Proof for the Ascorbate Oxidase Activity of Ceruloplasmin.

The ascorbate oxidase activity of ceruloplasmin has been a subject of controversy because of (a) the relatively low oxidase activity of this serum copper protein toward ascorbic acid and (b) the different conclusions reached by several investigators. In 1951, Holmberg and Laurel1 (1) reported that ascorbic acid was a substrate of ceruloplasmin although it was less active as a substrate than p-phenylenediamine. Humoller et al. (2) concluded that ceruloplasmin has ascorbate oxidase activity, primarily because they had observed that ceruloplasmin and Cu(I1) had a different susceptibility toward albumin. In 1962, Walter (3) described numerous kinetic features of ascorbate oxidation by ceruloplasmin which were different from the catalysis by Cu(II), and Frieden (4) suggested an authentic ascorbic acid oxidase activity of ceruloplasmin. More& Aisen, and Scheinberg (5) re-examined this problem with the use of a ceruloplasmin sample that was treated with a chelating resin, Chelex 100, to eliminate nonceruloplasmin copper, and concluded that ceruloplasmin had no ascorbate oxidase activity. At this point, we decided to examine this question by using all possible experimental criteria to reach a decisive conclusion.1 The data presented here show an indisputable difference between the oxidation of ascorbate by ceruloplasmin and by Cu(II), and indicate that ceruloplasmin has definite ascorbate oxidase activity.