Mechanisms of biosynthesis of mammalian copper/zinc superoxide dismutase

Copper/zinc superoxide dismutase (SOD1) is an abundant intracellular enzyme with an essential role in antioxidant defense. The activity of SOD1 is dependent upon the presence of a bound copper ion incorporated by the copper chaperone for superoxide dismutase, CCS. To elucidate the cell biological mechanisms of this process, SOD1 synthesis and turnover were examined following Cu-64 metabolic labeling of fibroblasts derived from CCS+/+ and CCS-/- embryos. The data indicate that copper is rapidly incorporated into both newly synthesized SOD1 and preformed SOD1 apoprotein, that each process is dependent upon CCS and that once incorporated, copper is unavailable for cellular exchange. The abundance of apoSOD1 is inversely proportional to the intracellular copper content and immunoblot and gel filtration analysis indicate that this apoprotein exists as a homodimer that is distinguishable from SOD1. Despite these distinct differences, the abundance and half-life of SOD1 is equivalent in CCS+/+ and CCS-/- fibroblasts, indicating that neither CCS nor copper incorporation has any essential role in the stability or turnover of SOD1 in vivo. Taken together, these data provide a cell biological model of SOD1 biosynthesis that is consistent with the concept of limited intracellular copper availability and indicate that the metallochaperone CCS is a critical determinant of SOD1 activity in mammalian cells. These kinetic and biochemical findings also provide an important framework for understanding the role of mutant SOD1 in the pathogenesis of familial amyotrophic lateral sclerosis.