The Use of a GroEL-BLI Biosensor to Rapidly Assess Preaggregate Populations for Antibody Solutions Exhibiting Different Stability Profiles

An automated method using biotinylated GroEL-streptavidin biosensors with biolayer interferometry (GroEL-BLI) was evaluated to detect the formation of transiently formed, preaggregate species in various pharmaceutically relevant monoclonal antibody (mAb) samples. The relative aggregation propensity of various IgG1 and IgG4 mAbs was rank ordered using the GroEL-BLI biosensor method, and the least stable IgG4 mAb was subjected to different stresses including elevated temperatures, acidic pH, and addition of guanidine HCl. The GroEL-BLI biosensor detects mAb preaggregate formation mostly before, or sometimes concomitantly with, observing soluble aggregates and subvisible particles using size-exclusion chromatography and microflow imaging, respectively. A relatively unstable bispecific antibody (Bis-3) was shown to bind the GroEL biosensor even at low temperatures (25^°C). During thermal stress (50°C, 1 h), increased Bis-3 binding to GroEL-biosensors was observed prior to aggregation by size-exclusion chromatography or microflow imaging. Transmission electron microscopy analysis of Bis-3 preaggregate GroEL complexes revealed, in some cases, potential hydrophobic interaction sites between the Fc domain of the Bis-3 and GroEL protein. The automated BLI method not only enables detection of transiently formed preaggregate species that initiate protein aggregation pathways but also permits rapid mAb formulation stability assessments at low volumes and low protein concentrations.

Comparison of proteins from thermophilic and nonthermophilic sources in terms of structural parameters inferred from amino acid composition

The amino acid composition of 14 different proteins from thermophilic bacteria were compiled along with the amino acid compositions of 56 corresponding proteins from nonthermophilic sources. A comparison was made between proteins serving the same catalytic function, and significant differences in composition were noted for those proteins from thermophilic bacteria. However, no consistent pattern was evident and the differences were often small. The two data pools were treated as two distinct classes and a thermophilic versus non-thermophilic comparison of amino acid composition was made using the Student's t-test. Significant differences in composition were found for Asx (sum of Asp and Asn, if known), Ser, and Arg. Both classes of data have similar standard deviations for the mean of any single amino acid, suggesting a similar tolerance of variation in the two classes of proteins. This would argue against the hypothesis that thermophiles exhibit a greater frequency of errors in protein synthesis. The amino acid compositions were used to calculate structural parameters (% helix, % beta, % turn, hydrophobicity, and melting temperatures) for the two classes of proteins. Of these, only the predicted % beta content was significantly lower for proteins of thermophilic origin. No differences in hydrophobicity or predicted melting temperature were observed for the two classes of proteins. This study supports the hypothesis that while small differences may occur in the amino acid composition of thermophilic proteins, they are quite varied and often are very subtle.

Determining the identity and purity of recombinant proteins by UV absorption spectroscopy

Because each protein (gene product) has a unique amino acid sequence, the particular aromatic amino acid content of each protein results in a unique spectrum in the near-UV (250 to 350 nm) region. The highly specific microenvironment experienced by each aromatic residue in the three-dimensional protein matrix results in fine shifts in a protein's spectrum. This unit provides protocols for the detection and analysis of UV spectra of recombinant proteins and their peptide fragments. The unique UV spectral properties of proteins can in turn be used to assess their purity. This application is inherent in the use of a diode array detector to monitor the effluent from a high-performance liquid chromatography (HPLC) column. A protocol using this technique to assess the purity of recombinant proteins is presented.

