Yelena Margolin

Graduate Student

Guanine, having the lowest ionization potential of all four DNA bases, is the major target of oxidation in the DNA, and reacts with a variety of oxidants to form primary and secondary oxidation products.  Many of these products have been shown to be mutagenic, producing G⇒T and G⇒C transversions, possibly leading to cancer.

Not all guanines within double-stranded DNA are equally susceptible to oxidation.  It has long been established that a variety of oxidizing agents, such as anthraquinones, ethidium, riboflavin-mediated photooxidation, etc., are selective for 5’ guanines within GG sequences and middle guanines within GGG sequences.  Theoretical calculations of sequence-specific ionization potentials of guanines revealed that 5’-GG-3’ and 5’-GGG-3’ sites have the lowest ionization potentials of all guanine sequence contexts.  During the process of charge transfer, which results from one-electron oxidation of double-stranded DNA, the electron hole created in the initial oxidation step is capable of migrating to these lowest ionization potential sites, where it becomes trapped, ultimately resulting in a damage hotspot.

We focus our attention on biological oxidants that are produced during the process of chronic inflammation.  Facile recombination of macrophage-produced nitric oxide with superoxide, and then with carbon dioxide, yields nitrosoperoxycarbonate, which in turn, decomposes to give nitrogen dioxide and carbonate radical anion.  The latter is a very strong oxidant that produces site-selective guanine oxidations in double-stranded DNA.

Using short, double-stranded oligonucleotides I have demostrated that nitrosoperoxycarbonate, unlike previously studied oxidants,  is not selective for 5’-GG-3’ and 5’-GGG-3’ runs, but instead oxidizes 5’-AGC-3’, 5’-TGC-3’, and 5’-CGC-3’, sequences that have the highest calculated ionization potentials of all guanine sequence contexts.  We are currently working to elucidate the mechanism of this unusual sequence selectivity of nitrosoperoxycarbonate.

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