physiological response. Endogenous InsP3-specific phosphatase activity was present in smooth muscle and had about 35-fold greater activity than that in the skeletal-muscle preparation. Caged InsP3 was not susceptible to phosphatases in either preparation.

Oxygen exchange between Pi in the medium and water during ATP hydrolysis mediated by skinned fibers from rabbit skeletal muscle. Evidence for Pi binding to a force-generating state

Oxygen exchange between (18O4)Pi in the medium and water accompanies ATP hydrolysis catalyzed by the calcium-regulated MgATPase of vertebrate skeletal muscle. Exchange was observed in chemically skinned fibers from rabbit psoas muscle held isometrically and activated by 30 microM free Ca2+. The rate of exchange was approximately proportional to Pi concentration (up to 10 mM) and was characterized by an apparent second order rate constant greater than or equal to 475 M-1 S-1 (pH 7.1, ionic strength 0.2 M, 22 degrees C). Much less exchange occurred in the absence of Ca2+ or when ATP was replaced by ADP. It has been inferred from mechanical experiments that Pi can bind to a force-generating ADP-bound state of actomyosin with resultant suppression of force (Hibberd, M. G., Dantzig, J. A., Trentham, D. R., and Goldman, Y. E. (1985) Science 228, 1317-1319). The oxygen exchange results support this inference by providing direct evidence that Pi in the medium binds at the ATPase catalytic site in activated isometric fibers. The inter-relationship of these two effects involving Pi on mechanochemical coupling in muscle is discussed.

Relationships between chemical and mechanical events during muscular contraction

In this review we have attempted a synthesis of ideas from cross-bridge theories of muscle contraction with biochemical mechanisms of the actomyosin ATPase. This synthesis of ideas has been based on experimental approaches that permit mechanical and biochemical investigations on the same system. We have formulated an example of how biochemical processes may be influenced by strain in the cross-bridge and have highlighted how much has yet to be learned about the biochemistry (and protein structure) of the working stroke of the cross-bridge. Processes that do not appear to be related to the working stroke such as ATP-induced dissociation of actomyosin or protein-bound ATP hydrolysis appear to be similar kinetically in fibers and isolated actomyosin. But, as might be expected, this is not the case in those processes that involve force production and the performance of mechanical work. There appears to be a sound base from which the mechanochemistry of individual processes within the cross-bridge cycle can be analyzed in detail. There is a need for the development of spectroscopic techniques, particularly those that might detect the rate of Pi and ADP dissociation from cross-bridges into the medium. The combination of pulse photolysis of caged ATP and time-resolved structure analysis by use of synchrotron radiation (53) should lead to better understanding of the structure of cross-bridge states in relation to the chemistry and mechanics of transient intermediates.

The ATPase kinetics of insect fibrillar flight muscle myosin subfragment-1

Myosin subfragment-1 (S1) has been prepared from the fibrillar flight muscles of the giant water bug Lethocerus by chymotryptic digestion of myofibrillar suspensions in the absence of magnesium ions. The S1 obtained has a single light chain and a heavy chain with molecular weights of about 18 kDa and 90 kDa respectively. The kinetics of the elementary steps of the magnesium-dependent ATPase of insect S1 and rabbit S1 are similar, both with ATP and with ATP analogues as substrates. However, the presence of variable amounts of inactive protein within our preparation means that several rate constants cannot be obtained with as much precision in the case of insect S1. The most striking differences between the rabbit and insect S1 are values for the Vmax and the Km of actin during actin-activation of the MgATPase activity, which are up to an order of magnitude lower and greater in the insect than in the rabbit, respectively. The mechanical properties of strain activation and of capacity to do extended oscillatory work are unique to insect fibrillar flight muscle and distinguish it from vertebrate striated muscle. It is likely that these properties reflect differences in the organization of actin and myosin within the respective filament lattices rather than intrinsic differences in the ATPase mechanisms of the isolated myosin molecules from the two types of muscle.

Phosphate release and force generation in skeletal muscle fibers

Rapid laser pulse-induced photolysis of an adenosine triphosphate precursor in muscle fibers abruptly initiated cycling of the cross-bridges. The accompanying changes in tension and stiffness were related to elementary mechanochemical events of the energy-transducing mechanism. When inorganic phosphate was present at millimolar concentrations during liberation of adenosine triphosphate in the absence of calcium, relaxation was accelerated. Steady active tension in the presence of calcium was decreased but the approach to final tension was more rapid. These results suggest that, during energy transduction, formation of the dominant force-generating cross-bridge state is coupled to release of inorganic phosphate in a reaction that is readily reversible.