Highly Concentrated Monoclonal Antibody Solutions: Direct Analysis of Physical Structure and Thermal Stability

Virtually all current analytical methods employed in the development of highly concentrated monoclonal antibody (MAb) formulations require dilution of the sample before acquiring data. Thus, there is an unmet need for methods to study proteins directly at high concentration, since extrapolation of stability indicating parameters obtained from dilute studies may not be representative of the high concentration solution. Only slight or no modifications of biophysical methods including fluorescence, UV absorbance, circular dichroism, and FTIR (ATR and transmittance) spectroscopies as well as differential scanning calorimetry (DSC) are described here that permit the direct study of immunoglobulins (and other proteins) at high concentrations. Although FTIR spectra show differences that are dependent upon sampling geometry, other spectroscopic data from two different recombinant MAbs suggests that structure of each antibody exists in a physically similar state in the concentration range of 0.1-190 mg/mL in 40 mM pH 6 citrate-phosphate buffer. Upon thermally stressing these proteins, spectroscopic techniques that probe tertiary structure demonstrate a decrease in the apparent thermal melting temperature of approximately 5-20 degrees C of both proteins with increasing concentration. In contrast, DSC thermograms and CD thermal experiments suggest a minor degree of stabilization (approximately 2 degrees C) for both antibodies although protein association could be responsible for these observations. Empirical phase diagrams produced from spectroscopic data also suggest (1) the existence of similar structural states at low temperatures independent of concentration and (2) a decrease in the temperature at which phase changes are observed with increasing concentration. The decrease in structural stability observed in these studies is probably the result of aggregation or self-association of the recombinant MAbs upon heating in crowded solutions and not due to a decrease in the intrinsic structural stability of the MAbs.

The 1st annual meeting of the American Society of Gene Therapy

The inaugural meeting of the American Society of Gene Therapy (ASGT) attracted over 1700 participants to the Pacific gateway city of Seattle, Washington for a multifaceted 4 day meeting organised into a series of symposia, workshops, poster sessions and educational opportunities representative of gene therapy's immense diversity. Presentations from the international assemblage of industrial and academic scientists covered a blend of data from cutting edge research to current clinical investigations across a spectrum of therapeutic targets. Unique educational sessions allowed participants to gain basic information regarding areas of gene therapy research in which they lacked familiarity, whereas several 'Meet the Investigator' sessions allowed participants to interact directly with experts in small group settings in order to obtain a more sophisticated perspective through informal dialogue. A large majority of the symposia and poster presentations showcased the development and current successes of viral-mediated gene therapy although non-viral approaches received their share of attention. Despite the fact that promising results were presented from ongoing clinical trials using viral-mediated gene therapy, much of the symposia addressed the accompanying problems of immunogenicity, low levels of gene expression and lack of control of gene expression that are presently limiting the clinical success of virally mediated gene transfer.

Recent advances in our understanding of protein conformational stability from a pharmaceutical perspective

The marginal conformational stability of proteins has made them in some cases less than ideal candidates for pharmaceutical agents. Recent progress in our understanding of protein structure and stability has provided the opportunity to design the desired degree of stability into protein drug candidates. Modifications such as the optimisation of interior side-chain packing, the introduction of new ion-pairs, as well as the design of stabilising disulfide bridges and ligand binding sites, all offer the opportunity to produce proteins with enhanced stability properties.

Secondary structure of a dynamic type I'beta-hairpin peptide

A spontaneously folding beta-hairpin peptide (Lys-Lys-Tyr-Thr-Val-Ser-Ile-Asn-Gly-Lys-Lys-Ile-Thr-Val-Ser-Ile) and related cyclic (cyclo-Gly-Lys-Tyr-Ile-Asn-Gly-Lys-Ile-Ile-Asn) and linear (Ser-Ile-Asn-Gly-Lys) controls were studied to determine the effects of various factors on secondary structure. Secondary structure was evaluated using circular dichroism (CD) and 1D and 2D (1)H nuclear magnetic resonance (NMR). The effects of chemical modifications in the peptide and various solution conditions were investigated to determine their impact on peptide structure. The beta-hairpin peptide displayed a CD minimum at 216 nm and a TOCSY i + 1 - i + 2 and i + 2 -i + 3 interaction, confirming the expected structure. Using NMR alpha-proton (H(alpha)) chemical shifts, the extents of folding of the beta-hairpin and linear control were estimated to be 51 and 25% of the cyclic control (pH 4, 37 degrees C), which was taken to be maximally folded. Substitution of iso-aspartic acid for Asn reduced the secondary structure dramatically; substitution of aspartic acid for Asn also disrupted the structure. This result suggests that deamidation in unconstrained beta-turns may have adverse effects on secondary structure. N-terminal acetylation and extreme pH conditions also reduced structure, while the addition of methanol increased structure.

