Identification of an apo-superoxide dismutase (Cu,Zn) pool in human lymphoblasts

Copper incorporation ( ⁶⁴Cu(II)) into Cu,Zn-superoxide dismutase (SOD) was studied in human lymphoblasts. Rapid incorporation of copper with a proportionate increase in SOD activity was detected. No copper incorporation or SOD activation was detected when ⁶⁴Cu(II) was added to cell cytosols rather than to intact cells. Thus, incorporation of ⁶⁴Cu was not due to isotopic exchange. Cycloheximide had no significant effect on copper incorporation and activation of SOD when the data were corrected for total cell copper. Thus, the data were consistent with copper incorporation into a preexisting apoSOD pool rather than newly synthesized SOD, and no new SOD synthesis was detected over a 15-h incubation period. The size of the apoSOD pool was estimated to be ≃35% of the total SOD in lymphoblasts. When cells were preincubated for 15 h with excess copper (15 μM Cu(II)), the size of the apo pool markedly decreased but was not eliminated, suggesting that the apoSOD was not doe to copper deficiency. These experiments also indicated that newly arrived copper was preferentially incorporated into the apoSOD pool, while the function(s) of an apoSOD pool remains unknown. Copper binding to apoSOD may provide a rapid protective response against copper toxicity.