Radiation inactivation of (Na,K)-ATPase, an enzyme showing multiple radiation-sensitive domains

Depth of the Steroid Core Location Determines the Mode of Na,K-ATPase Inhibition by Cardiotonic Steroids

Cardiotonic steroids (CTSs) are specific inhibitors of Na,K-ATPase (NKA). They induce diverse physiological effects and were investigated as potential drugs in heart diseases, hypertension, neuroinflammation, antiviral and cancer therapy. Here, we compared the inhibition mode and binding of CTSs, such as ouabain, digoxin and marinobufagenin to NKA from pig and rat kidneys, containing CTSs-sensitive (α1S) and -resistant (α1R) α1-subunit, respectively. Marinobufagenin in contrast to ouabain and digoxin interacted with α1S-NKA reversibly, and its binding constant was reduced due to the decrease in the deepening in the CTSs-binding site and a lower number of contacts between the site and the inhibitor. The formation of a hydrogen bond between Arg111 and Asp122 in α1R-NKA induced the reduction in CTSs’ steroid core deepening that led to the reversible inhibition of α1R-NKA by ouabain and digoxin and the absence of marinobufagenin’s effect on α1R-NKA activity. Our results elucidate that the difference in signaling, and cytotoxic effects of CTSs may be due to the distinction in the deepening of CTSs into the binding side that, in turn, is a result of a bent-in inhibitor steroid core (marinobufagenin in α1S-NKA) or the change of the width of CTSs-binding cavity (all CTSs in α1R-NKA).

Effects of palytoxin on cation occlusion and phosphorylation of the (Na+,K+)-ATPase

Palytoxin (PTX) inhibits the (Na(+) + K+)-driven pump and simultaneously opens channels that are equally permeable to Na+ and K+ in red cells and other cell membranes. In an effort to understand the mechanism by which PTX induces these fluxes, we have studied the effects of PTX on: 1) K+ and Na+ occlusion by the pump protein; 2) phosphorylation and dephosphorylation of the enzyme when a phosphoenzyme is formed from ATP and from P(i); and 3) p-nitro phenyl phosphatase (p-NPPase) activity associated with the (Na+, K+)-ATPase. We have found that palytoxin 1) increases the rate of deocclusion of K+(Rb+) in a time- and concentration-dependent manner, whereas Na+ occluded in the presence of oligomycin is unaffected by the toxin; 2) makes phosphorylation from P(i) insensitive to K+, and 3) stimulates the p-NPPase activity. The results are consistent with the notion that PTX produces a conformation of the Na+, K(+)-pump that resembles the one observed when ATP is bound to its low-affinity binding site. Further, they suggest that the channels that are formed by PTX might arise as a consequence of a perturbation in the ATPase structure, leading to the loss of control of the outside "gate" of the enzyme and hence to an uncoupling of the ion transport from the catalytic function of the ATPase.

Na+/K(+)-ATPase with a blocked E1ATP site still allows backdoor phosphorylation of the E2ATP site

The role of simultaneously existing ATP-binding sites in the catalytic process of Na+/K(+)-ATPase is unclear. In order to learn whether blocking the E1ATP site affects the properties of the E2ATP site, the E1ATP site was inactivated by either fluorescein 5'-isothiocyanate, the non-phosphorylating Cr(H2O)4AdoPP[CH2]P or the phosphorylating Cr(H2O)4ATP. The properties of the remaining E2ATP site were studied by measuring 'backdoor phosphorylation' in the presence of ouabain, or K(+)-activated hydrolysis of p-nitrophenyl phosphate. The involvement of the E2ATP site was further tested by the effects of Co(NH3)4ATP, a specific inactivator of this site. When the E1ATP site was inactivated by fluorescein 5'-isothiocyanate or the non-phosphorylating Cr(H2O)4AdoPP[CH2]P, backdoor phosphorylation and the activity of K(+)-activated p-nitrophenylphosphatase remained unchanged. Both processes were lost, however, when the E2ATP site was additionally inactivated by Co(NH3)4ATP. Inactivation of the E1ATP site by fluorescein 5'-isothiocyanate or Cr(H2O)4AdoPP[CH2]P decreased the affinity of the p-nitrophenylphosphatase activity of the E2ATP site for the substrate p-nitrophenyl phosphate by four times. This is consistent with a former report showing that dephosphorylation in a fluorescein 5'-isothiocyanate-inactivated Na+/K(+)-ATPase has a lowered sensitivity for ATP [Scheiner-Bobis, G., Antonipillai, J. & Farley, R. A. (1993) Biochemistry 32, 9592-9599]. Inactivation of the E1ATP site by the phosphorylating Cr(H2O)4ATP, however, led to a loss of the property of the E2ATP site to hydrolyse K(+)-dependent p-nitrophenyl phosphate and to achieve backdoor phosphorylation. Evidently, ATP sites coexist in Na+/K(+)-ATPase, and binding of ATP to one site affects the property of the other site [Scheiner-Bobis, G., Esmann, M. & Schoner, W. (1989) Eur. J. Biochem. 183, 173-178]. Although the enzyme can be phosphorylated from both ATP sites, phosphorylation of the E1ATP site excludes the phosphorylation of the E2ATP site.

