Indication of the metarhodopsin I-II transition by absorption-changes of Eriochromblack T

The dye eriochromblack T (erio T), added to an aqueous suspension of bovine retinal outer segments solubilized by digitonin, shows a light-induced absorption-increase at lambda = 645 nm. Erio T is shown to directly interact with miscellar metarhodopsin I and metarhodopsin II. The absorption-changes of erio T can be regarded as an indication of the transition from the metarhodopsin I conformation (with associated Ca2+) to the metarhodopsin II conformation (with associated H+).