Interactions of a reversible ryanoid (21-amino-9alpha-hydroxy-ryanodine) with single sheep cardiac ryanodine receptor channels

The binding of ryanodine to a high affinity site on the sarcoplasmic reticulum Ca2+-release channel results in a dramatic alteration in both gating and ion handling; the channel enters a high open probability, reduced-conductance state. Once bound, ryanodine does not dissociate from its site within the time frame of a single channel experiment. In this report, we describe the interactions of a synthetic ryanoid, 21-amino-9alpha-hydroxy-ryanodine, with the high affinity ryanodine binding site on the sheep cardiac sarcoplasmic reticulum Ca2+-release channel. The interaction of 21-amino-9alpha-hydroxy-ryanodine with the channel induces the occurrence of a characteristic high open probability, reduced-conductance state; however, in contrast to ryanodine, the interaction of this ryanoid with the channel is reversible under steady state conditions, with dwell times in the modified state lasting seconds. By monitoring the reversible interaction of this ryanoid with single channels under voltage clamp conditions, we have established a number of novel features of the ryanoid binding reaction. (a) Modification of channel function occurs when a single molecule of ryanoid binds to the channel protein. (b) The ryanoid has access to its binding site only from the cytosolic side of the channel and the site is available only when the channel is open. (c) The interaction of 21-amino-9alpha-hydroxy-ryanodine with its binding site is influenced strongly by transmembrane voltage. We suggest that this voltage dependence is derived from a voltage-driven conformational alteration of the channel protein that changes the affinity of the binding site, rather than the translocation of the ryanoid into the voltage drop across the channel.

Structural components of ryanodine responsible for modulation of sarcoplasmic reticulum calcium channel function

Comparative molecular field analysis (CoMFA) was used to analyze the relationship between the structure of a group of ryanoids and the modulation of the calcium channel function of the ryanodine receptor. The conductance properties of ryanodine receptors purified from sheep heart were measured using the planar, lipid bilayer technique. The magnitude of the ryanoid-induced fractional conductance was strongly correlated to specific structural loci on the ligand. Briefly, electrostatic effects were more prominent than steric effects. The 10-position of the ryanoid had the greatest influence on fractional conductance. Different regions of the ligand have opposing effects on fractional conductance. For example, steric bulk at the 10-position is correlated with decreased fractional conductance, whereas steric bulk at the 2-position (isopropyl position) is correlated with increased fractional conductance. In contrast to fractional conductance, the 3-position (the pyrrole locus) had the greatest influence on ligand binding, whereas the 10-position had comparatively little influence on binding. Two possible models of ryanodine action, a direct (or channel plug) mechanism and an allosteric mechanism, were examined in light of the CoMFA. Taken together, the data do not appear to be consistent with direct interaction between ryanodine and the translocating ion. The data appear to be more consistent with an allosteric mechanism. It is suggested the ryanoids act by inducing or stabilizing a conformational change in the ryanodine receptor that results in the observed alterations in cation conductance.

The pharmacology of ryanodine and related compounds

The goal of this review has been to describe the current state of the pharmacology of ryanodine and related compounds relative to the vertebrate RyRs. Resolution of questions concerning the molecular properties of RyR channel function and the contributions made by the RyR isoforms to cellular signaling in a variety of tissues will require the production of new pharmacological agents directed against these proteins. Novel naturally occurring ryanodine congeners have been identified, and significant advances have been made in developing chemical approaches that permit the structure of ryanodine to be derivatized in selective ways. Moreover, several of these changes have yielded compounds that differ in their binding affinities and in their abilities to modify the properties of the RyR channels. These advances give substance to the possibility of designing the required pharmacological agents based on rational design changes of the structure ryanodine.

The pyrrole locus is the major orienting factor in ryanodine binding

Ryanodine, a natural product, is a complex modulator of a class of intracellular Ca2+ release channels commonly called the ryanodine receptors. Ryanodine analogs can cause the channel to persist in long-lived, subconductance states or, at high ligand concentrations, in closed, nonconducting states. In this paper, we further explore the relationship between structure and ryanodine binding to striated muscle. Ryanodine, 3-epiryanodine, and 10-ryanodine are three structural isomers of ryanodine. The dissociation constants of these compounds were measured using rabbit skeletal muscle ryanodine receptors. Placing the pyrrole carbonyl group at the 3-epi- and 10-positions of ryanodol largely restores the large loss of binding energy observed when ryanodine is hydrolyzed to ryanodol. Comparative molecular field analysis successfully predicts the enhanced binding and indicates that the pyrrole group controls the orientation of ligand binding. We propose that the ryanoids are reorientated in the binding site of the ryanodine receptors such that the pyrrole always occupies the same subsite. By applying this model, the binding constants of other ryanoids are predicted.

