Abstract
Purpose: :
Recently a novel retinoid cycle has been identified in the cone–dominated chicken retina. This novel retinoid cycle causes accumulation of 11–cis retinyl esters in the retina upon light adaptation. The purpose of this study is to investigate the biochemical pathway by which all–trans retinol is converted to 11–cis retinol.
Methods: :
Chicken eyes were dark adapted for two hrs before the anterior segment, lens, and vitreous were removed under dim light. Tritium labeled all–trans retinol (0.5 ml, 1.2 uCi, 11.3 nmol, 233 DPM/pmol; in Avian buffer with 500 ug/ml of BSA) was added to the eye cup and incubated at 4°C for 30 min. in dim light. Labeled retinol was then removed and the eye cup rinsed with Avian buffer before eye cups were incubated in the light (2000 Lux from incandescent lights) or in the dark for 30 min with 0.5 ml of Avian buffer at 4°C. Retinoids were extracted from retina/RPE and analyzed by HPLC. DPM of collected fractions from HPLC were analyzed by liquid scintillation counting. Experiments were repeated twice with similar results.
Results: :
Upon light adaptation, 11–cis retinyl esters and all–trans retinyl esters increased from 1.4 to 2.0, n=4, and from 0.2 to 1.3, n=4, nmol, respectively. Similarly, 11–cis retinol and all–trans retinol also increased from 0.081 to 0.096, n=4, and from 0.179 to 0.266, n=4 nmol/mg, respectively in response to light. Only all–trans retinyl ester, 11–cis retinol and all–trans retinol were labeled. The specific radioactivity of all–trans retinyl ester decreased from 95 to 27 DPM/pmol upon light adaptation. The specific activity of 11–cis retinol and all–trans retinol also decreased (from 137 to 126, n=4, and from 228 to 150, n=4 nmol/pmol, respectively) in response to light.
Conclusions: :
Light induced accumulation of 11–cis and all–trans retinyl esters in the chicken eye. Consequently, specific activity of labeled retinyl esters decreased upon light exposure. Our data show that exogenously applied labeled retinol is converted to both 11–cis retinol and all–trans retinyl esters. Based on the significantly lower specific activity of all–trans retinyl ester in comparison to the specific activity of 11–cis retinol, the former cannot serve as the immediate precursor for 11–cis retinol in the chicken eye.
Keywords: photoreceptors • retina • retinoids/retinoid binding proteins