Inhibition of cytotoxicity by intracellular superoxide dismutase supplementation

The role of intracellular oxyradicals in H2O2 and neutrophil-induced cytotoxicity is suggested by previous studies showing protection by inhibitors such as deferroxamine, dimethylthiourea, and dimethyl sulfoxide. In the current studies, the role of intracellular O2- is specifically examined by evaluating the effects of intracellular superoxide dismutase (SOD) supplementation on cytotoxicity of rat pulmonary artery endothelial cells induced by H2O2 and activated neutrophils. To minimize in vitro manipulation, supplementation was accomplished by incubating endothelial cells in the presence of SOD (1-20 mg/mL). Increases up to greater than 17-fold the baseline SOD activity were achievable using this approach, with uptake being maximal after 6 h of incubation. This increase was resistant to trypsin digestion, suggesting the intracellular location of SOD. Compared to controls, SOD-supplemented cells showed significantly increased resistance to killing by H2O2 and activated neutrophils. Inactive SOD failed to provide protection. The degree of protection was dependent on the dose of cytotoxic agent and the extent of SOD supplementation. The results provide new evidence that intracellular O2- participates in the killing process induced by these two stimuli. The intracellular source of O2- remains to be determined, although previous studies suggest xanthine oxidase as a likely candidate.