A STUDY ON THE STRUCTURE-ACTIVITY RELATIONSHIP OF THE CARDIOTONIC STEROIDS.
According to the prevailing opinion of today about the structure-activity relationship of cardenolides and bufadienolides, the structural requirements for the characteristic cardiotonic action of these compounds can be summarized as follows: cis-fusion of the C and D rings, hydroxyl groups in positions 3β and 14β, and a five- or six-membered unsaturated lactone ring having β-configuration at C14 (1). Most of the experimental data referred to as evidences supporting this opinion have come, however, from the determination of toxic doses in the whole animal, mainly in the cat, rather than from observations of the action of these compounds in isolated hearts. It follows as a natural consequence that the compounds with a rather weak toxicity have been condemned to be inactive, even if they have a strong cardiotonic action, in spite of the fact that such a kind of compounds may actually be more important from the clinical point of view. For example, the compound like dihydrodigitoxin of which the lactone ring is saturated, and the compound like resibufogenin, which have an epoxide ring between C14 and C15 instead of C14-OH, generally believed to be inactive based on the toxicity data in the cat, have recently been found to be fairly active when tested in the isolated heart (2-3). The authors also found that, different from the above-cited opinion, the presence of 3β-OH is not indispensable for the cardiotonic action of the cardenolides ; 3-epi-digitoxigenin was found to have a definite cardiotonic action upon the isolated frog's heart (See Fig. 8). These findings seem to warrant reexamination of the problem, on the basis of the action of these compounds in the isolated heart. In recent years several adrenocortical steroids and aldosterone antagonists have been reported to be cardiotonic (4-6), despite the fact that their stereochemical structures are essentially different from those of the cardiotonic steroids. In view of these, the authors took up the problem of the cis-fusion of the C and D rings out of the several important points in the structure of cardiotonic steroids as discussed above and reexamined its importance for the specific cardiotonic action using the isolated frog's heart and the heart-lung preparation of the dog.
Antiproliferative activity of Rhinella marina and Rhaebo guttatus venom extracts from Southern Amazon.
The venom of amphibians is a fascinating source of active substances. In view of their medical importance and aiming to explore the amazing Brazilian biodiversity, we conducted bioprospecting of antiproliferative activity in extracts of Rhinella marina and Rhaebo guttatus toads occurring in the Southern Amazon of Mato Grosso, Brazil. LC-MS and HPLC analysis of the venom extracts of R. marina revealed four bufadienolides (telocinobufagin, marinobufagin, bufalin and resibufogenin. R. guttatus venom extracts contained only marinobufagin. First, R. marina and R. guttatus venom extracts were evaluated for cytotoxicity against tumor cell lines by the MTT assay. All extracts revealed cytotoxicity, where R. marina extracts were comparable to doxorubicin (IC₅₀ values ranging from 0.01 to 0.23 μg/mL). Only extracts of R. guttatus toad venom caused membrane disruption of human erythrocytes. The extracts were investigated for selective activity by determining their effect on stimulated human peripheral blood mononuclear cells (PBMC) with the Alamar Blue™ assay. The extracts were up to 80-fold more selective against leukemia cells when compared to dividing leukocytes. Aiming to confirm these antiproliferative effects, BrdU incorporation into DNA was measured in HL-60 treated cells with R. marina venom extracts. These extracts decreased BrdU incorporation at both concentrations tested. In summary, nine extracts of R. marina and R. guttatus venom showed pronounced lethal and discriminating effects on tumor lines, especially those from R. marina, highlighting toad parotoid gland secretions as a promising source for novel lead anticancer chemicals.
Evaluation of antioxidants by a rapid polarographic method
A simple, rapid, precise, and reproducible method and an adaptable apparatus for polarographic measurement of lipid peroxidation and antioxidant activity is described. This method measures antioxidant activity within a few min time as compared to the hours or days needed for several widely used methods. Peroxidation rates were measured for different lipids as a function of amount of lipid, and amount of biocatalyst. The method is applicable to the detection and measurement of the antioxidant effects exhibited by weak antioxidants and has been satisfactorily used for the measurement of the antioxidant activity of different concentrations of powerful antioxidants.