The effect of near-UV light on Na-K-ATPase of the rat lens

The influence of in vitro near-UV radiation exposure on the physical state of the rat lens and on its membrane-bound Na-K-ATPase activity was investigated. Lens swelling was correlated to the appearance of opacities and the inactivation of the enzyme. The results show a significant decrease in the Na-K-ATPase activity which may be an early change leading to osmotic type cataracts. The dose-effect curves obtained for cortical and epithelial enzymes were different. Since the data do not follow a monoexponential function, the existence of two forms of Na-K-ATPase in the lens is discussed.

(Na+ + K+)-ATPase: on the number of the ATP sites of the functional unit

Questions concerning the number of the ATP sites of the functional unit of (Na+ + K+)-ATPase (i.e., the sodium pump) have been at the center of the controversies on the mechanisms of the catalytic and transport functions of the enzyme. When the available data pertaining to the number of these sites are examined without any assumptions regarding the reaction mechanism, it is evident that although some relevant observations may be explained either by a single site or by multiple ATP sites, the remaining data dictate the existence of multiple sites on the functional unit. Also, while from much of the data it is clear that the multiple sites of the unit enzyme represent the interacting catalytic sites of an oligomer, it is not possible to rule out the existence of a distinct regulatory site for ATP in addition to the interacting catalytic sites. Regardless of the ultimate fate of the regulatory site, any realistic approach to the resolution of the kinetic mechanism of the sodium pump should include the consideration of the established site-site interactions of the oligomer.

Characteristics of ethacrynic acid highly sensitive Mg2+-ATPase in microsomal fractions of the rat brain: functional molecular size, inhibition by SITS and stimulation by Cl-

Studies were performed to characterize ethacrynic acid (EA) highly sensitive Mg2+-ATPase isolated from microsomal fractions of the rat brain. The functional molecular sizes of the EA highly sensitive and EA less sensitive Mg2+-ATPases, estimated by a radiation inactivation method, were 480 and 80 kDa, respectively. An anion transport inhibitor, 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS) inhibited the EA highly sensitive Mg2+-ATPase activity. The type of inhibition was uncompetitive with respect to ATP, and the inhibition was suppressed by anions such as Cl-, Br- and I-. Chloride ions stimulated enzyme activity with an increase in Vmax, but not in Km, for ATP. Anions tested also increased the enzyme activity in the following order of decreasing potency: Cl- greater than Br- greater than CH3COO- = I- greater than SO4(2-) = HCO3- greater than SO3(2-). These results suggest that EA highly sensitive Mg2+-ATPase is a relatively large molecule with anion-sensitive sites that affect the ATP hydrolyzing activity and the SITS binding capacity through anions, with Cl- being the most potent.

Target molecular weights for red cell band 3 stilbene and mercurial binding sites

Radiation inactivation was used to measure the target sizes for binding of disulfonic stilbene anion transport inhibitor 4,4'-dibenzamido-2,2'-disulfonic stilbene (DBDS) and mercurial water transport inhibitor p-chloromercuribenzene sulfonate (pCMBS) to human erythrocytes. The measured target size for erythrocyte ghost acetylcholinesterase was 78 +/- 3 kDa. DBDS binding to ghost membranes was measured by a fluorescence enhancement technique. Radiation (0-26 Mrad) had no effect on total membrane protein and DBDS binding affinity, whereas DBDS binding stoichiometry decreased exponentially with radiation dose, giving a target size of 59 +/- 4 kDa. H2-4,4'-diisothiocyano-2,2'-disulfonic stilbene (H2-DIDS, 5 microM) blocked greater than 95% of DBDS binding at all radiation doses. pCMBS binding was measured from the time course of tryptophan fluorescence quenching in ghosts treated with the sulfhydryl reagent N-ethylmaleimide (NEM). Radiation did not affect the kinetics of tryptophan quenching, whereas the total amplitude of the fluorescence signal inactivated with radiation with a target size of 31 +/- 6 kDa. These results support the notion that DBDS and pCMBS bind to the transmembrane domain of erythrocyte band 3 in NEM-treated ghosts and demonstrate that radiation inactivation may probe a target significantly smaller than a covalently linked protein subunit. The small target size for the band 3 stilbene binding site may correspond to the intramembrane domain of the band 3 monomer (52 kDa), which is physically distinct from the cytoplasmic domain (42 kDa).

