Hao W, Fong H K
Department of Microbiology, University of Southern California School of Medicine, Los Angeles, USA.
Biochemistry. 1996 May 21;35(20):6251-6. doi: 10.1021/bi952420k.
The retinal pigment epithelium (RPE) contains an abundant opsin that is distinct from rhodopsin and cone visual pigments and is able to bind the retinaldehyde chromophore. The putative retinal G protein-coupled receptor (RGR) was isolated in digitonin solution from bovine RPE microsomes and copurified consistently with a minor 34-kDa protein. The absorption spectrum of RGR revealed endogenous pH-sensitive absorbance in the blue and near-ultraviolet regions of light. Membrane-bound RGR was incubated with exogenously added all-trans-retinal and formed two long-lived pH-dependent photopigments with absorption maxima of 469 +/- 2.4 and 370 +/- 7.3 nm. The effects of hydrogen ion concentration suggest that the blue and near-UV photopigments are tautomeric forms of RGR, in which an all-trans-retinal Schiff base is protonated or unprotonated, respectively. The RPE pigment was also demonstrable by its reactivity to hydroxylamine in the dark. The retinaldehyde-RGR conjugate at neutral pH favors the near-UV pigment and is a novel light-absorbing opsin in the vertebrate eye.
视网膜色素上皮（RPE）含有一种丰富的视蛋白，它不同于视紫红质和视锥视觉色素，并且能够结合视黄醛发色团。推测的视网膜G蛋白偶联受体（RGR）是从牛RPE微粒体的洋地黄皂苷溶液中分离出来的，并与一种34 kDa的小蛋白始终如一地共纯化。RGR的吸收光谱显示在蓝光和近紫外光区域存在内源性pH敏感吸光度。将膜结合的RGR与外源添加的全反式视黄醛一起孵育，形成了两种寿命较长的pH依赖性光色素，其吸收最大值分别为469±2.4和370±7.3 nm。氢离子浓度的影响表明，蓝光和近紫外光色素是RGR的互变异构形式，其中全反式视黄醛席夫碱分别被质子化或未被质子化。RPE色素在黑暗中对羟胺的反应性也得到了证实。中性pH下的视黄醛-RGR共轭物有利于近紫外光色素，是脊椎动物眼中一种新型的光吸收视蛋白。
