Hladky S B, Leung J C, Fitzgerald W J
Department of Pharmacology, University of Cambridge, England.
Biophys J. 1995 Nov;69(5):1758-72. doi: 10.1016/S0006-3495(95)80046-9.
Even though valinomycin has been employed and studied extensively for over 30 years, the attempts to explain its mechanism have not been entirely successful. The basic carrier model uses four rate constants that describe association of an ion and carrier, transfer of the complex across the membrane, dissociation of the complex, and transfer of the free carrier back across the membrane. If the basic model is correct all of these constants are independent of ion concentration. In previous work with rubidium the rate constants for transfer of free carrier, transfer of complexes, and dissociation were independent of the concentration, but the rate constant for association varied markedly. No satisfactory explanation for these observations was proposed. In this study current relaxations after charge pulses have been analyzed using digital data acquisition, a Bayesian algorithm, and inspection of linear plots of residuals. In agreement with previous results the relaxations for sufficiently high rubidium or potassium concentrations contain three exponential components, but the rate constants for association and dissociation decrease to similar extents as ion concentration increases. A simple extension of the carrier model to allow a more realistic description of association and dissociation is in good agreement with the rate constants fitted in the present study but not those for low ion concentrations found in previous work. At high ion concentrations the rate-limiting step in association appears to be a change in the conformation of the free carrier preceding the bimolecular association reaction. Transfer of neutral, free valinomycin between the surfaces is slower than the transfer of the charged ion-valinomycin complexes. Transfer of the complex may be hastened by deformation of the membrane, or transfer of the free carrier may be slowed by a need for conformation changes.
尽管缬氨霉素已被广泛应用和研究了30多年，但解释其作用机制的尝试尚未完全成功。基本的载体模型使用四个速率常数来描述离子与载体的结合、复合物跨膜转运、复合物的解离以及游离载体跨膜的反向转运。如果基本模型正确，所有这些常数都与离子浓度无关。在之前对铷的研究中，游离载体转运、复合物转运和解离的速率常数与浓度无关，但结合的速率常数变化显著。对于这些观察结果，没有提出令人满意的解释。在本研究中，使用数字数据采集、贝叶斯算法和残差线性图检查分析了电荷脉冲后的电流弛豫。与之前的结果一致，对于足够高的铷或钾浓度，弛豫包含三个指数成分，但随着离子浓度增加，结合和解离的速率常数下降程度相似。对载体模型进行简单扩展，以更真实地描述结合和解离，与本研究中拟合的速率常数相符，但与之前工作中低离子浓度下的速率常数不符。在高离子浓度下，结合的限速步骤似乎是双分子结合反应之前游离载体构象的变化。中性游离缬氨霉素在表面之间的转运比带电的离子 - 缬氨霉素复合物的转运慢。复合物的转运可能因膜的变形而加快，或者游离载体的转运可能因构象变化的需要而减慢。
