Membrane damage by Cerebratulus lacteus cytolysin A-III. Effects of monovalent and divalent cations on A-III hemolytic activity

The effects of monovalent and divalent cations on the hemolytic activity of Cerebratulus lacteus toxin A-III were studied. The activity of cytolysin A-III is remarkably increased in isotonic, low ionic strength buffer, the HC₅₀ (the toxin concentration yielding 50% lysis of a 1% suspension of erythrocytes after 45 min at 37°C) being shifted from 2 μg per ml in Tris or phosphate-buffered saline to 20-30 ng per ml in sucrose or mannitol buffered with Hepes, corresponding to a 50-100-fold increase in potency. On the contrary, hemolytic activity decreases progressively as the monovalent cation concentration in the medium increases for Na⁺, K⁺, or choline salts. The divalent cations Ca²⁺ and Zn²⁺ likewise inhibit the cytolysin A-III activity, but more strongly than do the monovalent cations specified above. Zn²⁺ at a concentration of 0.3 mM totally abolishes both toxin A-III-dependent hemolysis of human erythrocytes and toxin-induced leakage from liposomes. The observation of similar effects in both natural membranes and artificial bilayers suggests an effect of Zn²⁺ on the toxin A-III-induced membrane lesion, especially since Zn²⁺ does not alter binding of the cytolysin. The dose-response curve for toxin A-III exhibits positive cooperativity, with a Hill coefficient of 2 to 3. However, analysis of toxin molecular weight by analytical ultracentrifugation reveals no tendency to aggregate at protein concentrations up to 2 mg per ml. These data are consistent with a post-binding aggregational step which may be affected by the ionic strength of the medium.