Conformation similarities of the globular and tailed forms of acetylcholinesterase from Torpedo californica

The conformation of the globular dimer (G2), the tailed asymmetric dodecamer (A12, also containing some tailed octamer A8) and the globular tetramer (G4, prepared by removing the collagen-like tail from A12) of acetylcholinesterase (acetylcholine acetylhydrolase, EC 3.1.1.7) was studied by circular dichroism (CD) in the ultraviolet region. The G2 and G4 forms had similar conformation with about 40% alpha-helix, 35% beta-sheets and 4% beta-turns; the tailed form had a lower helicity (about 34%) and beta-form (about 25%) content probably because of the presence of the tail whose CD spectrum resembles that of an unordered form, but it had about the same amount of beta-turns as the other two forms. All three forms also had similar CD spectra in the near-ultraviolet region due to their non-peptide chromophores. The pH, thermal and urea denaturation of the three acetylcholinesterase forms was also similar to each other. The pH-dependency of both the enzymatic activity and CD intensity of the three forms showed bell-shaped curves with a plateau at pH 7-8. The activity was completely lost at pH below 5 or above 10, but the corresponding CD spectra retained 70-80% of the original magnitudes. Thermal denaturation of the three forms at pH 7.5 showed a conformational transition and loss of activity between 30 and 40 degrees C, but the CD intensity of the helical band at 222 nm was reduced by only 20-30%. Urea denaturation of the three forms began at 1 M urea; it was protein concentration- and time-dependent. Again, the activity disappeared faster than the decreasing CD intensity. Thus, the overall conformation of the three acetylcholinesterase forms appears to be relatively stable, but their active site is easily perturbed by changing the environment. The loss of activity correlated well with the disappearance of the CD band of tryptophan(s) in the near-ultraviolet region, suggesting that the Trp residue(s) might be at or near the active center of the enzyme.     

Electrogenic behavior of synaptic vesicles from Torpedo californica.

Electrical potential changes in pure synaptic vesicles from Torpedo californica were monitored with the fluorescent dye 3,3'-dipropylthiadicarbocyanine iodide. Vesicles resuspended in variable external sodium ion in the presence of gramicidin established sodium ion membrane diffusion potentials. Vesicles resuspended in choline or acetylcholine chloride became hyperpolarized upon addition of gramicidin. Hyperpolarization was subsequently partially reversed spontaneously by choline or acetylcholine influx, which was confirmed by gel filtration, to yield a new, less negative, stable membrane potential. Thus, acetylcholine and choline are taken up electrogenically by synaptic vesicles.     

Irreversible thermal denaturation of Torpedo californica acetylcholinesterase.

Thermal denaturation of Torpedo californica acetylcholinesterase, a disulfide-linked homodimer with 537 amino acids in each subunit, was studied by differential scanning calorimetry. It displays a single calorimetric peak that is completely irreversible, the shape and temperature maximum depending on the scan rate. Thus, thermal denaturation of acetylcholinesterase is an irreversible process, under kinetic control, which is described well by the two-state kinetic scheme N-->D, with activation energy 131 +/- 8 kcal/mol. Analysis of the kinetics of denaturation in the thermal transition temperature range, by monitoring loss of enzymic activity, yields activation energy of 121 +/- 20 kcal/mol, similar to the value obtained by differential scanning calorimetry. Thermally denatured acetylcholinesterase displays spectroscopic characteristics typical of a molten globule state, similar to those of partially unfolded enzyme obtained by modification with thiol-specific reagents. Evidence is presented that the partially unfolded states produced by the two different treatments are thermodynamically favored relative to the native state.     

Affinity partitioning of membranes. Cholinergic receptor-containing membranes from Torpedo californica.

