Odde D
Department of Chemical Engineering, Michigan Technological University, Houghton 49931, USA.
Eur Biophys J. 1998;27(5):514-20. doi: 10.1007/s002490050161.
Recent high-resolution analysis of tubulin's structure has led to the prediction that the taxol binding site and a tubulin acetylation site are on the interior of microtubules, suggesting that diffusion inside microtubules is potentially a biologically and clinically important process. To assess the rates of transport inside microtubules, predictions of diffusion time scales and concentration profiles were made using a model for diffusion with parameters estimated from experiments reported in the literature. Three specific cases were considered: 1) diffusion of alpha beta-tubulin dimer, 2) diffusion/binding of taxol, and 3) diffusion/binding of an antibody specific for an epitope on the microtubule's interior surface. In the first case tubulin is predicted to require only approximately 1 min to reach half the equilibrium concentration in the center of a 40 microns microtubule open at both ends. This relatively rapid transport occurs because of a lack of appreciable affinity between tubulin and the microtubule inner surface and occurs in spite of a three-fold reduction in diffusivity due to hindrance. By contrast the transport of taxol is much slower, requiring days (at nM concentrations) to reach half the equilibrium concentration in the center of a 40 microns microtubule having both ends open. This slow transport is the result of fast, reversible taxol binding to the microtubule's interior surface and the large capacity for taxol (approximately 12 mM based on interior volume of the microtubule). An antibody directed toward an epitope in the microtubule's interior is predicted to require years to approach equilibrium. These results are difficult to reconcile with previous experimental results where substantial taxol and antibody binding is achieved in minutes, suggesting that these binding sites are on the microtubule exterior. The slow transport rates also suggest that microtubules might be able to serve as vehicles for controlled-release of drugs.
最近对微管蛋白结构的高分辨率分析预测，紫杉醇结合位点和微管蛋白乙酰化位点位于微管内部，这表明微管内的扩散可能是一个生物学和临床上重要的过程。为了评估微管内的运输速率，使用一个扩散模型并根据文献报道的实验估计参数，对扩散时间尺度和浓度分布进行了预测。考虑了三种具体情况：1）αβ-微管蛋白二聚体的扩散，2）紫杉醇的扩散/结合，3）针对微管内表面表位的抗体的扩散/结合。在第一种情况下，预测微管蛋白在两端开口的40微米微管中心达到平衡浓度的一半仅需约1分钟。这种相对快速的运输是由于微管蛋白与微管内表面之间缺乏明显的亲和力，并且尽管由于阻碍扩散系数降低了三倍，但运输仍会发生。相比之下，紫杉醇的运输要慢得多，在两端开口的40微米微管中心达到平衡浓度的一半需要数天（在纳摩尔浓度下）。这种缓慢的运输是由于紫杉醇与微管内表面快速、可逆的结合以及紫杉醇的大容量（基于微管内部体积约为12 mM）。针对微管内部表位的抗体预计需要数年才能接近平衡。这些结果很难与之前的实验结果相协调，在之前的实验中，数分钟内就能实现大量的紫杉醇和抗体结合，这表明这些结合位点在微管外部。缓慢的运输速率还表明微管可能能够作为药物控释的载体。
