Evidence that human ceruloplasmin molecule consists of homologous parts

The products of spontaneous and induced proteolysis of human ceruloplasmin (Cp) were studied. Some physico-chemical properties of the six fragments with electrophoretically determined Mr 130,000 (F1), 110,000 (F2), 66,000 (F3), 48,000 (F4) 22,000 (F5) and 18,000 (F6) were compared. The amino acid compositions and N-terminal amino acid sequences coincide in F1-F5, but differ from those of F6. Limited tryptic proteolysis of Cp causes the accumulation of polypeptide fragment with Mr 22,000, the N-terminal primary structure of which is identical to that of F5 produced by spontaneous proteolysis. Electrophoretic fragments of Cp were extracted from polyacrylamide gel, treated with 125I and then studied by peptide mapping with subsequent radioautography. The comparison of the "finger prints" showed the identity of F1 to F2 and F3 and gross similarity between F4 and F1-F3. It also revealed similar peptides in F5 and F6 hydrolyzates and almost perfect matching of the F4 map to the map of F5 + F6 mixture. On the basis of the obtained data general principles of Cp molecular organization are discussed and intramolecular homology is suggested to be a feature of the protein.