Affinity partitioning has been employed in the purification of membranes rich in cholinergic receptor from Torpedo californica electric organs. The procedure involves a modification of poly(ethylene oxide)-dextran aqueous phase partitioning systems where a ligand selective for the receptor is conjugated to the poly(ethylene oxide). Specific partitioning of the receptor-containing membranes into the poly(ethylene oxide)-rich phase occurs when bis-alpha,omega-trimethylamino poly(ethylene oxide) or bis-rho-tri-methylammonium phenylamino poly(ethylene oxide) was added to the phase system in low mole ratio. bis-alpha,omega-Methylamino poly(ethylene oxide), which should impart equivalent interfacial electromotive potential to the system but bind poorly to the receptor sites, was much less effective in producing phase distribution changes. The ligand-polymer-dependent phase distribution shifts were blocked by bisquaternary methonium ligands at concentrations consistent with their relative affinities for the cholinergic receptor. Titration or receptor sites with cobra alpha-toxin decreased the phase distribution changes in a linear fashion up to the point of stoichiometry. These observations are consistent with the phase distribution changes being consequent to ligand-polymer association with the pharmacologically important site on the receptor. The affinity partitioning procedure, when employed following an initial purification of the membranes by differential and density gradient centrifugation, yields membrane preparations with a high degree of morphological uniformity and a specific activity between 2.9 and 4.6 nmol of bound cobra alpha-toxin/mg of protein.     

Two partially unfolded states of Torpedo californica acetylcholinesterase.

Chemical modification with sulfhydryl reagents of the single, nonconserved cysteine residue Cys231 in each subunit of a disulfide-linked dimer of Torpedo californica acetylcholinesterase produces a partially unfolded inactive state. Another partially unfolded state can be obtained by exposure of the enzyme to 1-2 M guanidine hydrochloride. Both these states display several important features of a molten globule, but differ in their spectroscopic (CD, intrinsic fluorescence) and hydrodynamic (Stokes radii) characteristics. With reversal of chemical modification of the former state or removal of denaturant from the latter, both states retain their physiochemical characteristics. Thus, acetylcholinesterase can exist in two molten globule states, both of which are long-lived under physiologic conditions without aggregating, and without either intraconverting or reverting to the native state. Both states undergo spontaneous intramolecular thioldisulfide exchange, implying that they are flexible. As revealed by differential scanning calorimetry, the state produced by chemical modification lacks any heat capacity peak, presumably due to aggregation during scanning, whereas the state produced by guanidine hydrochloride unfolds as a single cooperative unit, thermal transition being completely reversible. Sucrose gradient centrifugation reveals that reduction of the interchain disulfide of the native acetylcholinesterase dimer converts it to monomers, whereas, after such reduction, the two subunits remain completely associated in the partially unfolded state generated by guanidine hydrochloride, and partially associated in that produced by chemical modification. It is suggested that a novel hydrophobic core, generated across the subunit interfaces, is responsible for this noncovalent association. Transition from the unfolded state generated by chemical modification to that produced by guanidine hydrochloride is observed only in the presence of the denaturant, yielding, on extrapolation to zero guanidine hydrochloride, a high free energy barrier (ca. 23.8 kcal/mol) separating these two flexible, partially unfolded states.     

Comparison of asymmetric forms of acetylcholinesterase from the electric organ of Narke japonica and Torpedo californica

The asymmetric forms of acetylcholinesterase were purified from the electric organs of the electric rays Narke japonica and Torpedo californica, and their properties were compared. Asymmetric acetylcholinesterase was purified by immunoaffinity chromatography with a monoclonal antibody (Nj-601) to acetylcholinesterase. The MgCl2 extracts of these electric organs were applied to a column of Nj-601-Sepharose, and the bound acetylcholinesterase was eluted by lowering the pH of the eluent to 2.8. The purified asymmetric acetylcholinesterases gave peaks of 17 S (A12) and 13 S (A8) on sucrose density gradients. The enzyme from N. japonica contained more A8 than A12, while that of T. californica contained more A12. After treatment with collagenase, the enzymes gave three peaks on sedimentation; 20 S, 16 S and 11 S for N. japonica, and 19 S, 15 S and 11 S for T. californica, indicating the presence of collagen-like tails. On polyacrylamide gel electrophoresis in sodium dodecyl sulfate, the asymmetric acetylcholinesterase from N. japonica gave bands of Mr 140 000, 100 000, 70 000 and 60 000, while that from T. californica gave bands of Mr 140 000, 100 000, 70 000 and 55 000. The bands of Mr 70 000 and 140 000 were monomers and non-reducible dimers, respectively, of the catalytic subunits. The bands of Mr 60 000 and 55 000 were the tail subunits, since collagenase treatment of the purified enzymes markedly decreased the amounts of these components. The Mr 100 000 subunit constituted less than 3% of the total asymmetric acetylcholinesterase from N. japonica but 18% of that from T. californica. The tail subunits constituted 6-8% of the two preparations. The catalytic subunits and the Mr 100 000 subunits bound concanavalin A, indicating that they are glycoproteins. The amino acid compositions of the enzymes from N. japonica and T. californica were very similar. Both contained hydroxyproline and hydroxylysine, characteristic of the collagen-like tails. The enzyme required divalent metal ions for activity, but only Mn2+, Mg2+ and Ca2+ were effective. Mn2+ was effective at the lowest concentrations, while Mg2+ gave the highest activity.     

