MutT-related proteins, including Escherichia coli MutT and the human MTH1 
(NUDT1), degrade 8-oxo-7, 8-dihydrodeoxyguanosine triphosphate (8-oxo-dGTP) to 
8-oxo-dGMP and thereby prevent mutations caused by the misincorporation of 
8-oxoguanine into DNA. The human NUDT5, which has an intrinsic activity to 
cleave ADP sugars to AMP and sugar phosphate, possesses the ability to degrade 
8-oxo-dGDP to the monophosphate. Since 8-oxo-dGDP and 8-oxo-dGTP are 
interconvertible by cellular enzymes, NUDT5 has the potential to prevent errors 
during DNA replication. The two activities associated with NUDT5 exhibit 
different pH dependencies; the optimum for the cleavage of ADP ribose is pH 7-9, 
while that for 8-oxo-dGDPase is around pH 10. The kinetic parameters for the two 
types of reactions indicated that ADP ribose is a better substrate for NUDT5 
compared with oxidized guanine nucleotides. The 8-oxo-dGDP cleavage was 
competitively inhibited by ADP ribose and its reaction product, AMP, and in 
reverse, the cleavage of ADP ribose was inhibited by 8-oxo-dGDP. These results 
imply that the two types of substrates may share the same binding site for 
catalysis.