Biomechanical characterization of cartilages by a novel approach of blunt impact testing

The present article introduces a novel method of characterizing the macromechanical cartilage properties based on dynamic testing. The proposed approach of instrumented impact testing shows the possibility of more detailed investigation of the acting dynamic forces and corresponding deformations within the wide range of strain rates and loads, including the unloading part of stress-strain curves and hysteresis loops. The presented results of the unconfined compression testing of both the native joint cartilage tissues and potential substitute materials outlined the opportunity to measure the dissipation energy and thus to identify the initial mechanical deterioration symptoms and to introduce a better definition of material damage. Based on the analysis of measured specimen deformation, the intact and pathologically changed cartilage tissue can be distinguished and the differences revealed.

Mechanical response of porous scaffolds for cartilage engineering

Mechanical properties of scaffolds seeded with mesenchymal stem cells used for cartilage repair seem to be one of the critical factors in possible joint resurfacing. In this paper, the effect of adding hyaluronic acid, hydroxyapatite nanoparticles or chitosan nanofibers into the cross-linked collagen I on the mechanical response of the lyophilized porous scaffold has been investigated in the dry state at 37 oC under tensile loading. Statistical significance of the results was evaluated using ANOVA analysis. The results showed that the addition of hyaluronic acid significantly (p<<0.05) reduced the tensile elastic modulus and enhanced the strength and deformation to failure of the modified cross-linked collagen I under the used test conditions. On the other hand, addition of hydroxyapatite nanoparticles and chitosan nanofibers, respectively, increased the elastic modulus of the modified collagen ten-fold and four-fold, respectively. Hydroxyapatite caused significant reduction in the ultimate deformation at break while chitosan nanofibers enhanced the ultimate deformation under tensile loading substantially (p<<0.05). The ultimate tensile deformation was significantly (p<<0.05) increased by addition of the chitosan nanofibers. The enhanced elastic modulus of the scaffold was translated into enhanced resistance of the porous scaffolds against mechanical load compared to scaffolds based on cross-linked neat collagen or collagen with hyaluronic acid with similar porosity. It can be concluded that enhancing the rigidity of the compact scaffold material by adding rigid chitosan nanofibers can improve the resistance of the porous scaffolds against compressive loading, which can provide more structural protection to the seeded mesenchymal stem cells when the construct is implanted into a lesion. Moreover, scaffolds with chitosan nanofibers seemed to enhance cell growth compared to the neat collagen I when tested in vitro as well as the scaffold stability, extending its resorption to more than 10 weeks.

Composite hyaluronate-type I collagen-fibrin scaffold in the therapy of osteochondral defects in miniature pigs

The potential of novel scaffold containing sodium hyaluronate, type I collagen, and fibrin was investigated in the regeneration of osteochondral defects in miniature pigs. Both autologous chondrocyte-seeded scaffolds and non-seeded scaffolds were implanted into two defects located in the non-weight-bearing zone of the femoral trochlea (defect A was located more distally and medially, defect B was located more proximally and laterally). Control defects were left untreated. Twelve weeks after the operation, the knees were evaluated in vivo using MRI. Six months after the implantation, the defects were analyzed using MRI, histological, and immunohistochemical analysis. In the A defects of chondrocyte-seeded scaffold group, hyaline cartilage and fibrocartilage was formed, containing type II collagen, acidic and neutral glycosaminoglycans while the non-seeded scaffold group was predominantly filled with fibrocartilage. Defects in the control group were predominantly filled with fibrous tissue. Histomorphometric analysis of photomicrographs revealed a significantly higher amount of hyaline cartilage in the cell-seeded scaffold group in A defects than in other groups. Both scaffold groups in A defects showed significantly less fibrous tissue than cell-seeded defects B and the control group. Both histological and MRI analysis proved that the novel composite scaffold has a potential to regenerate osteochondral defects within six months.

Comparative modeling of the H4-H5-loop of the alpha2-isoform of Na+/K+-ATPase alpha-subunit in the E1 conformation

Restraint-based comparative modeling was used for calculation and visualization of the H4-H5-loop of Na+/K+-ATPase from mouse brain (Mus musculus, adult male brain, alpha2-isoform) between the amino acid residues Cys 336 and Arg 758 in the E1 conformation The structure consists of two well separated parts. The N-domain is formed by a seven-stranded antiparallel beta-sheet with two additional beta-strands and five alpha-helices sandwiching it, the P-domain is composed of a typical Rossman fold. The ATP-binding site was found on the N-domain to be identical in both alpha2- and alpha1-isoforms. The phosphorylation Asp 369 residue was found in the central part of the P-domain, located at the C-terminal end of the central beta-sheet. The distance between the alpha-carbon of Phe 475 at the ATP-binding site and the alpha-carbon of Asp 369 at the phosphorylation site is 3.22 nm. A hydrogen bond between the oxygen atom of Asp 369 and the nitrogen atom of Lys 690 was clearly detected and assumed to play a key role in maintaining the proper structure of the phosphorylaton site in E1 conformation.

Reconstruction of the anterior cruciate ligament: dynamic strain evaluation of the graft

The study is focused on the biomechanical aspects of the anterior cruciate ligament (ACL) reconstruction procedures with an emphasis on evaluating the dynamic strain of materials commonly used for this purpose. Separate and multiple, equally tensioned strands of hamstring grafts used for the reconstruction of the ACL were biomechanically tested and compared to original ACL and bone-patellar tendon-bone (BPTB) grafts, using tissue samples from cadavers. The study was focused on measuring such material properties as the strength, stiffness, maximum load, and elongation at maximum load of the original ACL, BPTB graft, and single tendon hamstring (gracilis and semitendinosus) grafts, continued by double strands and finally by four-strand graft (STG) evaluation. Fresh-frozen cadaveric knees were used, which had been clamped and tensioned equally. The measurement was performed by drop-weight testing, using a Laser Doppler Vibrometer as a basic sensor of the dynamic movements of the gripping clamps, with parallel correlation by a piezoelectric transducer. The grafts for experiments were obtained from 21-paired knees. The measurement was performed at room temperature (21 degrees C) after 24 h of thawing at 4 degrees C. All the specimens were measured for their response to the dynamic tensile load. The maximum strength values were obtained and calculated for the appropriate section area of the specimen. The tensioned strands of the original ACL showed a maximum average load of 1,246 +/- 243 N in the section area of about 30 mm(2) (max. stress 41.3 MPa); the strands of BPTB grafts showed values of 3,855 +/- 550 N in the section area of 80 mm(2) (max. stress 40.6 MPa); the gracilis tendons showed 925 +/- 127 N in the section area of 10 mm(2) (max. stress 95.1 MPa) and the semitendinosuss yielded a result of 2,050 +/- 159 N in the area of 20 mm(2) (max. stress 88.7 MPa). Of all the materials, the original ACL have the lowest strength and stiffness in respect of their biomechanical properties. BPTB grafts showed a slightly higher value of maximum stress, while both the gracilis and semitendinosus tendons showed double the value of maximum load per section area-tensile stress. Two- and four- combined hamstring strands clamped together and equally tensioned with a drop-weight had the combined tensile strength properties of the individual strands within the estimated range of measurement errors. No significant changes in maximum loads/stresses were observed under impact loading conditions. The results of this study demonstrate that equally tensioned four-strand hamstring-tendon grafts have higher initial tensile properties than those in other varieties of samples. From a biomechanical point of view, they seem to be a reasonable alternative procedure for ACL reconstruction.

