High Affinity Ca 2 + - stimulated Mg 2 + - dependent ATPase in Rat Brain Synaptosomes , Synaptic Membranes

High affinity Ca2+-stimulated Mga+-dependent ATPase activity of nerve ending particles (!vnaphmm) from rat brain tissue appears to be associated primarily with isolated synaptic plasma membranes. The synaptic membrane (Ca2+ + Mga+)-ATPase activity was found to exhibit strict dependence on Mg+ for the presence of the activity, a high affinity for Ca2+ (&.a = 0.23 NM), and relatively high affinities for both M e + and ATP = 6.0 plrr for Mgz* and KM = 18.9 NM for ATP). These kinetic constants were determined in incubation media that were buffered with the divalent cation chelator t ans-cyclohexane 1,2-diamineN,N,N',N'-tetraacetic acid. The enzyme activity was not inhibited by ouabain or oligomycin but was sensitive to low concentrations of vanadate. The microsomal membrane subfraction was the other b ain subcellular fraction with a high affinity (Ca" + M6+)-ATPase activity which approximated that of the synaptic plasma membranes. The two membrane-related high affinity (Ca2+ + Mg+)-ATPase activities could be distinguished on the basis of their differential sensitivity to vanadate at concentrations below 10 PM. Only the synaptic plasma membrane (Ca2+ + Mg2*)-ATPase was inhibited by 0.25-10 NM vanadate. The studies described here indicate the possible involvement of both the microsomal and the neuronal plasma membrane (Ca" + Mg+)-ATPase in high affinity Ca" transport across membranes of brain neurons. In addition, they suggest a means by which the relative contributions of each transport system might be evaluated based on their differential sensitivity to inhibition by vanadate.