Study on the differential modifications of (Na+ plus K+)-ATPase and its partial reactions by dimethylsulfoxide

Dimethyl sulfoxide inhibits renal Na(+)-K(+)-ATPase at a site different from ouabain and atrial peptides

The present investigation was designed to determine 1) if dimethyl sulfoxide (DMSO) inhibits renal cortical and medullary Na(+)-K(+)-ATPase at the ouabain binding site and 2) if this inhibition of Na(+)-K(+)-ATPase by. DMSO involves atrial natriuretic peptides or prostaglandins. DMSO (10%) inhibited renal cortical and medullary Na(+)-K(+)-ATPase 31% and 29.5%, respectively. Ouabain (0.5 mM) inhibited renal cortical and medullary Na(+)-K(+)-ATPase 32% and 35%, respectively. When DMSO and ouabain were added together the inhibition of renal cortical and medullary Na(+)-K(+)-ATPase was 55% and 59%, respectively. Atrial natriuretic peptides consisting of amino acids 1-30 (i.e., long acting natriuretic peptide), 31-67 (vessel dilator), 79-98 (kaliuretic peptide) and 99-126 (atrial natriuretic factor, ANF) of the 126 amino acid ANF prohormone inhibited renal cortical Na(+)-K(+)-ATPase 27.5%, 20%, 37.5% and 0% at a 10(-11)M concentration. The addition of DMSO caused a doubling (P < 0.05) of this inhibition. Likewise, these same atrial peptides inhibited renal medullary Na(+)-K(+)-ATPase 27.8%, 19.2%, 39.5%, and 0% with DMSO doubling (P < 0.05) their Na(+)-K(+)-ATPase inhibition. There was not any additive effect of any of the atrial peptides with ouabain. Naproxen, a prostaglandin inhibitor, completely blocked atrial peptides and ouabain's inhibition of Na(+)-K(+)-ATPase but not DMSO's. Each of the atrial peptides except ANF increased prostaglandin E2 synthesis while DMSO did not increase prostaglandin E2 synthesis. This investigation suggests that DMSO has its inhibiting effect at a site different from the ouabain binding site on renal Na(+)-K(+)-ATPase since it has an additive effect with ouabain in inhibiting Na(+)-K(+)-ATPase while the atrial peptides appear to have their effect at the ouabain binding site. As opposed to the atrial peptides, DMSO's mechanism of inhibiting Na(+)-K(+)-ATPase does not appear to involve prostaglandin E2.

The role of Mg2+ and K+ in the phosphorylation of Na+,K(+)-ATPase by ATP in the presence of dimethylsulfoxide but in the absence of Na+

We have previously demonstrated that Na+,K(+)-ATPase can be phosphorylated by 100 microM ATP and 5 mM Mg2+ and in the absence of Na+, provided that 40% dimethylsulfoxide (Me2SO) is present. Phosphorylation was stimulated by K+ up to a steady-state level of about 50% of Etot (Barrabin et al. (1990) Biochim. Biophys. Acta 1023, 266-273). Here we describe the time-course of phosphointermediate (EP) formation and of dephosphorylation of EP at concentrations of Mg2+ from 0.1 to 5000 microM and of K+ from 0.01 to 100 mM. The results were simulated by a simplified version of the commonly accepted Albers-Post model, i.e. a 3-step reaction scheme with a phosphorylation, a dephosphorylation and an isomerization/deocclusion step. Furthermore it was necessary to include an a priori, Mg(2+)- and K(+)-independent, equilibration between two enzyme forms, only one of which (constituting 14% of Etot) reacted directly with ATP. The role of Mg(2+) was two-fold: At low Mg2+, phosphorylation was stimulated by Mg2+ due to formation of the substrate MgATP, whereas at higher concentrations it acted as an inhibitor at all three steps. The affinity for the inhibitory Mg(2+)-binding was increased several-fold, relative to that in aqueous media, by dimethylsulfoxide. K+ stimulated dephosphorylation at all Mg(2+)-concentrations, but at high, inhibitory [Mg2+], K+ also stimulated the phosphorylation reaction, increasing both the rate coefficient and the steady-state level of EP. Generally, the presence of Me2SO seems to inhibit the dephosphorylation step, the isomerization/deocclusion step, and to a lesser extent (if at all) the phosphorylation reaction, and we discuss whether this reflects that Me2SO stabilizes occluded conformations of the enzyme even in the absence of monovalent cations. The results confirm and elucidate the stimulating effect of K+ on EP formation from ATP in the absence of Na+, but they leave open the question of the molecular mechanism by which Me2SO, inhibitory Mg2+ and stimulating K+ interact with the Na+,K(+)-ATPase.

