Structure of a complex between yeast hexokinase A and glucose: II. Detailed comparisons of conformation and active site configuration with the native hexokinase B monomer and dimer

Detailed comparison of the refined crystal structures of the hexokinase A: glucose complex (HKA · G) and native hexokinase B shows that, in addition to the 12 ° rotation of one lobe of the enzyme relative to the other as described previously (Bennett & Steitz, 1978) there are small systematic differences in the conformation of the polypeptide backbones of the two structures adjacent to the glucose binding site and crystal packing contacts. In the HKA · G complex, the cleft between the two lobes of the hexokinase molecule is narrowed, substantially reducing the accessibility of the active site to solvent. The HKA · G structure suggests specific contacts with a bound glucose molecule that cannot form in the more open native structure. The closed conformation of the HKA · G complex can be formed by either subunit in the heterologous dimer configuration of hexokinase B (Anderson et al. 1974); new or different interactions between subunits, or with ligands bound to the intersubunit ATP site, may be made when the upper subunit of the dimer is in the closed conformation and may contribute to the cooperative interactions observed in the crystalline dimer and in solution.     

Equilibrium and kinetic studies on the binding of des-N-tetramethyltriostin A to DNA

The interaction between TANDEM (a des-methyl analogue of triostin A) and poly(dA-dT) results in extension of the helix by 6.8 Å for each ligand molecule bound, exactly as predicted for a bis-intercalation reaction. Cooperativity is evident in Scatchard plots for the interaction at ionic strengths of 0.2 and 1.0, where the binding constant is diminished compared to that which pertains at low salt concentration. Binding to a natural DNA (calf thymus), already considerably weaker than binding to poly(dA-dT), is also sensitive to increased ionic strength. With a self-complementary octanucleotide d(G-G-T-A-T-A-C-C) the binding curve indicates the presence of a single des-N-tetramethyltriostin A binding site per helical fragment with a non-cooperative association constant about 6·10⁶ M^?1. Detergent-induced dissociation of des-N-tetramethyltriostin A-poly(dA-dT) complexes results in a simple exponential decay at all levels of binding, but the time constant of decay is dependent upon the initial binding ratio. This behaviour cannot directly explain the cooperativity of equilibrium binding isotherms but suggests the occurrence of relatively long-lived perturbations of the helical structure by binding of the ligand. [Ala³, Ala⁷]des-N-tetramethyltriostin A, which has a more flexible octapeptide ring lacking the disulphide cross-bridge, dissociates from poly(dA-dT) much faster than des-N-tetramethyltriostin A. Dissociation of des-N-tetramethyltriostin A from calf thymus DNA is more rapid than dissociation of triostin A or other quinoxaline antibiotics, which may account for its low antimicrobial activity.     

Kinetic and hydrodynamic studies of the complex of proflavine with poly A·poly U

Relaxation kinetic experiments reveal general similarity between the mechanism of binding of proflavine to poly A·poly U and DNA. There are differences in detail, however. For example, the rate constants are roughly an order of magnitude smaller for the former, and the thermodynamic parameters of the individual steps are also different. The total heat and free energy for intercalation of free dye are quite similar in the two cases. As was the case with DNA, considerable dye (up to 25% of the bound form) is attached externally to the double helix, even in the strong binding region of the isotherm. Sedimentation measurements on small, rodlike fragments of poly A·poly U reveal a length increase on binding proflavine of a magnitude similar to that found with DNA. This length increase seems to become smaller under conditions (high temperature) where the relaxation measurements indicate a higher fraction of externally bound dye.     

DNA-ethidium reaction kinetics: demonstration of direct ligand transfer between DNA binding sites.

