The Interaction of Cytochrome C with Phospholipids

Summary

The reactions of the hydrated electron (e_aq⁻), produced during pulse radiolysis, have been used to study the binding of phosphatidyl choline (PC), phosphatidyl serine (PS), phosphatidyl ethanolamine (PE), and phosphatidyl inositol (PI) vesicles with horse-heart cytochrome c. An interaction could only be detected between cytochrome c and either PS or PI. An apparent equivalence point in the binding was reached for both phospholipids at a molar ratio of phospholipid : protein of 6 : 1. At this point, the reactivity of (e_aq⁻) towards the cytochrome c was very markedly reduced. Indeed, the rate of disappearance of (e_aq⁻) under such conditions was the same as the rate of e_aq⁻ disappearance in triply-distilled water. The inclusion of cholesterol at a molar ratio of 1 : 1 within the phospholipid vesicles changed the stoichiometry of the interaction. Evidence that protonated ϵ-amino groups of lysine residues are involved in the interaction is presented. Possible models for the complexes formed are discussed.