Deep-sea photodynamic vision at low light level - Which is more important, prosthetic retinal or apo-rhodopsin...

The special case of far-red vision of mesopelagic dragonfish Malacosteus niger facilitated by the presence in the rod outer segment of photosensitizer chlorin e6 from diet has drawn considerable attention both in vision research and in photodynamic action. Rhodopsin binding of Ce6 from either the extracellular or intracellular loops may exert different effects. Theoretical works predict that the extracellularly bound Ce6 upon absorption of red light produces a singlet oxygen, which could via an oxygen tunnel reach the Lys-tethered 11-cis-retinal, by way of peroxy-dioxetane intermediates, to enhance 11-cis- to all-trans-retinal isomerization, therefore triggering the ultrafast phototransduction process. Recent works on the permanent photodynamic activation of some A-class G protein-coupled receptors suggest that the singlet oxygen generated by Ce6 photodynamic action might also oxidize the scotopsin moiety of rhodopsin, leading to direct oxidative rhodopsin activation. More attention needs to be paid to the latter respects of the far-red vision process of the deep-sea dragonfish, with potential translational values.