Effects of quin2 acetoxymethyl ester on H2O2-induced DNA single-strand breakage in mammalian cells: H2O2-concentration-dependent inhibition of damage and additive protective effect with the hydroxyl-radical scavenger dimethyl sulphoxide.

The cell-membrane-permeable calcium probe quin2 acetoxymethyl ester (quin2 AM) was ineffective, in comparison with o-phenanthroline, in protecting cells against H2O2-induced DNA single-strand breakage at H2O2 concentrations of about, and higher than, 0.5 mM. The present study shows that quin2 actually potentiated intracellular DNA damage at high H2O2 concentrations. H2O2-induced DNA breakage appeared within 5 min after exposure, and quin2 affected the induction of DNA breaks at both 0 degree C and 37 degrees C. Aurintricarboxylic acid, an endonuclease inhibitor, or a decrease in extracellular Ca2+, did not reduce DNA damage. These facts strongly suggest that the breaks were not produced by a Ca(2+)-dependent nuclease. We showed previously that, in the presence of Fe3+ and H2O2, quin2 strongly potentiated the formation of oxidizing species as well as plasmid DNA breakage, and, as could be expected for a transition-metal chelator, quin2 inhibited the Fenton reaction when Cu2+ was tested instead of Fe3+ [Sandström, Granström and Marklund (1994) Free Radicals Biol. Med. 16, 177-185]. In the present work with cultured cells, titration with quin2 AM showed that, despite the fact that Cu2+ has a three-to-four-orders-of-magnitude higher affinity for quin2 than has Fe3+, both inhibition and potentiation of H2O2-induced DNA damage occurred at quin2 AM concentrations of about 100 nM. Thus inhibition appeared not to involve Cu2+. The combination of quin2 AM and dimethyl sulphoxide (DMSO) gave an additive effect on H2O2-induced DNA damage compared with the effect of quin2 AM or DMSO alone, whereas the combination of o-phenanthroline and DMSO gave about the same effect as o-phenanthroline alone. In conclusion, our results do not support a role for Ca2+ in the inhibiting effect of quin2 on H2O2-induced DNA damage. Instead, it is likely that inhibition and potentiation by quin2 involves interaction with Fe ions.