da Cruz Fresco P, Kortenkamp A
Department of Toxicology, School of Pharmacy, University of London, UK.
Carcinogenesis. 1994 Sep;15(9):1773-8. doi: 10.1093/carcin/15.9.1773.
A detailed study of the ability of chromate in combination with ascorbate to induce DNA single-strand breaks in the absence of iron(II) and copper(II) has been carried out. In solutions containing 1 mM ascorbate and chromate in the range 0.1-1 mM extensive DNA cleavage occurred. Chromate alone or the final product of the chromate/ascorbate reaction were not responsible for the cleavage observed. Evidence is presented that an intermediate generated during the reduction of chromate is the reactive species. No strand breaks occurred upon addition of catalase, pointing to a role for peroxidic species in the steps leading to the generation of the cleaving species. The exclusion of oxygen led to a substantial decrease in the number of strand breaks. Furthermore, the formation of strand breaks declined with decreasing concentrations of phosphate in the phosphate buffers used as the incubation medium. No DNA strand breaks were induced in medium containing HEPES. These observations rule out chromium(V) as the agent directly responsible for the DNA degradation, as chromium(V) is formed during the reduction of chromate by ascorbate in HEPES buffer. Our results lead us to suggest that the DNA-damaging ability of chromate upon reduction by ascorbate arises from the activation of oxygen exacerbated by phosphate and points to a peroxo or superoxo complex involving chromium(V) or chromium(IV) as a possible candidate.
已对铬酸盐与抗坏血酸盐在不存在亚铁离子（II）和铜离子（II）的情况下诱导DNA单链断裂的能力进行了详细研究。在含有1 mM抗坏血酸盐和浓度范围为0.1 - 1 mM铬酸盐的溶液中，发生了广泛的DNA切割。单独的铬酸盐或铬酸盐/抗坏血酸盐反应的最终产物都不是观察到的切割的原因。有证据表明，铬酸盐还原过程中产生的一种中间体是反应性物种。加入过氧化氢酶后未发生链断裂，这表明过氧化物物种在导致切割物种产生的步骤中起作用。排除氧气导致链断裂数量大幅减少。此外，随着用作孵育介质的磷酸盐缓冲液中磷酸盐浓度的降低，链断裂的形成也减少。在含有HEPES的培养基中未诱导出DNA链断裂。这些观察结果排除了铬（V）是直接导致DNA降解的试剂，因为在HEPES缓冲液中抗坏血酸盐还原铬酸盐的过程中会形成铬（V）。我们的结果使我们提出，抗坏血酸盐还原铬酸盐时其DNA损伤能力源于磷酸盐加剧的氧活化，并指出涉及铬（V）或铬（IV）的过氧或超氧络合物可能是候选物。
