Binding properties of Pyrularia thionin and Naja naja kaouthia cardiotoxin to human and animal erythrocytes and to murine P388 cells

Pyrularia thionin and snake venom cardiotoxin are strongly basic peptides which induce hemolysis, depolarization of muscle cells and activation of endogenous phospholipase A 2. An earlier study of the hemolysis reaction indicated that the two peptides bind to and compete for the same site on human erythrocytes. A recent study examined the hemolysis induced by both peptides as the phosphate and Ca 2+ content of the reaction mixture was varied. The results of the recent study ( Vernon, L. P. and Rogers, A., Toxicon 30, 701–709) agree with this companion study on the binding of 125I-labeled pyrularia thionin and cardiotoxin to erythrocytes under the same conditions. Added phosphate ion at 5 mM and removal of membrane-bound Ca 2+ by treatment with 10 mM EGTA make more binding sites of the same affinity available to both peptides, which are shown to bind in a competitive fashion to the same site. Addition of 10 mM Ca 2+ to the medium decreases peptide binding due to competitive binding of Ca 2+ to the same site on the membrane. For human erythrocytes the number of binding sites/cell for the thionin ranged from 0.7 to 1.7 × 10 5 and for cardiotoxin from 0.82 to 1.6 × 10 5. The calculated dissociation constants ( K d) from the Scatchard plots ranged from 0.43 to 1.1 μM for the thionin and from 0.40 to 0.98 μM for the cardiotoxin. The binding sites for thionin and cardiotoxin with sheep erythrocytes were 1.7 and 2.0 × 10 4 sites/cell, respectively, and both cow and horse erythrocytes demonstrated 2.7 × 10 4 sites/cell for the thionin. Binding studies with murine P388 cells showed 7.0 and 9.5 × 10 6 sites per cell for Pyrularia thionin and cardiotoxin, respectively.