Seoh S A, Busath D
Section of Physiology, Brown University, Providence, Rhode Island 02912.
Biophys J. 1993 Apr;64(4):1017-28. doi: 10.1016/S0006-3495(93)81467-X.
The conductance properties of organic cations in single gramicidin A channels were studied using planar lipid bilayers. From measurements at 10 mM and at 27 mV the overall selectivity sequence was found to be NH4+ > K+ > hydrazinium > formamidinium > Na+ > methylammonium, which corresponds to Eisenman polyatomic cation sequence X'. Methylammonium and formamidinium exhibit self block, suggesting multiple occupancy and single filing. Formamidinium has an apparent dissociation constant (which is similar to those of alkali metal cations) for the first ion being 22 mM from the Eadie-Hofstee plot (G0 vs. G0/C), 12 mM from the rate constants of a three-step kinetic model. The rate-limiting step for formamidinium is translocation judging from supralinear I-V relations at low concentrations. 1 M formamidinium solutions yields exceptionally long single channel lifetimes, 20-fold longer than methylammonium, which yields lifetimes similar to those found with alkali metal cations. The average lifetime in formamidinium solution significantly decreases with increasing voltage up to 100 mV but is relatively voltage independent between 100 and 200 mV. At lower voltages (< or = 100 mV), the temperature and concentration dependences of the average lifetime of formamidinium were steep. At very low salt concentrations (0.01 M, 100 mV), there was no significant difference in average lifetime from that formed with 0.01 M methylammonium or hydrazinium. We conclude that formamidinium very effectively stabilizes the dimeric channel while inside the channel and speculate that it does so by affecting tryptophan-reorientation or tryptophan-lipid interactions at binding sites.
利用平面脂质双层研究了单个短杆菌肽A通道中有机阳离子的电导特性。通过在10 mM和27 mV下的测量，发现总体选择性顺序为NH4+ > K+ > 肼鎓 > 甲脒鎓 > Na+ > 甲铵，这与艾森曼多原子阳离子序列X'相对应。甲铵和甲脒鎓表现出自阻断，表明存在多重占据和单通道填充。根据伊迪-霍夫斯泰因图（G0对G0/C），甲脒鎓的第一个离子的表观解离常数（与碱金属阳离子的相似）为22 mM，根据三步动力学模型的速率常数为12 mM。从低浓度下的超线性I-V关系判断，甲脒鎓的限速步骤是转运。1 M甲脒鎓溶液产生异常长的单通道寿命，比甲铵长20倍，甲铵产生的寿命与碱金属阳离子相似。在高达100 mV的电压下，甲脒鎓溶液中的平均寿命随电压升高而显著降低，但在100至200 mV之间相对与电压无关。在较低电压（≤100 mV）下，甲脒鎓平均寿命的温度和浓度依赖性很强。在非常低的盐浓度（0.01 M，100 mV）下，平均寿命与0.01 M甲铵或肼鎓形成的平均寿命没有显著差异。我们得出结论，甲脒鎓在通道内部能非常有效地稳定二聚体通道，并推测它是通过影响结合位点处色氨酸的重新取向或色氨酸-脂质相互作用来实现的。
