23Na+-NMR studies of cation binding to multi-chain and single-chain glycosaminoglycan peptides

Specific Buffer Effects on the Intermolecular Interactions among Protein Molecules at Physiological pH

BSA and lysozyme molecular motion at pH 7.15 is buffer-specific. Adsorption of buffer ions on protein surfaces modulates the protein surface charge and thus protein-protein interactions. Interactions were estimated by means of the interaction parameter k_D obtained from plots of diffusion coefficients at different protein concentrations (D_app = D₀[1 + k_DC_protein]) via dynamic light scattering and nuclear magnetic resonance. The obtained results agree with recent findings confirming doubts regarding the validity of the Henderson-Hasselbalch equation, which has traditionally provided a basis for understanding pH buffers of primary importance in solution chemistry, electrochemistry, and biochemistry.

Anionic biopolyelectrolytes of the syndecan/perlecan superfamily: physicochemical properties and medical significance

In the review article presented here, we demonstrate that the connective tissue is more than just a matrix for cells and a passive scaffold to provide physical support. The extracellular matrix can be subdivided into proteins (collagen, elastin), glycoconjugates (structural glycoproteins, proteoglycans) and glycosaminoglycans (hyaluronan). Our main focus rests on the anionic biopolyelectrolytes of the perlecan/syndecan superfamily which belongs to extracellular matrix and cell membrane integral proteoglycans. Though the extracellular domain of the syndecans may well be performing a structural role within the extracellular matrix, a key function of this class of membrane intercalated proteoglycans may be to act as signal transducers across the plasma membrane and thus be more appropriately included in the group of cell surface receptors. Nevertheless, there is a continuum in functions of syndecans and perlecans, especially with respect to their structural role and biomedical significance. HS/CS proteoglycans are receptor sites for lipoprotein binding thus intervening directly in lipid metabolism. We could show that among all lipoproteins, HDL has the highest affinity to these proteoglycans and thus instals a feedforward forechecking loop against atherogenic apoB100 lipoprotein deposition on surface membranes and in subendothelial spaces. Therefore, HDL is not only responsible for VLDL/IDL/LDL cholesterol exit but also controls thoroughly the entry. This way, it inhibits arteriosclerotic nanoplaque formation. The ternary complex 'lipoprotein receptor (HS/CS-PG) - lipoprotein (LDL, oxLDL, Lp(a)) - calcium' may be interpreted as arteriosclerotic nanoplaque build-up on the molecular level before any cellular reactivity, possibly representing the arteriosclerotic primary lesion combined with endothelial dysfunction. With laser-based ellipsometry we could demonstrate that nanoplaque formation is a Ca(2+)-driven process. In an in vitro biosensor application of HS-PG coated silica surfaces we tested nanoplaque formation and size in clinical trials with cardiovascular high-risk patients who underwent treatment with ginkgo or fluvastatin. While ginkgo reduced nanoplaque formation (size) by 14.3% (23.4%) in the isolated apoB100 lipid fraction at a normal blood Ca(2+) concentration, the effect of the statin with a reduction of 44.1% (25.4%) was more pronounced. In addition, ginkgo showed beneficial effects on several biomarkers of oxidative stress and inflammation. Besides acting as peripheral lipoprotein binding receptor, HS/CS-PG is crucially implicated in blood flow sensing. A sensor molecule has to fulfil certain mechanochemical and mechanoelectrical requirements. It should possess viscoelastic and cation binding properties capable of undergoing conformational changes caused both mechanically and electrostatically. Moreover, the latter should be ion-specific. Under no-flow conditions, the viscoelastic polyelectrolyte at the endothelium - blood interface assumes a random coil form. Blood flow causes a conformational change from the random coil state to the directed filament structure state. This conformational transition effects a protein unfurling and molecular elongation of the GAG side chains like in a 'stretched' spring. This configuration is therefore combined with an increase in binding sites for Na(+) ions. Counterion migration of Na(+) along the polysaccharide chain is followed by transmembrane Na(+) influx into the endothelial cell and by endothelial cell membrane depolarization. The simultaneous Ca(2+) influx releases NO and PGI2, vasodilatation is the consequence. Decrease in flow reverses the process. Binding of Ca(2+) and/or apoB100 lipoproteins (nanoplaque formation) impairs the flow sensor function. The physicochemical and functional properties of proteoglycans are due to their amphiphilicity and anionic polyelectrolyte character. Thus, they potently interact with cations, albeit in a rather complex manner. Utilizing (23)Na(+) and (39)K(+) NMR techniques, we could show that, both in HS-PG solutions and in native vascular connective tissue, the mode of interaction for monovalent cations is competition. Mg(2+) and Ca(2+) ions, however, induced a conformational change leading to an increased allosteric, cooperative K(+) and Na(+) binding, respectively. Since extracellular matrices and basement membranes form a tight-fitting sheath around the cell membrane of muscle and Schwann cells, in particular around sinus node cells of the heart, and underlie all epithelial and endothelial cell sheets and tubes, a release of cations from or an adsorption to these polyanionic macromolecules can transiently lead to fast and drastic activity changes in these tiny extracellular tissue compartments. The ionic currents underlying pacemaker and action potential of sinus node cells are fundamentally modulated. Therefore, these polyelectrolytic ion binding characteristics directly contribute to and intervene into heart rhythm.

