| Abstract: | Abstract—Light absorption by rhodopsin in receptor cell membranes initiates the excitation of the receptor cell. Rhodopsin-phospholipid membrane vesicles were studied to localize initial transduction events. Rhodopsin-phospholipid recombinant membranes are thermally stable and light sensitive and may be chemically regenerated after bleaching in the same manner as receptor cell membranes. Rhodopsin-containing vesicles prepared from unsaturated phosphatidylcholine (PCho) or PCho and phosphatidylethanolaminc display kinetics for the metarhodopsin I to II transition which are comparable to those of receptor cell membranes. NMR spectroscopy was used to examine the permeability of the membrane vesicles to added shift (Eu3+) or relaxation reagents (Mn2+, Co2+). Unexposed rhodopsin-phospholipid vesicles are sealed to ion movement and become permeable after light exposure. Selected ions (Ca2+, Mn2+, Co2+) may be photoreleased from the interior of loaded membrane vesicles. The quantity released is proportional to the initial ionic concentration. The number of ions released/rhodopsin bleached is dependent on the light intensity, and high yields (40–160) of Ca2+/rhodopsin bleached are observed at low levels of light bleaching. The present results indicate that rhodopsin spans the phospholipid bilayer membrane, and are consistent with an increase in the permeability of the membrane initiated by light excitation of rhodopsin. |