Purification of Torpedo californica post-synaptic membranes and fractionation of their constituent proteins.

A rapid methof for preparation of membrane fractions highly enriched in nicotinic acetylcholine receptor from Torpedo californica electroplax is described. The major step in this purification involves sucrose-density-gradient centrifugation in a reorienting rotor. Further purification of these membranes can be achieved by selective extraction of proteins by use of alkaline pH or by treatment with solutions of lithium di-idosalicylate. The alkali-treated membranes retain functional characteristics of the untreated membranes and in addition contain essentially only the four polypeptides (mol.wts. 40000, 50000, 60000 and 65000) characteristic of the receptor purified by affinity chromatography. Dissolution of the purified membranes or of the alkali-treated purified membranes in sodium cholate solution followed by sucrose-density-gradient centrifugation in the same detergent solution yields solubilized receptor preparations comparable with the most highly purified protein obtained by affinity-chromatographic procedures.     

Three-dimensional structure of a complex of E2020 with acetylcholinesterase from Torpedo californica

The 3D structure of a complex of the anti-Alzheimer drug, E2020, also known as Aricep^®, with Torpedo californica acetylcholinesterase is reported. The X-ray structure, at 2.5 Å resolution, shows that the elongated E2020 molecule spans the entire length of the active-site gorge of the enzyme. It thus interacts with both the ‘anionic’ subsite, at the bottom of the gorge, and with the peripheral anionic site, near its entrance, via aromatic stacking interactions with conserved aromatic residues. It does not interact directly with either the catalytic triad or with the ‘oxyanion hole’. Although E2020 is a chiral molecule, and both the S and R enantiomers have similar affinity for the enzyme, only the R enantiomer is bound within the active-site gorge when the racemate is soaked into the crystal. The selectivity of E2020 for acetylcholinesterase, relative to butyrylcholinesterase, can be ascribed primarily to its interactions with Trp279 and Phe330, which are absent in the latter.     

A neutral molecule in a cation-binding site: specific binding of a PEG-SH to acetylcholinesterase from Torpedo californica

The crystal structure of acetylcholinesterase from Torpedo californica complexed with the uncharged inhibitor, PEG-SH-350 (containing mainly heptameric polyethylene glycol with a terminal thiol group) is determined at 2.3 A resolution. This is an untypical acetylcholinesterase inhibitor, since it lacks the cationic moiety typical of the substrate (acetylcholine). In the crystal structure, the elongated ligand extends along the whole of the deep and narrow active-site gorge, with the terminal thiol group bound near the bottom, close to the catalytic site. Unexpectedly, the cation-binding site (formed by the faces of aromatic side-chains) is occupied by CH(2) groups of the inhibitor, which are engaged in C-H...pi interactions that structurally mimic the cation-pi interactions made by the choline moiety of acetylcholine. In addition, the PEG-SH molecule makes numerous other weak but specific interactions of the C-H...O and C-H...pi types.     

Residues in Torpedo californica acetylcholinesterase necessary for processing to a glycosyl phosphatidylinositol-anchored form

Acetylcholinesterase from Torpedo californica (TcAChE) can be found as a glycosyl phosphatidylinositol (GPI)-anchored, membrane associated form. The C-terminal amino-acid sequence of the precursor protein resembles the signal peptide sequence found in proteins and enzymes destined for GPI-modification. Characteristics of such a signal peptide are a relatively polar stretch of amino acids, separating a cleavage- and modification-site (ω-site) residue from a hydrophobic C-terminus. We have introduced mutations, both at putative ω-sites and in the hydrophobic region, and analysed their effects on GPI-anchoring of TcAChE. Our results show that substitution of all three Ser residues in the region Ser⁵⁴²-Ser⁵⁴⁴ prevents GPI-modification and membrane anchoring. Individual substitution of each of these residues resulted in no or only a minor effect on the modification. We therefore conclude that more than one residue within this sequence can be utilised as the ω-site. Our analyses of double substitutions indicate that Ser-543 and Ser-544 are the preferred residues for GPI-modification. Moreover, the hydrophobic region is shown to be essential for GPI-anchoring of TcAChE.     