Blood-flow sensing by anionic biopolymers

Using 23Na-NMR techniques we could show that the polyanion proteoheparan sulfate integrated into the membrane of endothelial cells may serve as "flow sensor'. Based on its viscoelastic properties, heparan sulfate proteoglycan is present as a random coil under "no flow' conditions, whereby most of its polyanionic sites undergo intramolecular hydrogen bonding. With increasing flow the macromolecule becomes unfolded into a filamentous structure. Additional anionic binding sites to which Na+ ions from the blood bind are released by this shear stress-dependent conformational change. The Na+ binding triggers the signal transduction chain for a vasodilatory vessel reaction. Decrease in flow effects, for reasons of the intramolecular elastic recoil forces of the macromolecules, an entropic coiling, the release of Na+ ions and thus an interruption of the signal chain. Proteoheparan sulfate adsorbed onto a hydrophobic surface in physiological Krebs solution at pH 7.3 demonstrated clearly its characteristic as a Na+ sensor. While Ca2+ ions modulated the adsorption (promotion with increasing Ca2+ concentrations) by changing the conformation of the sensor molecule, the adsorbed amount was determined preferably by the Na+ concentration. K+ and Mg2+ ions showed slightly desorbing properties with increasing concentrations. Thus, it may be concluded that Na+ ions play the role as "first messenger' in flow-dependent vasodilation.

Anionic biopolymers as blood flow sensors

The finding of flow-dependent vasodilation rests on the basic observation that with an increase in blood flow the vessels become wider, with a decrease the vascular smooth muscle cells contract. Proteoheparan sulphate could be the sensor macromolecule at the endothelial cell membrane-blood interface, that reacts on the shear stress generated by the flowing blood, and that informs and regulates the vascular smooth muscle cells via a signal transduction chain. This anionic biopolyelectrolyte possesses viscoelastic and specific ion binding properties which allow a change of its configuration in dependence on shear stress and electrostatic charge density. The blood flow sensor undergoes a conformational transition from a random coil to an extended filamentous state with increasing flow, whereby Na+ ions from the blood are bound. Owing to the intramolecular elastic recoil forces of proteoheparan sulphate the slowing of a flow rate causes an entropic coiling, the expulsion of Na+ ions and thus an interruption of the signal chain. Under physiological conditions, the conformation and Na+ binding proved to be extremely Ca(2+)-sensitive while K+ and Mg2+ ions play a minor role for the susceptibility of the sensor. Via counterion migration of the bound Na+ ions along the sensor glycosaminoglycan side chains and following Na+ passage through an unspecific ion channel in the endothelial cell membrane, the signal transduction chain leads to a membrane depolarization with Ca2+ influx into the cells. This stimulates the EDRF/NO production and release from the endothelial cells. The consequence is vasodilation.

