Creamer L K, Richardson T
Arch Biochem Biophys. 1984 Nov 1;234(2):476-86. doi: 10.1016/0003-9861(84)90295-9.
Electrophoresis in the presence of sodium dodecyl sulfate (SDS) provides a relatively simple means of determining molecular weights of proteins. This technique relies on the validity of a correlation between some function of Mr and the mobility of the protein through the gel matrix. However, bovine caseins (especially alpha s1-casein) have lower mobilities than expected on the basis of their known Mr. The binding of SDS to both alpha s1-casein (Mr 23,600) and beta-casein (Mr 24,000) reached a maximum at the slightly low value of 1.3 g SDS/g protein. Gel-filtration chromatography showed, however, that the alpha s1-casein:SDS complex was larger than the beta-casein:SDS complex at pH 6.8 or 7.0, but that they were similar in size at pH 2.9 or 3.0. Circular dichroism spectra indicated that the low helical structure content of both alpha s1- and beta-casein increased with the addition of SDS and/or decreasing the pH to 1.5. 13C NMR results showed that SDS bound to alpha s1- and beta-casein in the same way as it did to bovine serum albumin. Either esterification or dephosphorylation followed by amidation of alpha s1-casein increased its mobility in SDS-gel electrophoresis, but neither modification affected beta-casein mobility. These and other results indicate that the low electrophoretic velocity of alpha s1-casein in SDS-gel electrophoresis results from its unexpectedly large hydrodynamic size. This is caused by localized high negative charges on certain segments of alpha s1-casein, which would induce a considerable amount of inter- and intrasegmental electrostatic repulsion, leading to an expanded or extended structure for portions of the alpha s1-casein molecule in the presence of SDS. It is clear that the conformation, and hence the equivalent radius, of an SDS:protein complex is determined by the sequence of amino acids in the protein and that, a priori, it cannot be anticipated that the electrophoretic mobility of such a complex will bear more than a casual relationship to the Mr of the protein.
在十二烷基硫酸钠（SDS）存在的情况下进行电泳，为测定蛋白质分子量提供了一种相对简单的方法。该技术依赖于相对分子质量（Mr）的某种函数与蛋白质在凝胶基质中的迁移率之间相关性的有效性。然而，牛乳酪蛋白（尤其是αs1-酪蛋白）的迁移率比根据其已知Mr预期的要低。SDS与αs1-酪蛋白（Mr 23,600）和β-酪蛋白（Mr 24,000）的结合在1.3 g SDS/g蛋白质这个略低的值时达到最大值。然而，凝胶过滤色谱显示，在pH 6.8或7.0时，αs1-酪蛋白：SDS复合物比β-酪蛋白：SDS复合物大，但在pH 2.9或3.0时它们的大小相似。圆二色光谱表明，随着SDS的加入和/或将pH降低至1.5，αs1-和β-酪蛋白的低螺旋结构含量均增加。13C核磁共振结果表明，SDS与αs1-和β-酪蛋白的结合方式与它与牛血清白蛋白的结合方式相同。αs1-酪蛋白的酯化或去磷酸化后再酰胺化会增加其在SDS-凝胶电泳中的迁移率，但这两种修饰都不影响β-酪蛋白的迁移率。这些以及其他结果表明，αs1-酪蛋白在SDS-凝胶电泳中电泳速度较低是由于其出乎意料的大流体力学尺寸。这是由αs1-酪蛋白某些片段上局部的高负电荷引起的，这会诱导相当数量的片段间和片段内静电排斥，导致在SDS存在下αs1-酪蛋白分子的部分结构膨胀或伸展。很明显，SDS：蛋白质复合物的构象以及等效半径由蛋白质中的氨基酸序列决定，并且事先无法预料这样一个复合物的电泳迁移率与蛋白质的Mr之间会有超过偶然的关系。