Positive effect of oral supplementation with glycosaminoglycans and antioxidants on the regeneration of osteochondral defects in the knee joint

The effect of oral supplementation with glycosaminoglycans (GAG) and radical scavengers (vitamin E/selenium) on the regeneration of osteochondral defects was investigated in rabbits. After introduction of defined osteochondral defects in the knee joint, groups of ten animals were given a GAG/vitamin E/selenium mixture or a placebo (milk sugar) for 6 weeks. Following sacrifice, histological and histochemical analysis was performed. The amount of synovial fluid was increased in the placebo group, while the viscosity of the synovial fluid was significantly enhanced in the GAG group. The amount of sulfated GAG in the osteochondral regenerates (8.8 +/- 3.6 % vs. 6.0 +/- 5.6 %; p <0.03) was significantly higher in the GAG group. In both groups, the GAG amount in the cartilage of the operated knee was significantly higher than in the non-involved knee (p <0.05). Histological analysis of the regenerates in the GAG group was superior in comparison with the placebo group. For the first time, a biological effect following oral supplementation with GAG was demonstrated in healing of osteochondral defects in vivo. These findings support the known positive clinical results.

Fluorescent advanced glycation end products in the detection of factual stages of cartilage degeneration

Patients treated for knee disorders were included in this study. They were examined clinically (Lequesne and Tegner scores) and by standard X-ray investigation. Patients underwent a surgical procedure, either arthroscopy or knee replacement. At the initial phase of surgery, a sample of cartilage was taken for laboratory examination. Progression of the disorder and the clinical examination was correlated with the actual state of the cartilage using a novel fluorescence approach. The intrinsic fluorescence of cartilages was shown as a suitable and sensitive method for detection of the actual state of cartilages because the correlation with X-ray examination and clinical status was found. Intrinsic fluorescence properties of cartilages from patients with chondropathy and osteoarthritis were described and found to be age-dependent. We also observed a higher concentration of advanced glycation end products due to inflammatory and/or degenerative processes in the cartilage. In addition, acute pathological changes due to diseases such as meniscal lesions or anterior cruciate ligament rupture caused a significant increase of formation of advanced glycation end products even in the group of young patients. In fact, such an observation could be crucial and important for the detection of knee conditions suspected of early meniscal and/or ACL lesions especially among young patients.

Protein structure and dynamics determined by protein modeling combined with spectroscopic techniques

Beside of the protein crystals, another attractive option in protein structure analysis has recently appeared: computer modeling of the protein structure based on homology and similarity with proteins of already known structures. We used the combination of computer modeling with spectroscopic techniques, such as steady-state or time-resolved fluorescence spectroscopy or Raman spectroscopy, and with molecular biology techniques. This method could achieve reliable results comparable with resolution obtained from crystal structures. Molecular modeling of the ATP site within the H4-H5-loop revealed eight amino acids residues, namely besides the previously reported amino acids Asp443, Lys480, Lys501, Gly502 and Arg544, also Glu446, Phe475 and Gln482, which form the complete ATP recognition site. Moreover, we proved that a hydrogen bond between Arg423 and Glu472 supported the connection of two opposite halves of the ATP-binding pocket. Similarly, the conserved residue Pro489 is important for the proper interaction of the third and fourth-strands, which both contain residues that take part in the ATP-binding (Ref. 34).

Short-lived fluorescence component of DPH reports on lipid--water interface of biological membranes

Fluorescence measurements of 1,6-diphenyl-1,3,5-hexatriene (DPH) in large unilamellar phospholipid vesicles were performed to characterize the influence of the membrane physical properties on the short-lived lifetime component of the fluorescence decay. We have found that the short-lived component of DPH significantly shortens when the membrane undergoes a temperature-induced phase transition as it is known for the long-lived component of DPH. We induced membrane phase transitions also by alcohols, which are reported to be distributed different way in the membrane--ethanol close to the membrane-water interface and benzyl alcohol in the membrane core. A different effect of the respective alcohol on the short and long decay component was observed. Both the time-resolved fluorescence spectra of DPH taken during lipid vesicle staining and the lifetime dependences caused by changes of temperature and/or induced by the alcohols show that the short-lived fluorescence originates from the population of dye molecules distributed at the membrane-water interface.

The phosphatase activity of the isolated H4-H5 loop of Na+/K+ ATPase resides outside its ATP binding site

The structural stability of the large cytoplasmic domain (H(4)-H(5) loop) of mouse alpha(1) subunit of Na(+)/K(+) ATPase (L354-I777), the number and the location of its binding sites for 2'-3'-O-(trinitrophenyl) adenosine 5'-triphosphate (TNP-ATP) and p-nitrophenylphosphate (pNPP) were investigated. C- and N-terminal shortening revealed that neither part of the phosphorylation (P)-domain are necessary for TNP-ATP binding. There is no indication of a second ATP site on the P-domain of the isolated loop, even though others reported previously of its existence by TNP-N(3)ADP affinity labeling of the full enzyme. Fluorescein isothiocyanate (FITC)-anisotropy measurements reveal a considerable stability of the nucleotide (N)-domain suggesting that it may not undergo a substantial conformational change upon ATP binding. The FITC modified loop showed only slightly diminished phosphatase activity, most likely due to a pNPP site on the N-domain around N398 whose mutation to D reduced the phosphatase activity by 50%. The amino acids forming this pNPP site (M384, L414, W411, S400, S408) are conserved in the alpha(1-4) isoforms of Na(+)/K(+) ATPase, whereas N398 is only conserved in the vertebrates' alpha(1) subunit. The phosphatase activity of the isolated H(4)-H(5) loop was neither inhibited by ATP, nor affected by mutation of D369, which is phosphorylated in native Na(+)/K(+) ATPase.