Study of the relaxation kinetics of the interaction of ethidium and DNA reveals a novel and potentially important general binding mechanism, namely direct transfer of the ligand between DNA binding sites without requiring dissociation to free ligand. The measurable relaxation spectrum shows three relaxation times, indicating that three bound dye species are present at equilibrium; about 80% of the dye is in the major intercalated form. For each relaxation the reciprocal relaxation time varies linearly with concentration up to very high DNA concentrations. The failure of the longer relaxation times to plateau at high concentration can be accounted for by including a bimolecular pathway for conversion from one complex form to another. This we envisage as direct transfer of an ethidium molecule, bound to one DNA molecule, to an empty binding site on another DNA molecule. Additional evidence for this direct transfer mechanism was obtained from an experiment showing that DNA (which binds ethidium relatively rapidly) accelerates the binding of ethidium to poly(rA) · poly(rU), presumably by first forming a DNA-ethidium complex and then transferring the ethidium to RNA. The bimolecular rate constant for transfer is found to be about four times larger than the constant for intercalating the free dye. The transfer pathway thus provides a highly efficient means for the ligand to equilibrate over its DNA binding sites, especially at high polymer concentration. The potential importance of direct transfer for DNA-binding regulatory proteins is emphasized.     

Binding isotherms and the interaction between proflavine and a DNA of high G · C content

Binding isotherms corresponding to several situations of ligand binding to a linear polymer are calculated, including cases of cooperativity or anticooperativity between the bound ligand states, multiple binding modes that are competitive or non competitive, and possible exclusion of an arbitrary number of adjacent sites upon occupancy of a site by a single ligand. The sequence generating function method of Lifson and Bradley is used, requiring the assumption that no end effects are involved. The case of strong binding of the dye proflavine to a DNA of high G. C content, that of M. lysodeikticus, is considered in detail, and a single model capable of reconciling the available kinetic and equilibrium data on this system, involving two competing binding modes, is discussed.     

Kinetic and thermodynamic studies on nitrobenzylthioinosine binding to the nucleoside transporter of chinese hamster ovary cells

The binding of [G-³H]nitrobenzylthioinosine to intact Chinese hamster ovary cells has been studied kinetically and thermodynamically. The association of nitrobenzylthioinosine with cells is a second-order process which proceeds at 24°C with a rate constant of 2·10⁷ M^?1·s^?1. Dissociation of the complex was characterized as a simple first-order process with rate constant on the order of 7·10^?3 s^?1. The quotient of these is comparable to the dissociation constant as measured in equilibrium binding studies, 2.2·10^?10 M. The temperature dependence of the rate of association indicated an Arrhenius activation energy of 8.4 kcal·mol^?1, while that of the equilibrium constant for dissociation indicated a standard enthalpy change of 8.8 kcal·mol^?1. The large increase in affinity of nitrobenzylthioinosine as compared to natural nucleosides is attributable to an entropy-driven interaction with the binding site. Thymidine, dipyridamole and papaverine each decrease the apparent dissociation constant for the nitrobenzylthioinosine-cell complex; the latter, inhibitors of nucleoside transport, decrease the rate of dissociation of the complex.     

Reaction of carbon monoxide with hemocyanin: Stereochemical effects of a non-bridging ligand

The carbon monoxide binding equilibria and kinetics of a number of molluscan and arthropodal hemocyanins have been investigated employing the visible luminescence of the carbon monoxide-copper complex.Proteins from both phyla, in oligomeric and monomeric form, bind carbon monoxide non-co-operatively; the reaction is largely enthalpy driven is associated with a small unfavourable entropy change.Molluscan hemocyanins display a carbon monoxide affinity (p₅₀ = 1 to 10mm Hg) higher than that of arthropodal hemocyanins (p₅₀ = 100 to 700mm Hg), and only Panulirus interruptus hemocyanin, among those studied here, exhibits a small Bohr effect. The observed differences in equilibrium constant are kinetically reflected in differences in the carbon monoxide dissociation rate constant, which ranges from 20 to 70 s^?1 for molluscan hemocyanins and from 200 to 9000 s^?1 for arthropodal hemocyanins; on the other hand the differences in the combination rate constants between the two phyla are considerably smaller. A comparison of the equilibrium and kinetic results shows some discrepancies between the two sets of data, suggesting that carbon monoxide binding may be governed by a complex mechanism.The correlation between the ligand binding properties and the stereochemistry of the active site is discussed in the light of the knowledge that, while oxygen is bound to both copper atoms in a site, carbon monoxide is a “non-bridging” ligand, being bound to only one of the metals.     

