8-hydroxyguanine (oh8Gua) is a major form of oxygen free radical-induced DNA damage. The oh8Gua nucleotide can pair with cytosine (C) and adenine (A) nucleotides which can cause G:C to T:A transversions. It is known that multiple repair systems for the correction of the oh8Gua exist in both mammalian and bacterial cells. Using the technique of gel mobility shift assay, protein(s) bound to the oh8Gua:C base pair in short fragments of DNA was detected in cell-free extracts of a human small-cell lung cancer cell line. This DNA binding activity was specific, since it was poorly detected with an unmodified G:C base pair containing oligonucleotide duplex and was affected by neither the unmodified G:C base pair nor an oh8Gua:A base pair containing oligonucleotide duplex. The partially purified protein which selectively binds to the oh8Gua:C base pair was shown by gel filtration column chromatography to have an apparent molecular mass of 52 kDa. The column fraction which showed the highest binding activity to the oh8Gua:C base pair was found to possess an enzymatic activity that specifically cleaves the oh8Gua containing oligonucleotide strand at both the 5' and 3' sides of the oh8Gua residue. These results indicate the presence of a protein(s) that is involved in a DNA repair pathway for the correction of the oh8Gua residue in human cells.
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