The hydrogen bonds between Arg423 and Glu472 and other key residues, Asp443, Ser477, and Pro489, are responsible for the formation and a different positioning of TNP-ATP and ATP within the nucleotide-binding site of Na(+)/K(+)-ATPase

Mutation of Arg(423) at the N-domain of Na(+)/K(+)-ATPase resulted in a large decrease of both TNP-ATP and ATP binding. Thus, this residue, localized outside the binding pocket, seems to play a key role in supporting the proper structure and shape of the binding site. In addition, mutation of Glu(472) also caused a large decrease of both TNP-ATP and ATP binding. On the basis of our computer model, we hypothesized that a hydrogen bond between Arg(423) and Glu(472) supports the connection of two opposite halves of the ATP-binding pocket. To verify this hypothesis, we have also prepared the construct containing both these mutations. Binding of neither TNP-ATP nor ATP to this double mutant differed from binding to any of the single mutants. This strongly supported the existence of the hydrogen bond between Arg(423) and Glu(472). Similarly, the conserved residue Pro(489) seems to be substantial for the proper interaction of the third and fourth beta-strands of the N-domain, which both contain residues that take part in ATP binding. Mutation of Asp(443) affected only ATP, but not TNP-ATP, binding, suggesting that these ligands adopt different positions in the nucleotide-binding pocket. On the basis of a recently published crystal structure [Håkansson, K. O. (2003) J. Mol. Biol. 332, 1175-1182], we improved our model and computed the interaction of these two ligands with the N-domain. This model is in good agreement with all previously reported spectroscopic data and revealed that Asp(443) forms a hydrogen bond with the NH(2) group of the adenosine moiety of ATP, but not TNP-ATP.

Substrate evokes translocation of both domains in the mitochondrial processing peptidase alpha-subunit during which the C-terminus acts as a stabilizing element

All three tryptophan residues in alpha-subunit of mitochondrial processing peptidase (MPP) were subsequently substituted. While substitutions of Trp223 led to misfolded non-functional protein, mutations of Trp147 and/or Trp481 did not affect the enzyme processing activity. Thus, fluorescence properties of the mutants with fewer tryptophans were used for observation of both alpha-MPP domain translocation and visualization of conformational changes in the interdomain linker evoked by substrate. We found that in the presence of substrate the C-terminal penultimate Trp481 was approaching Trp223, which is localized at the border of N-terminal domain and interdomain linker. Also, excision of the alpha-MPP C-terminal 30 amino acid residues (DeltaC30) led to a complete loss of protein function. Even shorter deletions of the alpha-MPP C-terminus destabilized the protein slightly (DeltaC2) or dramatically (DeltaC17). It suggests that the extreme C-terminus of alpha-MPP provides mechanical support to the C-terminal domain during its extensive conformational change accompanying the substrate recognition process.

Fluorescence Competition Assay for the Assessment of ATP Binding to an Isolated Domain of Na+, K+-ATPase

An equation allowing estimation of the dissociation constant for binding of a non-fluorescent ligand to the enzyme is presented that is based on the competitive replacement of the ligand by its fluorescent analog. We derived an explicit formula for the probe fluorescence intensity, which is suitable for nonlinear least-squares analysis. We used this formula to evaluate the binding of ATP to the large cytoplasmic loop of Na+,K(+)-ATPase. The estimated value of KD (6.2+/- 0.7 mM) is comparable with the results from other laboratories for similar constructs obtained by a different method.

Fluorescence competition assay for the assessment of ATP binding to an isolated domain of Na+, K(+)-ATPase

An equation allowing estimation of the dissociation constant for binding of a non-fluorescent ligand to the enzyme is presented that is based on the competitive replacement of the ligand by its fluorescent analog. We derived an explicit formula for the probe fluorescence intensity, which is suitable for nonlinear least-squares analysis. We used this formula to evaluate the binding of ATP to the large cytoplasmic loop of Na+,K(+)-ATPase. The estimated value of KD (6.2+/- 0.7 mM) is comparable with the results from other laboratories for similar constructs obtained by a different method.

Protein modeling combined with spectroscopic techniques: an attractive quick alternative to obtain structural information

Beside of the protein crystallography or NMR, another attractive option in protein structure analysis has recently appeared: computer modeling of the protein structure based on homology and similarity with proteins of already known structures. We have used the combination of computer modeling with spectroscopic techniques, such as steady-state or time-resolved fluorescence spectroscopy, and with molecular biology techniques. This method could provide useful structural information in the cases where crystal or NMR structure is not available. Molecular modeling of the ATP site within the H4-H5-loop revealed eight amino acids residues, namely besides the previously reported amino acids Asp443, Lys480, Lys501, Gly502 and Arg544, also Glu446, Phe475 and Gln482, which form the complete ATP recognition site. Moreover, we have proved that a hydrogen bond between Arg423 and Glu472 supports the connection of two opposite halves of the ATP-binding pocket. Similarly, the conserved residue Pro489 is important for the proper interaction of the third and fourth beta-strands, which both contain residues that take part in the ATP-binding. Alternatively, molecular dynamics simulation combined with dynamic fluorescence spectroscopy revealed that 14-3-3 zeta C-terminal stretch is directly involved in the interaction of 14-3-3 protein with the ligand. Phosphorylation at Thr232 induces a conformational change of the C-terminus, which is presumably responsible for observed inhibition of binding abilities. Phosphorylation at Thr232 induces more extended conformation of 14-3-3zeta C-terminal stretch and changes its interaction with the rest of the 14-3-3 molecule. This could explain negative regulatory effect of phosphorylation at Thr232 on 14-3-3 binding properties.

Limitations in linearized analyses of binding equilibria: binding of TNP-ATP to the H4-H5 loop of Na/K-ATPase

Binding of TNP-ATP [2',3'- O-(2,4,6-trinitrophenyl)adenosine 5'-triphosphate, a fluorescent analogue of ATP] to the K605 protein was studied. This is an isolated N-domain in the cytoplasmic loop of the Na/K-ATPase alpha-subunit, lying between membrane-spanning segments 4 and 5 (sequence L(354)-I(604)). A titration equation is derived that explicitly makes it possible to relate the fluorescence of TNP-ATP and K605 solutions to total probe concentration in the sample. Using this, it is possible to obtain the value of the dissociation constant from the titration experiment without resorting to the Scatchard plot, which is far from optimal from the statistical point of view. Using the new formula with non-linear regression analysis, a value of the dissociation constant K(D) for TNP-ATP binding to the K605 protein of 3.03 +/- 0.28 microM at 22 degrees C was obtained. A series of fits to simulated data with added noise demonstrated clearly the advantage of non-linear regression using the new formula over the commonly employed linear regression using the Scatchard plot. The procedure presented is generally applicable to binding assays using changes in the fluorescence of ligands on binding.

