Oxidative Nucleobase Modifications Leading to Strand Scission

In this survey, emphasis was placed on the main photoreactions of nucleic acid components, involving both direct and indirect effects. The main UVB- and UVA-induced DNA photoproducts, together with the mechanisms of their formation, are described. Information on the photoproduct distribution within cellular DNA is also provided, taking into account the limitations of the different analytical methods applied to monitor the formation of the DNA damage. Thus, the formation of the main DNA dimeric pyrimidine lesions produced by direct absorption of UVB photons was assessed using a powerful HPLC-tandem mass spectrometry assay. In addition, it was found that UVA photooxidation damage mostly involves the guanine residues of cellular DNA as the result of singlet oxygen generation by still unknown endogenous photosensitizers.

Direct and indirect effects of UV radiation on DNA and its components.

Endogenous and exogenous sources cause free radical-induced DNA damage in living organisms by a variety of mechanisms. The highly reactive hydroxyl radical reacts with the heterocyclic DNA bases and the sugar moiety near or at diffusion-controlled rates. Hydrated electron and H atom also add to the heterocyclic bases. These reactions lead to adduct radicals, further reactions of which yield numerous products. These include DNA base and sugar products, single- and double-strand breaks, 8,5′-cyclopurine-2′-deoxynucleosides, tandem lesions, clustered sites and DNA-protein cross-links. Reaction conditions and the presence or absence of oxygen profoundly affect the types and yields of the products. There is mounting evidence for an important role of free radical-induced DNA damage in the etiology of numerous diseases including cancer. Further understanding of mechanisms of free radical-induced DNA damage, and cellular repair and biological consequences of DNA damage products will be of outmost importan...

Mechanisms of free radical-induced damage to DNA

Oxidative DNA damage is a major cause of cell death and mutagenesis in all aerobic organisms, and several new oxidative base lesions have been identified in recent years. Improved chemistry for the synthesis of oligonucleotides with modified base residues at defined positions has allowed detailed studies of repair, replication, transcription and mutagenesis at specific lesions in vitro and in vivo. The aim of this review is to present the structure of all the various known oxidised DNA base lesions known to date and to summarise the present knowledge about the mutagenic and toxic effects of oxidised base modifications and their repair.

Mutagenicity, toxicity and repair of DNA base damage induced by oxidation.

The trap depth (in DNA) of 8-oxo-7,8-dihydro-2'deoxyguanosine as derived from electron-transfer equilibria in aqueous solution

Calf thymus DNA and 8-oxo-7,8-dihydro-2‘-deoxyguanosine (8-oxodGuo) were photooxidized in the dark by triplet-excited ketones generated in the thermal decomposition of 3-(hydroxymethyl)-3,4,4-trimethyl-1,2-dioxetane (HTMD). The oxidation of DNA led to 8-oxodGuo and the type I photooxidation product 2,2-diamino[2-deoxy-β-d-erythro-pentofuranosyl)-4-amino]-5(2H)-oxazolone (oxazolone). While the yield of oxazolone progressively increased, 8-oxodGuo was substantially consumed in DNA on successive exposure to HTMD. The oxidation of authentic 8-oxodGuo by HTMD and established photosensitizers such as benzophenone (mainly type I) and Rose Bengal (predominantly type II) was studied in detail in regard to the concentration and time dependence and the influence of D2O versus H2O. The singlet-oxygen-derived 4R* and 4S* diastereomers of 4-hydroxy-8-oxo-4,8-dihydro-2‘-deoxyguanosine (4-HO-8-oxodGuo) and oxazolone were the major products. A substantial kinetic D2O effect (ca. 10-fold) in the Rose Bengal-photosensitized...

Photooxidation of 8-Oxo-7,8-dihydro-2‘-deoxyguanosine by Thermally Generated Triplet-Excited Ketones from 3-(Hydroxymethyl)-3,4,4-trimethyl-1,2-dioxetane and Comparison with Type I and Type II Photosensitizers

The nucleoside 2‘-deoxyguanosine (dGuo) was treated with 3-(hydroxymethyl)-3,4,4-trimethyl-1,2-dioxetane (HTMD), the latter generates efficiently triplet-excited carbonyl products on thermal decomp...

