Ecto‐ATPase of Mammalian Synaptosomes: Identification and Enzymic Characterization

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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 × 10−5M), 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 × 10−4M 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 μM) inhibit the ecto‐ATPase, whereas ouabain (1 mM) and oligomycin (3.5 μg × 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.