title: The binding of calcium to a salivary phosphoprotein, protein A, common to human parotid and submandibular secretions.
abstract: The binding of Ca2+ to a previously described phosphoprotein from human parotid saliva, protein A [Bennick (1975) Biochem J. 145, 557-567] was studied by means of equilibrium dialysis. In 5 mM-Tris/HC1 buffer, pH7.5, protein A bound 664nmol of Ca/mg of protein. Km was determined to be 181 muM and the binding of Ca2+ to the protein was non-co-operative. The binding of Ca2+ apparently occurs to side-chain carboxyl groups in the protein, but protein phosphate is of minor if any importance in calcium binding. Hydrolysis of protein A by trypsin and collagenase or heating of the protein at 60 degrees or 100 degrees C did not affect Ca2+ binding. The Ca2+ binding decreases with increased concentration of the dialysis buffer and on the addition of SrCl2, or MgCl2 or MnCl2 to the dialysis buffer. Protein A does not aggregate in the presence of Ca2+, since the s20,w was identical when determined in the presence (1.30S) and absence (1.35S) of CaCl2. By use of a specific antiserum to protein A it was found that protein C [Bennick & Connell (1971) Biochem. J. 123, 455-464] and perhaps minor related components cross-reacted with protein A. No other salivary proteins showed immunological similarity. Proteins A and C were also present in submandibular saliva. The possible functions of protein A are discussed.
mesh_terms: D001667:Binding, Competitive; D002118:Calcium / Q000378:metabolism*; D003429:Cross Reactions; D003956:Dialysis; D006358:Hot Temperature; D006801:Humans; D005779:Immunodiffusion; D007122:Immunoelectrophoresis; D003012:Microbial Collagenase; D010306:Parotid Gland / Q000378:metabolism; D010750:Phosphoproteins / Q000378:metabolism*; D011485:Protein Binding; D012463:Saliva / Q000378:metabolism*; D013363:Submandibular Gland / Q000378:metabolism; D014357:Trypsin; D014461:Ultracentrifugation
chemical_list: D010750:Phosphoproteins; D014357:Trypsin; D003012:Microbial Collagenase; D002118:Calcium