8-Oxo-7,8-dihydroguanine (8-oxoGua) is generated in nucleic acids as well as in 
their precursors due to the actions of oxygen radicals produced through a normal 
cellular metabolism. Since oxidized guanine can pair with both cytosine and 
adenine, it causes alterations in the phenotypic expression when it is present 
in RNA. To prevent such an outcome, organisms must have some mechanism for 
eliminating such oxidized guanine nucleotides from RNA and its precursors. In 
mammalian cells, MTH1 and NUDT5 proteins degrade 8-oxoGTP and 8-oxoGDP to 
8-oxoGMP, which is an unusable form for RNA synthesis. In a search for proteins 
functioning at the RNA level, polynucleotide phosphorylase (PNP) protein has 
been suggested to be a good candidate for such a role. The human PNP protein has 
an ability to bind specifically to RNA containing 8-oxoGua. When human cells are 
exposed to agents that induce oxidative stress, such as hydrogen peroxide and 
menadion, the amounts of PNP protein decrease rapidly while amounts of other 
proteins in the cells do not change after such treatments. No specific decrease 
in the PNP protein level is observed when cells are treated with ACNU and 
cycloheximide at doses sufficient to provide the same degree of growth 
suppression. These results imply that the PNP protein might thus play a role in 
excluding oxidized forms of RNA from the translation mechanism.