Lead ion activates phosphorylation of electroplax Na+- and K+-dependent adenosine triphosphatase ((NaK)-ATPase) in the absence of sodium ion

Aluminum-induced alterations in [3H]ouabain binding and ATP hydrolysis catalyzed by the rat brain synaptosomal (Na(+)+K+)-ATPase

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.

Control of the Na+,K(+)-ATPase under normal and pathological conditions

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.

Alterations of cerebromicrovascular Na+,K(+)-ATPase activity due to fatty acids and acute hypertension

Acute hypertension, induced in rats by intravenous injection of angiotensin II, previously has been shown to increase cerebrovascular permeability to macromolecules. The purpose of this study was to examine the effect of acute hypertension on Na+,K(+)-ATPase, the enzyme responsible for controlling ionic permeability of the cerebromicrovascular endothelium. The K(+)-dependent p-nitrophenylphosphatase activity of the cerebromicrovascular Na+,K(+)-ATPase was determined using microvessels prepared from hypertensive and normotensive rats. When compared to controls, a 70% decrease (P < 0.02) in the maximum rate (Vmax) of the Na+,K(+)-ATPase from hypertensive rats was evident with no change in the Michaelis constant (KM). In contrast, gamma-glutamyltranspeptidase, a marker enzyme for cerebral endothelial cells, was not significantly affected. Sodium arachidonate (1-100 microM) inhibited the phosphatase activity of the Na+,K(+)-ATPase in microvessels isolated from both normotensive and hypertensive rats in a dose-dependent manner. Furthermore, poly-unsaturated fatty acids (sodium linoleate and arachidonate) evoked the greatest inhibition of the enzyme, while sodium oleate and sodium palmitate inhibited the Na+,K(+)-ATPase to lesser extents. This regulation of enzyme activity by fatty acids was comparable in control and hypertensive groups. In summary, the data indicate that the cerebromicrovascular Na+,K(+)-ATPase was altered as a consequence of acute hypertension and that poly-unsaturated fatty acids can modulate this enzyme in microvessels derived from hypertensive or control rats.

Pb2+ and imidazole-activated phosphorylation by ATP of (Na+ + K+)-ATPase

In order to study whether Pb2+ and imidazole increase the ATP phosphorylation level of (Na+ + K+)-ATPase by the same mechanism, the effects of both compounds on phosphorylation and dephosphorylation reactions of the enzyme have been studied. Imidazole in the presence of Mg2+ increases steady-state phosphorylation of (Na+ + K+)-ATPase by decreasing, in a competitive way, the K+-sensitivity of the formed phospho-enzyme (E-P . Mg). If Pb2+ is present during phosphorylation, the rate of phosphorylation increases and a K+- and ADP-insensitive phosphointermediate (E-P . Pb) is formed. Pb2+ has no effect on the K+-sensitivity of E-P . Mg and EDTA is unable to affect the K+-insensitivity of E-P . Pb. These effects indicate that Pb2+ acts before or during phosphorylation with the enzyme. Binding of Na+ to E-P . Pb does not restore K+-sensitivity either. However, increasing Na+ during phosphorylation in the presence of Pb2+ leads to formation of the K+-sensitive intermediate (E-P . Mg), indicating that E-P . Pb is formed via a side path of the Albers-Post scheme. ATP and ADP decrease the dephosphorylation rate of both E-P . Mg and E-P . Pb. Above optimal concentration, Pb2+ also decreases the steady-state phosphorylation level both in the absence and in the presence of Na+. This inhibitory effect of Pb2+ is antagonized by Mg2+.

Inhibitory characteristics of lead chloride in sodium- and potassium- dependent adenosinetriphosphatase preparations derived from kidney, brain, and heart of several species

Inhibition of adenosinetriphosphatase (ATPase) by lead chloride (PbCl2) was studied in microsomal fractions or tissue homogenates of kidney, brain, and heart of several species, including humans. The concentration of PbCl2 causing 50% inhibition (I50) of Na+ + K+ ATPase activity varied from 8 X 10(-6) to 8 X 10(-5) M, depending on the species and organ of origin of the enzyme. The enzyme preparations derived from various parts of the kidney showed no differential sensitivity to PbCl2. These differences in sensitivity to lead were not related to specific activity of the enzyme or to the protein content of the preparations studied. Mg2+ ATPase, which contaminated the enzyme preparations to a variable degree, was 10--100 times more resistant to PbCl2 than was Na+ + K+-activated ATPase. The following more detailed studies were performed on the dog brain and/or kidney enzyme. The inhibition of microsomal Na+ + K+ ATPase was characterized by reversible kinetics. The inhibitory effect was antagonized by Na+, increased by Mg2+, and not altered by K+. ATP alone, or together with Mg2+, antagonized the inhibition. Disodium edetate prevented or reversed the inhibition. These inhibitory characteristics suggest that Pb2+ inhibits Na+ + K+ ATPase at the Na+-dependent phosphorylation site, and that ATP chelates Pb2+ in competition with Mg2+. Combining Pb2+ with ATP may not only result in a reduction of ATPase activity but also cause a relative ATP deficiency if lead is present in sufficiently high concentration.

Lead actions on sodium-plus-potassium-activated adenosinetriphosphatase from electroplax, rat brain, and rat kidney

Inorganic lead ion, in micromolar concentrations, reversibly inhibits the sodium-plus-potassium-activated adenosinetriphosphatase (ATPase) and potassium-activated p-nitrophenylphosphatase (NPPase) activities of microsomal fractions from electric organ, rat kidney, and rat brain. In the presence of 3 mM MgC12 and 3 mM ATP, the concentrations of PbC12 producing half-maximal inhibition of the ATPase from these tissues are 4 X 10(-6) M, 20 X 10(-6) M, and 55 X 10(-6) M, respectively. The corresponding values for inhibition of the NPPase are 10(-6) M, 53 X 10(-6) M, and 22 X 10(-6) M. PbC12 also stimulates the phosphorylation by [gamma-32P]ATP of a microsomal protein from all three tissues in the absence of added sodium ion. This reaction was extensively studied with electroplax microsomes. In common with the well-known Na+-dependent phosphorylation of (Na+ + K+)-ATPase, the Pb2 -dependent reaction is inhibited by ouabain, specific for ATP, dependent on Mg2+, and yields and acid-stable phosphoprotein with a molecular weight of 98,000 in sodium dodecylsulfate. The Pb2+-dependent phosphoprotein, however, is not sensitive to K+. These observations are pertinent to the biochemistry and toxicity of inorganic lead in tissues and to the molecular mechanism of the cation transport enzyme.