Kefalov V J, Carter Cornwall M, Crouch R K
Department of Physiology, Boston University School of Medicine, Boston, Massachusetts 02118, USA.
J Gen Physiol. 1999 Mar;113(3):491-503. doi: 10.1085/jgp.113.3.491.
The retinal analogue beta-ionone was used to investigate possible physiological effects of the noncovalent interaction between rod opsin and its chromophore 11-cis retinal. Isolated salamander rod photoreceptors were exposed to bright light that bleached a significant fraction of their pigment, were allowed to recover to a steady state, and then were exposed to beta-ionone. Our experiments show that in bleach-adapted rods beta-ionone causes a decrease in light sensitivity and dark current and an acceleration of the dim flash photoresponse and the rate constants of guanylyl cyclase and cGMP phosphodiesterase. Together, these observations indicate that in bleach-adapted rods beta-ionone activates phototransduction in the dark. Control experiments showed no effect of beta-ionone in either fully dark-adapted or background light-adapted cells, indicating direct interaction of beta-ionone with the free opsin produced by bleaching. We speculate that beta-ionone binds specifically in the chromophore pocket of opsin to produce a complex that is more catalytically potent than free opsin alone. We hypothesize that a similar reaction may occur in the intact retina during pigment regeneration. We propose a model of rod pigment regeneration in which binding of 11-cis retinal to opsin leads to activation of the complex accompanied by a decrease in light sensitivity. The subsequent covalent attachment of retinal to opsin completely inactivates opsin and leads to the recovery of sensitivity. Our findings resolve the conflict between biochemical and physiological data concerning the effect of the occupancy of the chromophore binding site on the catalytic potency of opsin. We show that binding of beta-ionone to rod opsin produces effects opposite to its previously described effects on cone opsin. We propose that this distinction is due to a fundamental difference in the interaction of rod and cone opsins with retinal, which may have implications for the different physiology of the two types of photoreceptors.
视黄醛类似物β-紫罗兰酮被用于研究视杆视蛋白与其发色团11-顺式视黄醛之间非共价相互作用可能产生的生理效应。将分离的蝾螈视杆光感受器暴露于强光下，使相当一部分色素被漂白，然后让其恢复到稳态，接着再暴露于β-紫罗兰酮中。我们的实验表明，在漂白适应的视杆细胞中，β-紫罗兰酮会导致光敏感度和暗电流降低，以及对弱闪光光反应、鸟苷酸环化酶和cGMP磷酸二酯酶的速率常数加速。这些观察结果共同表明，在漂白适应的视杆细胞中，β-紫罗兰酮在黑暗中激活了光转导。对照实验表明，β-紫罗兰酮对完全暗适应或背景光适应的细胞均无影响，这表明β-紫罗兰酮与漂白产生的游离视蛋白直接相互作用。我们推测，β-紫罗兰酮特异性地结合在视蛋白的发色团口袋中，形成一种比游离视蛋白本身更具催化活性的复合物。我们假设在色素再生过程中，完整视网膜中可能会发生类似反应。我们提出了一个视杆色素再生模型，其中11-顺式视黄醛与视蛋白的结合导致复合物激活，同时光敏感度降低。随后视黄醛与视蛋白的共价连接使视蛋白完全失活，并导致敏感度恢复。我们的研究结果解决了关于发色团结合位点占据对视蛋白催化活性影响的生化和生理数据之间的冲突。我们表明，β-紫罗兰酮与视杆视蛋白的结合产生的效应与其先前描述的对视锥视蛋白的效应相反。我们认为这种差异是由于视杆和视锥视蛋白与视黄醛相互作用的根本差异所致，这可能对两种光感受器的不同生理功能具有影响。
