Freeze-fracture study of cardiotoxin action on axonal membrane and axonal membrane lipid vesicles.

Freeze-fracture electron microscopy was used to follow morphological changes induced by Naja mossambica mossambica venom cardiotoxins on crab axonal membranes and thier lipids. It was shown that the extent of morphological changes depended drastically on the purity of cardiotoxin preparations and on their nature. Highly purified cardiotoxin induced mainly fusion of membrane or lipid vesicles. The extent of fusion and other morphological changes depended on the nature of cardiotoxin used: VII4 cardiotoxin induced only fusion while VII1 led to further modifications of membranes and liposomes. The most spectacular morphological changes were observed with axonal membranes treated with cardiotoxin containing traces of venom phospholipase A2. At low cardiotoxin concentration (10(-7)-10(-5) M) important intramembrane particle aggregation was observed and at higher concentrations (more than 10(-4) M) intramembrane particles disappeared from the membrane and were found in solution. The membrane vesicles, devoid of intramembrane particles, were observed to fuse rapidly into liposome-like aggregates. These morphological changes are interpreted as being due to the removal of intrinsic membrane proteins from the membrane by the combined action of cardiotoxin and phospholipase A2.