Some Properties of an Apoceruloplasmin-like Protein in Human Serum

Rabbit antibody to apoceruloplasmin has been used to demonstrate that two forms of ceruloplasmin occur in serum. In addition to the well known blue protein, a colorless component was found. The latter protein lacks oxidase activity, has a relatively low copper content, and is immunologically similar to apoceruloplasmin. It constitutes from 10 to 20% of the total ceruloplasmin found in normal serum. The electron paramagnetic resonance spectra of ceruloplasmin and its antibody precipitate were found to be identical. Therefore, antibodies could be used to isolate ceruloplasmin from serum for electron paramagnetic resonance studies. An antibody precipitate of all of the ceruloplasmin from a sample of serum contained two species of paramagnetic copper. The colorless ceruloplasmin appears to contain the type of copper which gives an electron paramagnetic resonance signal with a broad hyperfine structure.