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Mouse chorioallantoic placenta contains a specific calcium-binding protein (MCaBP). A procedure involving gel filtration and ion-exchange chromatography was developed to purify the MCaBP. The MCaBP activity increased as a function of embryonic gestation and was highly specific for Ca2+. The MCaBP is a monomeric protein of Mr 57000, with pI 4.7. Specific antibodies were prepared against the MCaBP and were used to localize the MCaBP to syncytiotrophoblasts of the chorionic villi of mouse chorioallantoic placenta. These properties suggest that the MCaBP may be involved in transplacental calcium transport.

The Ca2+-binding protein (HCaBP) of the human placenta was studied with respect to its biochemical properties, tissue and cellular distribution, and possible involvement in placental Ca2+ transport. Optimal Ca2+ binding by the HCaBP occurs at pH 7-8 and in 100 mM-Na+ and 3 mM-Ca2+. The HCaBP possesses at least 10 Ca2+-binding sites with a Kd of 5 × 10(-6) M ([Ca2+]). Highly specific rabbit-derived anti-HCaBP antibodies were used for HCaBP immunoquantification and immunohistochemistry, which revealed that the HCaBP is localized in the chorionic villi and is primarily associated with the trophoblastic cells of the placenta. In addition, an ‘in vitro’ cell-free assay system for Ca2+ uptake was constructed with microsomal membranes isolated from term placental tissues. Ca2+ uptake by the placental microsomal fraction exhibited characteristics indicative of active Ca2+ transport such as temperature-dependence, saturability and energetic requirement. In this system, preincubation of microsomal membranes with anti-HCaBP antibodies inhibited Ca2+ uptake, suggesting that the HCaBP is functionally involved in placental membrane Ca2+ uptake.

Carbonic anhydrase activity was demonstrated in the chick-embryonic chorioallantoic membrane and was correlated with the Ca2+-transport activity of the membrane. It is inhibited by sulphonamides and is expressed in the chorioallantoic membrane in an age-dependent fashion during embryonic development. Ca2+ uptake by the chorioallantoic membrane in vivo also increases in a similar age-dependent manner. The temporal increase in these activities is coincident with calcium deposition in the embryonic skeleton. Incubation of the chorioallantoic membrane in ovo with sulphonamides specifically inhibits both the carbonic anhydrase and the Ca2+ uptake activities of the membrane in vivo. Enzyme histochemistry revealed the carbonic anhydrase activity is localized in the Ca2+-transporting ectodermal cells of the chorioallantoic membrane. These results, taken together, indicate that carbonic anhydrase may be functionally important in the Ca2+-transport activity of the chorioallantoic membrane.