Hastie S B, Williams R C, Puett D, Macdonald T L
Department of Chemistry, State University of New York, Binghamton 13901.
J Biol Chem. 1989 Apr 25;264(12):6682-8.
Isocolchicine is a structurally related isomer of colchicine altered in the methoxytropone C ring. In spite of virtual structural homology of colchicine and isocolchicine, isocolchicine is commonly believed to be inactive in binding to tubulin and inhibiting microtubule assembly. We have found that isocolchicine does indeed bind to the colchicine site on tubulin, as demonstrated by its ability to competitively inhibit [3H]colchicine binding to tubulin with a KI approximately 400 microM. Isocolchicine inhibits tubulin assembly into microtubules with an I50 of about 1 mM, but the affinity of isocolchicine for the colchicine receptor site, 5.5 +/- 0.9 x 10(3) M-1 at 23 degrees C, is much less (approximately 500-fold) than that of colchicine. Unlike colchicine, isocolchicine binds rapidly, and the absorption and fluorescence properties of the complex are only modestly altered compared to free ligand. It is proposed that the binding of isocolchicine to tubulin may be rationalized either in terms of conformational states of colchicinoids when liganded to tubulin or by the structural requirements for C-10 substituents for high affinity binding to the colchicine receptor.
异秋水仙碱是秋水仙碱的一种结构相关异构体，其甲氧基色酮C环发生了改变。尽管秋水仙碱和异秋水仙碱在结构上几乎同源，但人们普遍认为异秋水仙碱在与微管蛋白结合及抑制微管组装方面无活性。我们发现，异秋水仙碱确实能与微管蛋白上的秋水仙碱位点结合，这可通过其以约400微摩尔的抑制常数（KI）竞争性抑制[3H]秋水仙碱与微管蛋白结合得以证明。异秋水仙碱以约1毫摩尔的半数抑制浓度（I50）抑制微管蛋白组装成微管，但其在23摄氏度时对秋水仙碱受体位点的亲和力为5.5±0.9×10³ M⁻¹，比秋水仙碱低得多（约500倍）。与秋水仙碱不同，异秋水仙碱结合迅速，且与游离配体相比，复合物的吸收和荧光特性仅略有改变。有人提出，异秋水仙碱与微管蛋白的结合可以根据配体结合到微管蛋白上时秋水仙碱类化合物的构象状态，或者根据C - 10取代基与秋水仙碱受体高亲和力结合的结构要求来解释。
