Erdahl W L, Chapman C J, Wang E, Taylor R W, Pfeiffer D R
Department of Medical Biochemistry, College of Medicine, Ohio State University, Columbus 43210, USA.
Biochemistry. 1996 Oct 29;35(43):13817-25. doi: 10.1021/bi961391q.
The cation transport selectivities of the Ca2+ ionophores A23187, Ionomycin, and 4-BrA23187 have been determined using a model system comprised of phospholipid vesicles loaded with the chelator/indicator Quin-2. At pH 7.00 and a 100 microM concentration of the cations, A23187 displays the transport selectivity sequence Zn2+ > Mn2+ > Ca2+ > Co2+ > Ni2+ > Sr2+, with the absolute rates of transport spanning approximately 3 orders of magnitude. Similar data are obtained with Ionomycin, although the relative transport rates of Zn2+ and Mn2+ are equivalent, and the range of absolute rates is decreased by a factor of approximately 3. When values are normalized to those of Ca2+, transport selectivity is seen to be only weakly related to complexation or extraction selectivity. It is also seen that, when used to manipulate Ca2+ (or Mg2+), both ionophores can be expected to alter the distribution of additional divalent cations which have known biological activities. 4-BrA23187 is a low-activity ionophore for Ca2+, compared to A23187 and Ionomycin, while retaining comparable activities as an ionophore for the other cations. As a consequence, 4-BrA23187 is highly selective for the transport of Zn2+ and Mn2+, compared to Ca2+, with selectivity ratios approaching that of valinomycin for K+ over Na+ when conditions are optimal. Plots of the log of the rate of cation transport vs the log of the ionophore concentration indicate that Ca2+ is transported primarily as a 2:1 complex by A23187 and 4-BrA23187, but Zn2+ and Mn2+ are transported, in part, as 1:1 complexes. These findings, together with a postulated low stability of 2:1, compared to 1:1 complexes between 4-BrA23187 and divalent cations, partially explain the novel transport selectivity of this compound. Unlike A23187 or Ionomycin, 4-BrA23187 may be useful for investigating cell regulation by Zn2+ and Mn2+, without interference by regulatory mechanisms which respond to Ca2+.
利用由装载螯合剂/指示剂喹啉-2的磷脂囊泡组成的模型系统，已测定了钙离子载体A23187、离子霉素和4-溴A23187的阳离子转运选择性。在pH 7.00和阳离子浓度为100 microM时，A23187表现出的转运选择性顺序为Zn2+>Mn2+>Ca2+>Co2+>Ni2+>Sr2+，其绝对转运速率跨越约3个数量级。用离子霉素获得了类似的数据，尽管Zn2+和Mn2+的相对转运速率相当，且绝对速率范围缩小了约3倍。当将数值归一化为Ca2+的数值时，可以看出转运选择性与络合或萃取选择性仅存在微弱的关联。还可以看出，当用于操纵Ca2+（或Mg2+）时，预计这两种离子载体都会改变具有已知生物活性的其他二价阳离子的分布。与A23187和离子霉素相比，4-溴A23187是一种低活性的Ca2+离子载体，而作为其他阳离子的离子载体，其活性相当。因此，与Ca2+相比，4-溴A23187对Zn2+和Mn2+的转运具有高度选择性，在最佳条件下，其选择性比率接近缬氨霉素对K+相对于Na+的选择性比率。阳离子转运速率的对数与离子载体浓度的对数的关系图表明，Ca2+主要以2:1络合物的形式被A23187和4-溴A23187转运，但Zn2+和Mn2+部分以1:1络合物的形式被转运。这些发现，连同与4-溴A23187和二价阳离子之间的1:1络合物相比，推测2:1络合物的稳定性较低，部分解释了该化合物新颖的转运选择性。与A23187或离子霉素不同，4-溴A23187可能有助于研究Zn2+和Mn2+对细胞的调节作用，而不会受到对Ca2+作出反应的调节机制的干扰。
