Burns R G, Islam K
Eur J Biochem. 1981 Jul;117(3):515-9. doi: 10.1111/j.1432-1033.1981.tb06367.x.
Microtubules reassembled in vitro from chick brain contain significant nucleosidediphosphate (NDP) kinase activity (EC 2.7.4.6) although the specific activity decreases with successive cycles of reassembly. However, while the recovery of microtubule protein, as a function of initial protein concentration, exhibits a critical concentration below which there is no polymerisation, the recovery of NDP kinase activity is, at low initial protein concentrations, directly proportional to the initial protein content indicating that microtubule protein and the kinase activity are independently recovered. This was confirmed by pelleting the reassembled microtubules through a sucrose cushion: the specific activity of the pelleted microtubules was reduced by approximately 90%. By contrast, when cold-dissociated microtubule protein, which is predominantly in the form of rings, is fractionated on a Biogel A 15 m column the microtubule protein and NDP kinase activity coeluted in the void volume and the specific activity remained constant throughout the ring fraction. Similarly, when microtubules were dissociated in the presence of NDP kinase the enzyme bound to the generated rings. These results suggest that NDP kinase binds preferentially to the rings compared with the microtubules, and a model is proposed to account for the presence of this enzyme in pellets of microtubule protein.
从鸡脑中体外重新组装的微管含有显著的核苷二磷酸（NDP）激酶活性（EC 2.7.4.6），尽管比活性会随着连续的重新组装循环而降低。然而，虽然微管蛋白的回收率作为初始蛋白浓度的函数呈现出一个临界浓度，低于该浓度则不会发生聚合，但在低初始蛋白浓度下，NDP激酶活性的回收率与初始蛋白含量成正比，这表明微管蛋白和激酶活性是独立恢复的。通过蔗糖垫层对重新组装的微管进行沉淀证实了这一点：沉淀后的微管的比活性降低了约90%。相比之下，当主要呈环形式的冷解离微管蛋白在Biogel A 15 m柱上进行分级分离时，微管蛋白和NDP激酶活性在空体积中共洗脱，并且在整个环级分中比活性保持恒定。同样，当微管在NDP激酶存在下解离时，该酶会与生成的环结合。这些结果表明，与微管相比，NDP激酶优先与环结合，并提出了一个模型来解释该酶在微管蛋白沉淀中的存在。