Multinuclear magnetic resonance studies of the interaction of inorganic cations with heparin

The interaction of Na+, Ca2+, Mg2+, Zn2+ and La3+ with heparin, a highly negatively charged glycosaminoglycan, was studied by 1H and 23Na nuclear magnetic resonance spectroscopy. 1H chemical shift and nuclear Overhauser effect (NOE) data indicate that the counter ions Na+, Ca2+ and Mg2+ interact with the low pH, carboxylic acid form of heparin by delocalized, long-range electrostatic interactions. At higher pH, 1H chemical shift and NOE data indicate that Na+ and Mg2+ continue to interact with heparin in the same manner, even upon deprotonation of the carboxylic acid group; however, there is a site-specific contribution to the binding of Ca2+, Zn2+ and La3+ under these conditions. Acid dissociation constants for heparin carboxylic acid groups and heparin-metal binding constants were determined from the pH dependence of 1H chemical shifts and 23Na spin-lattice (T1) relaxation times. Equilibrium constants for exchange of M2+ for heparin-bound Na+ were obtained from 23Na T1 data. The acid dissociation constants show a strong dependence on Na+ concentration due to the polyelectrolyte character of heparin.

Carotid artery mechanical properties of Dahl salt-sensitive rats

We evaluated the mechanical properties of the carotid artery in anesthetized Dahl rats with or without long-term treatment with the diuretic compound indapamide. The mechanical properties of the carotid artery were evaluated by establishing pressure-volume curves in situ in vivo before and after total relaxation of arterial smooth muscle by potassium cyanide. Dahl salt-sensitive and salt-resistant rats were fed either a low (0.4%) or high (7%) NaCl diet for 5 weeks. In each group, half the rats received for the same period of time oral treatment with indapamide (3 mg/kg per day). Blood pressure, heart rate, and pressure-volume curves were studied at the end of the 5-week period. In untreated Dahl salt-sensitive rats, the pressure-volume curve of the carotid artery was shifted to the right compared with that in untreated Dahl salt-resistant rats. The finding was observed even after potassium cyanide and regardless of the NaCl diet (P < .01 between Dahl salt-sensitive and -resistant rats). Indapamide was able to prevent the development of hypertension in Dahl salt-sensitive rats receiving a high NaCl diet (185 +/- 7 versus 146 +/- 8 mm Hg in untreated and treated Dahl salt-sensitive rats with a high NaCl diet, P < .0005). In the other groups, indapamide had no effect on blood pressure. Indapamide treatment increased carotid arterial static compliance in Dahl salt-sensitive rats with a high or low NaCl diet and to a lesser extent in Dahl salt-resistant rats. The increase was observed even after total relaxation of carotid arterial smooth muscle by potassium cyanide.(ABSTRACT TRUNCATED AT 250 WORDS)

Flow regulation of vascular tone. Its sensitivity to changes in sodium and calcium

Our hypothesis is that flow-through hydraulic drag or shear stresses the extracellular elements in the vascular wall. When the endothelium is intact, this results in the release of endothelium-derived relaxing factor and other substances, eg, prostanoids, from the endothelium. As in some reports, after inhibition of nitric oxide synthase, flow effects are still observed although diminished; the shear effect is extended mechanically to the subendothelial tissues. Shear causes conformational changes in the glycosaminoglycans by extending them from a randomly coiled aggregated state to a more elongated condition along the line of flow. This elongation and the consequent exposure of an increased number of cationic binding sites on the glycosaminoglycans lead to changes in sodium binding. The extent of the conformational change is influenced by the concentration of calcium, an ion that not only competes with sodium at specific binding sites but possibly cross-links the polysaccharide chains of the protein saccharide complex. These complex interactions might account for the cooperative, nonantagonistic interaction of sodium and calcium over the physiological concentration range. Sodium binding is influenced by changes in external sodium concentration, and this presumably accounts for the sodium sensitivity of the flow response. Although glycosaminoglycans are possibly the most studied in this regard, they are not the only candidates. Other extracellular proteins, either in conjunction with glycosaminoglycans or independently, might be involved. By mechanisms not yet identified, these changes are signaled to the cell. We have proposed that in part, at any rate, this may be related to the sodium concentration gradient.

