Interactions of cytochrome c with phospholipid membranes: Binding of cytochrome c to phospholipid liquid crystals

Cytochrome c added during the formation of lecithin-cardiolipin liquid crystals in 0.015 m KCl is readily bound. After successive washings with 0.15 m KCl, only about 50% of this bound cytochrome c is removed. The remaining cytochrome c is resistant to further salt extraction, and the amount of this cytochrome c that is bound varies with the concentration of added cytochrome c to a maximum binding ratio of 1:70, mole ratio cytochrome c to phospholipid. This binding appears to be electrostatic; it is competitively inhibited by increasing the initial molarity of KCl from 0.015 to 0.10 m. Binding of cytochrome c is insignificant in the absence of cardiolipin, and is affected by varying the pH. Electron microscope studies of osmium tetroxide-stained thin sections show that the liquid crystals consist of vesicles, each of which contains a large number of concentric, alternating light and dense lines. The dense lines have been identified by other workers with the polar head groups of the phospholipids on the surface of a bilayer, and the light area represents the hydrophobic interior. The addition of cytochrome c causes an average decrease in the number of lines per vesicle. It increases the center-to-center distance between two neighboring light or dense lines and the width of the dense lines. On the basis of this evidence and electrostatic binding, it is concluded that cytochrome c is binding on the polar surfaces of the phospholipid bilayers comprising the liquid crystalline vesicles. © 1970 Springer-Verlag New York Inc.SCOPUS: ar.j