Protease effects on the structure of acetylcholine receptor membranes from Torpedo californica

Protease digestion of acetylcholine receptor-rich membranes derived from Torpedo californica electroplaques by homogenization and isopycnic centrifugation results in degradation of all receptor subunits without any significant effect on the appearance in electron micrographs, the toxin binding ability, or the sedimentation value of the receptor molecule. Such treatment does produce dramatic changes in the morphology of the normally 0.5- to 2-microns-diameter spherical vesicles when observed by either negative-stain or freeze-fracture electron microscopy. Removal of peripheral, apparently nonreceptor polypeptides by alkali stripping (Neubig et al. 1979, Proc. Natl. Acad. Sci. U. S. A. 76:690-694) results in increased sensitivity of the acetylcholine receptor membranes to the protease trypsin as indicated by SDS gel electrophoretic patterns and by the extent of morphologic change observed in vesicle structure. Trypsin digestion of alkali- stripped receptor membranes results in a limit degradation pattern of all four receptor subunits, whereupon all the vesicles undergo the morphological transformation to minivesicles. The protein-induced morphological transformation and the limit digestion pattern of receptor membranes are unaffected by whether the membranes are prepared so as to preserve the receptor as a disulfide bridged dimer, or prepared so as to generate monomeric receptor.     

Active-site gorge and buried water molecules in crystal structures of acetylcholinesterase from Torpedo californica

Buried water molecules and the water molecules in the active-site gorge are analyzed for five crystal structures of acetylcholinesterase from Torpedo californica in the resolution range 2.2-2.5 A (native enzyme, and four inhibitor complexes). A total of 45 buried hydration sites are identified, which are populated with between 36 and 41 water molecules. About half of the buried water is located in a distinct region neighboring the active-site gorge. Most of the buried water molecules are very well conserved among the five structures, and have low displacement parameters, B, of magnitudes similar to those of the main-chain atoms of the central beta-sheet structure. The active-site gorge of the native enzyme is filled with over 20 water molecules, which have poor hydrogen-bond coordination with an average of 2.9 polar contacts per water molecule. Upon ligand binding, distinct groups of these water molecules are displaced, whereas the others remain in positions similar to those that they occupy in the native enzyme. Possible roles of the buried water molecules are discussed, including their possible action as a lubricant to allow large-amplitude fluctuations of the loop structures forming the gorge wall. Such fluctuations are required to facilitate traffic of substrate, products and water molecules to and from the active-site. Because of their poor coordination, the gorge water molecules can be considered as "activated" as compared to bulk water. This should allow their easy displacement by incoming substrate. The relatively loose packing of the gorge water molecules leaves numerous small voids, and more efficient space-filling by substrates and inhibitors may be a major driving force of ligand binding.     

A differential scanning calorimetry study of acetylcholine receptor-rich membranes from Torpedo californica

Various acetylcholine receptor-rich membrane preparations from Torpedo californica electroplax tissue were examined using the techniques of differential scanning calorimetry coupled with gel electrophoretic analysis of heat-denaturing material and functional assays following passage through discrete transitions. In unfractionated membranes, four irreversible calorimetric transitions were observed, one of which (Td = 59 degrees C) could be assigned to a complete loss of acetylcholine receptor function. A second lower temperature transition apparently corresponds to loss of certain peripheral membrane proteins including the Mr = 43,000 polypeptide and the acetylcholinesterase activity. Membrane preparations highly enriched in acetylcholine receptor polypeptides contained a major transition at 59 degrees C which could be shown to be sensitive to the presence of added ligands of the acetylcholine receptor, supporting its assignment to structural alterations of the receptor protein or its arrangement in the membrane.     

Raman spectroscopic study on the conformation of 11 S form acetylcholinesterase from Torpedo californica

Vibrational Raman spectroscopy has been used to study the conformation of the 11 S form of acetylcholinesterase from Torpedo californica. Secondary structure analysis by the method of Williams [(1983) J. Mol. Biol. 166, 581-603] shows 49% alpha-helical structure, 23% beta-sheets, 11% turns and 15% undefined structure. Secondary structure estimates obtained for this enzyme by Raman spectroscopy and circular dichroism have been analyzed.     

