Structure of a 3 alpha-(D-methylglycoside) of 7,8 beta-epoxysinogenin--a cardioactive steroid

C30H42O10, Mr = 562.66, orthorhombic, P2(1)2(1)2(1), a = 10.125 (1), b = 15.632 (2), c = 36.115 (4) A, V = 5716 (1) A3, Z = 8, Dm = 1.312 (2) (flotation), D chi = 1.308 g cm-3, Cu K alpha(lambda = 1.5418 A), mu = 7.18 cm-1, F(000) = 2416, T = 295 K. Final R(F) = 0.07 for 3348 significant reflections with I greater than or equal to 2.5 sigma(I). The A, B, C, D rings of the aglycone ring are found to be in cis-trans-cis fashion forming a buckled structure. The lactone is in C17 beta conformation. The molecules are stabilized by intermolecular hydrogen bonds. The longest direction of the steroid molecule is nearly parallel to the a axis. The conformational features exhibited by the molecule support proposals on activity.

Mechanisms of inhibition of (Na,K)-ATPase by hydrostatic pressure studied with fluorescent probes

To investigate the mechanisms by which hydrostatic pressure inhibits (Na,K)-ATPase, we measured enzyme activity, as a function of pressure and temperature, of purified (Na,K)-ATPase from dog kidney and eel electroplax, and we monitored protein conformation, possible subunit interactions, and the fluidity of the membrane with fluorescent probes. The (Na,K)-ATPase and p-nitrophenylphosphatase activities were inhibited reversibly by pressures below 1.5 kilobars (eel enzyme) and 2.5 kilobars (dog kidney enzyme). Above these pressures, the enzymes were inactivated irreversibly. The plots of 1n(activity) versus pressure were curvilinear; this indicates that the reversible inhibition by pressure involves two or more rate-limiting steps. The calculated activation volumes varied with temperature and pressure and were larger for the (Na,K)-ATPase activity compared to the p-nitrophenylphosphatase activity. The fluorescence polarization of three hydrophobic probes decreased with increasing temperature (10-36 degrees C) and increased with increasing pressure (10(-3)-1.5 kilobars), reversibly, without any evidence of a lipid phase transition. Plots of enzyme activity versus fluorescence polarization of the lipid probes showed an inverse relationship; this indicates that enzyme activity was directly related to the fluidity of the membrane as measured by the lipid probes. Pressure had no effect on the fluorescence polarization of two cardiac glycoside probes nor on the efficiency of resonance energy transfer between either donor and acceptor cardiac glycosides specifically bound to the ouabain sites of different alpha-subunits, or tryptophan and the bound cardiac glycoside probe. These results suggest that dissociation of dimeric alpha-subunits is not related to the inhibition by pressure, and that the cardiac glycoside-complexed enzyme conformation is stabilized by pressure. It is concluded that increased pressure decreases the membrane fluidity which hinders conformational transitions associated with rate-limiting steps of the (Na,K)-ATPase reaction. It is proposed that ion-bound or -occluded forms of (Na,K)-ATPase are stabilized by pressure because they occupy a smaller volume.

Interaction of cardiodigin, endogenous inhibitor of Na+,K+-ATPase, with antidigoxin and antidigitoxin antibodies

The present study was undertaken to further characterize the immunoreactivity of cardiodigin, digitalis-like factor present in mammalian tissues. Guinea-pig heart extracts purified by reverse-phase low pressure and high pressure liquid chromatographies were analysed for their cross-reactivity with antidigitoxin and antidigoxin antibodies. The putative digitalis-like factor showed an affinity about 10 times higher for antidigoxin antibodies than for antidigitoxin antibodies. EC50 ratios (digoxin/digitoxin assay) determined at two purification steps were different from those of digoxin and digitoxin. These results show that cardiodigin presents molecular determinants recognizable by antidigoxin and antidigitoxin antibodies but that it is a chemical entity different from these well known cardioactive steroids.

Quantitative structure-activity relationships of cardiotonic steroids using empirical molecular electrostatic potentials and semiempirical molecular orbital calculations

Empirical molecular electrostatic potentials and a rigid receptor H atom model are used to calculate differences in the interaction energy of cardiotonic steroids with the digitalis receptor. An attempt is made to map the receptor H binding sites for two different steroid conformations with respect to 17 beta side-chain orientation. The calculated interaction energies using single-crystal X-ray structure data indicate linear relationships with the Na+, K+-ATPase inhibitory activity. On the basis of these correlations, the activity of 8 so far pharmacologically not investigated steroids containing C = O in 17 beta substituents are estimated with the help of structural data determined by means of the semiempirical CNDO molecular orbital theory.

