Cytochrome b5 induced flip-flop of phospholipids in sonicated vesicles.

Cytochrome b5 induced flip-flop of phosphatidylethanolamine (PE) in sonicated vesicles prepared from a 9:1 mixture of phosphatidylcholine (PC) to phosphatidylethanolamine was determined as follows. First, vesicles having a nonequilibrium distribution of PE across the bilayer were prepared by amidinating the external amino groups with isethionyl acetimidate. Amidinated cytochrome b5 was then added, and after the protein was completely bound, the rate of appearance of fresh PE on the outer surface was determined by removing aliquots at timed intervals and titrating the external amino groups with trinitrobenzenesulfonic acid. The results show an initial rapid phase of flip-flop (especially in the presence of salt) followed by a very slow phase, at 25 degrees C. Similar results were obtained when cytochrome b5 was introduced into the amidinated vesicles by spontaneous transfer from PC donor vesicles. These results indicate that the accumulation of the transferable ("loose") form of cytochrome b5 on the outer surface of a vesicle causes a transient, global destabilization of the bilayer that is relieved by lipid flip-flop. We speculate that this mechanism may be a significant driving force for the transfer of amphipathic molecules across membranes.