Solvent effects on ouabain binding to the (Na+,K+)-ATPase of rat brain

Deuteration as a tool in investigating the role of protons in cell signaling

BACKGROUND

The mechanisms underlying the inhibitory effects of deuterium oxide (D₂O; heavy water) are likely to provide insight into the fundamental significance of hydrogen bonds in biological functions. Previously, to begin elucidating the effect of D₂O on physiological functions in living cells, we studied the effects of D₂O on voltage-sensitive Ca²(+) channels in AtT 20 cells and showed that actin distribution, Ca²(+) currents, and β-endorphin release were affected. However, the molecular mechanisms underlying the inhibitory effects of D₂O in whole animals and living cells remain obscure, especially in the effects of D₂O on the cell signaling.

METHODS

We investigated the molecular mechanisms underlying the inhibitory effects of D₂O on the IP₃-mediated Ca²(+) signaling pathway using Ca²(+) imaging and micro-calorimetric measurements in mGluR1-expressing CHO cells.

RESULTS

DHPG-induced Ca²(+) elevations were markedly reduced in D₂O. Moreover, the Ca²(+) elevations were completely suppressed in H₂O after receptor activation with DHPG in D₂O, recovering gradually in H₂O medium. Without prior stimulation in D₂O, however, DHPG-induced Ca²(+) elevations in H₂O were not affected. Micro-calorimetric measurements showed reduced total DHPG-evoked heat generation in D₂O, while initial heat production and absorption associated with receptor activation were found to be larger. The reduction of DHPG-induced Ca²(+) elevation and heat generation in D₂O medium may be due to decreased amount of IP₃ by the reduced hydrolysis of PIP₂.

GENERAL SIGNIFICANCE

Protein structure changes due to the replacement of hydrogen with deuterium will induce the inhibitory effects of D₂O by reduction of the frequency of -OH bonds.

Identification of hydroxyxanthones as Na/K-ATPase ligands

We have screened a chemical library and identified several novel structures of Na/K-ATPase inhibitors. One group of these inhibitors belongs to polyphenolic xanthone derivatives. Functional characterization reveals the following properties of this group of inhibitors. First, like ouabain, they are potent inhibitors of the purified Na/K-ATPase. Second, their effects on the Na/K-ATPase depend on the number and position of phenolic groups. Methylation of these phenolic groups reduces the inhibitory effect. Third, further characterization of the most potent xanthone derivative, MB7 (3,4,5,6-tetrahydroxyxanthone), reveals that it does not change either Na(+) or ATP affinity of the enzyme. Finally, unlike that of ouabain, the inhibitory effect of MB7 on Na/K-ATPase is not antagonized by K(+). Moreover, MB7 does not activate the receptor Na/K-ATPase/Src complex and fails to stimulate protein kinase cascades in cultured cells. Thus, we have identified a group of novel Na/K-ATPase ligands that can inhibit the pumping function without stimulating the signaling function of Na/K-ATPase.

Ethanol may stimulate or inhibit (Na+ + K+)-ATPase, depending upon Na+ and K+ concentrations

The influence of varying the ratios of [Na+]/[K+] on the effects of alcohol (500 mg/dl) on brain (Na+ + K+)-ATPase, using a commercial porcine enzyme preparation, showed that, generally, activity was stimulated by ethanol when [Na+] less than [K+], but inhibited when [Na+] greater than [K+] (with sum kept constant at 150 mM). In addition, when [Na+]/[K+] was 15/90 mM, representative of normal intracellular levels, ethanol (500 mg/dl) stimulated the porcine enzyme, but inhibited it when [Na+]/[K+] was 144/6 mM, representative of normal extracellular levels. Similarly, in freshly prepared enzyme from highly purified rat brain synaptic membranes, ethanol (100, 300, and 450 mg/dl) stimulated when [Na+]/[K+] was 15/88 mM (representing intracellular levels), but inhibited when [Na+]/[K+] was 142/4 mM (extracellular levels).

(Na+,K+)-ATPase of mammalian brain: effects of temperatures on cation and ATP interactions regulating phosphatase activity

The effects of temperature on interactions between univalent cations or ATP and the p-nitrophenylphosphatase activity associated with brain (Na+,K+)-ATPase were examined. The apparent affinity for K+ activation under conditions favoring the moderate affinity site was temperature dependent, increasing with decreasing temperature. A comparison of univalent cations showed that the negative apparent delta H and delta S for cation binding increased with increasing apparent cation affinity. In contrast to the case with the moderate affinity sites, apparent affinity for the high affinity K+ site was independent of temperature. As temperature decreased, properties of moderate affinity site binding approached those of the high affinity site. The temperature dependence of ATP inhibition was opposite to that for K+ activation, with positive apparent delta H and delta S. The apparent delta H and delta S for cation binding approached those for the overall conformational change to K+-sensitive enzyme as cation affinity increased. These data suggest that E2, the K+-sensitive form of (Na+,K+)-ATPase, is stabilized by forces that require a decrease in entropy, explaining the predominant existence of E1 at physiologic temperatures. A conformational change leading to stabilization of E2 at higher temperatures can be produced by binding of univalent cations to a moderate affinity, presumably intracellular, site. This effect is counteracted by ATP. ATP also appears to alter the selectivity of this site to favor Na+ over K+ binding.