Na+,K+-ATPase: on the nature of the "cross-linked" subunits obtained in the presence of o-phenanthroline and cupric ion

Oligomerization of the Na,K-ATPase in cell membranes

The higher order oligomeric state of the Na,K-ATPase alphabeta heterodimer in cell membranes is the subject of controversy. We have utilized the baculovirus-infected insect cell system to express Na,K-ATPase with alpha-subunits bearing either His(6) or FLAG epitopes at the carboxyl terminus. Each of these constructs produced functional Na,K-ATPase alphabeta heterodimers that were delivered to the plasma membrane (PM). Cells were simultaneously co-infected with viruses encoding alpha-His/beta and alpha-FLAG/beta Na,K-ATPases. Co-immunoprecipitation of the His-tagged alpha-subunit in the endoplasmic reticulum (ER) and PM fractions of co-infected cells by the anti-FLAG antibody demonstrates that protein-protein associations exist between these heterodimers. This suggests the Na,K-ATPase is present in cell membranes in an oligomeric state of at least (alphabeta)(2) composition. Deletion of 256 amino acid residues from the central cytoplasmic loop of the alpha-subunit results in the deletion alpha-4,5-loop-less (alpha-4,5LL), which associates with beta but is confined to the ER. Co-immunoprecipitation demonstrates that when this inactive alpha-4,5LL/beta heterodimer is co-expressed with wild-type alphabeta, oligomers of wild-type alphabeta and alpha-4,5LL/beta form in the ER, but the alpha-4,5LL mutant remains retained in the ER, and the wild-type protein is still delivered to the PM. We conclude that the Na,K-ATPase is present as oligomers of the monomeric alphabeta heterodimer in native cell membranes.

Role of the self-association of beta subunits in the oligomeric structure of Na+/K+-ATPase

The two subunits of Na(+)/K(+)-ATPase that are essential for function are alpha and beta. Previous cross-linking studies on the oligomeric structure of the membrane-bound enzyme identified alpha,beta and alpha,alpha associations, but only the former and not the latter could be detected after solubilization. To study the possibility of direct beta,beta association, the purified membrane enzyme and a trypsin-digested enzyme that occludes cations and contains an essentially intact beta and fragments of alpha were subjected to oxidative cross-linking in the presence of Cu(2+)-phenanthroline. Resolution of products on polyacrylamide gels, N-terminal analysis and reactivity with anti-beta antibody showed that, in addition to previously identified products (e.g. alpha,alpha and alpha,beta dimers), a beta,beta dimer, most likely linked through intramembrane Cys(44) residues of two chains, is also formed. This dimer was also noted when digitonin-solubilized intact enzyme, and the trypsin-digested enzyme solubilized with digitonin or polyoxyethylene 10-laurylether were subjected to cross-linking, indicating that the detected beta,beta association was not due to random collisions. In the digested enzyme, K(+) but not Na(+) enhanced beta,beta dimer formation. The alternative cross-linking of beta-Cys(44) to a Cys residue of a transmembrane alpha-helix was antagonized specifically by K(+) or Na(+). The findings (i) indicate the role of beta,beta association in maintaining the minimum oligomeric structure of (alpha,beta)(2), (ii) provide further support for conformation-dependent flexibilities of the spatial relations of the transmembrane helices of alpha and beta and (iii) suggest the possibility of significant differences between the quaternary structures of the P-type ATPases that do and do not contain a beta subunit.

Unitary model of cell activation, growth control, cancer and other diseases: 1. Activated oxygen species and arachidonic acid modulation of solute permeabilities, internal Ca, Na and AOS levels and DNA transcription and synthesis

A comprehensive model of cellular activation and proliferation is developed. The model has arachidonic acid (ARA) produced mainly from PLA2 on both sides of the membrane, and superoxide and other activated oxygen species (AOS) formed from O2 by electrons passing out through membrane NANPH and NADH oxidases, as the immediate stimulants of solute permeability. Both ARA and AOS interact with the various solute channel proteins especially their external thiols and disulfides, to increase influx of metabolic substrates, Na, Ca and O2. PLA2 and NADPH oxidase are turned on by growth factors at their receptors acting through tyrosine kinase phosphorylations of messenger proteins GP and ras p-21, stimulated proteases, and by Ca-calmodulin. The adenylate cyclase system has opposite, deactivating character as it increases efflux of Ca and desensitizes growth factor receptors by phosphorylation to shut down the increased solute permeability. Most cancer types are due to carcinogen binding to cell membrane channel and mitochondrial sites for increased solute influx with excessive AOS production inside the cell from mitochondria and other vesicles. High Ca, Na and AOS stimulate proliferation with extra high levels causing transformation to the autogenic, more embryonic-type cancer cell.