Complex effects of molecular chaperones on the aggregation and refolding of fibroblast growth factor-1

Fibroblast growth factor one (FGF-1) exists in a molten globule (MG)-like state under physiological conditions (neutral pH, 37 degrees C). It has been proposed that this form of the protein may be involved in its atypical membrane transport properties. Macromolecular chaperones have been shown to bind to MG states of proteins as well as to be involved in protein membrane transport. We have therefore examined the effect of such proteins on the aggregation and refolding of FGF-1 to evaluate whether they might play a role in FGF-1 transport. The proposed chaperone alpha-crystallin was found to strongly inhibit the aggregation of the MG state of FGF-1. Curiously, two other proteins of similar size and charge (thyroglobulin and a monoclonal IgM immunoglobulin) with no previously reported chaperone properties were also found to have a related effect. In contrast, the chaperone GroEL/ES induced further aggregation of MG-like FGF-1 but had no effect on the native conformation. Both chaperones stimulated refolding to the native state (25 degrees C) but had no detectable effect when FGF-1 was refolded to the MG state (37 degrees C). This suggests that disordered intermediates are present in the folding pathways of the native and MG-like FGF conformations which differ from the MG-like state induced under physiological conditions. FGF-1 does, therefore, interact with molecular chaperones, although this may involve both the MG and the native states of the protein.

The potential role of proteoglycans in cationic lipid-mediated gene delivery. Studies of the interaction of cationic lipid-DNA complexes with model glycosaminoglycans

Recent evidence supports a role for proteoglycans in polycation-mediated gene delivery. Therefore, the interaction of glycosaminoglycans with cationic lipid-DNA complexes (CLDCs) has been characterized using a combination of biophysical approaches. At low ionic strength, CLDCs bind to heparin-derivatized Sepharose particles, with the ratio of cationic lipid to DNA controlling the binding. Incorporation of the helper lipids cholesterol or 1,2-dioleoyl-phosphatidylethanolamine increases the amount of bound CLDC. Heparin also induces the aggregation of CLDCs, with cholesterol reducing this effect. Isothermal titration calorimetry demonstrates an endothermic heat for the binding of heparin to CLDCs at low ionic strength, whereas circular dichroism studies suggest a heparin-stimulated release of DNA from CLDCs at a greater than 20-fold charge excess. Increasing the ionic strength to 0.11 reduces CLDC binding to heparin beads, and greatly enhances the release of DNA from CLDCs by heparin. The ability of the cell surface glycosaminoglycan heparan sulfate to release DNA from CLDCs is more sensitive than heparin to the incorporation of the cholesterol or 1,2-dioleoyl-phosphatidylethanolamine. Titration calorimetry reveals an exothermic heat for the interaction glycosaminoglycans with CLDCs at higher ionic strength. These results are consistent with the direct involvement of proteoglycans in transfection.

Formulation and characterization of amphotericin B-polyethylenimine-dextran sulfate nanoparticles

A new aqueous nanoparticle system has been developed using complex coacervation employing the oppositely charged polymers polyethylenimine (PEI) and dextran sulfate (DS), with zinc sulfate as a stabilizing agent. Amphotericin B (AmB) was loaded into the nanoparticles as a model drug. The nanoparticles contained PEI and DS in the weight ratio of approximately 1:2. They possessed a zeta potential of approximately +30 mV and demonstrated a narrow size distribution in the range 100-600 nm with a polydispersity index of 0.2. Electron microscopy revealed spherical nanocapsules with a smooth surface. Very favorable drug entrapment and recovery efficiencies of up to 85% were routinely observed. Processing parameters, such as the pH of the PEI solutions, ratio of the two polymers, as well as the concentrations of DS and zinc sulfate, all played a significant role in controlling particle size. Dissolution studies demonstrated a fast release that is dependent on the model drug solubility. The AmB-loaded nanoparticles displayed no toxicity in tissue culture in contrast to free drug and were almost as efficacious as free drug in killing Candida albicans. Advantages of this simple technique are (1) ease of manufacturing and mild preparation conditions, (2) employment of completely aqueous processing conditions, (3) use of biocompatible polymers that can be prepared aseptically, (4) ability to control their size, and (5) a high level of drug entrapment.

