Light-dependent metabolic pathway of 3-hydroxyretinoids in the eye of a butterfly,Papilio xuthus

1.	After the intact compound eyes of the butterflyPapilio xuthus were adapted to darkness, white, blue ( max 460 nm) or orange light ( max 580 nm), the eyes were separated into the distal (primary pigment cells, the dioptric apparatus and ca. 30% of retinal tissue) and the proximal layers (the rest of the retinal tissues). Each layer was separated into a supernatant and a precipitate. Both in white and blue light-adapted eyes, the amount of 11-cis 3-hydroxyretinal increased in the supernatant of the distal layer (Sup-DL) much more than it did in dark-adapted eyes. No increase was observed in the Sup-DL of orange light-adapted eyes.
2.	When all-trans retinol (non-native chemical) was added to the Sup-DL, it was converted to all-trans retinal under the darkness, and to all-trans and 11-cis retinal by blue light irradiation. When all-trans retinal was added to the Sup-DL, the isomerization of all-trans retinal to 11-cis retinal was accelerated by the blue light.
3.	The Sup-DL was separated into ammonium sulfate soluble (AS-sup) and insoluble (AS-ppt) fractions. The AS-ppt fraction contained 3-hydroxyretinal but no 3-hydroxyretinol. Blue light irradiation to the AS-ppt fraction induced an increase in 11-cis 3-hydroxyretinal, with a concomitant decrease in all-trans 3-hydroxyretinal.

These results indicate that both the oxidation of all-trans 3-hydroxyretinol to all-trans 3-hydroxyretinal and the light-dependent isomerization of all-trans 3-hydroxyretinal to 11-cis isomer take place in the tissues of the distal layer of the eyes.