The effect of oxidative stress on the cellular uptake and nuclear translocation of extracellular superoxide dismutase (EC-SOD) was investigated. EC-SOD was incorporated from conditioned medium of stable EC-SOD expressing CHO-EK cells into 3T3-L1 cells within 15 min. The uptake was clearly inhibited by the addition of heparin at a concentration of 0.4 microg/ml. Treatment of the 3T3-L1 cells with H(2)O(2) (5 mM for 5 min), followed by incubation with CHO-EK medium downregulated the uptake of EC-SOD. Nuclear translocation of the incorporated EC-SOD was clearly enhanced by H(2)O(2) treatment following incubation with the CHO-EK medium. EC-SOD is the only anti-oxidant enzyme which is known at this time to be actively transported into nuclei. The results obtained here suggest that the upregulation of the nuclear translocation of EC-SOD by oxidative stress might play a role in the mechanism by which the nucleus is protected against oxidative damage of genomic DNA.