Calcium binding to placental plasma membranes as measured by rate of diffusion in a flow dialysis system |
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Authors: | Y. Shami H.H. Messer D.H. Copp |
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Affiliation: | Department of Physiology, University of British Columbia, Vancouver 8, B.C. Canada |
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Abstract: | The Ca2+-binding properties of placental plasma membranes were studied using a flow dialysis system.Ca2+-binding was not detectable at pH 4.0, but increased at higher pH values to a maximum binding at pH 11.0.Two types of Ca2+-binding sites were identified: high-affinity sites with dissociation constant Ks = 3.1 · 10−5 M and a capacity of 26 nmoles per mg protein; low-affinity sites with Ks = 1.1 · 10−3 M and a capacity of 266 nmoles per mg protein.The affinities of Mg2+ and Sr2+ for the high-affinity sites were 10-fold lower than that of Ca2+, and for the low-affinity sites were 4- and 8-fold lower, respectively.The placental plasma membranes contain sites for Ca2+ with capacity, specificity and affinity within the range reported for other membranes involved in an active transport of Ca2+ (mitochondria, sarcoplasmic reticulum, cardiac microsomes). The presence of high-affinity Ca2+ sites as well as Ca2+-ATPase implicates these membranes in Ca2+ transport from the maternal to the fetal circulation. |
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