Physical and Chemical Studies on Ceruloplasmin

The preparation of a particulate sialyl transferase from rat liver and its properties are described. The enzyme was used, in the presence of cytidine monophosphate-N-acetylneuraminic acid-1-14C and asialoceruloplasmin, to prepare substantive quantities of radioactive ceruloplasmin. With a completely desialylated preparation of the protein as acceptor, approximately 70% of the normal complement of N-acetylneuraminic acid was restored, and the reconstituted product was isolated and purified. Upon intravenous injection of this material into a rabbit, essentially all of the radioactivity disappeared from the circulation and was recoverable from the liver. With partially desialylated ceruloplasmin as acceptor, a radioactive preparation was obtained wherein the N-acetylneuraminic acid content approximated 85% that of the native protein. Upon injection of this material, 50% of the labeled ceruloplasmin survived in the serum with a normal half-life of 54 hours. These results are interpreted as indicating that exposure of only a small number of the total galactosyl residues of ceruloplasmin is sufficient to mark the molecule for removal from the circulation by the liver.