Stearns D M, Wetterhahn K E
Department of Chemistry, 6128 Burke Laboratory, Dartmouth College, Hanover, New Hampshire 03755-3564, USA.
Chem Res Toxicol. 1997 Mar;10(3):271-8. doi: 10.1021/tx9601521.
Ascorbate (vitamin C) is a biological reductant of the human carcinogen chromium(VI). The product of this reaction is presumed to be dehydroascorbate. However, we have found that chromium(VI) can also react with dehydroascorbate. This reaction was monitored by UV/ visible and electron paramagnetic resonance (EPR) spectroscopies. In sodium acetate buffer at pH 3.8, the reaction of chromium(VI) and excess dehydroascorbate produced chromium(V) and chromium(IV) intermediates. At high reaction concentration, the chromium(V) intermediate formed an EPR silent dimer, which dissociated upon dilution to lower concentration. UV/ visible experiments at pH 3.8 demonstrated that manganese(II) catalyzed the disproportionation of chromium(IV) to chromium(V) and chromium(III). The ability of the reaction intermediates to induce strand breaks in pBR322 DNA was determined at pH 3.8 and pH 5.8. At pH 3.8, chromium(IV) appeared to be the major species responsible for induction of strand breaks because the time course for formation of strand breaks did not parallel that of chromium(V), and strand breaks were decreased in the presence of the chromium(IV) scavenger manganese(II). At pH 5.8, fewer strand breaks were observed; however, the time course for their formation followed that of chromium(V). There has been much effort devoted to identification of the intermediate responsible for the induction of strand breaks during reactions of chromium(VI) with biological reductants. The current results suggest that it is not a single type of species that universally produces the DNA strand breaks observed in different chromium(VI) systems and that the reactivity of intermediates will depend on the chosen experimental conditions. Understanding this variability in chromium(VI) reactions may help to resolve the conflicting results from in vitro studies that are aimed at deciphering mechanisms of chromium(VI)-induced cancers.
抗坏血酸（维生素C）是人类致癌物六价铬的生物还原剂。该反应的产物推测为脱氢抗坏血酸。然而，我们发现六价铬也能与脱氢抗坏血酸发生反应。通过紫外/可见光谱和电子顺磁共振（EPR）光谱对该反应进行了监测。在pH 3.8的醋酸钠缓冲液中，六价铬与过量脱氢抗坏血酸的反应产生了五价铬和四价铬中间体。在高反应浓度下，五价铬中间体形成了一种EPR沉默二聚体，稀释至较低浓度时会解离。在pH 3.8下的紫外/可见实验表明，锰（II）催化了四价铬歧化为五价铬和三价铬。在pH 3.8和pH 5.8下测定了反应中间体诱导pBR322 DNA链断裂的能力。在pH 3.8时，四价铬似乎是导致链断裂的主要物质，因为链断裂形成的时间进程与五价铬的不同步，并且在存在四价铬清除剂锰（II）的情况下链断裂减少。在pH 5.8时，观察到的链断裂较少；然而，其形成的时间进程与五价铬的一致。人们致力于鉴定在六价铬与生物还原剂反应过程中负责诱导链断裂的中间体。目前的结果表明，并非单一类型的物质普遍导致在不同六价铬体系中观察到的DNA链断裂，并且中间体的反应性将取决于所选的实验条件。了解六价铬反应中的这种变异性可能有助于解决旨在破译六价铬诱导癌症机制的体外研究中相互矛盾的结果。