Deconjugation of glucuronides catalysed by UDPglucuronyltransferase

Evidence was found for UDPglucuronyltransferase-catalysed deconjugation of p-nitrophenol-, 4-methylumbelliferone- and phenolphthalein-glucuronides. The evidence is based on the following observations: 1, deconjugation is UDP-dependent and the reactions show Michaels-Menten kinetics with respect to UDP and glucuronide saturability; 2, UDP-glucuronic acid was identified as reaction product; 3, all studies were done in the presence of a beta-glucuronidase inhibitor; 4, induction profiles, using 3-methylcholanthrene and phenobarbital as inducing agents, were identical for conjugation and deconjugation reactions. Optimal deconjugation rates for p-nitrophenol- and 4-methylumbelliferone-glucuronides were at pH 5.1 and for phenolphthalein-glucuronide at pH 6.5. Only conjugation reactions showed latency; the corresponding deconjugation reactions were not latent. UDPglucuronyltransferase is a group of oligomeric isoenzymes with different molecular masses. The molecular masses of the isoenzyme species catalysing the forward and reverse reactions were determined by radiation-inactivation analysis. The molecular masses of the isoenzyme species mediating the catalyses of deconjugation reactions were significantly smaller than those mediating catalyses of conjugation reactions: 66 +/- 4 kDa vs. 109 +/- 7 kDa for p-nitrophenol; 82 +/- 8 kDa vs. 105 +/- 6 kDa for 4-methylumbelliferone; and 74 +/- 8 kDa vs. 159 +/- 14 kDa for phenolphthalein. This suggests that for catalyses of deconjugation reactions only part of a UDPglucuronyltransferase isoenzyme is needed, whereas for forward reactions the complete isoenzymes are required.

Radiation inactivation of multimeric enzymes: application to subunit interactions of adenylate cyclase

Radiation inactivation has been applied extensively to determine the molecular weight of soluble enzyme and receptor systems from the slope of a linear ln (activity) vs. dose curve. Complex nonlinear inactivation curves are predicted for multimeric enzyme systems, composed of distinct subunits in equilibrium with multimeric complexes. For the system A1 + A2----A1A2, with an active A1A2 complex (associative model), the ln (activity) vs. dose curve is linear for high dissociation constant, K. If a monomer, A1, has all the enzyme activity (dissociative model), the ln (activity) vs. dose curve has an activation hump at low radiation dose if the inactive subunit, A2, has a higher molecular weight than A1 and has upward concavity when A2 is smaller than A1. In general, a radiation inactivation model for a multistep mechanism for enzyme activation fulfills the characteristics of an associative or dissociative model if the reaction step forming active enzyme is an associative or dissociative reaction. Target theory gives the molecular weight of the active enzyme subunit or complex from the limiting slope of the ln (activity) vs. dose curve at high radiation dose. If energy transfer occurs among subunits in the multimer, the ln (activity) vs. dose curve is linear for a single active component and is concave upward for two or more active components. The use of radiation inactivation as a method to determine enzyme size and multimeric subunit assembly is discussed with specific application to the hormone-sensitive adenylate cyclase system. It is shown that the complex inactivation curves presented in the accompanying paper can be used select the best mechanism out of a series of seven proposed mechanisms for the activation of adenylate cyclase by hormone.

Occlusion of Rb+ by detergent-solubilized (Na+ + K+)-ATPase from shark salt glands

Occlusion of Rb+ by C12E8-solubilized (Na+ + K+)-ATPase from shark salt glands has been measured. The rate of de-occlusion at room temperature is about 1 s-1, which is the same as for the membrane-bound enzyme. The amount of Rb+ occluded is 3 moles Rb+ per mole membrane-bound shark enzyme, whereas only about 2 moles Rb+ are occluded by the C12E8-solubilized enzyme.