The ATPase cycle mechanism of the DEAD-box rRNA helicase, DbpA

DEAD-box proteins are ATPase enzymes that destabilize and unwind duplex RNA. Quantitative knowledge of the ATPase cycle parameters is critical for developing models of helicase activity. However, limited information regarding the rate and equilibrium constants defining the ATPase cycle of RNA helicases is available, including the distribution of populated biochemical intermediates, the catalytic step(s) that limits the enzymatic reaction cycle, and how ATP utilization and RNA interactions are linked. We present a quantitative kinetic and equilibrium characterization of the ribosomal RNA (rRNA)-activated ATPase cycle mechanism of DbpA, a DEAD-box rRNA helicase implicated in ribosome biogenesis. rRNA activates the ATPase activity of DbpA by promoting a conformational change after ATP binding that is associated with hydrolysis. Chemical cleavage of bound ATP is reversible and occurs via a γ-phosphate attack mechanism. ADP-P_i and RNA binding display strong thermodynamic coupling, which causes DbpA-ADP-P_i to bind rRNA with > 10-fold higher affinity than with bound ATP, ADP or in the absence of nucleotide. The rRNA-activated steady-state ATPase cycle of DbpA is limited both by ATP hydrolysis and by P_i release, which occur with comparable rates. Consequently, the predominantly populated biochemical states during steady-state cycling are the ATP- and ADP-P_i-bound intermediates. Thermodynamic linkage analysis of the ATPase cycle transitions favors a model in which rRNA duplex destabilization is linked to strong rRNA and nucleotide binding. The presented analysis of the DbpA ATPase cycle reaction mechanism provides a rigorous kinetic and thermodynamic foundation for developing testable hypotheses regarding the functions and molecular mechanisms of DEAD-box helicases.     

Electron transfer between horse heart and Candida krusei cytochromes c in the free and bound states

Electron transfer between horse heart and Candida krusei cytochromes c in the free and phosvitin-bound states was examined by difference spectrum and stopped-flow methods. The difference spectra in the wavelength range of 540–560 nm demonstrated that electrons are exchangeable between the cytochromes c of the two species. The equilibrium constants of the electron transfer reaction for the free and phosvitin-bound forms, estimated from these difference spectra, were close to unity at 20°C in 20 mM Tris-HCl buffer (pH 7.4). The electron transfer rate for free cytochrome c was (2–3) · 10⁴ M^?1 · s^?1 under the same conditions. The transfer rate for the bound form increased with increase in the binding ratio at ratios below half the maximum, and was almost constant at higher ratios up to the maximum. The maximum electron exchange rate was about 2 · 10⁶ M^?1 · s^?1, which is 60–70 times that for the free form at a given concentration of cytochrome c. The activation energy of the reaction for the bound cytochrome c was equal to that for the free form, being about 10 kcal/mol. The dependence of the exchange rate on temperature, cytochrome c concentration and solvent viscosity suggests that enhancement of the electron transfer rate between cytochromes c on binding to phosvitin is due to increase in the collision frequency between cytochromes c concentrated on the phosvitin molecule.     

Evidence That Antibiotics Bind to Human Mitochondrial Ribosomal RNA Has Implications for Aminoglycoside Toxicity

