Molecular Mechanism of Photoreception in Cones.

目前的研究主要集中在视觉色素的分子性质和光化学反应方面，以阐明视杆细胞和视锥细胞之间的光响应差异是如何基于视觉色素来解释的。利用基因克隆技术，我们获得了4个鸡视锥视色素基因克隆和2个夜间壁虎视色素基因克隆。根据视觉色素氨基酸序列的差异，推测了影响视觉色素光谱调谐的氨基酸残基。在氨基酸同源性的基础上构建了脊椎动物视觉色素的系统发育树。它表明，祖先的视觉色素首先进化成四组锥状视觉色素，而视紫红质组，即杆状视觉色素，后来从四组中的一组中分化出来。因此，这表明动物首先获得了辨别颜色的能力，然后获得了暗视。用低温光谱分析和激光光解等方法研究了碘化钠的漂白过程。结果表明，在其漂白过程中，Iodopsin具有与视紫红质的光敏、Batho-、BL、Lumi、Meta I、Meta II类似的中间体。此外，与视紫红质II一样，变紫红质II也具有与转导蛋白结合的能力，这表明它具有类似于变紫红质II的生理作用。视紫红质与视紫红质视锥视色素、碘质、鸡绿和鸡蓝的比较研究清楚地表明，视锥视色素在11顺式视网膜和视黄素的再生速度以及生理活性中间体(Meta II-中间体)的形成和衰变方面明显快于视紫红质。这些差异分别与锥体比杆状突起更快的暗适应、更快的光反应和更低的光敏性密切相关。

The present studies focused on the molecular properties and photochemical reactions of visual pigments in order to elucidate how the difference in light response between rods and cones can be explained on the basis of visual pigments.1. Using a cDNA cloning technique, we have isolated four kinds of cDNA clones encoding chicken cone visual pigments and two clones encoding nocturnal gecko visual pigments. The amino acid residues responsible for the spectral tuning of the visual pigments were speculated on the basis of the differences in amino acid sequence among the visual pigments. A phylogenetic tree of vertebrate visual pigments was then constructed on the basis of amino acid identity. It indicated that an ancestral visual pigment evolved first into four groups of cone visual pigments and that group of rhodopsins, the rod visual pigments, diverged later from one of the four groups. Thus, it is suggested that animals had acquired first the ability to distinguish color and then acquired scotopic vision.2. We have investigated the bleaching process of iodopsin by means of low temperature spectroscopy and laser photolyses. The results showed that iodopsin has similar intermediates corresponding to photo-, batho-, BL, lumi, meta I-, and meta II-intermediates of rhodopsin in its bleaching prosess. Furthemore, like metarhodopsin II, metaiodopsin II has a binding ability to transducin, suggesting that it has a physiological role similar to metarhodopsin II.Comparative studies of cone visual pigments, iodopsin, chicken green and chicken blue with the rod visual pigment rhodopsin clearly showed that cone visual pigments are faster than rhodopsin in rate of regeneration from 11-cis-retinal and opsin, and those of formation and decay of physiologically active intermediate (meta II-intermediate). These difference closely correlate to faster dark-adaptation, faster light response and less photosensitivity of cones than rods, respectively.