ATP-binding is stabilized by a stacking interaction within the binding site of Na+/K+ -ATPase

Site-directed mutagenesis was applied to modify phenylalanines (Phe(475)Trp, Phe(548)Tyr, and both) to generate mutants on the basis of molecular modeling of the ATP-binding domain of Na(+)/K(+)-ATPase, in order to characterize the forces that stabilize ATP in its binding pocket. Each of the mutants was examined by Raman difference spectroscopy, i.e., as a difference between the spectrum of the domain with and without bound ATP. It was shown that Phe(475) plays a key role in stabilizing ATP-binding by a stacking interaction. Phe(548) co-stabilizes ATP on the opposite site of the binding pocket and its type of interaction with ATP-binding differs from that of Phe(475).

Eight amino acids form the ATP recognition site of Na(+)/K(+)-ATPase

Point mutations of a part of the H(4)-H(5) loop (Leu(354)-Ile(604)) of Na(+)/K(+)-ATPase have been used to study the ATP and TNP-ATP binding affinities. Besides the previously reported amino acid residues Lys(480), Lys(501), Gly(502), and Cys(549), we have found four more amino acid residues, viz., Glu(446), Phe(475), Gln(482), and Phe(548), completing the ATP-binding pocket of Na(+)/K(+)-ATPase. Moreover, mutation of Arg(423) has also resulted in a large decrease in the extent of ATP binding. This residue, localized outside the binding pocket, seems to play a key role in supporting the proper structure and shape of the binding site, probably due to formation of a hydrogen bond with Glu(472). On the other hand, only some minor effects were caused by mutations of Ile(417), Asn(422), Ser(445), and Glu(505).

Frequency-domain lifetime fluorometry of double-labeled creatine kinase

Myofibril-bound creatine kinase EC 2.7.3.2 (CK), a key enzyme of muscle energy metabolism, has been selected for studies of conformational changes that underlie the cellular control of enzyme activity. For fluorescence spectroscopy measurements, the CK molecule was double-labeled with IAF (5-iodoacetamidofluorescein) and ErITC (erythrosin 5'-isothiocyanate). Measurement of fluorescence resonance energy transfer (FRET) from fluorescein to erythrosin was used to obtain information about the donor-acceptor pair distance. Frequency-domain lifetime measurements evaluate the donor-acceptor distance in the native CK molecule as 7.8 nm. The Förster radius equals 5.3 nm with the resolution range from 0.2 to 1.0 nm. Erythrosin-fluorescein labeling (EFL) was tested for artificial conformational changes of the CK molecule with high-salt concentration treatment. The transition distance, defined by His-97 and Cys-283 and derived from a 3D model equals 0.766 nm for the open (inactive) form and 0.277 nm for the closed (reactive) form of the CK molecule. In this way, the resolution range of the used spectroscopy method is significant, concerning the difference of 0.489 nm. Nevertheless, the CK enzyme activity, assessed by the hexokinase-coupled assay, was diminished down to 1 % of the activity of the native enzyme. EFL is suitable for description of conformational behavior implied from the regulation of creatine kinase. However, the observed inhibition restricts EFL to studies of conformational changes during natural catalytic activity.

Phe(475) and Glu(446) but not Ser(445) participate in ATP-binding to the alpha-subunit of Na(+)/K(+)-ATPase

The ATP-binding site of Na(+)/K(+)-ATPase is localized on the large cytoplasmic loop of the alpha-subunit between transmembrane helices H(4) and H(5). Site-directed mutagenesis was performed to identify residues involved in ATP binding. On the basis of our recently developed model of this loop, Ser(445), Glu(446), and Phe(475) were proposed to be close to the binding pocket. Replacement of Phe(475) with Trp and Glu(446) with Gln profoundly reduced the binding of ATP, whereas the substitution of Ser(445) with Ala did not affect ATP binding. Fluorescence measurements of the fluorescent analog TNP-ATP, however, indicated that Ser(445) is close to the binding site, although it does not participate in binding.

Secondary and tertiary structure of nucleotide-binding domain of alphasubunit of Na+/K+-ATPase

The nucleotide-binding domain of the alpha subunit of mouse brain Na+/K+-ATPase was expressed and isolated from Escherichia coli cells. A model structure was constructed by comparative modeling with and without docked ATP. This was compared with the secondary structure determination from UV circular dichroism and Raman spectroscopy. Thus, we support the quality of the model and the correct folding of the recombinant protein. ATP binding was followed by Raman difference spectroscopy, and its influence on the secondary structure of the N domain seems to not be significant.

Substrate binding changes conformation of the alpha-, but not the beta-subunit of mitochondrial processing peptidase

Lifetime analysis of tryptophan fluorescence of the mitochondrial processing peptidase (MPP) from Saccharomyces cerevisiae clearly proved that substrate binding evoked a conformational change of the alpha-subunit while presence of substrate influenced neither the lifetime components nor the average lifetime of the tryptophan excited state of the beta-MPP subunit. Interestingly, lifetime analysis of tryptophan fluorescence decay of the alpha-MPP subunit revealed about 11% of steady-state fractional intensity due to the long-lived lifetime component, indicating that at least one tryptophan residue is partly buried at the hydrophobic microenvironment. Computer modeling, however, predicted none of three tryptophans, which the alpha-subunit contains, as deeply buried in the protein matrix. We conclude this as a consequence of a possible dimeric (oligomeric) structure.

Effects of fluorescent pseudo-ATP and ATP-metal analogs on secondary structure of Na(+)/K(+)-ATPase

The secondary structure of Na(+)/K(+)-ATPase after modification of the ATP-binding sites was analyzed. Consistently with recent reports, we found in trypsin-treated Na(+)/K(+)-ATPase additionally to alpha-helix also beta-sheet structures in the transmembrane segments. However, binding of fluorescein 5'-isothiocyanate (FITC), the pseudo-ATP analog, to the ATP-binding site did not affect the secondary structure of undigested Na(+)/K(+)-ATPase. Consequently, fluorescence intensity changes of FITC-labeled Na(+)/K(+)-ATPase commonly used to observe conformational transitions of the enzyme reflect physiological changes of the native structure. The metal complex analogues of ATP, Cr(H(2)O)(4)ATP and Co(NH(3))(4)ATP, on the other hand, affected the secondary structure of Na(+)/K(+)-ATPase. We propose that these changes in the secondary structure are responsible for inhibition of backdoor phosphorylation.

Different cation binding to the I domains of alpha1 and alpha2 integrins: implication of the binding site structure

In the present work, we studied the interactions of recombinant alpha1 and alpha2 integrin I domains with cations Tb(3+), Mn(2+), Mg(2+) and Ca(2+). We observed that alpha1 and alpha2 I domains bind these cations with significantly different characteristics. The binding of Mg(2+) by the alpha1 I domain was accompanied by significant changes of tryptophan fluorescence which could be interpreted as a conformational change. Comparison of the alpha1 integrin I domain structure obtained by comparative modeling with a known structure of the alpha2 integrin I domain shows distinct differences in the metal ion binding sites which could explain the differences in cation binding.

