Osses L, Condrescu M, DiPolo R
Biochim Biophys Acta. 1986 Sep 11;860(3):583-91. doi: 10.1016/0005-2736(86)90557-2.
Purified plasma membrane vesicles from the optic nerve of the squid Sepiotheutis sepioidea accumulate calcium in the presence of Mg2+ and ATP. Addition of the Ca2+ ionophore A23187 to vesicles which have reached a steady state of calcium-active uptake induces complete discharge of the accumulated cation. Kinetic analysis of the data indicates that the apparent Km for free Ca2+ and ATP are 0.2 muM and 21 muM, respectively. The average Vmax is 1 nmol Ca2+/min per mg protein at 25 degrees C. This active transport is inhibited by orthovanadate in the micromolar range. An Na+-Ca2+ exchange mechanism is also present in the squid optic nerve membrane. When an outwardly directed Na+ gradient is imposed on the vesicles, they accumulate calcium in the absence of Mg2+ and/or ATP. This ability to accumulate Ca2+ is absolutely dependent on the Na+ gradient: replacement of Na+ by K+, or passive dissipation of the Na+ gradient, abolishes transport activity. The apparent Km for Ca2+ of the Na+-Ca2+ exchange is more than 10-fold higher than that of the ATP-driven pump (app. Km=7.5 muM). While the apparent Km for Na+ is 74 mM, the Vmax of the exchanger is 27 nmol Ca2+/min per mg protein at 25 degrees C. These characteristics are comparable to those displayed by the uncoupled Ca pump and Na+-Ca2+ exchange previously described in dialyzed squid axons.
来自乌贼（Sepiotheutis sepioidea）视神经的纯化质膜囊泡在Mg2+和ATP存在的情况下积累钙。向已达到钙主动摄取稳态的囊泡中添加Ca2+离子载体A23187会导致积累的阳离子完全释放。对数据的动力学分析表明，游离Ca2+和ATP的表观Km分别为0.2 μM和21 μM。在25℃时，平均Vmax为每毫克蛋白质1 nmol Ca2+/分钟。这种主动运输在微摩尔范围内受到原钒酸盐的抑制。乌贼视神经膜中也存在Na+-Ca2+交换机制。当向囊泡施加外向的Na+梯度时，它们在没有Mg2+和/或ATP的情况下积累钙。这种积累Ca2+的能力绝对依赖于Na+梯度：用K+替代Na+或Na+梯度的被动消散会消除运输活性。Na+-Ca2+交换的Ca2+表观Km比ATP驱动泵的Ca2+表观Km高10倍以上（约Km = 7.5 μM）。虽然Na+的表观Km为74 mM，但在25℃时，交换器的Vmax为每毫克蛋白质27 nmol Ca2+/分钟。这些特性与先前在透析乌贼轴突中描述的未偶联Ca泵和Na+-Ca2+交换所显示的特性相当。
