Timonin I M, Dvoryantsev S N, Petrov V V, Ruuge E K, Levitsky D O
USSR Cardiology Research Center, Academy of Medical Sciences, Moscow.
Biochim Biophys Acta. 1991 Jul 1;1066(1):43-53. doi: 10.1016/0005-2736(91)90248-7.
The analysis of the 23Na-NMR signal shape variations in the presence of vesicles of light sarcoplasmic reticulum (SR) shows the existence of sodium sites on the membranes with Kd values of about 10 mM. Other monovalent cations displace Na+ from SR fragments in a competitive manner according to the row K+ greater than Rb+ greater than Cs+ greater than Li+. Calcium ions also reduce Na+ binding, the Na+ desorption curve being of a two-stage nature, which, as suggested, indicates the existence of two types of Ca(2+)-sensitive Na+ binding sites (I and II). Sites of type I and II are modified by Ca2+ in submicromolar and millimolar concentrations, respectively. Analysis of sodium (calcium) desorption produced by calcium (sodium) allowed us to postulate the competition of these two cations for sites I and identity of these sites to high-affinity Ca(2+)-binding ones on the Ca(2+)-ATPase. Sites I weakly interact with Mg2+ (KappMg approximately 30 mM). Reciprocal effects of sodium and calcium on binding of each other to sites II cannot be described by a simple competition model, which indicates nonhomogeneity of these sites. A portion of sites I (approximately 70%) interacts with Mg2+ (KappMg = 3-4 mM). The pKa value of sites II is nearly 6.0. The number of sites II is three times greater than that of sites I. In addition, sites with intermediate affinity for Ca2+ were found with Kd values of 2-5 microM. These sites were revealed due to the reducing of the sites II affinity for Na+ upon Ca2+ binding to SR membranes. It can thus be concluded that in nonenergized SR there are binding sites for monovalent cations of at least three types: (1) sites I (which also bind Ca2+ at low concentrations), (2) magnesium-sensitive sites II and (3) magnesium-insensitive sites II.
对存在轻度肌浆网（SR）囊泡时23Na-NMR信号形状变化的分析表明，膜上存在钠位点，其解离常数（Kd）值约为10 mM。其他单价阳离子以竞争方式从SR片段中取代Na+，顺序为K+＞Rb+＞Cs+＞Li+。钙离子也会减少Na+的结合，Na+解吸曲线呈两阶段性质，这表明存在两种对Ca(2+)敏感的Na+结合位点（I和II）。I型和II型位点分别在亚微摩尔和毫摩尔浓度的Ca2+作用下发生改变。对钙（钠）产生的钠（钙）解吸分析使我们推测这两种阳离子在I型位点存在竞争，且这些位点与Ca(2+)-ATPase上的高亲和力Ca(2+)结合位点相同。I型位点与Mg2+的相互作用较弱（表观解离常数KappMg约为30 mM）。钠和钙对彼此与II型位点结合的相互影响不能用简单的竞争模型来描述，这表明这些位点具有非均质性。一部分I型位点（约70%）与Mg2+相互作用（KappMg = 3 - 4 mM）。II型位点的pKa值接近6.0。II型位点的数量比I型位点多三倍。此外，还发现了对Ca2+具有中等亲和力的位点，其Kd值为2 - 5 microM。这些位点是由于Ca2+与SR膜结合后II型位点对Na+的亲和力降低而被揭示出来的。因此可以得出结论，在无能量的SR中存在至少三种类型的单价阳离子结合位点：（1）I型位点（在低浓度时也结合Ca2+），（2）对镁敏感的II型位点和（3）对镁不敏感的II型位点。
