Characterization of a High‐Affinity Mg2+‐Independent Ca2+‐ATPase from Rat Brain Synaptosomal Membranes

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Abstract
A high-affinity Mg2+-independent Ca2+-ATPase (Ca2+-ATPase) has been differentiated from the Mg2+ dependent, Ca2+-stimulated ATPase (Ca2+, Mg2+-ATPase) in rat brain synaptosomal membranes. Using ATP as a substrate, the K0.5 of Ca2+ for Ca2+-ATPase was found to be 1.33 .mu.M with a Km for ATP of 19 .mu.M and a Vmax of 33 nmol/mg/min. Using Ca-ATP as a substrate, the Km for Ca-ATP was found to be 0.22 .mu.M. Unlike Ca2+, Mg2+-ATPase, Ca2+-ATPase was not inhibited by N-ethylmaleimide, trifluoperazine, lanthanum, zinc, or vanadate. La3+ and Zn2+, in contrast, stimulated the enzyme activity. Unlike Ca2+, Mg2+-ATPase activity, ATP-dependent Ca2+ uptake was negligible in the absence of added Mg2+, indicating that the Ca2+ transport into synaptosomal endoplasmic reticulum may not be a function of the Ca2+-ATPase described. Ca2+-ATPase activity was not stimulated by the monovalent cations Na+ or K+. Ca2+,Mg2+-ATPase demonstrated a substrate preference for ATP and ADP, but not GTP, whereas Ca2+-ATPase hydrolyzed ATP and GTP, and to a lesser extent ADP. The results presented here suggest the high-affinity Mg2+-independent Ca2+-ATPase may be a separate form from Ca2+,Mg2+-ATPase. The capacity of Mg2+-independent Ca2+-ATPase to hydrolyze GTP suggests this protein may be involved in GTP-dependent activities within the cell.