A superoxide anion induced DNA strand-break metabolic pathway in human leukocytes: effects of vanadate

Abstract

Superoxide anion (O₂⁻) is an active oxygen species found in virtually all cells grown in the presence of oxygen. In vivo, the highest concentration of this oxygen radical is found after granulocytes have been exposed to particles or the tumor promoter, phorbol myristate acetate. O₂⁻ is released from the cell as a "respiratory burst," which is followed shortly by the appearance of strand breaks in the DNA of the producing cell. In the present report, we have continued our investigation into the mechanism by which extracellular O₂⁻ causes breakage of intracellular DNA. Although hydrogen peroxide is present and could also cause strand breaks, its effects are eliminated by the addition of catalase. When the amount of O₂⁻ is increased threefold by adding glucose to the medium, the number of breaks increases only slightly, suggesting that the number of breaks that could be induced is limited. The strand-break process is abruptly interrupted by the addition of metabolic poisons such as ionophore A23187, fluoride, or 2-deoxyglucose, but ATP does not appear to be involved. The number of O₂⁻-induced strand breaks is increased in the presence of sodium orthovanadate and decreased by A23187. Orthovanadate prevents the inhibition caused by A23187. Reaction of O₂⁻ with orthovanadate itself appears not to be responsible for the enhancement of breaks by orthovanadate. We propose that orthovanadate exerts its effect by acting as an inhibitor of a phosphoprotein phosphatase and that A23187 acts to deplete intracellular Ca²⁺. These data support our hypothesis that the O₂⁻ radical causes strand breaks not by attacking the DNA but rather by activating a specific metabolic DNA strand-break pathway.