Exchange of cytochrome b5 between phospholipid vesicles

Detergent-extracted cytochrome b5, which binds readily to phosphatidylcholine vesicles, has been shown to exchange between vesicles by the following experiments. When preformed cytochrome b5-vesicle complexes are incubated with fresh vesicles a new species arises which sediments in the analytical ultracentrifuge with a rate intermediate between those of the original components. When the same mixture is subjected to free boundary electrophoresis no slow moving component corresponding to pure vesicles is seen. The results of sucrose density gradient centrifugation are also consistent with the conclusion that cytochrome b5 exchanges between vesicles. The possibility that the above results were produced by vesicle fusion or aggregation was eliminated by light-scattering studies and a direct assay for vesicle fusion. The ability of cytochrome b5 to exchange between lipid structures has important implications not only in the cytochrome b5-lipid system but also in enzymatic reactions in which cytochrome b5 is involved.