Kinetic study of photoregeneration process of digitonin-solubilized squid rhodopsin.

In the photoregeneration process of squid rhodopsin, an intermediate has been found at neutral pH values (phosphate buffer) with a flash light (lambda greater than 540 nm). An intermediate R430, with the 11-cis retinal as chromophore, is produced from metarhodopsin in light and is converted to rhodopsin through the processes R430 leads to P380 and P380 leads to rhodopsin. The pH dependence of the velocity of the conversions suggests that processes R430 leads to P380 and P380 leads to rhodopsin involve a protolytic reaction and that the ionized group is a histidine residue of opsin. Kinetic parameters show that the largest conformational change in opsin occurs in the conversion of R430 leads to P380.