A new function for ceruloplasmin as an acute-phase reactant in inflammation: a scavenger of superoxide anion radicals.

In summary, purified human ceruloplasmin inhibits several reactions mediated by superoxide anion in a fashion consistent with an ability to scavenge this free radical. It must be pointed out, however, that on a weight basis, the superoxide-scavenging activity of ceruloplasmin is substantially less than that of purified human erythrocyte superoxide dismutase. Nevertheless, since superoxide dismutase is almost exclusively an intracellular enzyme, ceruloplasmin probably represents the major circulating scavenger of superoxide anion radicals. The level of superoxide dismutase in human plasma has been reported to be 0.7 microgram/ml. It is not clear, however, how this was measured. We have found that concentrations of plasma exceeding 10% (v/v) interfere significantly with the assays routinely employed for detecting superoxide-scavenging activity. Consequently, we have not yet been able to quantify the superoxide-scavenging activity of either ceruloplasmin or superoxide dismutase in whole human plasma. Thus, we can only speculate that under conditions where levels of ceruloplasmin are markedly elevated, as during pregnancy, during acute infections, or in association with inflammatory diseases (such as rheumatoid arthritis), this acute-phase reactant may play a major role as a circulating scavenger of oxygen-derived free radicals.