Aminoglycosides are a well known antibiotic family used to treat bacterial infections in humans and animals, but which can be toxic. By binding to the decoding site of helix44 of the small subunit RNA of the bacterial ribosome, the aminoglycoside antibiotics inhibit protein synthesis, cause misreading, or obstruct peptidyl-tRNA translocation. Although aminoglycosides bind helix69 of the bacterial large subunit RNA as well, little is known about their interaction with the homologous human helix69. To probe the role this binding event plays in toxicity, changes to thermal stability, base stacking, and conformation upon aminoglycoside binding to the human cytoplasmic helix69 were compared with those of the human mitochondrial and Escherichia coli helix69. Surprisingly, binding of gentamicin and kanamycin A to the chemically synthesized terminal hairpins of the human cytoplasmic, human mitochondrial, and E. coli helix69 revealed similar dissociation constants (1.3–1.7 and 4.0–5.4 μm, respectively). In addition, aminoglycoside binding enhanced conformational stability of the human mitochondrial helix69 by increasing base stacking. Proton one-dimensional and two-dimensional NMR suggested significant and specific conformational changes of human mitochondrial and E. coli helix69 upon aminoglycoside binding, as compared with human cytoplasmic helix69. The conformational changes and similar aminoglycoside binding affinities observed for human mitochondrial helix69 and E. coli helix69, as well as the increase in structural stability shown for the former, suggest that this binding event is important to understanding aminoglycoside toxicity.     

Iodination significantly influences the binding of human transferrin to the transferrin receptor

The human transferrin receptor (TfR) and its ligand, the serum iron carrier transferrin, serve as a model system for endocytic receptors. Although the complete structure of the receptor's ectodomain and a partial structure of the ligand have been published, conflicting results still exist about the magnitude of equilibrium binding constants, possibly due to different labeling techniques. In the present study, we determined the equilibrium binding constant of purified human TfR and transferrin. The results were compared to those obtained with either iodinated TfR or transferrin. Using an enzyme-linked assay for receptor-ligand interactions based on the published direct calibration ELISA technique, we determined an equilibrium constant of Kd=0.22 nM for the binding of unmodified human Tf to surface-immobilized human TfR. In a reciprocal experiment using soluble receptor and surface-bound transferrin, a similar constant of Kd=0.23 nM was measured. In contrast, covalent labeling of either TfR or transferrin with 125I reduced the affinity 3-5-fold to Kd=0.66 nM and Kd=1.01 nM, respectively. The decrease in affinity upon iodination of transferrin is contrasted by an only 1.9-fold decrease in the association rate constant, suggesting that the iodination affects rather the dissociation than the association kinetics. These results indicate that precautions should be taken when interpreting equilibrium and rate constants determined with covalently labeled components.     

Linkage of EcoRI dissociation from its specific DNA recognition site to water activity, salt concentration, and pH: separating their roles in specific and non-specific binding.

We have measured the dependencies of both the dissociation rate of specifically bound EcoRI endonuclease and the ratio of non-specific and specific association constants on water activity, salt concentration, and pH in order to distinguish the contributions of these solution components to specific and non-specific binding. For proteins such as EcoRI that locate their specific recognition site efficiently by diffusing along non-specific DNA, the specific site dissociation rate can be separated into two steps: an equilibrium between non-specific and specific binding of the enzyme to DNA, and the dissociation of non-specifically bound protein. We demonstrated previously that the osmotic dependence of the dissociation rate is dominated by the equilibrium between specific and non-specific binding that is independent of the osmolyte nature. The remaining osmotic sensitivity linked to the dissociation of non-specifically bound protein depends significantly on the particular osmolyte used, indicating a change in solute-accessible surface area. In contrast, the dissociation of non-specifically bound enzyme accounts for almost all the pH and salt-dependencies. We observed virtually no pH-dependence of the equilibrium between specific and non-specific binding measured by the competition assay. The observed weak salt-sensitivity of the ratio of specific and non-specific association constants is consistent with an osmotic, rather than electrostatic, action. The seeming lack of a dependence on viscosity suggests the rate-limiting step in dissociation of non-specifically bound protein is a discrete conformational change rather than a general diffusion of the protein away from the DNA.     

