Ecto‐ATPase of Mammalian Synaptosomes: Identification and Enzymic Characterization

Abstract

Intact synaptosomes isolated from mammalian brain tissues (rat, mouse, gerbil, and human) have an ATP hydrolyzing enzyme activity on their external surface. The synaptosomal ecto-ATPase(s) possesses characteristics consistent with those that have been described for ecto-ATPases of various other cell types. The enzyme has a high affinity for ATP (the apparent Km values are in the range of 2-5 .times. 10-5 M), and is apparently stimulated equally well by either Mg2+ or Ca2+ in the absence of any other cations. The apparent activation constant for both divalent cations is approximately 4 .times. 10-4 M in all mammalian brain tissues studied. The involvement of a nonspecific phosphatase in the hydrolysis of externally added ATP is excluded. ATP hydrolysis is maximal in the pH range 7.4-7.8 for both divalent cation-dependent ATPase activities. Dicyclohexylcarbodiimide, 2,4-dinitrophenol, trifluoperazine, chlorpromazine, and p-chloromercuribenzoate (50 .mu.M) inhibit the ecto-ATPase, whereas ouabain (1 mM) and oligomycin (3.5 .mu.g .cntdot. mg-1 protein) show little or no inhibition of this enzyme activity. Inhibitor data suggest that the Mg2+- and Ca2+-dependent ecto-ATPase may represent two different enzymes on the surface of synaptosomes.