Effects of digitalis on cell biochemistry: sodium pump inhibition

It is now generally agreed that Na+-K+ adenosine triphosphatase (ATPase), a transport enzyme derived from the sarcolemmal sodium pump, is the primary site at which digitalis exerts its effects on the myocardial cell. Inhibition of the ability of this ion transport enzyme to catalyze Na+ efflux from the cell in exchange for K+ leads to both the therapeutic and toxic effects of the cardiac glycosides. The mechanism by which digitalis inhibits the sodium pump has been established in studies of Na+-K+ ATPase which show that the ability of cardiac glycosides to inhibit adenosine triphosphate (ATP)-supported transport of Na+ is reduced in the presence of elevated levels of K+. These studies explain the ability of hypokalemia to potentiate the effects of cardiac glycosides on the heart, and of high K+ concentrations to overcome the inhibition of sodium pump activity by the cardiac glycosides. Recent demonstrations that the positive inotropic effect of the cardiac glycosides is correlated with an increased intracellular Na+ provide strong evidence that these effects of digitalis to impair sodium efflux are responsible for the increased myocardial contractility caused by digitalis.

Molecular size of the high-affinity glutamate-binding site on synaptic membranes from rat brain

We have used radiation inactivation as a means of determining the molecular size of the high-affinity glutamate-binding site on rat brain synaptic membranes. The molecular size was 75,000 +/- 15,000 in the absence of glutamate and 263,000 +/- 34,000 in the presence of glutamate. These data may be interpreted as suggesting that the high-affinity glutamate-binding site is comprised of a number of subunits. The minimum sub-unit size detected by this method was 75,000 +/- 15,000.

The molecular pharmacology and structural features of calcium channels

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Minimal functional unit for transport and enzyme activities of (Na+ + K+)-ATPase as determined by radiation inactivation

Frozen aqueous suspensions of partially purified membrane-bound renal (Na+ + K+)-ATPase have been irradiated at -135 degrees C with high-energy electrons. (Na+ + K+)-ATPase and K+-phosphatase activities are inactivated exponentially with apparent target sizes of 184 +/- 4 kDa and 125 +/- 3 kDa, respectively. These values are significantly lower then found previously from irradiation of lyophilized membranes. After reconstitution of irradiated (Na+ + K+)-ATPase into phospholipid vesicles the following transport functions have been measured and target sizes calculated from the exponential inactivation curves: ATP-dependent Na+-K+ exchange, 201 +/- 4 kDa; (ATP + Pi)-activated Rb+-Rb+ exchange, 206 +/- 7 kDa and ATP-independent Rb+-Rb+ exchange, 117 +/- 4 kDa. The apparent size of the alpha-chain, judged by disappearance of Coomassie stain on SDS-gels, lies between 115 and 141 kDa. That for the beta-glycoprotein, though clearly smaller, could not be estimated. We draw the following conclusions: (1) The simplest interpretation of the results is that the minimal functional unit for (Na+ + K+)-ATPase is alpha beta. (2) The inactivation target size for (Na+ + K+)-dependent ATP hydrolysis is the same as for ATP-dependent pumping of Na+ and K+. (3) The target sizes, for K+-phosphatase (125 kDa) and ATP-independent Rb+-Rb+ exchange (117 kDa) are indistinguishable from that of the alpha-chain itself, suggesting that cation binding sites and transport pathways, and the p-nitrophenyl phosphate binding site are located exclusively on the alpha-chain. (4) ATP-dependent activities appear to depend on the integrity of an alpha beta complex.

Sizes of opioid receptor types in rat brain membranes

The sizes of the receptors binding opiates and enkephalins in rat brain membranes were investigated by the radiation inactivation technique. By comparison with enzymes of known size added as internal standards, the mu and delta binding sites both gave a molecular weight of about 110000. Other opiate-binding components, which may include the kappa site, showed a much lower rate of inactivation when in reducing conditions, implying a subunit molecular weight of the order of 30000 for such a site.

Target size analysis of rhodopsin in retinal rod disk membranes

Radiation inactivation of rhodopsin in situ using high-energy electrons gave a value for Mr of 20,200 by spectral assay, but 47,100 by assay of rhodopsin regeneration from opsin and 11-cis-retinal (sequence Mr = 38,840). No light/dark differences were seen. We conclude: (a) radiation inactivation measures the size of the functional unit, and the single hit hypothesis does not hold in our experiments; (b) 500 nm absorbance requires only about half the rhodopsin molecule to be intact, but reconstitution of rhodopsin from opsin requires the whole molecule; (c) we find no evidence for functional interactions between rhodopsin monomers in darkness or light.