Peroxynitrite mobilizes calcium ions from ryanodine-sensitive stores, a process associated with the mitochondrial accumulation of the cation and the enforced formation of species mediating cleavage of genomic DNA

Peroxynitrite does not directly cause strand scission of genomic DNA. Rather, as we previously reported, the DNA cleavage is largely mediated by H2O2 resulting from the dismutation of superoxide generated in the mitochondria upon peroxynitrite-dependent inhibition of complex III. The present study demonstrates that this process is strictly controlled by the availability of Ca2+ in the mitochondrial compartment. Experiments using intact as well as permeabilized U937 cells showed that the DNA-damaging response evoked by peroxynitrite is enhanced by treatments causing an increase in mitochondrial Ca2+ uptake and remarkably reduced under conditions leading to inhibition of mitochondrial Ca2+ accumulation. An additional, important observation was that the source of the Ca2+ mobilized by peroxynitrite is the ryanodine receptor; preventing the mobilization of Ca2+ with ryanodine suppressed the mitochondrial formation of reactive oxygen species and the ensuing DNA strand scission. Identical results were obtained using PC12, C6, and THP-1 cells. These results, along with our previous findings indicating that the DNA damage induced by peroxynitrite is also suppressed by inhibition of the electron flow through complex I, e.g., by rotenone, or by the respiration-deficient phenotype, demonstrate that the mitochondrial formation of DNA-damaging species is critically regulated by the inhibition of complex III and by the availability of Ca2+.