Käser-Glanzmann R, Jakábová M, George J N, Lüscher E F
Biochim Biophys Acta. 1978 Sep 11;512(1):1-12. doi: 10.1016/0005-2736(78)90213-4.
Human blood platelets are capable of removing Ca2+ from the cytoplasm by means of an active, ATP-dependent and cyclic AMP-stimulated transport system. Calcium-accumulating vesicles are obtained by sonicating platelets. On density gradient centrifugation, this activity is found in the heavier of two membrane fractions. Concentrated in this fraction are also the Ca2+-stimulated Mg2+-ATPase and glucose-6-phosphatase, believed to be a marker for internal membrane systems. When the isolated vesicles are loaded with Ca2+, a third band separates from the two vesicular fractions in the density gradient. This band C contains virtually all the Ca2+-accumulating activity. Evidence that this activity is due to an active uptake and not to surface binding or adsorption is presented. Whereas electron microscopy does not reveal striking differences between active and inactive fractions, differences in protein composition are revealed by sodium dodecyl sulphate-polyacrylamide gel electrophoresis. Furthermore, this band contains an enzyme system which converts arachidonic acid to malondialdehyde and therefore this fraction must be the site of prostaglandin synthesis. Membranes prepared by loading platelets with glycerol, followed by osmotic lysis are unable to accumulate calcium. In sodium dodecyl sulphate-polyacrylamide gel electrophoresis such membranes show significant differences in their protein pattern as compared to the actively Ca2+-accumulating vesicular membranes of band C. All preparations with Ca2+-accumulating activity also contain markers for plasma membranes and the question whether this activity is due exclusively to an intracellular structural element equivalent to the sarcoplasmic reticulum of muscle or whether an "extrusion pump" expelling Ca2+ to the outside of the cell is also involved, cannot yet be ;nswered.
人血血小板能够通过一种活跃的、依赖ATP且受环磷酸腺苷刺激的转运系统从细胞质中去除Ca2+。通过对血小板进行超声处理可获得积累钙的囊泡。在密度梯度离心时，这种活性存在于两个膜组分中较重的那个组分里。Ca2+刺激的Mg2+-ATP酶和葡萄糖-6-磷酸酶也集中在该组分中，它们被认为是内膜系统的标志物。当分离出的囊泡装载上Ca2+后，在密度梯度中会从两个囊泡组分中分离出第三条带。这条带C几乎包含了所有的Ca2+积累活性。文中给出了证据表明这种活性是由于主动摄取而非表面结合或吸附所致。虽然电子显微镜未揭示活性组分和无活性组分之间有明显差异，但十二烷基硫酸钠-聚丙烯酰胺凝胶电泳显示出蛋白质组成上的差异。此外，这条带含有一个能将花生四烯酸转化为丙二醛的酶系统，因此该组分必定是前列腺素合成的场所。通过用甘油装载血小板然后进行渗透裂解制备的膜无法积累钙。在十二烷基硫酸钠-聚丙烯酰胺凝胶电泳中，与带C中具有活跃Ca2+积累活性的囊泡膜相比，这类膜在蛋白质图谱上显示出显著差异。所有具有Ca2+积累活性的制剂也都含有质膜的标志物，而这种活性是否仅归因于一种等同于肌肉肌浆网的细胞内结构元件，或者是否还涉及一个将Ca2+排出细胞外的“外排泵”，目前尚无法回答。