Effects of ATP and monovalent cations on Mg2+ inhibition of (Na,K)-ATPase

The hydrolysis of ATP catalyzed by purified (Na,K)-ATPase from pig kidney was more sensitive to Mg2+ inhibition when measured in the presence of saturating Na+ and K+ concentrations [(Na,K)-ATPase] than in the presence of Na+ alone, either at saturating [(Na,Na)-ATPase] or limiting [(Na,0)-ATPase] Na+ concentrations. This was observed at two extreme concentrations of ATP (3 mM where the low-affinity site is involved and 3 microM where only the catalytic site is relevant), although Mg2+ inhibition was higher at low ATP concentration. In the case of (Na,Na)-ATPase activity, inhibition was barely observed even at 10 mM free Mg2+ when ATP was 3 mM. When (Na,K)-ATPase activity was measured at different fixed K+ concentrations the apparent Ki for Mg2+ inhibition was lower at higher monovalent cation concentration. When K+ was replaced by its congeners (Rb+, NH+4, Li+), Mg2+ inhibition was more pronounced in those cases in which the dephosphorylating cation forms a tighter enzyme-cation complex after dephosphorylation. This effect was independent of the ATP concentration, although inhibition was more marked at lower ATP for all the dephosphorylating cations. The K0.5 for ATP activation at its low-affinity site, when measured in the presence of different dephosphorylating cations, increased following the sequence Rb+ greater than K+ greater than NH+4 greater than Li+ greater than none. The K0.5 values were lower with 0.05 mM than with 10 mM free Mg2+ but the order was not modified. The trypsin inactivation pattern of (Na,K)-ATPase indicated that Mg2+ kept the enzyme in an E1 state. Addition of K+ changed the inactivation into that observed with the E2 enzyme form. On the other hand, K+ kept the enzyme in an E2 state and addition of Mg2+ changed it to an E1 form. The K0.5 for KCl-induced E1-to-E2 transformation (observed by trypsin inactivation profile) in the presence of 3 mM MgCl2 was about 0.9 mM. These results concur with two mechanisms for free Mg2+ inhibition of (Na,K)-ATPase: "product" and dead-end. The first would result from Mg2+ interaction with the enzyme in the E2(K) occluded state whereas the second would be brought about by a Mg2+-enzyme complex with the enzyme in an E1 state.

Membrane stabilization during freezing: the role of two natural cryoprotectants, trehalose and proline

The relative effectiveness of two natural cryoprotectants, proline and trehalose, in preserving membrane structure and function during freezing was studied. Isolated vesicles of sarcoplasmic reticulum (SR) from lobster muscle (Homarus americanus) were employed to study changes in structure and function during rapid freeze-thaw conditions. Both proline and trehalose were shown to effectively preserve the structure (assessed with freeze fracture) and function (assessed by the ability of the membranes to transport calcium) in the frozen vesicles. As a first step toward determining the mechanism of cryoprotection by these compounds, we have investigated their effectiveness in inhibiting freezing induced fusion between phospholipid vesicles. Pamiltoyloleoyl-phosphatidylcholine: phosphatidylserine (85:15 mole ratio) small unilamellar vesicles (SUVs) were made incorporating one of the following fluorescent probes, and energy donor, cholesteryl anthracene-9-carboxylate, or an energy acceptor, nitrobenzo-2-oxa-1,3-diazole phosphatidylethanolamine to investigate the amount of membrane mixing during rapid freeze-thaw cycles, and storage at -20 degrees C. Membrane mixing was measured as an energy transfer from donor to acceptor when donor vesicles and acceptor vesicles were mixed before a particular freezing treatment. Membrane mixing was correlated with structural changes in these membranes by freeze-fracture analysis. Both trehalose and proline were found to be more effective in preventing membrane mixing between SUVs than the standard protectants, glycerol and dimethylsulfoxide.

Na+-like effect of imidazole on the phosphorylation of (Na+ + K+)-ATPase

A high basal level of phosphorylation (approx. 70% of the optimal Na+-dependent phosphorylation level) is observed in 50 mM imidazole-HCl (pH 7.0), in the absence of added Na+ and K+ and the presence of 10-100 microM Mg2+. In 50 mM Tris-HCl (pH 7.0) the basal level is only 5%, irrespective of the Mg2+ concentration. Nevertheless, imidazole is a less effective activator of phosphorylation than Na+ (Km imidazole-H+ 5.9 mM, Km Na+ 2 mM under comparable conditions). Imidazole-activated phosphorylation is strongly pH dependent, being optimal at pH less than or equal to 7 and minimal at pH greater than or equal to 8, while Na+-activated phosphorylation is optimal at pH 7.4. This suggests that imidazole-H+ is the activating species. Imidazole facilitates Na+-stimulated phosphorylation. The Km for Na+ decreases from 0.63 mM at 5 mM imidazole-HCl to 0.21 mM at 50 mM imidazole-HCl (pH 7; 0.1 mM Mg2+ in all cases). Imidazole-activated phosphorylation is more sensitive to inhibition by K+ (I50 = 12.5 microM) than Na+-activated phosphorylation (I50 = 180 microM). Mg2+ antagonizes activation by imidazole-H+ and also inhibition by K+. The Ki value for Mg2+ (approx. 0.3 mM) is the same for the two antagonistic effects. Tris buffer (pH 7.0) inhibits imidazole-activated phosphorylation with an I50 value of 30 mM in 50 mM imidazole-HCl (pH 7.0) plus 0.1 mM Mg2+. We conclude that imidazole-H+, but not Tris-H+, can replace Na+ as an activator of ATP-dependent phosphorylation, primarily by shifting the E2----E1 transition to the right, leading to a phosphorylating E1 conformation which is different from that in Tris buffer.

