Photoinduced isochromic rearrangement in rhodopsin

Two tests have been used to detect and to study conformational rearrangements of cattle rhodopsin, occurring in the process of rhodopsin photolysis and resulting in no change in the visual pigment absorption spectrum. The first test concerns the ability of retinal to react with hydroxylamine. This ability occurs after photoisomerization of retinal with a time constant of 0.3 s at 20 degrees C reflecting this way a conformational transition demasking the retinal-opsin NC-bond. The other test takes advantage of the ability of rhodopsin to modulate the conductance of artificial lipid membranes. After a bleaching flash such a rhodopsin containing membrane shows a transient change in conductance. One of its characteristic time constants is that of NC-bond demasking. It shows that the "demasking" rearrangement is not an artefact due to presence of hydroxylamine and that it occurs in native rhodopsin. It has been shown that the "demasking" rearrangement is isochromic, not associated with known rhodopsin conformational transitions and, judging by its time characteristics, it may be of a functionally importance. The common scheme of rhodopsin photolysis has been modified to include a new conformational transition.