Identification and characterization of a calcium-binding protein from human placenta

The human placenta has been found to contain a specific calcium-binding protein (HCaBP) which increases in amount as a function of gestation. A procedure is described for the purification of the HCaBP to chromatographic and electrophoretic homogeneity. The HCaBP is (a) a high molecular weight protein with a native Mr of 150 000 and appears to be a dimer of subunits of Mr = 70 000; and (b) an acidic protein with a pI of 4.6. Amino acid analysis of the HCaBP revealed an abundance of acidic amino acid residues (27 per cent = Asp and Glu). A specific antiserum has been prepared against purified HCaBP. With Ouchterlony double immunodiffusion, the HCaBP appears to be specific for the human placenta, and is not cross-immunoreactive with the CaBP of the chick embryonic chorio-allantoic membrane.