PHOTOREACTIONS AND RELATED CHARGE DISPLACEMENTS IN THE RHODOPSIN FROMSEPIA-OFFICINALIS

The photochemical cycle and the accompanying charge displacements of r hodopsin were investigated in membrane fragments and solubilized rhodo psin from the retina of Sepia officinalis. Absorption changes were mea sured following excitation by a short laser flash. Light excitation pr oduces successive forms with absorption maxima at 536 nm and 474 nm as signed to lumirhodopsin and mesorhodopsin, respectively. The lifetimes of these forms were 8 +/- 3 mu s and 96 +/- 5 mu s, respectively. The absorption spectrum of the acid meta form was found to be almost iden tical to that of rhodopsin. Membrane fragments were also oriented by a d.c. electric field and fixed in gel. Charge displacements following light excitation were measured as a displacement current. The electric al signals from the oriented gels were separated into two exponential components with time constants identical within errors to those obtain ed from the absorption kinetic experiments. It is shown that the prote in conformational changes indicated by the absorption changes are acco mpanied by intramolecular charge displacements. The transitions are as signed to the decay of the lumi and meso states.