Synaptosomal apyrase in the hypothalamus of adult rats

1. The synaptosomal fraction isolated from hypothalamus of adult rats on a sucrose density gradient hydrolyzes the labile phosphates from ATP and ADP, thereby satisfying the general definition of apyrase activity. 2. The parallel behavior of ATPase and ADPase activities under different reaction conditions suggests the presence of a "true" apyrase enzyme. The optimum conditions for the reaction are the same for both nucleotides: pH 8.0, 0.6 mM nucleotide and 1.5 mM cation. At temperatures between 10 and 40 degrees C, both activities increase with no change in the ATP/ADP hydrolysis ratio. Thermal inactivation or inhibition of the enzyme activity by iodoacetamide, p-hydroxymercuribenzoate or 2-mercaptoethanol affected the hydrolysis of both substrates in a similar manner. 3. Adenylate kinase and pyrophosphatase activities were not detected in the preparation. 4. The enzyme is located on the outer surface of the synaptosomal membrane: intact and lysed synaptosomes have similar activity and the supernatant obtained by centrifugation of intact synaptosomal preparations does not hydrolyze ATP or ADP.