Priority Paper The metabolic pathway of visual pigment chromophore formation in Drosophila melanogaster

Abstract

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 remained constant and the content of 3‐hydroxyretinal in a head was three logarithmic units less than that of normal flies reared on medium containing yellow corn grits. When all‐ trans retinal was supplied as the sole source of retinoids, the flies formed and accumulated all‐ trans 3‐hydroxyretinal in the dark. To examine the metabolic pathway to produce (3 S )‐3‐hydroxyretinal in Drosophila , all‐ trans retinal was supplemented 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 initial formation of (3 R )‐3‐hydroxyretinal followed by isomerization into the 3 S enantiomer. In another set of experiments, the membrane fraction was obtained from the head homogenate of retinoid‐depleted flies and an in vitro assay of 3‐hydroxyretinal formation from retinal was performed. The 3‐hydroxyretinal produced was the 3 R enantiomer, supporting the result obtained from the in vivo experiment whereby (3 S )‐3‐hydroxyretinal is produced from retinal via (3 R )‐3‐hydroxyretinal. Addition of NADPH enhanced 3‐hydroxyretinal formation and the presence of carbon monoxide inhibited it, suggesting that hydroxylation at the C3 position of retinal occurred via the monooxygenase activity of cytochrome P ‐450.