Evidence for a Mg2+-induced conformational change at the ATP-binding site of (Na+ + K+)-ATPase demonstrated with a photoreactive ATP-analogue

1. The 3'-ribosyl ester of ATP with 2-nitro-4-azidophenyl propionic acid has been prepared and its ability to act as a photoaffinity label of (Na+ + K+)-ATPase has been tested. 2. In the dark 3'-O-[3-(2-nitro-4-azidophenyl)-propionyl]adenosine triphosphate (N3-ATP) is a substrate of (Na+ + K+)-ATPase and a competitive inhibitor of ATP hydrolysis. 3. Upon irradiation by ultraviolet light, N3-ATP photolabels the high-affinity ATP-binding site and is covalently attached to the alpha-subunit and an approximately 12000-Mr component. 4. Photolabeling of the alpha-subunit by N3-ATP irreversibly inactivates (Na+ + K+)-ATPase. 5. Photoinactivation is strictly Mg2+-dependent. Na+ enhances the inactivation. ATP or ADP and K+ protect the enzyme against inactivation. 6. Mg2+, in concentrations required for photoinactivation, protects (Na+ + K+)-ATPase against inactivation by tryptic digestion under controlled conditions. 7. It is assumed that a conformational change of the ATP-binding site of (Na+ + K+)-ATPase occurs upon binding of Mg2+ to a low-affinity site.

Stimulatory and inhibitory effects of dimethyl sulfoxide and ethylene glycol on ATPase activity and calcium transport of sarcoplasmic membranes

1. The effect of dimethyl sulfoxide (Me2SO) and ethylene glycol on two different preparations of the sarcoplasmic reticulum, i.e. native membranes and membranes whose phospholipids were hydrolyzed by phospholipase A, were investigated using ATP and p-nitrophenylphosphate as substrates. 2. Me2SO and ethylene glycol inhibit both calcium-dependent ATP hydrolysis and ATP-supported calcium transport by native vesicles. 3. In contrast, calcium-dependent p-nitrophenylphosphatase activity as well as p-nitrophenyl-phosphate-supported calcium transport are activated by both agents at concentrations lower than 30% (v/v). 4. Me2SO strongly stimulates p-nitrophenylphosphate activity of vesicles treated with phospholipase A, but has relatively little effect on p-nitrophenylphosphatase activity of native vesicles. 5. Up to a concentration of approximately 40% Me2SO (v/v) the inhibiting effect on the calcium-dependent ATPase is fully reversible, but only partially reversible on calcium transport. 6. In the concentration range where Me2SO inhibits ATP hydrolysis and calcium transport, it does not affect ATP binding to the membranes nor calcium-dependent formation of phospho-protein. 7. The rate of dephosphorylation as well as the rate of Pi exchange between ATP and ADP are markedly reduced by the presence of 30% Me2SO (v/v). 8. While Me2SO inhibits passive calcium efflux, ethylene glycol produces a considerable activation. 9. ADP-dependent calcium efflux and ATP synthesis are activated by 15% Me2SO (v/v). Ethylene glycol reduces both activities. 10. The results suggest that the respective substrate-enzyme complexes are differently affected by the agents, resulting either in inhibition or stimulation

Na+-dependent phosphorylation of the rat brain (Na+ + K+)-ATPase. Possible non-equivalent activation sites for Na+

The steady state levels of Na+-dependent phosphoenzyme (E-P) in the (Na+ + K+)-ATPase (EC 3.6.1.3) of rat brain, obtained from a time course study of phosphoenzyme formation at 4 degrees C, were dependent on the concentration of Na+ in the reaction and were maximal in the presence of 64 mM Na+. The plot of phosphoenzyme vs. Na+ concentration gave a curve which on conversion to a double reciprocal plot (1/E-P vs. 1/Na+) gave a line with two breaks, yielding apparently three linear segments. This may be taken to indicate the presence of multiple Na+ sites for the formation of the phosphoenzyme. To test this hypothesis further, the following approach was taken. By making the assumption that the phosphoenzyme may represent bound Na+, it was possible to subject the data to rigorous multiple-site analysis by utilizing steady-state binding equations described by Klotz and Hunston (1971) (Biochemistry 10, 3065-3069), and by Scatchard (1949) (Ann. N.Y. Acad. Sci. 51, 660-672). The analysis of the data by these methods suggests that there may be three non-equivalent Na+ activation sites for the formation of Na+-dependent phosphoenzyme in the (Na+ + K+)-ATPase. The estimated intrinsic association constants (Ka) for activation by Na+ at each of the three sites were 3.4, 0.295, and 0.025 mM-1, respectively.