Electrophysiological effects of ryanodine derivatives on the sheep cardiac sarcoplasmic reticulum calcium-release channel

We have examined the effects of a number of derivatives of ryanodine on K+ conduction in the Ca2+ release channel purified from sheep cardiac sarcoplasmic reticulum (SR). In a fashion comparable to that of ryanodine, the addition of nanomolar to micromolar quantities to the cytoplasmic face (the exact amount depending on the derivative) causes the channel to enter a state of reduced conductance that has a high open probability. However, the amplitude of that reduced conductance state varies between the different derivatives. In symmetrical 210 mM K+, ryanodine leads to a conductance state with an amplitude of 56.8 +/- 0.5% of control, ryanodol leads to a level of 69.4 +/- 0.6%, ester A ryanodine modifies to one of 61.5 +/- 1.4%, 9,21-dehydroryanodine to one of 58.3 +/- 0.3%, 9 beta,21beta-epoxyryanodine to one of 56.8 +/- 0.8%, 9-hydroxy-21-azidoryanodine to one of 56.3 +/- 0.4%, 10-pyrroleryanodol to one of 52.2 +/- 1.0%, 3-epiryanodine to one of 42.9 +/- 0.7%, CBZ glycyl ryanodine to one of 29.4 +/- 1.0%, 21-p-nitrobenzoyl-amino-9-hydroxyryanodine to one of 26.1 +/- 0.5%, beta-alanyl ryanodine to one of 14.3 +/- 0.5%, and guanidino-propionyl ryanodine to one of 5.8 +/- 0.1% (chord conductance at +60 mV, +/- SEM). For the majority of the derivatives the effect is irreversible within the lifetime of a single-channel experiment (up to 1 h). However, for four of the derivatives, typified by ryanodol, the effect is reversible, with dwell times in the substate lasting tens of seconds to minutes. The effect caused by ryanodol is dependent on transmembrane voltage, with modification more likely to occur and lasting longer at +60 than at -60 mV holding potential. The addition of concentrations of ryanodol insufficient to cause modification does not lead to an increase in single-channel open probability, such as has been reported for ryanodine. At concentrations of > or = 500 mu M, ryanodine after initial rapid modification of the channel leads to irreversible closure, generally within a minute. In contrast, comparable concentrations of beta-alanyl ryanodine do not cause such a phenomenon after modification, even after prolonged periods of recording (>5 min). The implications of these results for the site(s) of interaction with the channel protein and mechanism of the action of ryanodine are discussed. Changes in the structure of ryanodine can lead to specific changes in the electrophysiological consequences of the interaction of the alkaloid with the sheep cardiac SR Ca2+ release channel.

High affinity C10-Oeq ester derivatives of ryanodine. Activator-selective agonists of the sarcoplasmic reticulum calcium release channel

The plant alkaloids ryanodine and dehydroryanodine are specific and potent modulators of the sarcoplasmic reticulum calcium release channel. In the present study, acidic, basic, and neutral side chains esters of these diterpene compounds were prepared and their pharmacologic activities were assessed. Binding affinities of the novel C10-Oeq ester derivatives for the sarcoplasmic reticulum Ca2+ release channel were evaluated with sarcoplasmic reticular vesicles prepared from rabbit skeletal muscle. Kd values of the derivatives varied 500-fold, ranging from 0.5 to 244 nM. In comparison, Kd values for ryanodine and dehydroryanodine were 4.4 nM and 5.4 nM, respectively. Basic substituents at the C10-Oeq side chain terminus produced the highest affinity derivatives (Kd values from 0.5 to 1.3 nM). Neutral and/or hydrophobic side chain derivatives exhibited intermediate affinities for the high affinity ryanodine receptor site (Kd values from 2.5 to 39 nM), whereas a derivative with a terminal acidic group had the lowest affinity (Kd value > 100 nM). Certain of the higher affinity C10-Oeq derivatives were evaluated more extensively for their pharmacologic activity on the sarcoplasmic reticular Ca2+ release channel. Both channel activating (opening) and deactivating (closing) actions were assessed from the ability of the ryanoids to alter Ca2+ efflux rates from skeletal junctional sarcoplasmic reticular vesicles that had been passively loaded with Ca2+. The natural Ryania secondary metabolites ryanodine, dehydroryanodine and esters E and F, all exhibit antithetical concentration-effect curves, indicating both activator and deactivator actions. In contrast, the semi-synthetic C10-Oeq esters selectively activate the Ca2+ release channel. Half-maximal concentrations for such activation (EC50 act) ranged from 0.87 microM to 4.2 microM, compared with an EC50 act of 1.3 microM for ryanodine. These derivatives were also evaluated for their ability to augment ATP-dependent CA2+ accumulation by cardiac junctional sarcoplasmic reticular vesicles, an effect that results from deactivation of the Ca2+ release channels. None of the derivatives tested was able to significantly augment Ca2+ accumulation, further substantiating their inability to deactivate the sarcoplasmic reticular Ca2+ release channel. Additionally, these derivatives functionally antagonized the action of ryanodine to close the Ca2+ release channel. The results presented demonstrate that these C10-Oeq ester derivatives of ryanodine and dehydroryanodine bind specifically to the SR Ca2+ release channel, selectively activate the channel, and, although they fail to effect channel closure, they nevertheless functionally compete with ryanodine at its low affinity (deactivator) site(s).

