High-affinity, calcium-stimulated ATPase in brush border membranes of the human term placenta.

ATPase activity which is stimulated by submicromolar concentrations of calcium (Ca2+) was identified in human placental microvillous brush border membranes. The high-affinity enzyme has an apparent K0.5 for free Ca2+ of 18.3 +/- 3.7 nM and a Vmax of 233.0 +/- 30.0 nmol/min/mg protein. Studies using trans-1,2-diaminocyclohexane-N,N,N1,N1-tetraacetic acid (CDTA) show that this enzyme requires submicromolar concentrations of Mg2+ for maximal activity, but that it appears to have a low basal activity in the absence of this cation. The high-affinity Ca2+-ATPase was unaffected by up to 100 microM concentrations of vanadate, but was sensitive to trifluoperazine inhibition (I50 less than 50 microM). It was not found to be stimulated by the addition of up to 10 micrograms calmodulin, but this lack of effect may be related to the endogenous calmodulin content of the membrane preparation. A low-affinity, non-specific divalent cation ATPase was also identified in this membrane preparation. In contrast to the high-affinity enzyme, it has an apparent K0.5 for calcium of 99.7 +/- 22.1 microM, and a Vmax of 1.54 +/- 0.17 mumol/min/mg protein. The characteristics of the high-affinity Ca2-ATPase are similar to those of other Ca2+- ATPases known to transport and regulate intracellular calcium concentrations in other tissues. By analogy, we suggest that the high-affinity Ca2+-ATPase described here could play an important role in cellular calcium homeostasis in the human placenta.