Torpedo marmorata acetylcholinesterase; a comparison with the Electrophorus electricus enzyme. Molecular forms, subunits, electron microscopy, immunological relationship.

Electron microscopy, sequential degradation by hydrolytic enzymes and the physical-chemical properties of the molecular forms of Torpedo acetylcholinesterase indicate that these molecules are structurally related to each other in the same way as the molecular forms of Electrophorus acetylcholinesterase: all are derived from a complex structure in which three tetrameric groups of subunits are associated with a rod-like 'tail'. In aged preparations the catalytic subunits are split into fragments in a manner similar to those of Electrophorus acetylcholinesterase. Immunological cross-reaction between both enzymes demonstrates the occurrence of common antigenic sites. The enzymes from the two sources, however, are different in their molecular weights and susceptibility to hydrolytic enzymes. Also, Torpedo acetylcholinesterase does not precipitate with either isologous or heterologous antibodies.     

Preparation of right-side-out, acetylcholine receptor enriched intact vesicles from Torpedo californica electroplaque membranes.

Intact vesicles enriched in acetylcholine receptor from Torpedo californica electroplaque membranes can be separated from collapsed or leaky vesicles and membrane sheets on sucrose density gradients. alpha-Bungarotoxin binding in intact vesicles reveals that approximately 95% of the acetylcholine receptor containing vesicles are formed outside-out (with the synaptic membrane face exposed on the vesicle exterior). The binding data also indicated that only 5% or less of the sites for alpha-bungarotoxin binding to synaptic membranes are located on the interior, cytoplasmic face. Intact vesicles are stable to gentle pelleting and resuspension but are easily osmotically shocked. The vesicles are impermeable to sucrose and Ficoll, but glycerol readily transverses to membrane barrier. Intact vesicles provide a sealed, oriented membrane preparation for studies of vectorial acetylcholine receptor mediated processes.     

Interaction of partially unfolded forms of Torpedo acetylcholinesterase with liposomes.

A water-soluble dimeric form of acetylcholinesterase from electric organ tissue of Torpedo californica was obtained by solubilization with phosphatidylinositol-specific phospholipase C of the glycophosphatidylinositol-anchored species, followed by purification by affinity chromatography. The water-soluble species, in its catalytically active native conformation, did not interact with unilamellar vesicles of dimyristoylphosphatidylcholine. We previously showed that either chemical modification or exposure to low concentrations of guanidine hydrochloride converted the native enzyme to compact, partially unfolded species with the physicochemical characteristics of the molten globule state. In the present study, it was shown that such molten globule species, whether produced by mild denaturation or by chemical modification, interacted efficiently with small unilamellar vesicles. Binding was not accompanied by significant vesicle fusion, but transient leakage occurred at the time of binding. The bound acetylcholinesterase reduced the transition temperature of the vesicles slightly, and NMR data suggested that it interacted primarily with the head-group region of the bilayer. The effects of tryptic digestion of the bound acetycholinesterase were monitored by gel electrophoresis under denaturing conditions. It was found that a single polypeptide, of mass approximately 5 kDa, remained associated with the vesicles. Sequencing revealed that this is a tryptic peptide corresponding to the sequence Glu 268-Lys 315. This polypeptide contains the longest hydrophobic sequence in the protein, Leu 274-Met 308, as identified on the basis of hydropathy plots. Inspection of the three-dimensional structure of acetylcholinesterase reveals that this hydrophobic sequence is largely devoid of tertiary structure and is localized primarily on the surface of the protein. It is suggested that this hydrophobic sequence is aligned parallel to the surface of the vesicle membrane, with nonpolar residues undergoing shallow penetration into the bilayer.     