Interaction of cardiac glycosides and Na,K-ATPase is regulated by effector-controlled equilibrium between two limit enzyme conformers

The paper describes the dissociation parameters of the complexes between [3H]-digitoxin and Na,K-ATPase (Na+ + K+-activated, Mg2+-dependent ATP phosphohydrolase, E.C. 3.6.1.3) from pig cardiac muscle and brain cortex formed and dissociated in the presence of different combinations and concentrations of the enzyme effectors ATP, Mg2+, Na+ and K+. Systematic variation of effector-ligation of Na,K-ATPase allowed production of glycoside complexes with two enzyme conformers only, which showed either rapid or slow dissociation kinetics. Appropriate changes of enzyme ligation allowed the interconversion of the two conformer types. Biphasic, rapid and slow glycoside release was not bound with the presence of two Na,K-ATPase isozymes, but caused by the enzyme ligation-determined coexistence of the two conformers of Na,K-ATPase. The rate constants for the rapid and slow glycoside release were within the complexes of each dissociation type much alike indicating uniform isomerization kinetics of the two conformers even when differently liganded. Taken together, the observations indicated the effector-controlled isomerizations of two conformers of Na,K-ATPase possessing different geometries of the glycoside binding domain. Present findings and relevant literature data were integrated in a circular, consecutive and simultaneous model for induced conformation changes that accounted for the regulation of the interaction of cardiac glycosides and Na,K-ATPase through an effector-controlled equilibrium between two limit enzyme conformers.

A comparative dose-effect study with cardiac glycosides assessing cardiac and extracardiac responses in normal subjects

We tested the hypothesis that differences exist in the pharmacodynamic pattern of different cardiac glycosides. We conducted a randomized, placebo-controlled study in normal volunteers and evaluated the effects of weekly increased oral dosing of digoxin (n = 10; from 0.25 to 1.0 mg/day), meproscillarin (n = 10; from 0.5 to 2.0 mg/day), and placebo (n = 5). To determine the glycoside effects, corrected electromechanical systole (QS2c) was used to measure inotropy and the PQ interval to test dromotropy. Red-green discrimination and critical flicker fusion (CFF) assessed visual functions. Subjective complaints were collected using rating lists. Both glycosides dose dependently shortened QS2c and prolonged PQ interval. PQ prolongations over +20 ms occurred in seven of 10 digoxin subjects, in two of 10 meproscillarin, and in one of five placebo. Equi-inotropic response, identified at 12 ms mean QS2c shortening, revealed the relative potency of digoxin to be 2.4 times higher than meproscillarin; this ratio increased to sevenfold for equi-effective negative dromotropic effects at 12 ms mean PQ prolongation. Each drug was associated with a dominant subjective complaint: digoxin with anergy and meproscillarin with diarrhea. Red-green discrimination was better under meproscillarin and CFF was depressed by digoxin. The results indicate that pharmacodynamic differences exist between cardiac glycosides. A differential use of various glycosides should be considered and tested clinically.

Comparison of the inotropic effects of some 5 alpha-cardenolides on guinea pig left atria

The inotropic activity of eleven 5 alpha-cardenolides and six 5 beta-cardenolides was determined using guinea pig left atria and the results were compared with published data obtained from cat toxicity studies. The guinea pig atrial studies showed that the configuration of the A/B ring junction did not influence significantly the cardiotonic potency of digitoxigenin and its 5 alpha-epimer, uzarigenin, but was of significance with respect to the influence of substituent groups. Glucosidation decreased the potency of uzarigenin but increased that of digitoxigenin. By contrast, conjugation with rhamnose increased the activity of both uzarigenin and digitoxigenin. Cat toxicity data did not correlate well with that obtained using guinea pig atria, possibly because of the variable influence of pharmacokinetic factors applicable to the cat toxicity studies.

[Theoretical study of a structure-activity relationship of cardiotonic steroids]

The calculations have been performed to determine conformations and assess electronic structure of 44 compounds with known cardiotonic activity. Atoms O(3), C(13) and C(18) of all conformations were superimposed on the same atoms of digitoxigenin, and the relationship between the spatial disposition of the side chain carbonyl oxygen and biological activity of the respective compounds was examined. The most active cardiotonics are shown to share very similar topography. B- or A-conformations are active for cardenolides or bufadienolides, respectively. Some correlations between the structure and cardiotonic activity were disclosed. The principles for cardiotonic drug design are formulated and some formulas of new potentially active compounds are suggested.

