Base oxidation: The plasmid model also reveals DNA base damages when agarose gel electrophoresis is combined with the use of specific DNA repair enzymes. Formamidopyrimidine DNA N-glycosylase (Fpg) reveals modified purine bases including 8-OH-dGuo, 2,4-diamino-6-hydroxy-5-formamidopyrimidine and 4,6-diamino-5-formamidopyrimidine. Endonuclease III (Endo III) is almost specific for oxidized pyrimidine bases such as thymine and cytosine glycols or 5-hydroxy-5-methylhydantoin. T<>T are evidenced by T4 endonuclease V or Micrococcus luteus endonuclease. Indeed, LEV (45) and OFL (39,45), as well as 6,8 dihalogenated FQs LOM and BAY (81,106) show a higher potential in plasmid DNA base oxidation of both purine and pyrimidine, whereas methoxy substitution (MOX) tends to reduce this effect (45). T<>T formation has been evidenced for ENO and PEF (18), as well as for NOR (18,27). OFL (18,39) and RUF (18) are unable to photoinduce this lesion, according to the low triplet energy value of these two drugs, as reported above (18,27). RUF induces formation of 8-OH-dGuo on plasmid DNA (70) with a yield similar to that achieved in sonicated calf thymus DNA (63). Its occurrence in these two biomodels can be attributed to one-electron oxidation reactions (107).
Cleavage: Almost all FQs have been reported to induce SSB on plasmid DNA: ENO (18,37,52,87,108–111), PEF (18,108,109), BAY (40,81,106), LOM (37,40,81,87,108,109,111–113), NOR (18,29,87,108,109,111), SPA (37,87,108,109,112,114), OFL (37,39,45,87,108,109,111), CIP (37,40,87,108,109,111–113), LEV (45,112), RUF (70,108,109), FLE (108,109,111), TOS (87,111), ORB, CLI (108,109), MOX (40,45) and GAT (87). As observed for the other FQs DNA targets, most of 6,8 dihalogenated FQs (LOM, FLE, ORB, BAY and SPA) show a higher potential as DNA cleaving agents; also in this case methoxy substitution (MOX) tends to reduce this effect (40,45). Experiments carried out with some additives are useful to confirm the participation of ROS in the FQs mediated photosensitized DNA cleavage in plasmids (37,52,70,110,111). Nevertheless, the formation of anionic radicals from the pyrimidine bases is one of the key steps that result in DNA breakage by elimination of phosphate ester in C′3 (57). In any case, photoinduced damages leading to photocleavage are different from photo-oxidative base damages (70).