Labeling of functionally sensitive sulfhydryl-containing domains of acetylcholine receptor from Torpedo californica membranes

N-(1-Pyrene)maleimide, a fluorescent, lipophilic, alkylating agent, was used as a probe for the nicotinic acetylcholine receptor (AChR). Preincubation with N-(1-pyrene)maleimide under nonreducing conditions inhibits agonist-induced cation permeability of AChR-enriched membranes. This inhibition is dependent on the concentration of N-(1-pyrene)maleimide used. This correlation was also exhibited by resonance energy transfer of tryptophan fluorescence to N-(1-pyrene)maleimide and by the labeling stoichiometries. However, agonist-induced desensitization, as based on the time-dependent inhibition of alpha-bungarotoxin binding upon preincubation with the agonist carbamylcholine, was unaffected by N-(1-pyrene)maleimide. Alkylation of the AChR by N-(1-pyrene)maleimide is pH-dependent with an apparent pKa of 7.5 and is unaffected by preincubation with carbamylcholine, alpha-bungarotoxin, tubocurarine, or decamethonium. Preincubation with a 25-fold molar excess of N-ethylmaleimide partially protects against N-(1-pyrene)maleimide, yet simultaneous incubation with an equimolar concentration does not protect. In contrast, simultaneous incubation with equimolar concentrations of phenylmaleimide or naphthylmaleimide inhibited N-(1-pyrene)maleimide alkylation by 52 and 67%, respectively. Each AChR subunit is labeled by N-(1-pyrene)maleimide. Prior alkylation with N-ethylmaleimide does not alter the labeling profile but lowers the amount of labeling of all subunits. Reductive methylation of membranes under conditions which dimethylate all or most protein amino groups does not inhibit alkylation by N-(1-pyrene)maleimide. The above results, as well as amino acid analysis of N-(1-pyrene)maleimide-alkylated receptor, indicate that a homologous class of cysteines, which reside in each subunit within the AChR domain embedded in the membrane, are involved in the reaction with N-(1-pyrene)maleimide.     

Purification of active synaptic vesicles from the electric organ of Torpedo californica and comparison to reserve vesicles

At least two distinguishable forms of synaptic vesicles exist, the active and reserve, but the reserve form is studied most because it has been difficult to purify the active vesicles. In the work reported here the active vesicles (termed VP₂) were highly enriched from the electric organ of Torpedo californica by an improved method developed for the reserve vesicles (termed VP₁) with the addition of density gradient centrifugation based on Percoll. No significant differences between the vesicular types were found in the amounts of SV1, SV2, and SV4 epitopes and P-type and V-type ATPase activities. The buoyant densities (g/ml) of VP₁ and VP₂ vesicles were determined by centrifugation in isosmotic sucrose (1.051, 1.069), Percoll (1.034, 1.040), and glycerol (1.087, 1.090) gradients. The radii were determined by dynamic quasi-elastic laser light-scattering to be (56.6 ± 10.8) nm and (55.0 ± 12.7) nm. For both vesicular types the volume of excluded sucrose is only about 37% of the volume of excluded Percoll, indicating that the surfaces are rough. Approx. 51% of the VP₁ and 32% of the VP₂ vesicular volumes are ‘osmotically active’ water that is exchangeable with glycerol. The different buoyant densities and amounts of osmotically active water in VP₁ and VP₂ vesicles probably are due to the different internal solutes. Previously observed differences in acetylcholine active transport and vesamicol binding by VP₁ and VP₂ synaptic vesicles cannot be explained by major alterations in the protein composition or conformation of the membranes in the two types of vesicles.     

Stabilization of a metastable state of Torpedo californica acetylcholinesterase by chemical chaperones

Chemical modification of Torpedo californica acetylcholinesterase by the natural thiosulfinate allicin produces an inactive enzyme through reaction with the buried cysteine Cys 231. Optical spectroscopy shows that the modified enzyme is "native-like," and inactivation can be reversed by exposure to reduced glutathione. The allicin-modified enzyme is, however, metastable, and is converted spontaneously and irreversibly, at room temperature, with t(1/2) approximately 100 min, to a stable, partially unfolded state with the physicochemical characteristics of a molten globule. Osmolytes, including trimethylamine-N-oxide, glycerol, and sucrose, and the divalent cations, Ca(2+), Mg(2+), and Mn(2+) can prevent this transition of the native-like state for >24 h at room temperature. Trimethylamine-N-oxide and Mg(2+) can also stabilize the native enzyme, with only slight inactivation being observed over several hours at 39 degrees C, whereas in their absence it is totally inactivated within 5 min. The stabilizing effects of the osmolytes can be explained by their differential interaction with the native and native-like states, resulting in a shift of equilibrium toward the native state. The stabilizing effects of the divalent cations can be ascribed to direct stabilization of the native state, as supported by differential scanning calorimetry.