Catterall W A
J Biol Chem. 1975 Jun 10;250(11):4053-9.
The activation of the action potential Na+ ionophore by veratridine and batrachotoxin is time- and concentration-dependent and completely reversible. Batrachotoxin acts more slowly than veratridine. The concentration dependence of activation at equilibrium suggests reversible interaction of each toxin with a single class of independent sites having dissociation constants at physiologic ion concentrations of 80 plus or minus 13 muM for veratridine and 0.4 plus or minus muM for batrachotoxin. The maximum velocity of Na+ uptake at 50 mM Na+ is 128 plus or minus 12 nmol/min/mg in the presence of batrachotoxin compared to 48 plus or minus 4 nmol/min/mg in the presence of veratridine. Treatment of cells with excess veratridine in addition to batrachotoxin inhibits batrachotoxin-dependent 22-Na+ uptake. The concentration dependence of this inhibition suggests that it reflects competitive displacement of batrachotoxin from its binding site by veratridine. The activation by veratridine and batrachotoxin is inhibited in a competitive manner by divalent cations. The inhibition by divalent cations exhibits significant ion specificity with Mn-2+ greater than Co-2+ greater than Ni-2+ greater than Ca-2+ greater than Mg-2+ greater than Sr-2+. The inhibition constants (KI) for Ca-2+ are 0.84 mM for veratridine-dependent 22-Na+ uptake and 1.2 mM for batrachotoxin-dependent 22-Na+ uptake. The activation by veratridine and batrachotoxin is inhibited in a noncompetitive manner by tetrodotoxin. The apparent KD for tetrodotoxin as 11 plus or minus 1 nM in the presence of 150 mM Na+ and approximately 8.5 nM in 50 mM Na+. Divalent cations do not affect the apparent KD for tetrodotoxin. A hypothesis is presented which suggests that batrachotoxin, veratridine, and divalent cations interact with an activation site associated with the action potential Na+ ionophore, whereas tetrodotoxin interacts with a physically and functionally independent site involved in the transport of monovalent cations by the ionophore.
藜芦碱和蟾毒素对动作电位钠离子载体的激活具有时间和浓度依赖性，且完全可逆。蟾毒素的作用比藜芦碱慢。平衡时激活的浓度依赖性表明，每种毒素与一类独立位点发生可逆相互作用，在生理离子浓度下，藜芦碱的解离常数为80±13 μM，蟾毒素的解离常数为0.4± μM。在蟾毒素存在下，50 mM Na+时Na+摄取的最大速度为128±12 nmol/分钟/毫克，而在藜芦碱存在下为48±4 nmol/分钟/毫克。除蟾毒素外，用过量藜芦碱处理细胞会抑制蟾毒素依赖性的22-Na+摄取。这种抑制的浓度依赖性表明，它反映了藜芦碱从其结合位点竞争性取代蟾毒素。藜芦碱和蟾毒素的激活被二价阳离子以竞争性方式抑制。二价阳离子的抑制表现出显著的离子特异性，Mn2+＞Co2+＞Ni2+＞Ca2+＞Mg2+＞Sr2+。Ca2+对藜芦碱依赖性22-Na+摄取的抑制常数（KI）为0.84 mM，对蟾毒素依赖性22-Na+摄取的抑制常数为1.2 mM。藜芦碱和蟾毒素的激活被河豚毒素以非竞争性方式抑制。在150 mM Na+存在下，河豚毒素的表观KD为11±1 nM，在50 mM Na+中约为8.5 nM。二价阳离子不影响河豚毒素的表观KD。本文提出了一个假说，即蟾毒素、藜芦碱和二价阳离子与动作电位钠离子载体相关的激活位点相互作用，而河豚毒素与离子载体转运单价阳离子所涉及的物理和功能上独立的位点相互作用。
