Chemical and physical characterization of a phosphoprotein, Protein C, from human saliva and comparison with a related protein A

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Abstract
The isolation of a highly purified phosphoprotein, previously named protein C, from human parotid saliva is described. A chemical and physical characterization of protein C was undertaken and the properties of protein C were compared with those of a related protein A. The content of glycine, proline and dicarboxylic amino acids accounts for 83% of the total residues of protein C and it contains 2.0 mol of P/mol of protein, most likely as phosphoserine. The protein also contains 1.2% glucose, but no hexosamine. The N-terminus is blocked and the proposed C-terminal sequence is -Ser(Gly,Pro)Gln. The MW determined from ultracentrifugation is 16,300. Circular dichroism and NMR fail to demonstrate the presence of polyproline structure, and there are no conformational changes under a variety of conditions. With specific antisera to protein C the protein can be detected in submandibular as well as in parotid saliva, but there is only reaction of partial identity of proteins A and C. At least part of the difference between proteins A and C may be due to the presence of an additional length of peptide at the C-terminus of protein C.