Age-related change of plasma extracellular-superoxide dismutase.

Extracellular-superoxide dismutase (EC-SOD) is a secretory protein that is the major SOD isozyme in extracellular fluids. Plasma EC-SOD mass concentrations in individuals were distributed in two discrete groups with the rare group (2.9%) having a variant of the enzyme with about 15-fold higher plasma levels. The EC-SOD level of the common phenotype in children/youths was significantly higher than that in their parents, and decreased with age, with an average decrease of about 2% per year to age 20. On the other hand, the parents' EC-SOD increased slightly with age. The cord EC-SOD level was the lowest. We observed no significant age-dependent changes in mass concentrations of copper- and zinc-containing SOD (Cu,Zn-SOD) or manganese-containing SOD (Mn-SOD). The total SOD activity in children/youths was significantly higher than that in parents and decreased with age. Three SOD isozymes function together in a complementary manner as a consequence of different cellular and subcellular distributions. However, the results suggest that the changes in EC-SOD levels may be the most sensitive biomarker for age-related changes in the antioxidant capacity.