Chang H W, Neumann E
Proc Natl Acad Sci U S A. 1976 Oct;73(10):3364-8. doi: 10.1073/pnas.73.10.3364.
Interaction of Ca and acetylcholine (AcCh) ions with purified acetylcholine receptor (AcChR) from Torpedo californica and Electrophorus electricus has been investigated in view of these ions' role proposed in bioelectricity. Spectrophotometric Ca titration using murexide as an indicator and an ultrafiltration method with 45Ca show that AcChR proteins have a high binding capacity for Ca ions. Per macromolecule of 260,000 daltons, up to 60 Ca ions can be bound with at least three Ca dissociation constants. A linear inhibition of AcCh binding to AcChR by Ca was observed in the 0.1-1 mM Ca range, indicating competition of AcCh and Ca for AcChR. The addition of AcCh to a Ca-AcChR solution at 1.2 mM Ca causes release of four to six bound Ca ions from AcChR when a maximum of two AcCh ions are bound per 260,000 dalton macromolecule. The subsequent addition of alpha-bungarotoxin causes reuptake of up to six Ca ions by AcChR. These results suggest that the neural activator AcCh and the inhibitor alpha-bungarotoxin induce opposing shifts between different conformational states of isolated AcChR.
鉴于钙离子和乙酰胆碱(AcCh)离子在生物电中所起的作用,研究了它们与来自加州电鳐和电鳗的纯化乙酰胆碱受体(AcChR)的相互作用。使用紫脲酸铵作为指示剂的分光光度法钙滴定以及采用⁴⁵Ca的超滤方法表明,AcChR蛋白对钙离子具有高结合能力。每260,000道尔顿的大分子,最多可结合60个钙离子,且至少有三个钙解离常数。在0.1 - 1 mM钙范围内观察到钙对AcCh与AcChR结合的线性抑制,表明AcCh和钙对AcChR存在竞争。当每260,000道尔顿大分子最多结合两个AcCh离子时,向1.2 mM钙的钙 - AcChR溶液中添加AcCh会导致四个至六个结合的钙离子从AcChR中释放。随后添加α - 银环蛇毒素会使AcChR重新摄取多达六个钙离子。这些结果表明,神经激活剂AcCh和抑制剂α - 银环蛇毒素在分离的AcChR的不同构象状态之间诱导相反的转变。