Calcium‐Activated ATPases in Presynaptic Nerve Endings

We studied the properties of calcium‐activated ATPases present in preparations of isolated presynaptic nerve ending (synaptosome) and its sub‐fractions from mouse brain. ATPase activity in the preparation was stimulated by Ca 2+ and by Mg 2+ , but not by Na + and K + , when each was added alone. The substrate specificities were found to be similar. The ATPases hydrolyzed only the high‐energy phosphate bond and similar activity was exhibited for all nucleoside triphosphates tested (ATP, CTP, GTP, UTP). Moreover, the enzymes were insensitive to mitochondrial markers and to ouabain, but were inhibited by La 3+ . La 3+ produced uncompetitive inhibition of Ca 2+ ‐ATPase in intact synaptosomes. Inhibition by La 3+ was greatly increased after lysis of the synaptosomes, suggesting that the active sites of the enzymes may be on the cytosolic face of the membranes. The Ca 2+ ‐ATPase activity in synaptosomes was increased by increasing concentrations of external K + , suggesting that Ca 2+ influx may be involved. The Ca 2+ ‐ATPase in synaptosomal plasma membranes and synaptic vesicles had higher specific activities than those of intact synaptosomes and were activated, both in the presence and the absence of Mg 2+ , by Ca 2+ concentrations approximating the intracellular level (10 −7 M ). It is concluded that the nonmitochondrial synaptosomal Ca 2+ ‐ATPase may play an important role in the regulation of intracellular Ca 2+ .