The High-Affinity (Ca2+ —Mg2+ )ATPase of Rat Liver Plasma Membrane Hydrolyzes Extracellular ATP
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.
KeywordsATPase Activity Sarcoplasmic Reticulum Phospholipid Vesicle Liver Plasma Membrane Lactate Dehydrogenase Activity
Unable to display preview. Download preview PDF.