Vale M G, Carvalho A P
Biochim Biophys Acta. 1975 Dec 1;413(2):202-12. doi: 10.1016/0005-2736(75)90104-2.
The Ca2+ ionophore X-537A is employed as a tool to distinguish between intravesicular Ca2+ and surface membrane-bound Ca2+ in sarcoplasmic reticulum isolated from rabbit skeletal muscle. When sarcoplasmic reticulum is incubated in 20 mM Ca2+ in the absence of ATP, 10-12 h are necessary for measurable amount of Ca2+ to penetrate into the vesicular space, as determined by the fact that X-537A releases Ca2+ from 'loaded' vesicles only after this period of incubation. A fraction of Ca2+ of 50-60 nmol/mg protein, rapidly taken up by sarcoplasmic reticulum, exchanges with Mg2+ and K+ in the medium and is readily released by ethyleneglycol-bis-(beta-aminoethyl ether)-N,N'-tetraacetic acid, but it is not released by X-537A. The slow-penetrating fraction of Ca2+ (30-40 nmol/mg protein) is rapidly released X-537A. The results indicate that most of the Ca2+ retained by sarcoplasmic reticulum under conditions of passive uptake is bound to the external side of the membrane. The fraction of Ca2+ that slowly penetrates the vesicles remains essentially free inside the vesicles and only a small part is bound to the internal side of the membrane.
钙离子载体X-537A被用作一种工具，以区分从兔骨骼肌分离的肌浆网中囊泡内的Ca2+和表面膜结合的Ca2+。当肌浆网在无ATP的情况下于20 mM Ca2+中孵育时，需要10 - 12小时才能使可测量量的Ca2+渗透到囊泡空间中，这是由以下事实确定的：只有在这段孵育期后，X-537A才会从“加载”的囊泡中释放Ca2+。肌浆网迅速摄取的一部分Ca2+（50 - 60 nmol/mg蛋白质）与培养基中的Mg2+和K+交换，并很容易被乙二醇双（β-氨基乙醚）-N,N'-四乙酸释放，但它不会被X-537A释放。Ca2+的缓慢渗透部分（30 - 40 nmol/mg蛋白质）会被X-537A迅速释放。结果表明，在被动摄取条件下肌浆网保留的大部分Ca2+与膜的外侧结合。缓慢渗透到囊泡中的Ca2+部分在囊泡内基本上保持游离状态，只有一小部分与膜的内侧结合。