Structural determinants of high-affinity binding of ryanoids to the vertebrate skeletal muscle ryanodine receptor: a comparative molecular field analysis

Ryanodine binds to specific membrane proteins, altering the calcium permeability of intracellular membranes. In this study 19 ryanoids were isolated or synthesized and the structures correlated to the strength of binding to vertebrate skeletal muscle ryanodine receptors. Global minima were determined by employment of molecular mechanics and dynamics augmented by systematic searching of conformational space. Overall, steric and electrostatic factors contribute about equally to the differences in the experimentally determined dissociation constants. The dominant electrostatic interaction is localized to a hydroxyl group in an apolar region of the molecule. The pyrrole and isopropyl groups located together at one pole of the molecule have the greatest effect on steric interactions between ligand and receptor. We suggest ryanodine binds to the receptor with the pyrrole and isopropyl groups buried deep inside a cleft in the protein. This arrangement places special importance on the conformation of the pyrrole and isopropyl groups. In contrast, the opposite pole appears to be positioned at the entrance of the binding pocket because bulky adducts placed in the 9 position of ryanodine alter binding minimally. For example, a fluorescent ryanodine adduct was synthesized which has a dissociation constant close to that of ryanodine. Detailed examination reveals subtle interactions between ryanoid and receptor. In many cases, the major factors altering the strength of binding were found to be conformational alterations in the molecule remote from the site of covalent modification.

Differential activating and deactivating effects of natural ryanodine congeners on the calcium release channel of sarcoplasmic reticulum: evidence for separation of effects at functionally distinct sites

Two novel natural ryanoids from extracts of the wood of Ryania speciosa Vahl were evaluated with sarcoplasmic reticulum (SR) vesicles for their binding affinities and their activating and deactivating effects on Ca2+ release channels. The new ryanoids, which are more polar than the known Ryania constituents ryanodine and didehydro-(9,21)-ryanodine, were purified using silica gel column chromatography and reverse phase high performance liquid chromatography. The new ryanoids were designated ester E and ester F, in keeping with nomenclature previously used in the literature. These compounds were identified by NMR spectroscopy and mass spectroscopy as C9ax-hydroxyryanodine and C8ax-hydroxy-C10-epi-dehydroryanodine, respectively. Binding of esters E and F to the high affinity (nanomolar Kd) site on SR Ca2+ release channels was determined from relative binding affinity assays using 6.7 nM [3H]ryanodine. Apparent Kd values of ryanodine, ester E, and ester F for binding to this domain on the skeletal muscle ryanodine receptor/SR Ca2+ release channel were 4.4 +/- 0.8, 65 +/- 10, and 257 +/- 53 nM, respectively (mean +/- standard deviation, four or more experiments). Apparent Kd values for cardiac muscle receptors were 0.51 +/- 0.01, 12 +/- 0.4, and 57 nM, respectively. As a functional indication of the effects of the ryanoids, channel-opening (activator) and channel-closing (deactivator) actions were assessed from the ability of the ryanoids to alter the rate of Ca2+ efflux from passively loaded skeletal muscle junctional sarcoplasmic reticular vesicles (JSRV). Activator actions among the ryanoids were similar, in that they exhibited apparently parallel concentration-effect curves, having a slope of 40% Ca2+ loss/decade increment in ryanoid concentration. Half-maximal values for activation (EC50 values) were 2.5, 63, and 43 microM for ryanodine, ester E, and ester F, respectively. Maximal channel opening by ester E was significantly less than that produced by the other ryanoids. The deactivator actions of the compounds on skeletal JSRV were dissimilar, in that their concentration-effect curves appeared not to be parallel. The quotient of the EC50 for deactivation and that for activation was taken as the concentration-coupling ratio (CCR). The CCR for ryanodine was 114 and that for ester F was 72, but the CCR for ester E was only 21. ATP-dependent Ca2+ accumulation by cardiac JSRV provided a second means to evaluate deactivator actions of the ryanoids. Results from cardiac JSRV assays were in general similar to those from skeletal JSRV assays.(ABSTRACT TRUNCATED AT 400 WORDS)

