Flannery J G, O'Day W, Pfeffer B A, Horwitz J, Bok D
Jules Stein Eye Institute UCLA School of Medicine 90024.
Exp Eye Res. 1990 Dec;51(6):717-28. doi: 10.1016/0014-4835(90)90057-2.
Upon absorption of a photon, the 11-cis retinaldehyde chromophore of rhodopsin is isomerized and reduced to all-trans retinol (vitamin A) in the photoreceptor outer segments, whereupon it leaves the photoreceptors, and moves to the retinal pigment epithelium (RPE). To clarify the function of the RPE in the regeneration of 11-cis retinaldehyde, we delivered all-trans retinol to monolayer cultures of human RPE. During delivery the retinol was associated with its putative natural carrier, interphotoreceptor retinoid binding protein (IRBP). IRBP has been proposed as a carrier protein involved in the exchange of retinoids between the photoreceptors and the retinal pigment epithelium. The retinoid composition of RPE cells and culture medium was analyzed by HPLC following several incubation periods. The RPE monolayer was found to process all-trans retinol into two distinct end-products: all-trans retinyl palmitate, which remained within the RPE monolayer: and 11-cis retinaldehyde which was released into the culture medium. These results demonstrate retinoid isomerase, retinol oxidoreductase and retinyl ester synthetase activity in human RPE cells cultured under the appropriate conditions. They show that IRBP can serve as a carrier of retinol through an aqueous medium to the RPE, and they illustrate that the visual cycle can be studied in vitro.
视紫红质的11-顺式视黄醛发色团吸收光子后，在光感受器外段异构化并还原为全反式视黄醇（维生素A），随后它离开光感受器，转移至视网膜色素上皮（RPE）。为阐明RPE在11-顺式视黄醛再生中的作用，我们将全反式视黄醇递送至人RPE单层培养物中。递送过程中，视黄醇与其假定的天然载体——光感受器间类视黄醇结合蛋白（IRBP）相结合。IRBP被认为是一种参与光感受器与视网膜色素上皮之间类视黄醇交换的载体蛋白。在几个孵育期后，通过高效液相色谱法分析RPE细胞和培养基的类视黄醇组成。发现RPE单层将全反式视黄醇加工成两种不同的终产物：全反式视黄醇棕榈酸酯，其保留在RPE单层内；以及11-顺式视黄醛，其释放到培养基中。这些结果证明了在适当条件下培养的人RPE细胞中存在类视黄醇异构酶、视黄醇氧化还原酶和视黄醇酯合成酶活性。它们表明IRBP可以作为视黄醇通过水性介质至RPE的载体，并且它们说明了可以在体外研究视觉循环。
