Calcium uptake and binding by membrane fractions of human placenta: ATP-dependent calcium accumulation

An ATP-dependent calcium (Ca2+) sequestration activity was demonstrated in membrane vesicles prepared from the human term placenta. Microsomal and brush border membrane fractions accumulated Ca2+ within a vesicular space by a saturable process requiring Mg2+ and ATP. The "uptake" activity was enriched six-fold in a microsomal membrane fraction and was only 1.5-fold enriched in purified brush border membranes compared to the activity present in the filtered homogenate. Mitochondrial inhibitors such as azide and oligomycin did not inhibit Ca2+ uptake in these preparations. The process was temperature dependent and displayed Michaelis-Menten-like kinetics with respect to free Ca2+ concentrations. At 30 degrees C, the Vmax was 1.05 nmole/mg/min; Km = 74 nM for free Ca2+ in the microsomal fraction. Oxalate and phosphate enhanced uptake in both fractions. Ca2+ uptake activity was not associated with Ca2+-stimulated ATPase, alkaline phosphatase, or other brush border markers during cell fractionation. The characteristics of the Ca2+ uptake process contrasted sharply with those of Ca2+-stimulated ATPase, and a Ca2+-stimulated, Mg2+-dependent ATPase activity could not be identified in these membrane vesicle preparations.