Mg super(2+)/Ca super(2+)-ATPase Activity Is Not Enriched in Synaptic Vesicles Isolated from Rat Cerebral Cortex

Neuronal ATPases comprise a wide variety of enzymes which are not uniformly distributed in different membrane preparations. Since purified vesicle fractions have Mg super(2+)/Ca super(2+)-ATP ase, the purpose of the present study was to know whether such enzyme activities have a preferential concentration in a synaptic vesicle fraction in order to be used as markers for these organelles. Resorting to a procedure developed in this Institute, we fractionated the rat cerebral cortex by differential centrifugation following osmotic shock of a crude mitochondrial fraction and separated a purified synaptic vesicle fraction over discontinuous sucrose gradients. Mg super(2+)/Ca super(2+)-ATP ase activities and ultrastructural studies of isolated fractions were carried out. It was observed that similar specific activities for Mg super(2+)/Ca super(2+)-ATP ases were found in all fractions studied which contain synaptic vesicles and/or membranes. Although the present results confirm the presence of Mg super(2+) and Ca super(2+)-ATPase activities in synaptic vesicles preparations, they do not favor the contention that Mg super(2+)/Ca super(2+)-ATP ase is a good marker for synaptic vesicles.