Photoreactions and related charge displacements in the rhodopsin from Sepia officinalis

Abstract

The photochemical cycle and the accompanying charge displacements of rhodopsin were investigated in membrane fragments and solubilized rhodopsin from the retina of Sepia officinalis. Absorption changes were measured following excitation by a short laser flash. Light excitation produces successive forms with absorption maxima at 536 nm and 474 nm assigned to lumirhodopsin and mesorhodopsin, respectively. The lifetimes of these forms were 8 ± 3 μs and 96 ± 5 μs, respectively. The absorption spectrum of the acid meta form was found to be almost identical 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 electrical signals from the oriented gels were separated into two exponential components with time constants identical within errors to those obtained from the absorption kinetic experiments. It is shown that the protein conformational changes indicated by the absorption changes are accompanied by intramolecular charge displacements. The transitions are assigned to the decay of the lumi and meso states.