Amino- and guanidinoacylryanodines: basic ryanodine esters with enhanced affinity for the sarcoplasmic reticulum Ca(2+)-release channel

Amino- and guanidinoacyl esters of ryanodine were prepared to evaluate the effect of basicity on the binding affinity of these derivatives for the sarcoplasmic reticulum Ca(2+)-release channel (SR CRC). In the presence of DCC and DMAP Cbz-beta-alanine reacts with ryanodine in CH2Cl2 to give O10eq-Cbz-beta-alanylryanodine (3a), which on hydrogenolysis yields the beta-alanyl ester (4a). N,N'-bis-Cbz-S-methylthiourea reacts with 4a to yield beta-N,N'-bis-Cbz-guanidinopropionylryanodine (5a). O10eq-beta-guanidinopropionylryanodine (6a) is obtained on hydrogenolytic deprotection of 5a. The binding affinity of beta-alanine ester (4a) and its glycyl congener (4b) is 2-3-fold greater, and that of the beta-guanidinopropionyl ester (6a) and its acetyl congener (6b) 3-6-fold greater, than that of ryanodine. The effect of ryanodine on SR Ca2+ flux is of a biphasic nature: nanomolar levels open (activate) the channel, while micromolar levels close (deactivate) it. The base-substituted esters 4a and 6a both display a unidirectional effect: they only open the channel. An understanding of ryanodine's mode of action and the design of effective SR CRC activating and deactivating ryanoids for possible therapeutic application are major research objectives.

The Cotton Effects of Conjugated Homoannular Dienes. I. The Non-generality of the Rule of Moscowitz

The non-generality of the diene rule of Moscowitz, relating the helicity of a non-planar conjugated homoannular diene to the sign of its Cotton effect, is discussed. We report the preparation of 3β-bromoacetoxy-16α-ethyl-162-cyano-162,21-cyclo-5α-pregna-17,21-diene, the X-ray crystallographic analysis of which confirms, as reported by Ahmed and Pollard, the stereochemistry which we previously assigned to the parent alcohol and acetate. We also report the synthesis of the methyl ketonic analogues of these dienonitriles and show that their sodium borohydride reduction product still has a Cotton effect in contradiction to the diene rule. The implications of these and other experiments are discussed.

Cyanoethylations and Michael additions. V. The synthesis of allylic cyclohexenols by γ-cyanoethylation of α,β-unsaturated aldehydes and ketones. Part V

It is shown that the cyanoethylation of 3β-acetoxy-21-methyl-5α-pregn-17-en-21-one results in part in the addition of acrylonitrile to the γ-position, with concomitant cyclization, and that therefore the cyclo-γ-cyanoethylations of α,β-unsaturated carbonyl compounds reported previously were not confined to aldehydes. However, the main reaction products of the conjugated ketone were α-cyanoethylated derivatives. A variation of the reaction time did not affect the proportion of α- and γ-cyanoethylated products; the implications of this finding on mechanistic considerations are discussed; in particular, an explanation based on the assumption of an equilibrium between reactants and α- and γ-cyanoethylated products is ruled out. It is shown that the γ-additions observed, particularly in the case of α,β-unsaturated aldehydes, are not due to steric hindrance, neither to an abnormal charge distribution in the anion. It is further shown that in the presence of base the propionitrile moiety of oxygen-cyanoethylated α,β-unsaturated aldehydes with a suitable geometry is transferred to the γ-position of the original aldehyde and that cyclization occurs so that the same products are obtained from such cyano-enol ethers as in the direct cyanoethylation of the free aldehydes. On the basis of this finding, a mechanism for the cyclo-γ-cyanoethylations of α,β-unsaturated carbonyl compounds, involving such a transfer reaction, can be tentatively proposed.

Cyanoethylations and Michael additions. IV. The synthesis of allylic cyclohexenols by γ-cyanoethylation of α, β-unsaturated aldehydes and ketones. Part IV

The cyclo-γ-cyanoethylation of 3β-acetoxy-D-homo-5α-pregn-17a-en-21-al is reported. It is shown that the orientation of the cyanoethylation of 2-cycloalkyl-substituted ethenals does not significantly depend on the ring size of the cycloalkyl substituent. It is further shown that the stereochemistry of the cyclohexadienonitrile obtained by dehydration of the main reaction products seems to be, in the case of a fusion to a six-membered ring, in agreement with Moscowitz' rule relating the helicity of the chromophore to the sign of its Cotton effect, in contradistinction to the situation in the case of a system in which one of the double bonds of the chromophore is exocyclic to a five-membered ring.