DNA AND RNA STRAND SCISSION BY COPPER, ZINC AND MANGANESE SUPEROXIDE DISMUTASES

Copper/zinc (Cu/ZnSOD) and manganese (MnSOD) superoxide dismutases whi ch catalyze the dismutation of toxic superoxide anion, O-2(-), to O-2 and H2O2, play a major role in protecting cells from toxicity of oxida tive stress, However, cells overexpressing either form of the enzyme s how signs of toxicity, suggesting that too much SOD may be injurious t o the cell. To elucidate the possible mechanism of this cytotoxicity, the effect of SOD on DNA and RNA strand scission was studied, High pur ity preparations of Cu/ZnSOD and MnSOD were tested in an in vitro assa y in which DNA cleavage was measured by conversion of phage phi X174 s upercoiled double-stranded DNA to open circular and linear forms, Both types of SOD were able to induce DNA strand scission generating singl e- and double-strand breaks in a process that required oxygen and the presence of fully active enzyme. The DNA strand scission could be prev ented by specific anti-SOD antibodies added directly or used for immun odepletion of SOD. Requirement for oxygen and the effect of Fe(II) and Fe(III) ions suggest that cleavage of DNA may be in part mediated by hydroxyl radicals formed in Fenton-type reactions where enzyme-bound t ransition metals serve as a catalyst by first being reduced by superox ide and then oxidized by H2O2. Another mechanism was probably operativ e in this system, since in the presence of magnesium DNA cleavage by S OD was oxygen independent and not affected by sodium cyanide, It is po stulated that SOD, by having a similar structure to the active center of zinc-containing nucleases, is capable of exhibiting non-specific nu clease activity causing hydrolysis of the phosphodiester bonds of DNA and RNA, Both types of SOD were shown to effectively cleave RNA, These findings may help explain the origin of pathology of certain heredita ry diseases genetically linked to Cu/ZnSOD gene.