Phospholipid liposomes: Preparation, characterization, and uses

Rhodopsin and cyclic guanosine monophosphat (cGMP)‐dependent channel proteins are isolated from the rod outer segment disk membranes of dark‐adopted bovine retinae and incorporated in liposomes, prepared by the method of detergent removal dialysis. The ion channel does not lose its transport function (release of Ca 2+ ions by injection of cGMP) when incorporated in a liposome. Its activity depends on the degree of protein solubilization and the kind of detergent used. The highest activity is obtained by use of the detergent CHAPS. Shape, size, and size distribution of the liposomes are deduced from elastic and quasi‐elastic light scattering, the liposome number density by viscometry, and the photopigment or Ca 2+ content by optical absorbance. The liposomes are heterogeneous with respect to size and shape. Small unilamellar liposomes ( R h = 80 nm) and a narrow size distribution ( U D = 0.16) are obtained by using the detergent CHAPS. With increasing rhodopsin content per liposome, the hydrodynamic radius R h increases and at the same time the shape of a liposome converts from a sphere to a prolate ellipsoid. The amount of entrapped Ca 2+ per liposome reaches its maximum value when the Rhodopsin nearest‐neighbor distance approaches its minimum value. This suggests an intermembrane protein‐lipid‐protein lattice, which serves as barriere for Ca 2+ . The influence of temperature or total used Ca 2+ content is less profound. Increasing temperature yields slightly smaller liposomes.