The coexistence effects of multiple kinds of oxidized deoxyribonucleotides were 
examined using an SV40 origin-dependent in vitro replication system with a HeLa 
extract. Oxidized dGTP and dATP, 8-hydroxy-2'-deoxyguanosine 5'-triphosphate 
(8-OH-dGTP) and 2-hydroxy-2'-deoxyadenosine 5'-triphosphate (2-OH-dATP), were 
used in this study. The mutation frequency synergistically increased when the 
two oxidized deoxyribonucleotides were together in the reaction. 2-OH-dATP 
enhanced the mutagenicity of 8-OH-dGTP, since the induced mutations were A.T --> 
C.G transversions. The contribution of the highly error-prone DNA polymerase eta 
was unlikely, since similar results were observed with an XP-V cell extract. The 
possible involvement of 2-hydroxyadenine in the complementary (template) strand 
was excluded on the basis of experiments using plasmids containing 
2-hydroxyadenine as templates in the reactions with 8-OH-dGTP. 2-OH-dATP 
suppressed hydrolysis of 8-OH-dGTP, suggesting that the inhibition of the MTH1 
protein played the major role in the enhancement. These results highlight the 
importance of specific hydrolysis of 8-OH-dGTP for the suppression of its 
induced mutation.