Three recently isolated peptides, whose sequences have been determined-the corticotropin (adrenocorticotropic hormone)-releasing factor of ovine origin, sauvagine, from the skin of the frog Phyllomedusa sauvagei, and urotensin I from the teleost fish, Catostomu commersoni-show high (>50%) sequence homology. CD spectra of the three peptides in trifluoroethanol indicate predominantly helical character for these peptides. Analysis of the secondary structures by the Chou-Fasman method predicts that the overall structural organization of the peptides is the same. All three possess a long internal helix, spanning about 25 residues, connected by a turn region to a COOHterminal structural element that is an a-helix in corticotropin-releasing factor and urotensin I and a fl-sheet in sauvagine. The values for helical content estimated from the prediction method agree reasonably well with those computed from the CD spectra. This agreement as well as the CD spectra of corticotropin-releasing factor fragment 5-33 support the specific assignments of helical regions derived from the Chou-Fasman analysis. The three peptides exhibit significantly less helical structure in water than in trifluoroethanol as indicated by CD spectra. Hydrophilicity profiles provided comparison of the three peptides in terms of their overall hydrophilicity and the location of the regions of maximal hydrophilicity. A unique distribution of hydrophilic and hydrophobic residues within the internal helices is revealed by helical wheel analysis. Patches of both types of residues are formed following a heptad (four/three) rule. Since the two patches are shifted by one residue relative to one another, together they occupy only one face of the helical surface, a feature distinct from other amphiphilic structures.The presence of a corticotropin (adrenocorticotropic hormone)-releasing factor (CRF) in the hypothalamus was established more than 25 years ago (1-3). Its characterization from ovine hypothalami was achieved only very recently (4, 5). Synthetic CRF has been shown to release ACTH and 1-endorphin both in vitro and in vivo and to cause hypotensive and behavioral effects in rats (4, 6). Sauvagine, a 40-amino acid peptide, highly homologous with CRF, was isolated from the skin of the frog Phylonmedusa sauvagei (7) and displays, among other activities, a number of CRF-like effects, such as release of ACTH and , 1 endorphin (4, 8). The recently isolated peptides in this group, urotensin I from two species of teleost fisht, share significant homology with and possess similar biological activities to CRF and sauvagine (9).CRF, sauvagine, and urotensin I belong to a novel family of peptides from widely differing sources. Their high sequence homology is an indication of a possible ancestral relationship. The fact that they are equipotent in some assays indicates that functionally important parts of the molecule were highly conserved during evolution. It is therefore important to establish the secondary and tertiary structures of the three peptides to a...
The molecular mechanics program AMBER, assisted by CHEMLAB II, was used to model the covalent and noncovalent binding of anthramycin, tomaymycin, and neothramycin A to the hexanucleotide conformation. Structures covalently bonded at N2 of guanine gave excellent fits when placed in either direction in the minor groove. However, energy analysis showed a preference for the direction wherein the side chain points toward the 5' end of the covalently bound strand. This preference agrees with published NMR studies. Noncovalent binding of anthramycin in the minor groove near guanine gave good fits with almost no distortion in the helix, and the reactive center of the ligand was close enough to N2 for subsequent covalent bond formation. Anthramycin also gave a good noncovalent complex near adenine in the minor groove, but binding in the major groove had decreased dispersion attractions. Binding of tomaymycin was similar to that of anthramycin, although the smaller size of tomaymycin resulted in less binding energy. Neothramycin noncovalent binding was characterized by strong electrostatic interactions, partly involving the 3-OH group, and by part of the molecule lying outside the minor groove. AMBER was used for the exploratory design of an anthramycin analogue that theoretically would bind as well as anthramycin but not cause cardiotoxicity. A related study involving anthramycin, tomaymycin, and the pentanucleotide duplex d(AAGAA/TTCTT) was undertaken to evaluate further the ability of AMBER to predict sequence specificity. It indicated a preferred direction of binding toward 5' in the minor groove of the duplex, but rather weak interaction with the noncovalently bound strand. This prediction agreed with experiments on tomaymycin that showed separation of the duplex and alignment of the drug toward the 5' end of the covalently bound strand.
A new three-dimensional model for the relative binding mode of cassaine 1 and digitoxigenin 2 at the digitalis receptor site is proposed on the basis of the structural and conformational similarities among 1, 2 and its 14,15-seco analogues 3 and 4. Accordingly, the speculation that also 17alpha-substituted derivatives of the digitalis 5beta,14beta-androstane skeleton could efficiently bind to the Na+,K+-ATPase receptor is put forward and verified through the synthesis of some related compounds. The binding affinity shown by 2-(N,N-dimethylamino)ethyl 3beta, 14-dihydroxy-5beta,14beta-androstane-17alpha-acrylate 6 (IC50 = 5.89 microM) and, much more significantly, by the corresponding 14, 15-seco-14-oxo derivative 9 (IC50 = 0.12 microM) substantiates the new hypothesis and opens new prospects to the design of novel inhibitors of Na+,K+-ATPase as potential positive inotropic compounds.
A series of 17 beta-(hydrazonomethyl)-5 beta-androstane-3, beta,14 beta-diol derivatives was synthesized and evaluated in the displacement of [3H]ouabain binding from Na+,K(+)-ATPase. The data were explored with multiple linear regression and partial least-squares to find possible quantitatives structure-activity relationships. Good correlations were found between binding to the receptor and van der Waals volumes or molar refractivities of the 17 beta-hydrazonomethyl substituents and pKa values of the compounds. Equivalent results were obtained using the proton affinity (calculated using MOPAC) of the hydrazone residues instead of experimental pKa. As basicity or related electronic factors of the substituents explain a significant portion of the observed changes in the activity, an ion-pair interaction between a carboxylate residue of the enzyme and the protonated 17 beta-hydrazonomethyl group, as postulated by Thomas, plays an important role in the interaction of the ligand to the Na+,K(+)-ATPase receptor.
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