Tight binding of bulky fluorescent derivatives of adenosine to the low affinity E2ATP site leads to inhibition of Na+/K+-ATPase. Analysis of structural requirements of fluorescent ATP derivatives with a Koshland-Némethy-Filmer model of two interacting ATP sites

A Koshland-Némethy-Filmer model of two cooperating ATP sites has previously been shown to explain the kinetics of inhibition of Na+/K+-ATPase (EC 3.6.1.37) by dansylated ATP (Thoenges, D., and Schoner, W. (1997) J. Biol. Chem. 272, 16315-16321). The present work demonstrates that this model adequately describes all types of interactions and kinetics of a number of ATP analogs that differ in their cooperativity of the high and low affinity ATP binding sites of the enzyme. 2',3'-O(2,4,6-trinitrophenyl)ATP binds in a negative cooperative way to the E1ATP site (Kd = 0.7 microM) and to the E2ATP site (Kd = 210 microM), but 3'(2')-O-methylanthraniloyl-ATP in a positive cooperative way with a lower affinity to the E1ATP binding site (Kd = 200 microM) than to the E2ATP binding site (Kd = 80 microM). 3'(2')-O(5-Fluor-2,4-dinitrophenyl)-ATP, however, binds in a noncooperative way, with equal affinities to both ATP binding sites (Kd = 10 microM). In a research for the structural parameters determining ATP site specificity and cooperativity, we became aware that structural flexibility of ribose is necessary for catalysis. Moreover, puckering of the ring atoms in the ribose is essential for the interaction between ATP sites in Na+/K+-ATPase. A number of derivatives of 2'(3')-O-adenosine with bulky fluorescent substitutes bind with high affinity to the E2ATP site and inhibit Na+/K+-ATPase activity. Evidently, an increased number of interactions of such a bulky adenosine with the enzyme protein tightens binding to the E2ATP site.

Microenvironment of the high affinity ATP-binding site of Na+/K+-ATPase is slightly acidic

Fluorescein-5'-isothiocyanate (FITC) was used to study the high-affinity ATP-binding site of Na+/K+-ATPase. The molar ratio of specifically bound FITC per alpha-subunit of Na+/K+-ATPase was found to be 0.5 as followed from pretreatment experiments with another specific E1ATP-inhibitor Cr(H2O)4AdoPP[CH2]P. This indicated an existence of one high affinity ATP-binding site (E1ATP-binding site) in the native (alphabeta)2-diprotomer of Na+/K+-ATPase. Fluorescence dual-excitation ratio of specifically bound FITC revealed that at external pH 7.5, the pH value inside the E1ATP-binding site is 6.95 +/- 0.18. In addition, FITC fluorescence quenching by anti-fluorescein and by iodide choline indicated the limited access of water into the small pocket of the E1ATP-binding site.

Erythrosin 5'-isothiocyanate labels Cys549 as part of the low-affinity ATP binding site of Na+/K+-ATPase

The high-affinity E1ATP site of Na+/K+-ATPase labeled with fluorescein 5'-isothiocyanate and its E2ATP site labeled with erythrosin 5'-isothiocyanate (ErITC), as was shown recently [Linnertz et al. (1998) J. Biol. Chem. 273, 28813-28821], reside on separate and adjacent catalytic alpha subunits. This paper provides evidence that specific labeling of the E2ATP binding site with ErITC resulted in a modification of the Cys549 residue in the tryptic fragment with the sequence Val545-Leu-Gly-Phe-Cys549-His550. Hence, Cys549 is part of or close to the low-affinity E2ATP binding site of Na+/K+-ATPase.

Molecular distance measurements reveal an (alpha beta)2 dimeric structure of Na+/K+-ATPase. High affinity ATP binding site and K+-activated phosphatase reside on different alpha-subunits

ATP hydrolysis by Na+/K+-ATPase proceeds via the interaction of simultaneously existing and cooperating high (E1ATP) and low (E2ATP) substrate binding sites. It is unclear whether both ATP sites reside on the same or on different catalytic alpha-subunits. To answer this question, we looked for a fluorescent label for the E2ATP site that would be suitable for distance measurements by Förster energy transfer after affinity labeling of the E1ATP site by fluorescein 5'-isothiocyanate (FITC). Erythrosin 5'-isothiocyanate (ErITC) inactivated, in an E1ATP site-blocked enzyme (by FITC), the residual activity of the E2ATP site, namely K+-activated p-nitrophenylphosphatase in a concentration-dependent way that was ATP-protectable. The molar ratios of FITC/alpha-subunit of 0.6 and of ErITC/alpha-subunit of 0.48 indicate 2 ATP sites per (alpha beta)2 diprotomer. Measurements of Förster energy transfer between the FITC-labeled E1ATP and the ErITC-labeled or Co(NH3)4ATP-inactivated E2ATP sites gave a distance of 6.45 +/- 0.64 nm. This distance excludes 2 ATP sites per alpha-subunit since the diameter of alpha is 4-5 nm. Förster energy transfer between cardiac glycoside binding sites labeled with anthroylouabain and fluoresceinylethylenediamino ouabain gave a distance of 4.9 +/- 0.5 nm. Hence all data are consistent with the hypothesis that Na+/K+-ATPase in cellular membranes is an (alpha beta)2 diprotomer and works as a functional dimer (Thoenges, D., and Schoner, W. (1997) J. Biol. Chem. 272, 16315-16321).

Oligosaccharide organization of the beta-subunits of pig kidney Na+/K+-ATPase

The distance between the beta-subunits of Na+/K+-ATPase isolated from pig dark red kidney medulla was determined by Förster energy transfer. First, oligosaccharides of the beta-subunit were shown to be labelled with three fluorophores: Lucifer yellow (LY), Lissamine rhodamine B sulfonyl hydrazine (LRSH) and Cascade blue (CB). Further, LY and LRSH were used as the donor and the acceptor, respectively, for Förster energy transfer studies to determine the localization of the beta-subunit in the native enzyme which is known to be formed as a tetramer (alphabeta)2. It was found that the beta-subunits in the functional enzyme complex in the membrane are not localized next to each other but are spatially separated. The distance between fluorophores covalently attached to the beta-subunits was found to be 5.1 nm. This conclusion was confirmed by measurements with another donor-acceptor pair CB-LY. The results also support the idea of a direct interaction of the beta-subunit with the extracellular part of the alpha-subunit. These interactions were modified in the presence of millimolar concentrations of magnesium ions. This indicates a crucial role of magnesium in extracellular interactions between the alpha and beta subunits.

