Tachibanaki Shuji, Shimauchi-Matsukawa Yoshie, Arinobu Daisuke, Kawamura Satoru
Graduate School of Frontier Biosciences, Osaka University, Suita, Japan.
Photochem Photobiol. 2007 Jan-Feb;83(1):19-26. doi: 10.1562/2006-02-28-IR-823.
In the vertebrate retina, rods mediate twilight vision and cones mediate daylight vision. Their photoresponse characteristics are different. The light-sensitivity of a cone is 10(2)-10(3) times lower than that of a rod. In addition, the photoresponse time course is much faster in cones. The mechanism characterizing cone photoresponses has not been known mainly because of the difficulty in isolating cones in large quantities to perform biochemistry. Recently, we developed a method to purify cones from carp retina using a density gradient, which made it possible to analyze the differences in the molecular mechanism of phototransduction between rods and cones. The results showed that signal amplification in cones is less effective, which explains the lower light-sensitivity of cones. The results also showed that visual pigment phosphorylation, a quenching mechanism of light-activated visual pigment, is much more rapid in cones than in rods. The rapid phosphorylation in cones is attributed to a very high total kinase activity in cones. Because of this high activity, cone pigment is readily phosphorylated even at very high bleaching levels, which probably explains why cone photoresponses recover quickly. Based on these findings, the molecular mechanisms of the differences in the photoresponse characteristics between rods and cones are outlined.
在脊椎动物视网膜中，视杆细胞介导微光视觉，视锥细胞介导昼光视觉。它们的光反应特性不同。视锥细胞的光敏感度比视杆细胞低10² - 10³倍。此外，视锥细胞的光反应时间进程要快得多。视锥细胞光反应的特征机制一直不为人所知，主要是因为难以大量分离视锥细胞来进行生物化学研究。最近，我们开发了一种利用密度梯度从鲤鱼视网膜中纯化视锥细胞的方法，这使得分析视杆细胞和视锥细胞光转导分子机制的差异成为可能。结果表明，视锥细胞中的信号放大效果较差，这解释了视锥细胞较低的光敏感度。结果还表明，视觉色素磷酸化，即光激活视觉色素的淬灭机制，在视锥细胞中比在视杆细胞中要快得多。视锥细胞中的快速磷酸化归因于视锥细胞中非常高的总激酶活性。由于这种高活性，即使在非常高的漂白水平下，视锥色素也很容易被磷酸化，这可能解释了视锥细胞光反应快速恢复的原因。基于这些发现，概述了视杆细胞和视锥细胞光反应特性差异的分子机制。
