Prednisolone-3,20-bisguanylhydrazone: the mode of interaction with rat brain sodium and potassium-activated adenosine triphosphatase

Na+,K(+)-ATPase inhibition by an endogenous peptide, SPAI-1, isolated from porcine duodenum

SPAI-1, a peptide isolated from porcine duodenum, has been shown to inhibit Na+,K(+)-ATPase in vitro (Araki et al. (1989) Biochem. Biophys. Res. Commun. 164, 496-502). The characteristics of ATPase inhibition by this novel peptide were examined. SPAI-1 inhibited Na+,K(+)-ATPase preparations isolated from various organs of dog or rat or from sheep kidney with similar potency. Three isoforms of rat Na+,K(+)-ATPase had similar sensitivity to inhibition by SPAI-1 although these isoforms had remarkable differences in their sensitivity to the inhibitory effect of ouabain. Ca(2+)-ATPase isolated from the sarcoplasmic reticulum of rabbit skeletal muscle was insensitive to inhibition by SPAI-1. Ouabain-insensitive Mg(2+)-ATPase activity was unaffected by low concentrations of SPAI-1, but was stimulated at high concentrations. SPAI-1 inhibited H+,K(+)-ATPase from hog stomach in concentrations similar to that required for Na+,K(+)-ATPase inhibition. These results indicate that SPAI-1 is a specific inhibitor for monovalent cation transporting ATPases.

The lead structure in cardiac glycosides is 5 beta, 14 beta-androstane-3 beta 14-diol

The purpose of the present study was to determine the lead structure in cardiac glycosides at the receptor level, i.e. the minimal structural requirement for specific and powerful receptor recognition. Accordingly 73 digitalis-like acting steroids were characterized as to the concentration effecting half-maximum inhibition of Na,K-ATPase from human cardiac muscle under standardized turnover conditions. Since the Ki value equaled the apparent KD value, K'D was expressed in terms of the apparent standard Gibbs energy change delta G degrees' of steroid interaction with Na,K-ATPase. This allowed the use of the extrathermodynamic approach as a rational way of correlating in a quantitative manner, the potency and structure of the various steroidal compounds. The results of the present analysis taken in conjunction with relevant findings reported in the literature, favour the following conclusions. Cassaine, canrenone, prednisolone- and progesterone-3,20-bisguanylhydrazone, and chlormadinol acetate are compounds that are not congeneric with digitalis. The butenolide ring of cardenolides or the analogous side-chains at C17 beta of 5 beta, 14 beta-androstane-3 beta, 14-diol are not pharmacophoric substructures, but merely amplifiers of the interaction energy of the steroid lead. All modifications of the structure, geometry and spatial relationship between the steroid nucleus and butenolide side chain of digitoxigenin all at once weaken the close fit interaction with the steroid and butenolide binding subsites of the enzyme in such way that the cardenolide derivatives interact with the receptor binding site area in whatever orientation that will minimize the Gibbs energy of the steroid-receptor-solvent system. The "butenolide carbonyl oxygen distance model" (Ahmed et al. 1983) for the interpretation of the differences in potency of the cardenolide derivatives describes the change in interaction energy through structural modification as a function of the entire molecule. 5 beta, 14 beta-androstane-3 beta, 14-diol, the steroid nucleus of cardiac glycosides of the digitalis type, is the minimum structure for specific receptor recognition and the key structure for inducing protein conformational change and thus Na,K-ATPase inhibition. It is also the structural requirement for maximum contributions of the butenolide substituent at C17 beta and the sugar substituent at C3 beta-OH to the overall interaction energy, i.e. this steroid nucleus is the lead structure.(ABSTRACT TRUNCATED AT 400 WORDS)