Calcium dependence of flow-induced dilation. Cooperative interaction with sodium

The effect of changing extracellular calcium and sodium concentrations on flow, acetylcholine, and papaverine vasodilation and also on norepinephrine contraction was studied in a segment of a resistance branch of the rabbit central ear artery mounted in a myograph. Decreases in calcium to 80% of the normal physiological saline solution concentration (1.6 mM) reduced both flow- and acetylcholine-induced dilation. Increases of calcium to 120%, 140%, and 200% of normal decreased flow dilation responses, but not those to acetylcholine and papaverine. Thus, the optimum calcium concentration for flow dilation lies within the range of 1.4-1.9 mM. The concomitant proportionate reduction of sodium and calcium offsets the reduction in flow dilation that occurred with reduction in calcium alone. This was true whether sodium and calcium were reduced simultaneously or whether the effect of lowered sodium and then that of lowered sodium and calcium was studied. Emphasizing the uniqueness of this interaction between sodium and calcium are the observations that the depression of acetylcholine dilation by calcium reduction was not influenced by a concurrent reduction in sodium and that the depression of flow dilation caused by sodium reduction is increased by calcium increase, which by itself depresses flow dilation. None of these changes in sodium and calcium alters the responses of the artery segment to papaverine or norepinephrine. We propose that these interactions of sodium and calcium in relation to flow dilation may reflect the binding properties for sodium and calcium of a proposed flow sensor, the glycosaminoglycan polyanions.(ABSTRACT TRUNCATED AT 250 WORDS)

Physico-chemical properties of aqueous solutions of xanthan: an n.m.r. study

The conformations of xanthan in aqueous solution as a function of temperature have been studied. Measurements of optical activity indicate that the conformational transition, induced by varying the polymer concentration, is analogous to that induced by changes in ionic strength and pH. Within a certain range of concentrations, the low-temperature conformation has a molecular-weight-dependent stability, which shows the usual sigmoidal melting profile with increase in temperature. The 13C-n.m.r. data reflect the increase of the mobility of C-1 and the side-chain carbon atoms in the transition-temperature region. The 23Na relaxation behaviour changes on melting the ordered xanthan conformation. At least two correlation times are needed in order to describe the field-strength dependence of the longitudinal and transverse 23Na relaxation. At 25 degrees, a value of 6.8 ns is obtained for the largest correlation time for the fluctuation of the electric-field gradient. The high-temperature conformation also generates correlation times of the order of ns. From 17O relaxation measurements, a reduction of the mobility of water molecules in the presence of xanthan chains is also observed.

Diffusion coefficients of neurotransmitters and their metabolites in brain extracellular fluid space

Diffusion coefficients of catecholamine neurotransmitters, their metabolites and related species was measured in brain extracellular fluid using in vivo voltammetric techniques. Nanoliter volumes of the species were pressure-ejected into the rat caudate nucleus and their concentration profiles were determined at nearby voltammetric detector electrodes. Thorough testing was carried out to show that the present methodology gave results which agreed with brain diffusion coefficients measured previously by ion-selective microelectrode techniques. All of the species which are anionic at pH 7.4 have brain diffusion coefficients about one-third of their solution counterparts in accord with earlier studies of diffusion in tortuous media. However, the brain diffusion coefficients of all the cation species are about three-times slower than those of the anions. This phenomenon is believed to be caused by ion binding with the polyanionic glycosaminoglycans and related species in brain tissue. In vitro model experiments lend support to this interpretation. This new information on biogenic amines and their metabolites provides meaningful predictions of the spatio-temporal concentration distribution of these species in the extracellular fluid.

