Epipodophyllotoxin derivatives, such as etoposide (VP-16), constitute an important class of anticancer agents, the major cytotoxic effects of which are associated with trapping of the topoisomerase II/DNA cleavable complex and formation of protein-DNA cross-links and nicked DNA. VP-16, however, can be metabolized to several highly reactive products, including an ortho -quinone (VPQ). The inhibitory activity of VPQ against purified human topoisomerase II processing of supercoiled DNA was studied and compared with that of the parent compound, VP-16. Our results show that VPQ is a powerful inhibitor of topoisomerase II, which prevents DNA strand passage in the presence of ATP. As with VP-16, trapping of the cleavable complex is highly reversible upon removal of divalent ions, which indicating that VPQ alters the cleavage-reunion equilibrium of topoisomerase II and DNA mainly by noncovalent interactions, as does the parent compound. However, we observed several differences between the effects induced by VP-16 and VPQ, including a strong inhibition of the second DNA strand religation, which implies the involvement of additional (asymmetric) mode(s) of interactions of the VPQ, possibly by interference with ATP binding by the homodimeric enzyme, and/or involving covalent interactions. Reduced or oxidized glutathione prevented trapping of the topoisomerase/DNA cleavable complex by VPQ, but not by VP-16, probably by forming covalent adducts with the former.
表
鬼臼毒素衍
生物(如
依托泊苷(V
P-16))是一类重要的抗癌剂,其主要细胞毒性作用与拓扑异构酶 II/DNA 可裂解复合物的捕获以及蛋白质-DNA 交联和缺口 DNA 的形成有关。然而,V
P-16 可代谢为几种高活性产物,包括一种邻醌(
VPQ)。我们研究了
VPQ 对纯化的人类拓扑异构酶 II 处理超螺旋 DNA 的抑制活性,并将其与母体化合物 V
P-16 的抑制活性进行了比较。我们的研究结果表明,
VPQ 是拓扑异构酶 II 的强力
抑制剂,它能在
ATP 存在的情况下阻止 DNA 链通过。与 V
P-16 一样,移除二价离子后,可裂解复合物的捕获是高度可逆的,这表明
VPQ 与母体化合物一样,主要通过非共价相互作用改变拓扑异构酶 II 和 DNA 的裂解-重合平衡。然而,我们观察到 V
P-16 和
VPQ 诱导的效应之间存在一些差异,包括对 DNA 第二链重新连接的强烈抑制,这意味着
VPQ 参与了额外(不对称)的相互作用模式,可能是通过干扰同源二聚体酶的
ATP 结合,和/或涉及共价相互作用。还原或
氧化的
谷胱甘肽能阻止
VPQ 捕捉拓扑异构酶/DNA 可裂解复合物,但 V
P-16 却不能,可能是通过与前者形成共价加合物。