Antigen levels and antibody titers after DNA vaccination

DNA vaccination generates strong cellular and humoral immunity in animal models. The mechanisms by which plasmid DNA uptake and expression after intramuscular injection lead to immune responses are not well understood. In particular, the importance of antigen expression levels on subsequent antibody immune responses has not been established. We found that a chemiluminescent assay for alkaline phosphatase allows measurement of antigen levels of secreted alkaline phosphatase (SEAP) in vivo after intramuscular injection of a wide range of plasmid doses. The mice produced antibodies to the alkaline phosphatase reporter gene and both antigen levels and antibody titers were measured over time. We found that the correlation between initial antigen level and antibody response was high (r = 0.74, p < 0.001) and remained high even after accounting for the dose of plasmid injected (r = 0.61, p < 0.001). The correlation between DNA dose and antibody titer was statistically significant (r = 0.53, p < 0.001) but was reduced to almost zero after we accounted for initial antigen levels.

Infrared spectroscopic characterization of the interaction of cationic lipids with plasmid DNA

Fourier transform infrared spectroscopy was used to characterize the interaction of the cationic lipids 1,2-dioleoyl-3-trimethylammonium-propane and dioctadecyldimethylammonium bromide with plasmid DNA. The effect of incorporating the neutral colipids cholesterol and dioleoylphosphatidylethanolamine on this interaction was also examined. Additionally, dynamic and phase analysis light scattering were used to monitor the size and zeta potential of the resulting complexes under conditions similar to the Fourier transform infrared measurements. Results suggest that upon interaction of cationic lipids with DNA, the DNA remains in the B form. Distinct changes in the frequency of several infrared bands arising from the DNA bases, however, suggest perturbation of their hydration upon interaction with cationic lipids. A direct interaction of the lipid ammonium headgroup with and dehydration of the DNA phosphate is observed when DNA is complexed with these lipids. Changes in the apolar regions of the lipid bilayer are minimal, whereas the interfacial regions of the membrane show changes in hydration or molecular packing. Incorporation of helper lipids into the cationic membranes results in increased conformational disorder of the apolar region and further dehydration of the interfacial region. Changes in the hydration of the DNA bases were also observed as the molar ratio of helper lipid in the membranes was increased.

Isothermal Titration Calorimetric Analysis of the Interaction between Cationic Lipids and Plasmid DNA

The effects of buffer and ionic strength upon the enthalpy of binding between plasmid DNA and a variety of cationic lipids used to enhance cellular transfection were studied using isothermal titration calorimetry at 25.0 degrees C and pH 7.4. The cationic lipids DOTAP (1,2-dioleoyl-3-trimethyl ammonium propane), DDAB (dimethyl dioctadecyl ammonium bromide), DOTAP:cholesterol (1:1), and DDAB:cholesterol (1:1) bound endothermally to plasmid DNA with a negligible proton exchange with buffer. In contrast, DOTAP: DOPE (L-alpha-dioleoyl phosphatidyl ethanolamine) (1:1) and DDAB:DOPE (1:1) liposomes displayed a negative enthalpy and a significant uptake of protons upon binding to plasmid DNA at neutral pH. These findings are most easily explained by a change in the apparent pKa of the amino group of DOPE upon binding. Complexes formed by reverse addition methods (DNA into lipid) produced different thermograms, sizes, zeta potentials, and aggregation behavior, suggesting that structurally different complexes were formed in each titration direction. Titrations performed in both directions in the presence of increasing ionic strength revealed a progressive decrease in the heat of binding and an increase in the lipid to DNA charge ratio at which aggregation occurred. The unfavorable binding enthalpy for the cationic lipids alone and with cholesterol implies an entropy-driven interaction, while the negative enthalpies observed with DOPE-containing lipid mixtures suggest an additional contribution from changes in protonation of DOPE.

