Localization of an alkaline phosphatase and other synaptic vesicle proteins

The accessibility of the major vesicle proteins to trypsin digestion, to solubilization by detergents and lactoperoxidase catalysed iodination was examined in intact and in lysed synaptic vesicles obtained from bovine cerebral cortex. Three protein bands with apparent molecular weight values of 125,000, 70,000 and 47,000 were accessible only in lysed synaptic vesicles whereas most of the major vesicle proteins were accessible in intact vesicles as well. This suggests that the latter group are located at the external cytoplasmic surface of the membrane, whereas the first three are found at the membrane's inner surface. In a separate set of tests, the accessibility to trypsin digestion, the solubilization by detergents and modification by p-hydroxymercuribenzoate and phenylmethanesulfonyl fluoride were examined in an alkaline phosphatase present in the synaptic vesicles. These tests led to the conclusion that the active site of this enzyme is not exposed to the outer surface but is located at the interior surface of the synaptic vesicle membrane. On polyacrylamide gel electrophoresis in the presence of sodium dodecylsulfate the alkaline phosphatase comigrated with the 125,000 molecular weight vesicle protein.