Rhodopsin and Retinochrome in the Retina of a Tetrabranchiate Cephalopod, Nautilus pompilius

The nautilus retina contains retinochrome in addition to rhodopsin, just like the retinas of squid and octopus. Those photopigments of Nautilus pompilium were extracted to examine their chemical properties. The absorption maximum (λ_<max>) of rhodopsin is very short in wavelength, being at 465 nm. On irradiation with blue light, rhodopsin is changed to a photoequilibrium mixture with metarhodopsin (λ_<max>=510 nm), which is photoregenerated back to rhodopsin on reirradiation with orange light. Rhodopsin remains stable in the presence of 100 mM hydroxylamine (NH_2OH), whereas metarhodopsin is gradually decomposed forming retinaloxime. The molecular weight of nautilus rhodopsin is estimated to be 84,000 by SDS-PAGE, larger than that of squid rhodopsin. In the nautilus retina, retinochrome is present at a level of only about 4% of the rhodopsin content. When exposed to orange light, retinochrome (λ_<max>=510 nm) is readily bleached to metaretinochrome, which again yields retinochrome on addition of all-trans-retinal. Consequently, retinochrome catalyzes the isomerization of all-trans-retinal to the 11-cis form in the light. It is stable in 20 mM NH_2OH, but metaretinochrome is rapidly destroyed. The molecular weight of nautilus retinochrome is 26,000, similar to that of squid retinochrome. It was also suggested that the nautilus retina is provided with the same rhodopsin-retinochrome system as established in the squid retina.