Energy uptake in the first step of visual excitation.

Perception of light by the retina starts with the absorption of a photon by 11-cis retinal, which is covalently incorporated into the membrane-bound protein, rhodopsin. The initial result of photon capture is the very rapid formation of a red-shifted species, bathorhodopsin (also known as prelumirhodopsin), which is (meta-)stable at liquid nitrogen temperature but which decomposes at higher temperatures, in the dark, through a series of intermediate stages, resulting in the release of all-trans retinal from the apoprotein, opsin. Bathorhodopsin formation is the only photochemical step in the overall reaction and, therefore, merits investigation. Several models for the process have been proposed, and have been critically reviewed, although no consensus yet exists as to the nature or mechanism of formation of the batho intermediate. I report here on the first direct measurement of photon energy uptake during bathorhodopsin formation from bovine rhodopsin, and on its possible significance.