The High-Affinity (Ca²⁺ —Mg²⁺ )ATPase of Rat Liver Plasma Membrane Hydrolyzes Extracellular ATP

Abstract

The intracellular free Ca²⁺ concentration is very low (in the region of 0.1µM) in contrast to extracellular [Ca²⁺ ], which is usually in the millimolar range. An energy-dependent Ca²⁺ transporter in the plasma membrane is therefore important in maintaining the concentration gradient. Studies on the biochemical properties of a Ca²⁺ pump were first carried out with sarcoplasmic reticulum membrane that is enriched with a Ca²⁺ pump (for review, see Tadaet al., 1978). These studies established several properties of the sarcoplasmic reticulum Ca²⁺ pump: (1) The Ca²⁺ pump utilizes both Ca²⁺ -and Mg²⁺ -ATP as substrates. (2) It exhibits a high affinity for Ca²⁺ (in the submicromolar range). (3) It has a Ca²⁺ -stimulated ATPase activity. When the enzyme is incorporated into phospholipid vesicles, a coupled Ca²⁺ -stimulated ATP hydrolysis and Ca²⁺ transport can be demonstrated. (4) The mechanism of Ca²⁺ -stimulated ATP hydrolysis involves an enzyme phosphate intermediate. As a result of such a property, both the Ca²⁺ -stimulated ATP hydrolysis and Ca²⁺ transport can be inhibited by vanadate, a transition state analog of phosphate.