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. Author manuscript; available in PMC: 2022 Aug 1.
Published in final edited form as: Dev Biol. 2021 Mar 25;476:68–78. doi: 10.1016/j.ydbio.2021.03.013

Figure 5. Carotenoid uptake and vitamin A metabolism in Drosophila.

Figure 5.

(1) Dietary carotenoids are taken up in the Drosophila midgut by NinaD, the homolog of SR-BI. (2) Carotenoids reach the head via the hemolymph and are taken up in unknown extraretinal cells by another scavenger receptor, Santa-Maria. (3) Carotenoids are converted to zeaxanthin that is cleaved by NinaB, a homolog of BCO1 and RPE65, to 11-cis-3-hydroxyretinal and all-trans-3-hydroxyretinal. (4) The latter is reduced and isomerized to 11-cis-3-hydroxyretinol in a Pdh- and blue light-dependent manner. Oxidation by Rdhb generates the 11-cis-3-hydroxyretinal chromophore. (5) The chromophore covalently binds to the opsin protein and thereby forms the visual pigment Rhodopsin. (6) The absorption of light causes a photoisomerization from cis to trans and converts Rhodopsin to its activated form, Metarhodopsin. Note that all-trans-3-hydroxyretinal remains bound to the opsin. Metarhodopsin can be reconverted to Rhodopsin by absorption of a photon of a specific wavelength (7). Some Metarhodopsin is degraded and the released all-trans-3-hydroxyretinal (8) can be recycled to 11-cis-3-hydroxyretinal in pigment cells.