Purification and characterization of metal-binding proteins and the corresponding mRNA from human placentas.

Two metal-binding proteins, designated as PI and PII, were isolated and purified from normal term human placentas by gel filtration and ion-exchange chromatography. The molecular weights were determined to be 10,000 and 12,000 daltons, and isoelectric points (pI) were 4.8 and 5.9, respectively. The amino acid composition of these proteins was quite different from that of metallothionein. Total amount of acidic amino acid residues was in large excess over that of basic amino acid residues. Cadmium and zinc were the major metals bound to these proteins. The metal contents of cadmium and zinc in placental tissue were 39.34 and 22.23 ng/g placenta, respectively, as measured by flame atomic absorption spectrophotometry. The in vitro translated metal-binding proteins encoded by the corresponding mRNA were characterized by the purified rabbit antiserum against PI and PII. The demonstrated presence of these metal-binding proteins in human placenta suggests its possible role of detoxification activity and protective effect to the fetuses in utero.