Conformational origin of the aggregation of recombinant human factor VIII

Aggregation of proteins is a major problem in their use as drugs and is also involved in a variety of pathological diseases. In this study, biophysical techniques were employed to investigate aggregate formation in the pharmaceutically important protein, recombinant human factor VIII (rhFVIII). Recombinant human factor VIII incubated in solution at 37 degrees C formed soluble aggregates as detected by molecular sieve chromatography and dynamic light scattering. This resulted in a corresponding loss of biological activity. Fluorescence and CD spectra of the thermally stressed rhFVIII samples did not, however, suggest significant differences in protein conformation. To identify conformational changes in rhFVIII that may be involved in rhFVIII aggregation, temperature and solutes were used to perturb the native structure of rhFVIII. Far-UV CD and FTIR studies of rhFVIII as a function of temperature revealed conformational changes corresponding to an increase in intermolecular beta-sheet content beginning at approximately 45 degrees C with significant aggregation observed above 60 degrees C. Fluorescence and DSC studies of rhFVIII also indicated conformational changes initiating between 45 and 50 degrees C. An increase in the exposure of hydrophobic surfaces was observed beginning at approximately 40 degrees C, as monitored by increased binding of the fluorescent probe, bis-anilinonaphthalene sulfonic acid (bis-ANS). Perturbation by various solutes produced several transitions prior to extensive unfolding of rhFVIII. In all cases, a common transition, characterized by an increase in the wavelength of the fluorescence emission maximum of rhFVIII from approximately 330 to 335 nm, was observed during thermal and solute perturbation of factor VIII. Moreover, this transition was correlated with an increased association of factor VIII upon incubation at 37 degrees C in the presence of various solutes. These results suggest that association of rhFVIII in solution was initiated by a small transition in the tertiary structure of the protein which produced a nucleating species that led to the formation of inactive soluble aggregates.

Conformational lability of herpesvirus protein VP22

The herpesvirus protein VP22 traffics between cells, being exported from expressing cells in a non-Golgi-dependent manner and localizing in the nuclei of surrounding cells. This transport is retained in certain VP22 fusion proteins, making VP22 a candidate for use in macromolecular drug delivery. In an effort to understand the physical basis for this activity, we have initiated structural studies of VP22.C1, the C-terminal half of VP22, which possesses the full transport activity of the native protein. CD and Fourier transform infrared analyses indicate a secondary structure consisting of approximately 30% alpha-helix, 17% beta-sheet, and 51% disordered and turn structure. Unfolding studies conducted by CD, differential scanning calorimetry, and fluorescence reveal a series of discrete structural transitions in the range of 20-60 degrees C. CD and fluorescence studies of interactions between VP22.C1 and divalent cations and model polyanions indicate that Mg(2+), Zn(2+), oligonucleotides, and heparin interact with the protein, causing changes in secondary structure and thermal stability. Additionally, the interaction of VP22.C1 with model lipids was examined. Fluorescence titrations of the protein with trans-parinaric acid at various temperatures suggest the binding of one to two molecules of parinaric acid to VP22.C1 at temperatures >40 degrees C, suggesting the possibility of conformation dependent membrane interaction under physiological conditions.

Potential use of non-classical pathways for the transport of macromolecular drugs

Since an increasing number of drug delivery strategies utilising proteins and peptides exhibiting 'non-classical' transport activities have been proposed, studies have begun to establish underlying functional relationships between different vectors. These attempts to find common factors have been hampered by a lack of biophysical data for the various potential protein and peptide transporters, as well as by the structural and functional diversity of the group as a whole. We describe the various types of vectors being considered for use and the preliminary therapeutic successes that have been achieved. Additionally, the various models that have been proposed for non-classical import and export are outlined and discussed in relation to therapeutic delivery. Possible future developments are also discussed.

