Loutas J, Stevens A, Howlett G J, Augusteyn R C
National Vision Research Institute of Australia, Carlton.
Exp Eye Res. 1996 Jun;62(6):613-20. doi: 10.1006/exer.1996.0072.
Several models have been proposed for arrangement of the subunits in alpha-crystallin. These include the contrasting proposals that subunits are arranged in three layers and that subunits assemble into micelle-like structures. The validity of the micelle model was investigated by examining the effects of variations in protein concentration on the surface tension, conductivity, molecular weight and conformation of alpha-crystallin. The data were compared with those obtained for bovine serum albumin (BSA) and sodium dodecyl sulphate (SDS). Measurements of surface tension were conducted in the range, 10 micrograms ml-1 to 130 mg ml-1, in low and high ionic strength buffers. An apparent point of inflection, independent of ionic strength, was seen in alpha-crystallin's surface tension at around 1.9 mg ml-1 (95 microM). The surface tension did not plateau beyond this point, as is the case with surfactants, but continued to decrease up to 130 mg ml-1. BSA exhibited similar surface tension properties with an apparent inflection at 0.9 mg ml-1 (13 microM). The conductivity of alpha-crystallin and BSA solutions increased smoothly with no sign of any transition up to 96 mg ml-1 and 60 mg ml-1, respectively. In contrast, SDS showed a clear transition in this property at the concentration corresponding to its CMC. The aggregation state of the alpha-crystallin aggregates was examined by comparing molecular masses and Stokes radii. The size of the protein remained uniform over a wide concentration range and was unaffected by variations in ionic strength. Protein conformation, which was monitored by examining the microenvironment of tryptophan residues, was also found to be independent of protein concentration. It is concluded that over the concentration range that was investigated, alpha-crystallin does not exhibit any of the properties associated with classical micelles formed from small amphiphilic molecules.
关于α-晶状体蛋白中亚基的排列，已经提出了几种模型。其中包括相互矛盾的观点，即亚基排列成三层，以及亚基组装成胶束样结构。通过研究蛋白质浓度变化对α-晶状体蛋白的表面张力、电导率、分子量和构象的影响，对胶束模型的有效性进行了研究。将这些数据与牛血清白蛋白（BSA）和十二烷基硫酸钠（SDS）的数据进行了比较。在低离子强度和高离子强度缓冲液中，在10微克/毫升至130毫克/毫升的范围内进行了表面张力测量。在α-晶状体蛋白的表面张力中，在约1.9毫克/毫升（95微摩尔）处出现了一个明显的拐点，与离子强度无关。与表面活性剂的情况不同，在此点之后表面张力并未达到平稳状态，而是持续下降直至130毫克/毫升。BSA表现出类似的表面张力特性，在0.9毫克/毫升（13微摩尔）处有明显的拐点。α-晶状体蛋白和BSA溶液的电导率分别在高达96毫克/毫升和60毫克/毫升时平稳增加，没有任何转变的迹象。相比之下，SDS在对应于其临界胶束浓度（CMC）的浓度下，此特性出现了明显的转变。通过比较分子量和斯托克斯半径，研究了α-晶状体蛋白聚集体的聚集状态。蛋白质的大小在很宽的浓度范围内保持一致，并且不受离子强度变化的影响。通过检查色氨酸残基的微环境来监测蛋白质构象，发现其也与蛋白质浓度无关。得出的结论是，在所研究的浓度范围内，α-晶状体蛋白不表现出与由小的两亲分子形成的经典胶束相关的任何特性。