2'(3')-O-[N- [2- [3- [5-fluoresceinyl] thioureido] ethyl] carbamoyl] adenosine 5'-triphosphate and its Cr(H2O)4 and Co(NH3)4 complex derivatives are new fluorescent tools for labelling ATP binding sites of Na+/K+-ATPase

2'(3')-O-[N- [2- [3- [5-fluoresceinyl] thioureido] ethyl] carbamoyl] adenosine 5'-triphosphate (FEDA-ATP), a spectroscopic tool used for studying skeletal muscle myosin ATPase subfragment 1, was applied to Na+/K+-ATPase (EC 3.6.1.37). In contrast to the myosin subfragment, we found that FEDA-ATP is not a substrate for Na+/K+-ATPase. On the other hand, FEDA-ATP showed an affinity for both the low (E2, Kd=200 microM) and the high (E1, Kd=22 microM) affinity ATP-binding sites. When the microscopic affinities of FEDA-ATP were used for calculating the macroscopic affinity in the overall reaction according to Ki=(KdE1*KdE2)1/2, the experimentally measured inhibition constant of 66 microM was obtained. To evoke irreversible binding inhibitors, FEDA-ATP was transferred in its chromium(III) and cobalt(III) complex analogs, which are suitable tools for labelling the ATP binding sites of Na+/K+-ATPase in a specific way.

Modifications induced by plasma from insulin-dependent diabetic patients and by lysophosphatidylcholine on human Na+,K(+)-adenosine triphosphatase

To investigate the molecular mechanisms of the inhibition of Na+,K(+)-adenosine triphosphatase (Na+,K(+)-ATPase) in diabetes mellitus, we incubated Na+,K(+)-ATPase purified from human placenta of six healthy nondiabetic women with plasma from six insulin-dependent diabetic (IDDM) men and six healthy controls and with different concentrations of lysophosphatidylcholine (LPC). We determined the enzyme activity, anthroyl ouabain-binding capacity, dissociation constant (Kd), and average lifetime values (tau) by the static and dynamic fluorescence of anthroyl ouabain. The lipid annulus of the enzyme was studied by static and dynamic fluorescence of 1-(4-trimethylamino-phenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH). Moreover, we studied the lipid microenvironment surrounding the Na+,K(+)-ATPase purified from the placentas of six healthy women and six insulin-dependent diabetic women, determining the percent composition of phospholipids of the lipid annulus. The addition of total and protein-free IDDM plasma to normal Na+,K(+)-ATPase significantly inhibited the enzymatic activity even at the lowest concentration studied (1: 100), whereas the ouabain-binding capacity, Kd, and tau were not affected by IDDM plasma. The fluorescence polarization and lifetime values of TMA-DPH were significantly decreased by diabetic plasma. The incubation of Na+,K(+)-ATPase with LPC caused an inhibition of the enzymatic activity without modifications of the anthroyl ouabain-binding capacity and dissociation constant. The fluorescence polarization and lifetime values of TMA-DPH were significantly decreased by 5 mumol/L LPC. The study of the phospholipids surrounding Na+,K(+)-ATPase demonstrated a significant increase in the percent LPC content in IDDM patients compared with controls together with a concomitant decrease in phosphatidylcholine. These observations indicate that the inhibition caused by diabetic plasma on Na+,K(+)-ATPase is not dependent on a modification of the ouabain-binding site and that it seems to mimic the effect of LPC addition. A link between modification of the lipid moiety of the enzyme and Na+,K(+)-ATPase inhibition might be hypothesized.

The isolated H4-H5 cytoplasmic loop of Na,K-ATPase overexpressed in Escherichia coli retains its ability to bind ATP

The H4-H5 loop of the alpha-subunit of mouse brain Na,K-ATPase was expressed and isolated from E.coli cells. Using fluorescence analogues of ATP, this loop was shown to retain its capability to bind ATP. Isolation of a soluble H4-H5 loop with the native ATP binding site is a critical step for detailed studies of the molecular mechanism of ATP binding and utilisation.

The isolated H4-H5 cytoplasmic loop of Na,K-ATPase overexpressed in Escherichia coli retains its ability to bind ATP

The H4-H5 loop of the alpha-subunit of mouse brain Na,K-ATPase was expressed and isolated from Escherichia coli cells. Using fluorescence analogues of ATP, this loop was shown to retain its capability to bind ATP. Isolation of a soluble H4-H5 loop with the native ATP binding site is a crucial step for detailed studies of the molecular mechanism of ATP binding and utilisation.

Binding of pyrene isothiocyanate to the E1ATP site makes the H4-H5 cytoplasmic loop of Na+/K+-ATPase rigid

1-Pyreneisothiocyanate was shown to be an inhibitor of Na+/K+-ATPase. Reverse-phase HPLC and activity studies indicated binding of 1-pyreneisothiocyanate at the H4-H5 loop of the alpha subunit and competition with the fluorescein 5'-isothiocyanate for the E1ATP site. While fluorescein 5'-isothiocyanate, the fluorescent ATP pseudo-analog, was shown to be immobilized at the E1ATP site, there was no possibility to draw any conclusion about the flexibility of the E1ATP site due to its short lifetime. Employing 1-pyreneisothiocyanate as a long-lived fluorophore and a label for the E1ATP site, we found that the ATP-binding site of Na+/K+-ATPase and, in fact, the whole large intracellularly exposed H4-H5 loop of the catalytic alpha subunit is rigid and rotationally immobilized. This has important consequences for the molecular mechanism of the transport function.

Quenching of 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole-modified Na+/K+-ATPase reveals a higher accessibility of the low-affinity ATP-binding site

7-Chloro-4-nitrobenzo-2-oxa-1,3-diazole (NBD-Cl) labeled Na+/K+-ATPase covalently with two different inactivation constants (Ki = 2.5 microM; Ki' = 10 microM). It apparently modified the two different ATP-binding sites of the enzyme since it decreased the activity of the E2ATP site, i.e. the K+-activated para-nitrophenylphosphatase activity, in an enzyme whose high-affinity E1ATP site had been blocked by fluorescein 5'isothiocyanate (FITC). It also reduced the activity of the E1ATP site, i.e. the Na+-activated protein phosphorylation, in an enzyme whose low-affinity E2ATP site had been blocked by Co(NH3)4PO4. Fluorescence quenching experiments with KI, CsCl and MnCl2 of the NBD-Cl-labeled Na+/K+-ATPase revealed two differently accessible types of fluorophores depending on the ATP site: The E2ATP site apparently differs from the E1ATP site in that it is more open because the fluorophore labeling in the E2ATP site was sterically better accessible for quenchers.

