THE METABOLIC PATHWAY OF VISUAL PIGMENT CHROMOPHORE FORMATION IN DROSOPHILA-MELANOGASTER - ALL-TRANS (3S)-3-HYDROXYRETINAL IS FORMED FROM ALL-TRANS RETINAL VIA (3R)-3-HYDROXYRETINAL IN THE DARK

Carotenoid-depleted fruit flies, Drosophila melanogaster, were reared on yeast/glucose medium containing lipid-depleted white corn grits and cholesterol. After rearing for more than a year, the yield of flies r emained constant and the content of 3-hydroxyletinal in a head was thr ee logarithmic units less than that of normal flies reared on medium c ontaining yellow corn grits. When all-trans retinal was supplied as th e sole source of retinoids, the flies formed and accumulated all-trans 3-hydroxyretinal in the dark. To examine the metabolic pathway to pro duce (3S)-3-hydroxyretinal in Drosophila, all-trans retinal was supple mented for two hours to carotenoid-depleted flies in the dark, and the subsequent changes in the composition of 3-hydroxyretinal enantiomers were analyzed using a chiral column on HPLC. The results indicated in itial formation of (3R)-3-hydroxyretinal followed by isomerization int o the 3S enantiomer. Tn another set of experiments,the membrane fracti on was obtained From the head homogenate of retinoid-depleted flies an d an in vitro assay of 3-hydroxyretinal formation from retinal was per formed. The 3-hydroxyretinal produced was the 3R enantiomer, supportin g the result obtained from the in vivo experiment whereby ( 3S)-3-hydr oxyretinal is produced from retinal via (3R)3-hydroxyretinal. Addition of NADPH enhanced 3-hydroxyretinal formation and the presence of carb on monoxide inhibited it, suggesting that hydroxylation at the C3 posi tion of retinal occurred via the monooxygenase activity of cytochrome P-450.