The mechanism of regulation of actomyosin subfragment 1 ATPase

The mechanism of regulation of actin-subfragment 1 nucleoside triphosphatase is described in terms of the rate and equilibrium constants of a relatively simple kinetic scheme: (Formula: see text) where T, D, and Pi are nucleoside triphosphate, nucleoside diphosphate, and inorganic phosphate, respectively; Ka, Kb, and Kc are association constants; the ki are first-order rate constants; A is regulated actin (actin-tropomyosin-troponin); and M is subfragment 1. Calcium binding to regulated actin had little effect on step 2; k2 was almost unaffected, and k-2 increased, at most, 2-fold. k-1 and k3 increased 10-20-fold for ATP and 3-5-fold for 1-N6-ethenoadenosine triphosphate as substrates. Kb and Kc increased by less than 50%, whereas Ka increased 6-10-fold. The primary effect in regulation is on the rate of a conformational change which determines the rate of dissociation of ligands bound to the active site. The measurements probably underestimate the ratio of rate constants of product dissociation for active and relaxed states of actin because of heterogeneity. The kinetic evidence can be explained by a partial steric blocking mechanism or by a conformational (nonsteric) mechanism.     

The effects of E7 and E11 mutations on the kinetics of ligand binding to R state human hemoglobin

Association and dissociation rate constants for O2, CO, and methyl isocyanide binding to native and distal pocket mutants of R state human hemoglobin were measured using ligand displacement and partial photolysis techniques. Individual rate constants for the alpha and beta subunits were resolved by comparisons between the kinetic behavior of the native and mutant proteins. His-E7 was replaced with Gly and Gln in both alpha and beta subunits and with Phe in beta subunits alone. In separate experiments Val-E11 was replaced with Ala, Leu, and Ile in each globin chain. The parameters describing ligand binding to R state alpha subunits are sensitive to the size and polarity of the amino acids at positions E7 and E11. The distal histidine in this subunit inhibits the bimolecular rate of binding of both O2 and CO, sterically hinders bound CO and methyl isocyanide, and stabilizes bound O2 by hydrogen bonding. The Val-E11 side chain in alpha chains also appears to be part of the kinetic barrier to O2 and CO binding since substitution with Ala causes approximately 10-fold increases in the association rate constants for the binding of these diatomic ligands. However, substitution of Val-E11 by Ile produces only small decreases in the rates of ligand binding to alpha subunits. For R state beta subunits, the bimolecular rates of O2 and CO binding are intrinsically large, approximately 2-5-fold greater than those for alpha subunits, and with the exception of Val-E11----Ile mutation, little affected by substitutions at either the E7 or E11 positions. For the beta Val-E11----Ile mutant the association rate and equilibrium constants for all three ligands decreased 10-50-fold. All of these results agree with Shaanan's conclusions that the distal pocket in liganded beta subunits is more open whereas in alpha subunits bound ligands are more sterically hindered by adjacent distal residues (Shaanan, B. (1983) J. Mol. Biol. 171, 31-59). In the case of O2 binding to alpha subunits, the unfavorable steric effects are compensated by the formation of a hydrogen bond between the nitrogen atom of His-E7 and bound dioxygen.     

Neocarzinostatin chromophore binds to deoxyribonucleic acid by intercalation

The nonprotein chromophore of neocarzinostatin was found to share many of the characteristics of classical intercalators in its interaction with DNA. Viscosity studies with PM2 DNA indicated that the DNA helix unwinding induced by the chromophore was 0.82 times that of ethidium or 21 degrees. Electric dichroism of the chromophore--DNA complex showed that each bound chromophore molecule lengthened DNA by 3.3 A and that absorbance transitions of the chromophore at 315--385 nm were oriented approximately parallel to DNA bases, as expected for an intercalated aromatic ring. Binding to DNA induced strong hypochromicity and a pronounced red shift in the absorbance spectrum of the chromophore. Spectrophotometric titrations suggested at least two types of chromophore binding sites on DNA; one type of site was saturated at rb = 0.125 chromophore molecule/nucleotide, but binding to additional sites continued to at least rb = 0.3. These physical--chemical studies were performed at pH 4--5 in order to keep the chromophore stable, but chromophore bound to an excess of DNA at pH 7 showed a stable absorbance spectrum identical with that seen at pH 4--5, suggesting that a similar type of binding occurs at neutral pH. Chromophore which had spontaneously degraded in pH 8 buffer did not bind to DNA at all, as judged by absorbance spectroscopy. The degree of protection afforded by DNA against spontaneous chromophore degradation implied a dissociation constant of approximately 5 microM for the DNA--chromophore complex at neutral pH and physiological ionic strength. Supercoiled DNA was nearly twice as effective as relaxed DNA in protecting chromophore from degradation, providing additional evidence for intercalation at neutral pH. Comparison of absorbance, fluorescence, and dichroism spectra suggests that the naphthalene ring system is the intercalating moiety.     

