Fox K R, Waring M J
Biochemistry. 1984 Jun 5;23(12):2627-33. doi: 10.1021/bi00307a014.
The kinetics of association between the quinoxaline antitumor antibiotic echinomycin and DNA have been studied by stopped-flow methods. With natural DNAs, the reaction profile is completely described by a single exponential, the time constant for which varies linearly with the DNA concentration. This bimolecular rate constant is similar for both calf thymus and Micrococcus lysodeikticus DNA (k = 6 X 10(4) M-1 s-1 at 25 degrees C, I = 0.01) and is probably dominated by interaction with relatively weak but abundant binding sites from which the antibiotic dissociates fairly quickly. The observed single exponential suggests a molecular mechanism of binding in which both chromophores of the antibiotic become intercalated simultaneously rather than sequentially; no transition from a mono-intercalated state to a bis-intercalated state could be detected. The reaction is slowed by a factor of about 3 on raising the salt concentration from I = 0.01 to I = 0.5. Binding to poly(dA-dT) is also described by a single exponential, the time constant for which is about 3 times faster than that seen with natural DNAs. By contrast, the interaction with poly-(dG-dC) requires two exponentials for a proper description, the faster of which is similar to that seen with natural DNAs. This may reflect the initial interaction of the antibiotic with two types of sequences, tentatively identified as GpC and CpG, from which it dissociates at very different rates. The differences in kinetic behavior may be explicable on the basis of an alternating B structure for poly(dA-dT) and a more classical B form for poly(dG-dC).
采用停流法研究了喹喔啉类抗肿瘤抗生素棘霉素与DNA之间的缔合动力学。对于天然DNA，反应过程完全由单一指数描述，其时间常数随DNA浓度呈线性变化。小牛胸腺DNA和溶壁微球菌DNA的双分子速率常数相似（25℃，I = 0.01时，k = 6×10⁴ M⁻¹ s⁻¹），可能主要由与相对较弱但数量众多的结合位点相互作用主导，抗生素能较快地从这些位点解离。观察到的单一指数表明结合的分子机制是抗生素的两个发色团同时而非相继嵌入；未检测到从单嵌入状态到双嵌入状态的转变。将盐浓度从I = 0.01提高到I = 0.5时，反应速度减慢约3倍。与聚（dA - dT）的结合也由单一指数描述，其时间常数比天然DNA快约3倍。相比之下，与聚（dG - dC）的相互作用需要两个指数才能恰当描述，其中较快的一个与天然DNA的情况相似。这可能反映了抗生素与两种类型序列（暂定为GpC和CpG）的初始相互作用，抗生素从这两种序列解离的速率差异很大。基于聚（dA - dT）的交替B结构和聚（dG - dC）更经典的B形式，动力学行为的差异或许可以得到解释。