Studies on cardioactive steroids. V. Structure-activity relationships of derivatives of 16 alpha-gitoxin

The potency of 17 derivatives of 16 alpha-gitoxin was tested in the isolated atrium and heart of the guinea pig and the contractility-increasing activity of the 16 alpha-gitoxin 16 alpha-acetate was compared with that of 16 alpha-gitoxin in the anesthetized dog. The potency of 16 alpha-gitoxin was increased by the substitution of 16 alpha-OH for 16 alpha-methyl ether, 16 alpha-acetate and 16 alpha-nitrate. Substitution of the 16 alpha-acetyl group for substituents with a higher molar volume diminished this enhanced potency. Variation in the digitoxose moiety caused an increase or decrease in potency depending on the position and number of the substituted OH groups. In spite of changes in 16 alpha-OH, the low influence on rhythmicity persisted, as was found in experiments in the dog.

Elimination of cardiac glycosides through hemofiltration

Elimination of three different cardiac glycosides by hemofiltration was investigated using the flat bed RP-6 (Rhône-Poulenc, Paris). At a filtration rate of 59 +/- 9 ml/min the mean clearance of 3-H-g-strophanthin was 54.9 +/- 10.4, that of a 3-H-digoxin and unlabelled digoxin 36.7 +/- 6.6 and that of digitoxin 4.6 +/- 2.8 ml/min. It is concluded from these results that hemofiltration is able to eliminate more than 50% of the amount excreted during the same period of time by normal kidneys. Elimination of cardiac glycosides by continuous hemofiltration is high enough to justify its use in digitalis intoxication, particularly because of the excellent control of electrolyte balance with this new method of detoxification.

Steroid structure and steroid activity

The diverse biological effects associated with naturally occurring steroids are reviewed, with particular reference to the effect upon biological activity of changes in chemical structure. The motives for synthetic modification of the natural products are discussed and are illustrated by reference to the development of systemic and topical anti-inflammatory agents, with improved potency and reduced side-effects.

Increase in dissociation rate constants of cardiotonic steroid-brain (Na+ + K+)-ATPase complexes by reduction of the unsaturated lactone

Several cardiotonic steroids have been modified by reduction of the unsaturated lactone and their interactions with the sodium- and potassium-activated ATPase ((Na+ + K+)-ATPase) have been investigated. Reduction of the unsaturated lactone results in a decrease in binding affinity due primarily to an increase in the dissociation rate constant concomitant with a decrease in the activation free energy of dissociation. This decrease in activation free energy is about 2 to 4 kcal, which is approximately equal to the energy of one hydrogen bond. It is suggested that the increase in dissociation rate due to reduction of the unsaturated lactone may make possible the use of these compounds as affinity ligands for purification of the (Na+ + K+)-ATPase or an ouabain-binding fragment.

[Binding of Rb86 by a microsomal fraction of bovine cerebral cortex cells]

The maximal binding of 86Rb was noted when ions of Mg, ATP and Na were present simultaneously. The availability of olytoriside (1-10(-4)M), CaCl2 (1-10(-3)M) and phospholipase A (1-10(-5)M) in the medium and the removal of Na and substitution of ATP for ADP decreased to a different extent the binding of 86Rb.

Photoaffinity labeling of the ouabain-binding site on (Na+ plus K+) adenosinetriphosphatase

An ethyl diazomalonyl derivative of cymarin was synthesized in order to photoaffinity label the cardiac glycoside-binding site on (Na(+) + K(+)) adenosinetriphosphate (EC 3.6.1.3). When a noncovalent complex of the enzyme and this cardiac glycoside derivative was photolyzed, a covalent bond was formed between the ligand and the larger of the two polypeptide subunits of the enzyme. Several control experiments demonstrate that this photochemical reaction occured while the ligand was bound to the site at which it inhibits the enzyme activity. Another specific inhibitor, tentatively identified as the ethyl chloromalonyl derivative of cymarin, produced similar photoaffinity labeling of the larger subunit, demonstrating that the photolytic dissociation of the diazo group may not be responsible for the photochemical reaction. Since the cardiac glycoside-binding site, which is accessible from the outside surface of the plasma membrane, and the phosphorylation site, which is accessible from the inside surface, are both on the larger polypeptide subunit of (Na(+) + K(+)) adenosinetriphosphatase, this polypeptide has sequences exposed to both sides of the membrane.