Local regulation of blood flow

H+ and K+ ions participate decisively in the local regulation of blood flow. Variation of their extracellular concentration changes the membrane potential of vascular smooth muscle cells and tension via electromechanical coupling. The effect of K+ ions can be primarily attributed to a change of K+ equilibrium potential and electrogenic pump rate, the effect of H+ ions to a change of Na+ and K+ permeability of the cell membrane. Shifts of external proton and/or cation concentrations cause changes of the binding properties of the polyanionic macromolecules in vascular connective tissue. Thus, the extracellular concentration of various cation species can very fast and drastically in the tight mesh-work of connective tissue fibres close to the membrane of vascular smooth muscle cells. Especially, the K+ adsorption with extracellular acidification as well as the cooperative K+ binding as consequence of a conformational change induced by Mg++ ions are of great importance for membrane hyperpolarization and vasodilatation.

Dermatan sulfate: molecular conformations and interactions in the condensed state

The molecular conformations and manner of aggregation has been determined for three allomorphs of the connective tissue polysaccharide dermatan sulfate by analysis of X-ray diffraction from oriented, polycrystalline fibers of sodium salts. One allomorph is unique among glycosaminoglycans in having right-handed (8(3)) helical chains. Two such chains pack antiparallel in a tetragonal unit cell (a = b = 1.267 nm, c = 7.353 nm) with P4(3)2(1)2 space group symmetry. The 3(2) chains of the second allomorph are organized in a trigonal unit cell (a = b = 1.460 nm, c = 2.823 nm, space group symmetry P3(2)21) containing two left-handed antiparallel polysaccharide molecules. (The chirality of this allomorph has been assumed to be the same as in other 3-fold glycosaminoglycan helices, since discrimination between 3(1) and 3(2) symmetries was found not to be possible.) The archiral 2(1) helices of the third allomorph, pack probably in an orthorhombic unit cell (a = 1.151 nm, b = 1.065 nm, c = 1.878 nm, space group symmetry P2(1)2(1)2(1)) that contains again two antiparallel polymer molecules. Each dermatan sulfate chain is stabilized intramolecularly by O3-O5 hydrogen bonds across the beta (1 leads to 4) linkage. There are two intermolecular hydrogen bonds per tetrasaccharide repeat in the tetragonal structure and two per disaccharide in the trigonal structure. Fourier difference syntheses indicated equivalents of four sodium ions per tetrasaccharide and two sodium ions per disaccharide in the tetragonal and trigonal structures, respectively. The cations are either partially or fully hydrated and link dermatan sulfate chains either intra- or intermolecularly by involving besides other polyanion oxygen atoms, carboxylate and sulfate oxygen atoms. The probable mode of packing in the orthorhombic structure indicates a pair of hydrogen bonds between adjacent antiparallel polysaccharide chains and suggests plausible cationic sites in the unit cell.

Chondroitin 4-sulfate: comparison of the structures of the potassium and sodium salts

Analysis of the X-ray diffraction pattern from an oriented, polycrystalline fiber of a potassium chondroitin 4-sulfate proteoglycan shows that the polysaccharide chains have a left-handed 3-fold helical secondary structure stabilized by intra- and intermolecular hydrogen bonds. Two antiparallel chains pass through each trigonal unit cell, which has dimensions a = b = 1.385 nm, c = 2.776 nm and space group symmetry P3(2)21. The cations and water molecules in the crystals are not all periodic and only one potassium ion and four water molecules per disaccharide were located by difference Fourier methods. Sodium chondroitin 4-sulfate forms an analogous structure with polyanions of similar geometry. However, the packing arrangements in the two salts are quite different, presumably because of the different co-ordination preference of K+ and Na+. Thus the relatively small differences between these two cations are greatly amplified by the idiosyncratic polymer networks they promote.