Nonexclusive solute transport thru protein channels. Model of the Na,K ATPase complex and similar channels as general transport routes

In earlier work this author put forward a model of the Na,K ATPase complex as a general transport channel. Detailed treatment was limited to anion and monovalent cation transport. Here the functional mechanisms of the Na,K ATPase and similar protein channels as transport routes for all ionic fluxes and also amino acid, sugar and other solutes are presented. Anions, monosaccharide -OH groups and amino acid carboxyls bind to common arginyls and lose hydration water. They combine with cations which bind to adjacent side chain carboxyls, forming neutral ion pairs or positively charged complexes which have minimums in size, hydration and free polar groups. The smaller size and polarity facilitate entry into the tight, structured water channel of some 8-10 A outer bore. Solute fluxes depend on membrane redox activity which maintains channel sulfhydryls in reduced state required for proper transport. ATP binding at channels contributes to transport conformation while ATP hydrolysis gives high efflux of Na+, H+ and Ca2+ as phosphate ion pairs. This cation efflux current clears cations from inner membrane sites, increases negative potential and provides Na+ and H+ about the outer combining sites, while maintaining their inward gradients. Binding of many agents widens the outer bore to give larger, less selective influx.

Mode of action of the copper(I) complex of 2,9-dimethyl-1,10-phenanthroline on Mycoplasma gallisepticum

Various physiological important activities of Mycoplasma gallisepticum were inhibited by the copper(I) complex of 2,9-dimethyl-1,10-phenanthroline [Cu(DMP)2NO3]. The energy-yielding metabolism was inhibited because the conversion of pyruvate into lactate was found to be blocked by Cu(DMP)2NO3, indicating a selective inhibition of lactate dehydrogenase. Also, the production rate of acetate and the rate of oxygen uptake by whole cells of M. gallisepticum appeared to be strongly decreased. Experiments with crude cell extracts showed an inhibition of reduced nicotinamide adenine dinucleotide (NADH) oxidase by Cu(DMP)2NO3 and an even stronger inhibition of NADH oxidase and lactate dehydrogenase by CuSO4. No preferential inhibition of adenosine 5'-triphosphatase and pyruvate kinase was found. Investigations on the influence of Cu(DMP)2NO3 on deoxyribonucleic acid, ribonucleic acid, and protein synthesis with growing cells of M. gallisepticum showed a selective inhibition of the incorporation of [14C]thymidine into deoxyribonucleic acid. Cu(DMP)2NO3 induced a decrease in the total amount of accessible sulfhydryl groups of whole cells of M. gallisepticum, indicating that the observed diverse toxicity of Cu(DMP)2NO3 may be associated with the interaction of copper ions with protein sulfhydryl groups.

Studies on (Na+ + K+)-activated ATPase. XLVI. Effect of cation-induced conformational changes on sulfhydryl group modification

(1) (Na+ + K+)-ATPase (ATP phosphohydrolase, EC 3.1.6.3) contains 34 sulfhydryl groups on the catalytic subunit, and two on the glycoprotein subunit. Under native conditions, only sulfhydryl groups on the catalytic subunit are accessible to modifying reagents. (2) The degree of inhibition of (Na+ + K+)-ATPase activity by N-ethylmaleimide and 5,5'-dithiobis(2-nitrobenzoic acid) depends on the cations present in the reaction medium. Mg2+ strongly enchances the inhibitory effects of both sulfhydryl reagents. The effects of Mg2+ on the inhibition by 5,5'-dithiobis(2-nitrobenzoic acid) are counteracted by the addition of Na+ or K+. Na+ has no more effect than choline on the inhibition by 5,5'-dithiobis(2-nitrobenzoic acid), but it enhances the inhibitory effect of N-ethylmaleimide at low Na+ concentrations (less than 10 mM). Low concentrations of K+ (less than 10 mM) slightly protect the enzyme against modification. (3) Titration of residual sulfhydryl groups reveals that these ions do not only influence modification of essential sulfhydryl groups, but also that of sulfhydryl groups which are not essential for the enzyme activity. (4) These results indicate that Na+, K+ and Mg2+ have marked effects on the conformation of the catalytic subunit of (Na+ + K+)-ATPase. Various enzyme conformations can be induced, depending on the concentration and the kind of cation added. The largest effects are observed after addition of Mg2+.