Manipulating conformational equilibria in the lactose permease of Escherichia coli.

A mechanism proposed for lactose/H(+) symport by the lactose permease of Escherichia coli indicates that lactose permease is protonated prior to ligand binding. Moreover, in the ground state, the symported H(+) is shared between His322 (helix X) and Glu269 (helix VIII), while Glu325 (helix X) is charge-paired with Arg302 (helix IX). Substrate binding at the outer surface between helices IV (Glu126) and V (Arg144, Cys148) induces a conformational change that leads to transfer of the H(+) to Glu325 and reorientation of the binding site to the inner surface. After release of substrate, Glu325 is deprotonated on the inside due to re-juxtapositioning with Arg302. The conservative mutation Glu269-->Asp causes a 50-100-fold decrease in substrate binding affinity and markedly reduced active lactose transport, as well as decreased rates of equilibrium exchange and efflux. Gly-scanning mutagenesis of helix VIII was employed systematically with mutant Glu269-->Asp in an attempt to rescue function, and two mutants with increased activity are identified and characterized. Mutant Thr266-->Gly/Met267-->Gly/Glu269-->Asp binds ligand with increased affinity and catalyzes active lactose transport with a marked increase in rate; however, little improvement in efflux or equilibrium exchange is observed. In contrast, mutant Gly262-->Ala/Glu269-->Asp exhibits no improvement in ligand binding but a small increase in the rate of active transport; however, an increase in the steady-state level of accumulation, as well as efflux and equilibrium exchange is observed. Remarkably, when the two sets of mutations are combined, all translocation reactions are rescued to levels approximating those of wild-type permease. The findings support the contention that Glu269 plays a pivotal role in the mechanism of lactose/H(+) symport. Moreover, the results suggest that the two classes of mutants rescue activity by altering the equilibrium between outwardly and inwardly facing conformations of the permease such that impaired protonation and/or H(+) transfer is enhanced from one side of the membrane or the other. When the two sets of mutants are combined, the equilibrium between outwardly and inwardly facing conformations and thus protonation and H(+) transfer are restored.     

Theoretical Analysis of the Footprinting Experiment

Abstract

In the footprinting experiment, an end-radiolabeled DNA restriction fragment is subjected to digest by an endonuclease in the presence and absence of a ligand which alters the endonuclease cleavage rate at sites of ligand-DNA contact. The location of these sites, and the strength of the ligand binding, are then deduced from the measured concentrations of the different oligonucleotides produced by the digest. We analyze the experiment in terms of coupled kinetic equations which take into account the cutting rates of endonuclease for sites with ligand present and absent, and the rates of binding and dissociation of the ligand to a site. As long as the ligand concentration remains essentially constant (which occurs, for example, if digest is terminated early enough to assure that all fragments result from single cuts by the endonuclease), the oligonucleotide concentrations reflect only the ligand binding equilibrium constant (ratio of rate constants) and the cutting rates in the presence and absence of ligand. We also show how the measured oligonucleotide concentrations (from, e.g. an autoradiogram) can be used to deduce the ligand equilibrium binding constants for the various sites on the polymer.     

The dissociation of insulin from human insulin antibodies

The dissociation of insulin from human insulin antibodies has been investigated using a technique that is rapid and does not require addition of excess unlabelled insulin. A slow (k₁ = 2·1^?3 min^?1 and a fast (k₂ = 4·10^?2 min^?1) dissociating antibody component were identified in all studies. These have been shown to correspond, respectively, to the high and low affinity antibody components of equilibrium binding studies. The range of k₁ and k₂ values and their response to temperature change is small. Insulin resistance and stability of diabetes are not related to properties of antibody dissociation. Dissociation is faster in the presence of high (6–850 nM) insulin concentration due to increased binding to the fast dissociating component without change in the dissociation rate constants. When incubation time is increased beyond achivement of maximal binding there is a time-dependent rise in binding to the slow dissociating component, with a concomitant fall in k₁. The traditional concept that equilibrium is established at maximum binding requires further examination.     

