METHYLENE BLUE-MEDIATED PHOTOOXIDATION OF 7,8-DIHYDRO-8-OXO-2'-DEOXYGUANOSINE

One well known product of the methylene blue-mediated photosensitizati on of 2'-deoxyguanosine (dG) in oxygen saturated aqueous solution is 7 ,8-dihydro-8-oxo-2'-deoxyguanosine (8-oxodG). We observed that the rat e of 8-oxodG photodecomposition by methylene blue-mediated photosensit ization is approx. 3-times faster than for dG, The primary products of the methylene blue-mediated photosensitization of 8-oxodG are -D-eryt hro-pentofuranosyl)amino)-4H-imidazol-4-one (dIz), ta-D-eryrhro-pentof uranosyl)amino)-5(2H)-oxazolone (dZ), the 4R and 4S* diastereoisomers of 4,8-dihydro-4-hydroxy-8-oxo-2'-deoxyguanosine (dO), and an as yet unidentified product with a molecular weight of 287 (dX). Except for t he latter product, these compounds have all been identified following the methylene blue-mediated photooxidation of dG. Methylene blue-media ted photooxidation of 8-oxodG in D2O instead of H2O leads to a 4-fold increase in the rate of 8-oxodG photodecomposition while the addition of sodium azide retards the reaction, observations which imply that th e reaction occurs via a type II (singlet oxygen mediated) mechanism. L ike 8-oxodG, dIz and dZ are sensitive to hot piperidine and likely con tribute to strand breaks observed in double stranded DNA exposed to me thylene blue plus light followed by hot piperidine. Because 8-oxodG ge nerates predominately G-->T transversions, the photooxidation of 8-oxo dG to dIz, dO, and dX may explain the predominance of G-->C transversi ons in single-stranded M13mp2 bacteriophage DNA exposed to methylene b lue plus light and then transfected into SOS-induced Escherichia coli.