Fluoresceinyl-ethylenediamine-ouabain detects an acidic environment in the cardiac glycoside binding site of Na+/K+-ATPase

To probe the pH value in the microenvironment of the cardiac glycoside-binding site of Na+/K+-ATPase, pH-sensitive fluorescent derivatives of ouabain were synthesized. The fluoresceinyl derivative of ethylenediamino-ouabain (FEDO) had a pKs of 6.0 and showed a H+-dependent fluorescence change, when its ratio of excitation at 490 nm/450 nm was recorded at 530 nm. Binding of FEDO inactivated Na+/K+-ATPase at 37 degrees C and pH 7.25 in a slow time-dependent process under the conditions of backdoor phosphorylation with k(on) of 891 s(-1) M(-1). The complex dissociated with k(on) of 0.35 x 10(-3) s(-1) resulting in a Kd value of 0.4 microM for the FEDO x enzyme complex. Binding of FEDO was associated with a decrease of the excitatory fluorescence ratio at 490 nm/450 nm which could be used to convert this change into a pH value. A pH value of 5.1 +/- 0.2 was calculated to exist in the microenvironment of the FEDO x enzyme complex. This pH value was independent of the pH of the incubation medium used to form the FEDO x enzyme complex. Analysis of the accessibility of the fluorophore in the FEDO x enzyme complex to the dynamic quencher potassium iodide detected a decrease of the Stern-Volmer constant from 6.2 mM(-1) (free FEDO) to 1.5 mM(-1) (FEDO x enzyme complex) indicating thereby a limited accessibility of the fluorophore to anions. Analysis of the microenvironment of the fluorescein residue of the FEDO x enzyme complex by measurements of the anisotropy and the fluorescence half-life time revealed that both processes differed significantly when H2O was replaced by D2O. We conclude, therefore, that a pH of 5.1 +/- 0.2 exists in the vicinity of ouabain that is hidden in the depth of the receptor site when the ouabain receptor complex has been formed.

Intracellular Ca2+ signals induced by ATP and thapsigargin in glioma C6 cells. Calcium pools sensitive to inositol 1,4,5-trisphosphate and thapsigargin

In glioma C6 cells, extracellular ATP generates inositol 1,4,5-trisphosphate (InsP3), indicating the presence of purinergic receptors coupled to phosphoinositide turnover. To identify the effect of ATP (acting via InsP3) and thapsigargin (acting without InsP3 production as a specific inhibitor of the endoplasmic reticulum Ca(2+)-ATPase) on intracellular Ca2+ pools we used video imaging of Fura-2 loaded into single, intact glioma C6 cells. It has been shown that ATP and thapsigargin initiate Ca2+ response consistent with the capacitative model of Ca2+ influx. When the cells were stimulated by increasing concentrations of ATP (1, 10, 50 and 100 microM) the graded, quantal Ca2+ response was observed. In the absence of extracellular Ca2+ thapsigargin and ionomycin-releasable Ca2+ pools are overlapping, demonstrating that Ca2+ stores are located mainly in the endoplasmic reticulum. After maximal Ca2+ mobilization by ATP, thapsigargin causes further increase in cytosolic Ca2+ concentration, whereas emptying of thapsigargin-sensitive intracellular stores prevents any further Ca2+ release by ATP. Thus, the thapsigargin-sensitive intracellular pool of Ca2+ in glioma C6 cells seems to be larger than that sensitive to InsP3. Two hypothesis to explain this result are proposed. One postulates a presence of two different Ca2+ pools, sensitive and insensitive to InsP3 and both discharged by thapsigargin, and the other, the same intracellular pool of Ca2+ completely emptying by thapsigargin and only partially by InsP3. These results may contribute to understanding the mechanism of Ca2+ signalling mediated by ATP, the most potent intracellular Ca2+ mobilizing agonist in all types of glial cells.

Modifications induced by plasma of gestational hypertensive women on the Na+/K+-ATPase obtained from human placenta

In order to investigate the molecular mechanisms of the inhibition of Na+/K+-ATPase in Gestational Hypertension (GH), we incubated Na+/K+-ATPase purified from human placenta of 6 healthy normotensive women with plasma from 6 GH women and 6 healthy controls. We determined the enzyme activity by the method of Esman, and the anthroyl-ouabain-binding capacity, dissociation constant (Kd) and average lifetime values (tau) by the static and dynamic fluorescence of anthroyl-ouabain. The lipid annulus of the enzyme was studied by static and dynamic fluorescence of 1-(4-trimethylaminophenyl)-6-phenyl-1,3,5- hexatriene (TMA-DPH). The addition of total and protein-free GH plasma to normal Na+/K+-ATPase significantly inhibited the enzymatic activity even at the lowest concentration studied (1:100), as well as the ouabain-binding capacity, Kd and tau. GH plasma significantly decreased the fluorescence polarization and lifetime values of TMA-DPH. These observations indicate that the inhibition caused by GH plasma on Na+/K+-ATPase might be due to a reduction of the number of active molecules or a modification of the ouabain-binding site suggesting the existence of digitalis-like factor. A link between the modification of the lipid moiety of the enzyme and the Na+/K+-ATPase inhibition might be hypothesized.

Na+,K(+)-ATPase of human placenta during gestational hypertension: a biochemical-biophysical study

1. Na+,K(+)-ATPase is the membrane enzyme catalysing the active transport of Na+ and K+ across the plasma membrane of animal cells. A reduced activity of Na+,K(+)-ATPase has been described in gestational hypertension in a variety of cell types, in agreement with the hypothesis that gestational hypertension can induce membrane transport modifications similar to those reported for essential hypertension. The causes of the reduced Na+,K(+)-ATPase activity are still debated. 2. The aim of the present work was to investigate the molecular mechanism of the reduced enzymic activity in gestational hypertension using as a model Na+,K(+)-ATPase purified from human placenta. Na+,K(+)-ATPase obtained from term placentas of eight healthy pregnant women and eight age-matched women with gestational hypertension was purified as previously described. 3. We observed in gestational hypertension: (i) a significant increase in the activation energies above transition temperature; (ii) a significant decrease in the fluorescence polarization of 1-(4-trimethylaminophenyl)-6-phenyl-1,3,5-hexatriene (i.e. increased fluidity) and an increase in the mean lifetime (modified hydrophobicity); (iii) a lower Kq, suggesting an enzymic structural modification; and (iv) an increased mean lifetime and rotational relaxation time of pyrene isothiocyanate, indicating a modified ATP binding site.

Possible nystatin-protein interaction in yeast plasma membrane vesicles in the presence of ergosterol. A Förster energy transfer study

The Förster energy transfer from tryptophan residues of membrane proteins to nystatin was measured in reconstituted yeast plasma membrane vesicles free of, or doped with, ergosterol. We wanted to elucidate whether the functional change of membrane transport proteins from H+ symporters to facilitators, observed in ergosterol-containing plasma membrane vesicles on addition of nystatin [Opekarová and Tanner (1994) FEBS Lett. 350, 46-50], is reflected in altered protein-nystatin relations within the membrane. Both frequency-domain and time-domain time-resolved fluorescence spectroscopy showed that in the presence of ergosterol nystatin is located much closer to membrane proteins than in its absence.

