ISOLATION OF A CA2+ OR MG2+-ACTIVATED ATPASE (ECTO-ATPASE) FROM BOVINE BRAIN SYNAPTIC-MEMBRANES

An ATPase was isolated from synaptosomal plasma membranes derived from bovine cerebral cortex. The protein has an apparent molecular mass of 50 kDa and a pI of 5.3 to 5.9. It can be labelled by incubation of intact synaptosomes with azido-GTP or azido-ATP. The isolated ATPase can be activated to a similar extent in the presence of millimolar concentrations of Mg2+ or Ca2+. It does not hydrolyze ADP. Maximal activity is obtained between pH 7.5 and 8.5. Typical inhibitors of cytoplasmic ATPases do not affect enzyme activity. The enzyme is specifically inhibited after previous incubation of intact synaptosomes in the presence of the slowly membrane-permeable enzyme inhibitor diazotized sulfanilic acid. Incubation of intact synaptosomes with diazotized sulfanilic acid results in a small increase in the apparent molecular mass of the enzyme. Our results suggest that the active site of the membrane bound enzyme faces the extracellular medium. It thus would represent an ecto-ATPase.