Interactions of hemin with model erythrocyte membranes

To address the interactions of hemin with phospholipid bilayers, we introduce hemin to a solution of dimyristoylphosphatidylcholine (DMPC), a long chain phospholipid, and 3-(cholamidopropyl)(dimethylammonio)-2-hydroxy-1-propanesulfonate (CHAPSO), a detergent, in which vesicles form at around 37 degrees C. We show that vesicles composed of DMPC/CHAPSO form and grow, following a mechanism that does not trap solution and excludes larger solutes, such as hemin, from the vesicle interior. The existence of a limited number of patches of likely 2D crystalline hemin embedded in the phospholipid bilayer suggests that this layer is saturated with hemin molecules. We show that despite this saturation, even after prolonged contact with hemin-containing solution outside the vesicles, hemin is not released on the other side of the membrane; i.e., the phospholipid bilayer is impermeable to hemin. Comparison of the properties of the model membrane to those of the erythrocyte membrane suggests that the latter might also be impermeable to hemin and, given the absence of pores suitable to hemin in the erythrocyte membranes, that hemin might accumulate in erythrocytes after its release due to hemoglobin instability.