Kalinin E V, Orlova T G, Freidin A A
Institute of Biological Physics, U.S.S.R. Academy of Sciences, Pushchino, Moscow Region.
Biochim Biophys Acta. 1988 Jun 13;954(3):325-35. doi: 10.1016/0167-4838(88)90087-8.
Cyclic GMP-specific phosphodiesterase (3',5'-cyclic-nucleotide 5'-nucleotidohydrolase, EC 1.3.4.17) (PDE) is thought to be a key enzyme of the retinal-rod phototransduction cyclic nucleotide pathway. We attempted to investigate the properties and content of PDE in retinal-cone photoreceptors. The fractions obtained from cone-dominant ground squirrel retinas were analyzed for cone visual pigment content and PDE activity. The cone visual pigment content was estimated to be approx. 65 pmol per retina. The distribution of cone visual pigment coincided with that of the PDE activity through several steps of photoreceptor membrane purification by sucrose density gradient centrifugation. The ground squirrel retinal PDE was similar to the retinal-rod PDE by its kinetic properties, thermostability, sensitivity to tryptic activation, Stokes radius and pI values. The cone visual pigment enriched fractions contained the heat-stable trypsin-inactivated PDE inhibitor. Its functional properties seem to be similar to those of the retinal-rod PDE inhibitory subunit. The PDE content in ground squirrel retina was roughly estimated to be about five copies of enzyme per 100 cone visual pigment molecules. The obtained results indicated that the major portion of ground squirrel retinal cyclic GMP-specific PDE is the endogenous cone photoreceptor membrane enzyme and strongly supported the conception about the key role of PDE in cone phototransduction. The existence of essential differences between rod and cone systems rapidly returning cyclic GMP-specific amplification cascade components to the dark (or inactivated) states after photon absorption was suggested. If this suggestion is true, the well-known distinctions between response kinetics and light sensitivity of these two kinds of photoreceptor can be explained.
环磷酸鸟苷特异性磷酸二酯酶（3',5'-环核苷酸5'-核苷酸水解酶，EC 1.3.4.17）（PDE）被认为是视网膜视杆光转导环核苷酸途径的关键酶。我们试图研究视网膜视锥光感受器中PDE的性质和含量。对从以视锥细胞为主的地松鼠视网膜中获得的组分进行视锥视觉色素含量和PDE活性分析。估计每个视网膜的视锥视觉色素含量约为65皮摩尔。通过蔗糖密度梯度离心进行几步光感受器膜纯化后，视锥视觉色素的分布与PDE活性的分布一致。地松鼠视网膜PDE在动力学性质、热稳定性、对胰蛋白酶激活的敏感性、斯托克斯半径和pI值方面与视网膜视杆PDE相似。富含视锥视觉色素的组分含有热稳定的胰蛋白酶灭活的PDE抑制剂。其功能性质似乎与视网膜视杆PDE抑制亚基的相似。粗略估计地松鼠视网膜中的PDE含量约为每100个视锥视觉色素分子有五个酶分子。所得结果表明，地松鼠视网膜环磷酸鸟苷特异性PDE的主要部分是内源性视锥光感受器膜酶，并有力地支持了PDE在视锥光转导中起关键作用的概念。这表明在视杆和视锥系统之间存在本质差异，即在光子吸收后能迅速将环磷酸鸟苷特异性放大级联成分恢复到暗（或失活）状态。如果这一推测正确，那么这两种光感受器在反应动力学和光敏感性方面的众所周知的差异就可以得到解释。