The carbohydrate moieties of the beta-subunit of Na+, K(+)-ATPase: their lateral motions and proximity to the cardiac glycoside site

The beta-subunit associated with the catalytic (alpha) subunit of the mammalian Na+, K(+) -ATPase is a transmembrane glycoprotein with three extracellularly located N-glycosylation sites. Although beta appears to be essential for a functional enzyme, the role of beta and its sugars remains unknown. In these studies, steady-state and dynamic fluorescence measurements of the fluorophore lucifer yellow (LY) covalently linked to the carbohydrate chains of beta have demonstrated that the bound probes are highly solvent exposed but restricted in their diffusional motions. Furthermore, the probes' environments on beta were not altered by Na+ or K+ or ouabain-induced enzyme conformational changes, but both divalent cation and oligomycin addition evoked modest changes in LY fluorescence. Frequency domain measurements reflecting the Förster fluorescence energy transfer (FET) occurring between anthroylouabain (AO) bound to the cardiac glycoside receptor site on alpha and the carbohydrate-linked LY demonstrated their close proximity (18 A). Additional FET determinations made between LY as donor and erythrosin-5-isothiocyanate, covalently bound at the enzyme's putative ATP binding site domain, indicated that a distance of about 85 A separates these two regions and that this distance is reduced upon divalent cation binding and increased upon the Na+E1-->K+E2 conformational transition. These data suggest a model for the localization of the terminal moieties of the oligosaccharides that places them, on average, about 18 A from the AO binding site and this distance or less from the extracellular membrane surface.

The transmembrane gradient of the dielectric constant influences the DPH lifetime distribution

The fluorescence lifetime distribution of 1,6-diphenyl-1,3,5-hexatriene (DPH) and 1-[4-(trimethylamino)phenyl]-6-phenyl-1,3,5-hexatriene (TMA-DPH) in egg-phosphatidylcholine liposomes was measured in normal and heavy water. The lower dielectric constant (by approximately 12%) of heavy water compared with normal water was employed to provide direct evidence that the drop of the dielectric constant along the membrane normal shifts the centers of the distribution of both DPH and TMA-DPH to higher values and sharpens the widths of the distribution. The profile of the dielectric constant along the membrane normal was not found to be a linear gradient (in contrast to [1]) but a more complex function. Presence of cholesterol in liposomes further shifted the center of the distributions to higher value and sharpened them. In addition, it resulted in a more gradient-like profile of the dielectric constant (i.e. linearization) along the normal of the membrane. The effect of the change of dielectric constant on the membrane proteins is discussed.

Enzymically generated electron-excited species increase the membrane order parameter without changing the rotational relaxation time of TMA-DPH

The effect of enzymically generated triplet acetone produced from 60 mmol/1 2-methylpropanal (MPAL) in the presence of 2 mumol/l horseradish peroxidase on the properties of artificial unilamellar liposomes was studied. The concentration of malondialdehyde, membrane lateral pressure (membrane order parameter), lifetime distribution of the excited state of TMA-DPH incorporated into liposomes, and rotational relaxation time of TMA-DPH were determined. The concentration of MDA increased with the MPAL concentration, and the membrane lateral pressure increased linearly with the concentration of MDA. The mean lifetime values changed (p < 0.01) as did the limiting anisotropy (p < 0.005), the widths of the main peak of the lifetime distribution and the rotational relaxation time remaining unchanged. This indicates that there is no significant difference between the widths and rotational relaxation times in the control and triplet-acetone-treated sample; i.e. peroxidation by MPAL increased the lipid order of the bilayer but the effect of neighboring lipid molecules on the dynamics of the probe was negligible.

The effects of calcium and calcium channel blockers on sodium pump

The effects of 10 mM Ca2+ and Ca2+ channel blockers verapamil, diltiazem and flunarizine on the ouabain-sensitive electrogenic Na+, K+ pump activity of mouse diaphragm muscle fibres enriched with Na+ were compared with the changes in cytosolic [Ca2+]. The electrogenic Na+ pump activity produced by adding K+ to muscles previously bathed for 4 h in a K(+)-free, 2-mM [Ca2+] solution increased the resting membrane potential by about 18 mV. This hyperpolarization was completely inhibited after 10 min incubation in 10 mM Ca2+. Verapamil 10(-5) M, 10(-5) M diltiazem and 10(-7) M flunarizine effectively prevented the effect of elevated [Ca2+]. At these concentrations, these drugs did not affect the K(+)-induced hyperpolarization. In mouse diaphragm, the basal cytosolic [Ca2+] measured by the fluorescent indicator 1-[2-(5-carboxyoxazol-2-yl)-6- aminobenzofuran-5-oxy]2-(2'-amino-5'-methylphenoxy)ethane-N,N,N',N '- tetraacetic acid acetoxymethyl ester (fura-2/AM) was 261 +/- 6 nM. After 4 h in a Liley K(+)-free, 2 mM [Ca2+] solution, the cytosolic [Ca2+] increased to 314 +/- 28 nM. Increase in [Ca2+] from 2 to 10 mM caused a twofold increase of cytosolic [Ca2+] to 637 +/- 26 nM. This rise was, like the Ca(2+)-induced inhibition of electrogenic pump, prevented by 10(-5) M verapamil, 10(-5) M diltiazem and 10(-7) M flunarizine. The results suggest that substances which block Ca2+ entry into the cell prevent the Ca(2+)-induced inhibition of the Na+ pump.

Human hypertensive placenta contains an increased amount of Na,K-ATPase with higher affinity for cardiac glycosides

Placentas of women suffering from pregnancy-induced hypertension (PIH) were found to contain a greater amount of Na,K-ATPase molecules, estimated from anthroyl ouabain binding, than normotensive individuals. Both the microsomal fraction of placental cells and purified Na,K-ATPase showed an increased affinity for the specific inhibitor ouabain which, in the case of the microsomes, bound with a dissociation constant of 0.9 nM as compared with 3.4 nM in the controls. Likewise, the dissociation constant of the ouabain complex with purified Na,K-ATPase was about 3.5 times lower in the hypertensive patients. The differences are apparently caused by a different microenvironment of the ouabain-binding site, as reflected in the quantum yield of bound anthroyl ouabain. If an endogenous digitalis-like factor is present in the body fluids to regulate Na,K-ATPase activity, the present results render its role quite plausible.

Changes in Na+,K(+)-ATPase structure induced by cation binding. Approach by Raman spectroscopy

Raman analysis of Na+,K(+)-ATPase structural changes induced by cation binding reveals a slight decrease ( < 10%) of the alpha-helical content upon E1-E2 transition. Pronounced conformational changes of the enzyme are unlikely as the character of the environment of tyrosine residues remains unaltered. However, local changes can take place as evidenced by changes in tryptophan vibration at about 880 cm-1.