Analysis of plasmid DNA from a pharmaceutical perspective

The advent of gene therapy and polynucleotide-based vaccines has resulted in the use of plasmid DNA as a drug substance. Although biologically (cell or animal) based assays must currently be employed to establish the identity and potency of such drugs, we argue that in the future, a combination of microchip-based mutation detection devices combined with an array of chromatographic, electrophoretic, hydrodynamic, and spectroscopic methods can be employed to rigorously establish these properties. We review a variety of such methods in this context and also consider the issue of the chemical stability of plasmids. Extensive comparison is made to protein-based pharmaceuticals with the unique importance of polynucleotide sequence emphasized in comparison to protein tertiary structure.

Degradative covalent reactions important to protein stability

Commonly observed chemical modifications that occur in proteins during their in vitro purification, storage, and handling are discussed. Covalent modifications described include deamidation and isoaspartate formation, cleavage of peptide bonds at aspartic acid residues, cystine destruction and thiol-disulfide interchange, oxidation of cysteine and methionine residues, and the glycation and carbamylation of amino groups.

Size exclusion HPLC method for the determination of acidic fibroblast growth factor in viscous formulations

A size exclusion HPLC method has been developed to determine the protein concentration of pharmaceutical formulations of recombinant acidic fibroblast growth factor (aFGF). These topical aFGF formulations not only contain low levels of protein mass (50 micrograms ml-1), but also include buffer ions, polysaccharide polyanions to conformationally stabilize aFGF and 1% hydroxyethylcellulose to increase the solution's viscosity. A cesium chloride mobile phase is utilized during SEC-HPLC to dissociate aFGF from the pharmaceutical excipients and to minimize nonspecific interaction of the protein with the column matrix. The protein content of a viscous aFGF formulation is determined by comparison of aFGF peak areas to standards of known concentration. Fluorescence spectroscopy was utilized to directly demonstrate that the protein remains in its native conformation during sample preparation and analysis.

Size and conformational stability of the hepatitis A virus used to prepare VAQTA, a highly purified inactivated vaccine

A variety of biophysical techniques have been employed to examine the size and conformational integrity of highly purified hepatitis A virus (HAV) in solution (purified HAV particles are subsequently formalin-inactivated and adsorbed to aluminum salts for use as the vaccine VAQTA). The size of HAV particles was assessed by a combination of electron microscopy, sedimentation velocity, and dynamic light scattering. The effect of ionic strength and temperature on the overall conformational stability of HAV was determined by a combination of intrinsic HAV protein fluorescence, fluorescent probes of both RNA and protein, and UV-visible spectroscopy. A major structural change in HAV occurs near 60 degrees C with the addition of 0.2 M magnesium chloride enhancing the thermal stability of HAV by approximately 10 degrees C. Salt concentrations above 0.2 M, however, decrease the solubility of HAV. The effect of pH on the physical properties of HAV particles was monitored by dynamic light scattering, analytical size exclusion HPLC, and interaction with fluorescent dyes. HAV particles undergo a substantially reversible association/aggregation at pH values below 6 with the concomitant exposure of previously buried hydrophobic surfaces below pH 4. These results are in good agreement with previous studies of HAV thermal stability under extreme conditions in which the irreversible inactivation of the viral particles was measured primarily by the loss of viral infectivity. The wide variety of biophysical measurements described in this work, however, directly monitor structural changes as they occur, thus providing a molecular basis with which to monitor HAV stability during purification and storage.

Biochemical and biophysical comparison of native and chemically synthesized phospholamban and a monomeric phospholamban analog

