Structural changes of liposome phospholipid packing induced by cytotoxin of the Central Asia cobra venom

Liposomes and proteoliposomes obtained from rat brain were used; structural changes induced by Vc5 cytotoxin (CT) from Central Asia cobra venom have been studied by the EPR method using spin probes (5-, 10-, or 12-doxylstearic acid). The addition of CT to liposome samples, containing spin probes resulted in the appearance of a new EPR signal in the initial spectrum (samples without CT), typical of probes with strongly retarded mobility. The presence of hydrophobic interaction between the CT molecules and spin labelled fat acids permits the assumption that CT molecules in liposomes trap both lipid probes and phospholipids localized in the reach of action of hydrophobic forces. CT may be supposed to induce formation in membranes of liposomes with domain structures. As a result of hydrophobic interaction with CT molecules both the phospholipid and lipid probe mobility in the domain is substantially less than that in liposome regions free of CT molecules. Due to this, a new signal appears in the initial EPR spectrum of the spin probes. An analysis of the dependence of the probe order parameter value on CT concentration in samples has suggested that CT act uniformly along the membrane lipid profile with a certain CT concentration range. At high concentrations CT molecules cannot penetrate the lipid region deep enough, due to mutual electrostatic repulsion and steric factors at membrane surface. As a result, structural changes involve regions adjacent to the membrane surface only.