Laitko U, Hofmann K P
Institut für Medizinische Physik und Biophysik, Medizinische Fakultät Charité der Humboldt-Universität zu Berlin, Germany.
Biophys J. 1998 Feb;74(2 Pt 1):803-15. doi: 10.1016/S0006-3495(98)74005-6.
We propose a model for the recovery of the retinal rod photoresponse after a short stimulus. The approach describes the enzymatic deactivation of the photoactivated receptor, rhodopsin, by simple enzyme kinetics. An important feature of this description is that the R* deactivation obeys different time laws, depending on the numbers of R* formed per disc membrane and available enzyme molecules. If the enzyme works below substrate saturation, the rate of deactivation depends linearly on the number of R*, whereas for substrate saturation a hyperbolic relation--the well-known Michaelis-Menten equation--applies. This dichotomy is used to explain experimental finding that the relation between the saturation time of the photoresponse after short illumination and the flash strength has two sharply separated branches for low and high flash intensities (up to approximately 10% bleaching). By relating both branches to properties of the enzymatic rhodopsin deactivation, the new model transcends the classical notion of a constant characteristic lifetime of activated rhodopsin. With parameters that are plausible in the light of the available data and the additional information that the deactivating enzyme, rhodopsin kinase, and the signaling G-protein, transducin, compete for the active receptor, the slopes of the saturation function are correctly reproduced.
我们提出了一个用于描述短刺激后视网膜视杆光反应恢复的模型。该方法通过简单的酶动力学描述了光激活受体视紫红质的酶促失活过程。此描述的一个重要特征是，视紫红质*（R*）的失活遵循不同的时间规律，这取决于每个盘膜形成的R数量以及可用的酶分子数量。如果酶在底物饱和浓度以下起作用，失活速率与R数量呈线性关系，而在底物饱和时，则适用双曲线关系——即著名的米氏方程。这种二分法用于解释实验发现：短时间光照后光反应的饱和时间与闪光强度之间的关系在低闪光强度和高闪光强度（高达约10%漂白）时具有两个明显分开的分支。通过将两个分支与视紫红质酶促失活的特性相关联，新模型超越了激活视紫红质具有恒定特征寿命的经典概念。根据现有数据以及失活酶视紫红质激酶和信号转导G蛋白转导素竞争活性受体的额外信息，该模型合理设置参数，正确再现了饱和函数的斜率。