Phospholamban (PLB) was rapidly isolated from canine cardiac sarcoplasmic reticulum using immunoaffinity chromatography and prepared by solid phase peptide synthesis. The two proteins are indistinguishable when analyzed by SDS-polyacrylamide gel electrophoresis and exhibit pentameric oligomeric states. They are similarly detected on Western blots, are phosphorylation substrates, have identical amino acid compositions that directly reflect their predicted values, yield the same internal amino acid sequences upon CNBr digestion, and have molecular mass values agreeing with the expected value (approximately 6123 Da). Native and synthetic PLB reduced the calcium sensitivity of Ca2+ATPase, which is reversed by anti-PLB antibody. A Cys-to-Ser PLB analog, where the cysteines (36, 41, and 46) were substituted by serines, is monomeric on SDS-polyacrylamide gel electrophoresis, can be phosphorylated, and is recognized by polyclonal antisera. PLB migrates with a sedimentation coefficient of 4.8 S in sedimentation velocity ultracentrifugation experiments, whereas Cys-to-Ser PLB does not sediment, consistent with a monomeric state. Circular dichroism spectral analysis of PLB indicates about 70% alpha-helical structure, whereas Cys-to-Ser PLB manifests only about 30%. Because the physiochemical properties of native and synthetic PLB appear identical, the more readily available synthetic protein should be suitable for more extensive structural studies.

Epitope-labeled soluble human interleukin-5 (IL-5) receptors. Affinity cross-link labeling, IL-5 binding, and biological activity

The human receptor for the potent eosinophilopoietic cytokine interleukin-5 (IL-5) consists of two components: a 60-kDa ligand-binding alpha chain (IL-5 alpha R) and a 130-kDa beta chain (IL-5 beta R). Three ectodomain constructs of the alpha chain (alpha RED) bearing C-terminal epitope tags were engineered and expressed in baculovirus-infected Sf9 cells. Each recombinant alpha chain was secreted into the medium, maximum expression occurring 72 h post-infection. The various soluble alpha chains were shown by affinity cross-link labeling and competition with unlabeled IL-5 to bind recombinant human (rh) 125I-IL-5 specifically with an ED50 of 2-5 nM. The epitope tag provided a simple purification of the receptor from conditioned medium using immunoaffinity chromatography. The purified material had an apparent molecular mass of 43 kDa and was heterogeneously glycosylated. Sedimentation analysis revealed a 1:1 association of the purified epitope-tagged soluble receptor with its ligand, resulting in the formation of a 70-74-kDa complex. Circular dichroism analysis revealed that the soluble alpha chain existed with a significantly ordered structure consisting of 42% beta-sheet and 6% alpha-helix. Such analyses combined with fluorescence spectrometry suggested that ligand-receptor complex formation in solution resulted in minimal conformational changes, consistent with the suggestion that the membrane-associated form of the alpha chain itself has minimal signal transduction capability. Surface plasmon resonance studies of the interaction of the purified alpha RED with immobilized rhIL-5 revealed a specific, competable interaction with a dissociation constant of 9 nM. Preincubation of an IL-5-dependent cell line with the epitope-tagged alpha RED also dose-dependently neutralized rhIL-5-induced proliferation. These data demonstrate that biologically active epitope-tagged recombinant soluble IL-5 receptors are facile to produce in large quantities and may have therapeutic utility in the modulation of IL-5-dependent eosinophilia in man.

Interaction of partially structured states of acidic fibroblast growth factor with phospholipid membranes

Although acidic fibroblast growth factor (aFGF) lacks a conventional signal sequence, it is often found complexed to sulfated proteoglycans on the external surface of cells. The protein also forms a "molten globule"-like state at neutral pH and physiological temperatures as well as at acidic pH in the presence of physiological ionic strength or moderate quantities of polyanions. These states display a marked tendency to aggregate. Such observations suggest that related partially structured states might be involved in the membrane translocation of aFGF. To explore this hypothesis, we examined the interaction of this growth factor with lipid vesicles as well as the effect of such surfaces on the structure of the protein. We find that these states interact with negatively charged but not neutral phospholipid unilammelar vesicles at acidic pH, inducing bilayer disruption. The rate of leakage of a liposome-entrapped fluorescent probe is proportional to the logarithm of the aFGF concentration, suggesting competition between protein self-association and membrane binding. Liposome leakage can be also induced at neutral pH by partial unfolding of aFGF at or above physiological temperature in contrast to most control proteins. The importance of partially folded hydrophobic surfaces in aFGF self-association and membrane binding is further suggested by the fact that thermally unfolded aFGF does not aggregate, in contrast to states observed at intermediate temperatures or transiently during unfolding at high temperatures. In contrast to heparin, a polyanion which stabilizes the native structure of aFGF, negatively charged phospholipid membranes appear to enhance the disruption of aFGF tertiary structure at submicellar concentrations of sodium dodecyl sulfate but stabilize the remaining secondary structure.(ABSTRACT TRUNCATED AT 250 WORDS)

