An intermediate in the photolytic process of extracted squid rhodopsin

To clarify the formation process of acid metarhodopsin in the cephalopod rhodopsin cycle, changes in the difference spectrum of squid (Todarodes pacificus) rhodopsin in acid (pH 5.7, or pH 6.1) and alkaline (pH 10.2) solutions were studied at various temperatures by applying the flash photolytic technique. An intermediate with an absorption maximum at about 475 nm was transformed from lumirhodopsin prior to the formation of acid metarhodopsin independently of the pH of the solution. In alkaline solution, some of the converted acid metarhodopsins were then transformed to alkaline metarhodopsins. The conversion of lumirhodopsin to acid metarhodopsin seemed to be a first-order chain reaction.