The Contribution of Citrate to the Ferroxidase Activity of Serum

SummaryThe ferroxidase activity of ceruloplasmin has been demonstrated by other workers. In the present investigation, the ferroxidase activity of human serum was greater than that which could be accounted for by its ceruloplasmin content. The additional activity was due to a substance which was dialyzable, heat stable, separable from ceruloplasmin by immunoelectrophoresis, and not inhibitable by azide. The chromatographic properties of this substance were identical with those of citrate. Ferroxidase activity of citrate was demonstrated in the absence of other plasma factors. Furthermore, induced changes in serum citrate levels in vivo were correlated with changes in serum ferroxidase activity. Thus, citrate accounts for the nonceruloplasmin ferroxidase activity of serum.