Binding of the Activating Ion Co(II) to myo-Inositol Monophosphatase Monitored by Fluorescence and Phosphorescence Spectroscopy

Two extrinsic probes, pyrene-maleimide and eosin-maleimide, were used to label specific SH groups of the enzyme myo-inositol monophosphatase. The fluorescence of pyrene-monophosphatase is enhanced upon addition of the activating metal ions Co(II) and Mg(II). Co(II) ions bind with a dissociation constant of 4 μM, whereas the apparent activation constant K _a is 0.4 mM. Energy transfer measurements demonstrated that the pyrene chromophore, covalently linked to Cys-218, is within 9 Å of the metal ion Tb(III) coordinated to the metal-binding site. The phosphorescence emitted by eosin covalently linked to the protein is quenched by the addition of the activating cations Co(II) and Mg(II). Phosphorescence titrations conducted under anaerobic conditions were used to determine a dissociation constant of approximately 3 μM for the binding of Co(II) ions. The results are consistent with the hypothesis that two activating ions per monomeric subunit participate in the catalytic mechanism. The affinity of the tightly bound ion is at least 100-fold greater than the affinity of the weakly bound ion.     

Solubilization of the bovine cardiac sarcolemmal binding sites for calcium channel blockers

Nonionic and ionic detergents were used to solubilize the bovine cardiac sarcolemmal binding sites for nimodipine and (-)desmethoxyverapamil in the absence of added ligand. Only Chaps, digitonin and sucrose monolauryl ester were able to solubilize the binding sites in a form that bound radioligands. About 45% of each of the membrane-bound high-affinity site was solubilized by 0.4% Chaps (w/v) in the presence of 48% (w/v) glycerol. The solubilized binding sites were destroyed by trypsin or by a 10-min incubation at 50 degrees C. Calcium stimulated nimodipine binding slightly at 0.3 mM and inhibited (-)desmethoxyverapamil binding completely with an IC50 of 1.2 mM. Nimodipine binding was reduced by 20% in the presence of EGTA. The solubilized receptors sedimented in sucrose density gradients with an apparent s20,w of 21 S. An identical sedimentation value was obtained for the cardiac sarcolemmal and skeletal transverse tubulus receptor which were prelabeled with nitrendipine and solubilized by digitonin. Solubilization reduced the affinity of nimodipine for its high-affinity site slightly from 0.35 nM to 1.2 nM and that for its low-affinity site from 33 nM to 130 nM. Solubilization did not affect significantly the specific density of these sites. Binding of nimodipine to the low-affinity site was completely abolished by 0.1 microM nitrobenzylthioinosine. After solubilization only the high-affinity site for (-)desmethoxyverapamil could be measured with tenfold reduced affinity (Kd = 15.3 nM) but unchanged specific density. Binding to the solubilized high-affinity site for nimodipine and (-)desmethoxyverapamil was stereospecific and showed a similar rank order as the particulate binding sites. Binding of nimodipine was inhibited allosterically by phenylalkylamines. Similarly, (+)PN200-110 inhibited allosterically (-)desmethoxyverapamil binding. d-cis-Diltiazem stimulated nimodipine binding at 20 degrees C 1.2-fold, reduced the dissociation rate from 0.018 min-1 to 0.0083 min-1 and had no effect on the association rate (0.173 min-1. nM-1). The Kd calculated from the rate constants was 0.1 nM and in close agreement with the value of 0.49 nM measured under equilibrium conditions in the presence of nitrobenzylthioinosine. In contrast, desmethoxyverapamil increased the dissociation rate of nimodipine to 0.03 min-1. The association and dissociation rate constants for (-)desmethoxyverapamil were 0.024 min-1. nM-1 and 0.025 min-1, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)