The (Na(+)+K+)-ATPase is responsible for maintenance of the ionic milieu of cells. The objective of this study is to investigate the effect of aluminum, an ion implicated in several neurological disorders, on ATP hydrolysis catalyzed by the rat brain synaptosomal (Na(+)+K+)-ATPase and on the binding of [3H]ouabain to this enzyme. AlCl3 (25-100 microM) inhibits the phosphatase activity of the (Na(+)+K+)-ATPase in a dose-dependent manner. AlCl3 appears to act as a reversible, noncompetitive inhibitor of (Na(+)+K+)-ATPase activity by decreasing the maximum velocity of the enzyme without significantly affecting the apparent dissociation constant with respect to ATP. AlCl3 may affect Mg2+ sites on the (Na(+)+K+)-ATPase but does not appear to interact with Na+ or K+ sites on the enzyme. In contrast to this inhibitory effect on the phosphatase function of the enzyme, AlCl3 (1-100 microM) stimulates the binding of [3H]ouabain to the (Na(+)+K+)-ATPase. This effect is due to an increase in the maximum [3H]ouabain binding capacity of the enzyme with no change in the [3H]ouabain binding affinity. These data support the hypothesis that AlCl3 may stabilize the phosphorylated form of the synaptosomal (Na(+)+K+)-ATPase which increases [3H]ouabain binding while inhibiting the phosphatase activity of the enzyme.

The Na+,K(+)-ATPase is an important enzyme in determining the ionic milieu of the cerebromicrovasculature and neurons. The effect of hypertension or aging on this enzyme, as well as its susceptibility to regulation by fatty acids or aluminum, is the focus of this study. A significant increase (34%) in the apparent affinity constant (KD) but no change in the maximum binding capacity (Bmax) for [3H]ouabain binding to the cerebromicrovascular Na+,K(+)-ATPase occurs after induction of acute hypertension. In addition, long chain unsaturated fatty acids stimulate the binding of [3H]ouabain to the enzyme in microvessels from normotensive and hypertensive rats. The synaptosomal Na+,K(+)-ATPase is sensitive to aluminum. AlCl3 (1-100 microM) inhibits the K(+)-dependent-p-nitrophenylphosphatase (K(+)-NPPase) activity of the Na+,K(+)-ATPase in a dose-dependent manner. AlCl3 (100 microM) decreases the Vmax by 14% but does not alter the KM, suggestive of non-competitive inhibition. The enzyme from aged brain displays a greater Vmax, but shows the same susceptibility to AlCl3 as the enzyme from younger brain. In summary, disruption of the Na+,K(+)-ATPase may underlie, at least in part, abnormalities of nerve and vascular cell function in disorders where elevated concentrations of fatty acids or metal ions are involved.

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