Molecular Mechanism of Photoreception in Cones.

本文主要研究视色素的分子特性和光化学反应，以阐明视色素如何解释视杆细胞和视锥细胞的光响应差异。利用cDNA克隆技术，我们分离到了4个编码鸡锥视色素的cDNA克隆和2个编码夜行壁虎视色素的cDNA克隆。根据不同视色素氨基酸序列的差异，推测了视色素光谱调节的氨基酸残基。基于氨基酸特征，构建了脊椎动物视觉色素的系统发育树。这表明，一种祖先的视觉色素首先进化成四组锥状视觉色素，而这组视紫红质，即杆状视觉色素，后来从这四组中的一组中分化出来。因此，我们认为动物是先获得了辨别颜色的能力，然后才获得了暗视。采用低温光谱法和激光光解法研究了碘素的漂白过程。结果表明，碘视紫质在漂白过程中具有相似的光-、光-、光-、光-、间I-和间ii -中间体。此外，与后视紫红质II一样，后视紫红质II也具有与转导素的结合能力，提示其具有与后视紫红质II相似的生理作用。视锥视色素、碘视素、鸡绿、鸡蓝与杆状视色素视紫红质的对比研究表明，视锥视色素在11-顺式视网膜和视蛋白的再生速度以及生理活性中间体（后二中间体）的形成和衰变速度均快于视紫红质。这些差异分别与视锥细胞比杆状细胞更快地适应黑暗、更快地响应光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.