Rat Brain Synaptosomal ATP:AMP‐Phosphotransferase Activity

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Abstract
Adenylate kinase activity (ATPlAMP‐phospho‐transferase; EC 2.7.4.3) was studied in various subcellular fractions of rat brain tissues. Because of the presence of other adenosine nucleotide‐utilizing enzymes, adenylate kinase activity was assayed in both the forward and rejverse directions by using coupled enzyme systems and by bsing a specific adenylate kinase inhibitor, P1,P5‐di(adenasine‐5′) penta‐phosphate. As expected, the highest specific adenylate kinase activity (2.89 μumol/min/mg of protein) was detected in the cytosolic brain fraction. However, substantial enzyme activity (0.68 μumol/min/mg) was also found in the iintact synaptosomal fraction isolated on Percoll/sucrose gradients. The increased specific enzyme activity of purified sytfaptosomes and the differences found between the kinetic parameters of the membrane‐bound and cytosolic enzyme forms suggest that the synaptosomal adenylate kinase activity cannot be attributed to the small amount of contaminating cytosol present in our preparations. The adenylate kinase enzyme adhered to purified synaptic plasma membranes and was not released by washings with isoosmotic sucrose medium. The facts that the adenylate kinase enzyme activity could be measured in intact synaptosomal preparations and that both its substrates and its inhibitors do not cross intact plasma membranes support the possibility that the synaptosomal adenylate kinase is an ecto‐enzyme.