Type I and Type II Photosensitized Oxidative Modification of 2‘-Deoxyguanosine (dGuo) by Triplet-Excited Ketones Generated Thermally from the 1,2-Dioxetane HTMD

— Isolated calf thymus DNA was treated with the 1,2-dioxetanes 3-acetoxymethyl-3,4,4-tri-methyl-1,2-dioxetane, 2,3-dimethylbenzofuran dioxetane, 3-hydroxymethyl-3,4,4-trimethyl-1,2-dioxeta-ne (HTMD), 3,3,4,4-tetramethyl-1,2-dioxetane and 3,4,4-trimethyl-1,2-dioxetane (TrMD), which on thermal decomposition generate triplet-excited carbonyl products. To monitor quantitatively the formation of the mutagenic oxidation product 7,8-dihydro-8-oxoguanine (8-oxoGua), a sensitive and selective HPLC electrochemical assay was used after acidic hydrolysis (HF/pyridine) of the dioxetane-treated DNA. High yields of 8-oxoGua (up to ca 4% of the available guanine) were obtained for HTMD and TrMD. Both were investigated in detail with respect to effects of concentration, time and temperature. The oxidative reactivity of 1,2-dioxetanes was compared with several type I (benzophenone and riboflavin) and type II (methylene blue and rose bengal) photooxidants and disodium 1,4-etheno-2,3-ben-zodioxin-1,4-dipropionate as a chemical source of singlet oxygen. The persistence of 8-oxoGua towards oxidation by HTMD was examined in the reaction with 7,8-dihydro-8-oxo-2'-deoxyguanosine (8-oxodGuo) and with oxidized DNA. It was shown that, indeed, 8-oxoGua is consumed in the oxidized DNA on prolonged exposure to an excess of HTMD. The reaction of 8-oxodGuo with HTMD afforded the two 4R* and 4S* diastereomers of 9-(2-deoxy-s-D-erythropentofuranosyl)-4,8-dihydro-4-hydroxy-8-oxoguanine as main oxidation products. Trapping experiments with teft-butanol confirmed that hydroxyl radicals are not involved, whereas the use of the triplet quenchers sodium 9,10-dibromo-anthra-cene-2-sulfonate and 2,3-diazabicyclo[2.2.1]hept-2-ene established that triplet-excited states are mainly responsible for the observed DNA oxidation through type I action (electron transfer chemistry). The role of singlet oxygen was tested by means of deuterium isotope effects in D2O versus H2O, but no definitive conclusion could be reached in regard to the involvement of 102 in these oxidations. The present results reveal that 1,2-dioxetanes are efficient DNA oxidants and excellent tools to study photooxidation reactions of DNA in the dark.

Formation of 7,8-dihydro-8-oxoguanine in the 1,2-dioxetane-induced oxidation of calf thymus dna : evidence for photosensitized dna damage by thermally generated triplet ketones in the dark

https://opus.bibliothek.uni-wuerzburg.de/files/6007/Stopper07.pdf

André Schönberger

Papers

Photooxidation of 8-Oxo-7,8-dihydro-2‘-deoxyguanosine by Thermally Generated Triplet-Excited Ketones from 3-(Hydroxymethyl)-3,4,4-trimethyl-1,2-dioxetane and Comparison with Type I and Type II Photosensitizers

Type I and Type II Photosensitized Oxidative Modification of 2‘-Deoxyguanosine (dGuo) by Triplet-Excited Ketones Generated Thermally from the 1,2-Dioxetane HTMD

Formation of 7,8-dihydro-8-oxoguanine in the 1,2-dioxetane-induced oxidation of calf thymus dna : evidence for photosensitized dna damage by thermally generated triplet ketones in the dark

Genotoxicity studies of benzofuran dioxetanes and epoxides with isolated DNA, bacteria and mammalian cells.

Dimethyldioxirane oxidation of hydroquinones into quinones and 2,3-dihydroxycyclohexene-1,4-diones