Mode of action of copper complexes of some 2,2'-bipyridyl analogs on Paracoccus denitrificans

Copper complexes of 2,2'-bipyridyl and related compounds and CuSO4 inhibited the growth of paracoccus denitrificans. The copper(I) complex of 2,9-dimethyl-1,10-phenanthroline [Cu(DMP)2NO3] showed the highest activity, whereas the copper(II) complex of 1,10-phenanthroline and CuSO4 inhibited the growth to a lesser extent. The uncomplexed ligands (1,10-phenanthroline and 2,9-dimethyl-1,10-phenanthroline) showed little activity, but in the presence of noninhibitory amounts of CuSO4 this activity increased markedly. Copper ions therefore proved to be essential for the growth-inhibitor effect. The extent of inhibition appeared to be strongly dependent on the initial cell density and on the growth medium. No selective inhibition of deoxyribonucleic acid, ribonucleic acid, or protein synthesis was observed with Cu(DMP)2NO3. Respiratory electron transport of P. denitrificans appeared to be strongly inhibited by Cu(DMP)2NO3 and to a somewhat lesser extent by CuSO4. Both aerobic and anaerobic respirations were inhibited to the same extent, and from the cytochrome redox kinetics it is concluded that the site of this inhibition in the respiratory electron transport chain must be located before cytochrome b. Cu(DMP)2NO3 did not significantly influence the H+/O ratio with whole cells of P. denitrificans, suggesting that the efficiency of oxidative phosphorylation is not affected by CU(DMP)2NO3. Growing cultures of P. denitrificans showed a decrease in intracellular potassium ion content in the presence of increasing amounts of Cu(DMP)2NO3. It is concluded that interference with the cytoplasmic membrane, resulting in inhibition of respiratory electron transport, probably constitutes the main mode of action of copper complexes of 2,2'-bipyridyl analogs on P. denitrificans.

Characteristics of a copper-dependent cross-linking reaction between two forms of cytochrome P-450 in rabbit-liver microsomal membranes

1. In liver microsomal membranes from adult rabbits treated with beta-naphthoflavone, reaction with Cu2+ salts plus 1,10-phenanthroline leads to the cross-linking of the two specifically beta-naphthoflavone-inducible cytochrome P-450 species, form 4 and form 6, to form homo- and hetero-dimer species. 2. The cross-linking is not reversed by treatment with 2-mercaptoethanol, so that it can be observed conveniently and specifically on conventional reducing sodium dodecyl sulphate/polyacrylamide gels. 3. The reaction occurs rapidly, and significant cross-linking is observed after 30s at all temperatures from -10 to 40 degrees C. 4. The cross-linking can be brought about by Cu2+ alone at concentrations greater than 0.5 mM, but not by 1,10-phenanthroline alone; at low Cu2+ concentrations, 1,10-phenanthroline enhances the cross-linking reaction, but high concentrations of 1,10-phenanthroline are inhibitory; the optimal molar ratio of Cu2+ to 1,10-phenanthroline is 4:1.5. The effect of Cu2+ is not mimicked by Mn2+, Fe3+, Fe2+, Co2+, Ni2+, Zn2+ or Ag+; Cu+ is probably also ineffective. 6. The cross-linking reaction is inhibited by the prior addition of high concentrations of EDTA or thiol compounds, by sodium dodecyl sulphate at greater than or equal to 0.1% and by sodium deoxycholate and non-ionic detergents at greater than or equal to 1%; the reaction cannot be reversed by incubation with EDTA or with thiol compounds after reaction with cupric phenanthroline; the cross-linking reaction is not inhibited by prior treatment of microsomal membranes with N-ethylmaleimide. 7. The chemical nature of the cross-linking reaction is unknown, but it is most unlikely that it involves the formation of intermolecular disulphide bonds. 8. The great specificity of the reaction makes it a promising tool for the study of molecular interactions between cytochrome P-450 species in intact microsomal membranes.

(Na+ + K+)-ATPase: effects of detergents on the cross-linking of subunits in the presence of Cu2+ and o-phenanthroline

When (Na+ + K+)-ATPase is reacted with Cu2+ or Cu2+-phenanthroline, cross-linking of the two subunits (alpha and beta) occurs. The major products are alpha,beta- and alpha,alpha-dimers. The alpha,beta-dimer is unstable in the presence of EDTA, but becomes stable when it is first exposed to digitonin or Triton X-100. Conversion of alpha-CU2+-beta to alpha-S-S-beta is suggested. If the enzyme that is pretreated with these detergents is used, only the stable alpha,beta-dimer is obtained, and the formation of alpha,alpha-dimer is inhibited. The data are consistent with alpha 2 beta 2 quaternary structure of the enzyme.