Spectral dimorphism of crayfish visual pigment in solution

Abstract

Crayfish ( Procambarus) photoreceptor membranes were prepared by a new technique. Digitonin extracts made at 0° contain two rhodopsin-like pigments, P 562 and P 512, in a ratio of about 5:4. On exposure to light at 10°C, P 562 is converted to a metarhodopsin-like photoproduct with λ max at 515 nm (M 515), and P 512 is converted to a metarhodopsin-like photoproduct with λ max at 475 nm (M 475). When warmed to 22°C, both photoproducts bleach in minutes to retinal and opsin. Neither P 562 nor P 512 is attacked by hydroxylamine, but both are destroyed by the detergent Ammonyx LO and by reduction of disulfide bridges with 2-mercaptoethanol. Neither is altered by changing the concentration of Cl −. P 562 is more susceptible than P 512 to thermal denaturation and attack by OH −, and its spectrum is distinctly narrower than predicted by an Ebrey-Honig nomogram. P 562 and P 512 do not interconvert, either in the dark or in the presence of light. P 512 is distinct from M 515; it is substantially more stable at 22°C and has a 23% smaller molar extinction coefficient. On the other hand, M 515 is spectrally very similar to the single metarhodopsin that is formed by irradiation of rhabdoms from dark-adapted crayfish. As neither P 562 nor P 512 is found in crayfish rhabdoms, we conclude that both pigments are created in digitonin micelles from a spectrally and kinetically homogeneous population of rhodopsin molecules withλ max near 530 nm in the rhabdom.