Vale M G, Carvalho A P
Biochim Biophys Acta. 1981 Apr 22;643(1):168-76. doi: 10.1016/0005-2736(81)90229-7.
The Ca2+ actively accumulated by sarcoplasmic reticulum isolated from skeletal muscle is composed of two fractions; one represented by intravesicular free Ca2+ and another represented by Ca2+ selectively bound to the membranes. Both of these Ca2+ fractions depend on ATP, although it is not clear whether ATP hydrolysis is essential for accumulation of the second Ca2+ fraction. The existence of the membrane-bound Ca2+ induced by ATP is clearly shown in experiments in which the Ca2+ retention by sarcoplasmic reticulum is measured in the presence and in the absence of X-537A, a Ca2+ ionophore, which makes the membrane permeable to Ca2+. Thus, in the presence of X-537A all Ca2+ accumulated due to ATP is bound to the membranes. This membrane-bound Ca2+ represents about 30 nmol/mg protein in the range of external pCa values of 7 to 3.5. The magnitude of this Ca2+ fraction is slightly higher whether or not the experiments are performed in the presence of oxalate, which greatly increased the intravesicular Ca2+ accumulation. Furthermore, taking advantage of the impermeability of sarcoplasmic reticulum to EGTA, it is possible to show the existence of the membrane-bound Ca2+ as a distinct fraction from that which exists intravesicularly.
从骨骼肌分离出的肌浆网主动积累的Ca2+由两部分组成；一部分以囊泡内游离Ca2+为代表，另一部分以选择性结合于膜上的Ca2+为代表。这两种Ca2+部分都依赖于ATP，尽管尚不清楚ATP水解对于第二种Ca2+部分的积累是否必不可少。ATP诱导的膜结合Ca2+的存在在实验中得到了明确证明，在这些实验中，在存在和不存在Ca2+离子载体X-537A的情况下测量肌浆网对Ca2+的保留，Ca2+离子载体使膜对Ca2+具有通透性。因此，在存在X-537A的情况下，由于ATP积累的所有Ca2+都与膜结合。在外部pCa值为7至3.5的范围内，这种膜结合Ca2+约为30 nmol/mg蛋白质。无论实验是否在草酸盐存在下进行，这一Ca2+部分的量都略高，草酸盐会大大增加囊泡内Ca2+的积累。此外，利用肌浆网对EGTA的不渗透性，可以证明膜结合Ca2+作为与囊泡内存在的Ca2+不同的部分而存在。