Solution structure of the cytoplasmic domain of phopholamban: phosphorylation leads to a local perturbation in secondary structure

Peptides representing the N-terminal domain (Ia) of the cardiac sarcoplasmic reticulum protein phospholamban (residues 1-25 [PLB(1-25)] and a phosphorylated form [pPLB(1-25)]) were synthesized and their conformations examined using circular dichroism and nuclear magnetic resonance spectroscopy. In aqueous solution, both PLB(1-25) and pPLB(1-25) adopt a primarily disordered conformation. In 30% trifluoroethanol/10 mM phosphate, PLB(1-25) exhibits a CD spectrum consistent with 60% helical structure. This value decreases to 27% for the phosphorylated peptide. CD spectra in 2% SDS indicate 40% alpha-helix for PLB(1-25) and 20% for pPLB(1-25). Full chemical shift assignments were obtained by conventional homonuclear NMR methodologies for both PLB(1-25) and pPLB(1-25) in 30% trifluoroethanol/water and 300 mM SDS. The solution structure of PLB(1-25) in 30% TFE/water was determined from distance geometry calculations using 54 NOE distance constraints and 17 torsion angle constraints. In the family of 20 calculated conformers, the root mean square deviation from the mean structure is 0.79 A for backbone heavy atoms of residues 1-17. The structure comprises a regular alpha-helix extending from M1 to S16 with the remaining C-terminal residues disordered. The calculated structure is supported by analysis of C alpha H secondary shifts which are significantly negative for residues 1-16. Chemical shift degeneracy is substantially more extensive in the phospho form and precludes a direct comparison of calculated structures. However, the magnitudes of upfield secondary shifts are decreased by 20% in residues 1-11 and are not significantly helical for residues 12-16 according to the criteria of Wishart et al. [(1992) Biochemistry 31, 1647-1651]. 3JHN alpha coupling constants measured for I12, R13, A15, and S16 also suggest that residues 12-16 undergo a local unwinding of the helix upon phosphorylation. Similar results are obtained for PLB(1-25) and pPLB(1-25) in 300 mM perdeuterated sodium dodecyl sulfate except that differences in backbone dynamics for the helical and nonhelical regions of the peptide are evident in the DQF-COSY line shapes for fingerprint cross-peaks. This disruption of structure at the C-terminus of the helix suggests a model for phosphorylation-induced dissociation of the PLB/Ca(2+)-ATPase complex.

Spectroscopic characterization of tick anticoagulant peptide

Tick anticoagulant peptide (TAP) is a disulfide rich potent inhibitor of factor Xa. Although this peptide is of potential clinical utility, very little is known about its higher order structure. Therefore, the secondary structure of recombinant TAP (rTAP) has been examined by circular dichroism (CD) and Fourier transform infrared (FTIR) spectroscopy. Both techniques suggest that rTAP is rich in beta-sheet structure. Disulfide bonds play a significant role in the folding and structural stability of rTAP. This is apparent from the resistance of rTAP to fluorescence-detected unfolding by guanidinium chloride (Gdn-HCl), unless disulfides are first reduced. The protein's tryptophan and tyrosine residues exhibit greater solvent exposure upon reduction of the cystines as indicated by fluorescence spectra and second derivative UV spectroscopy. A considerable amount of beta-structure appears to be retained after reduction of disulfides, although the CD spectrum manifests an increased amount of disordered structure in the reduced peptide. While rTAP does not bind the hydrophobic fluorescence probe 2-p-toludinylnaphthalene-6-sulfonate (TNS) at neutral or acidic pH, the reduced peptide binds TNS at pH 2.0 but not at pH 7.0. The secondary structure of the reduced peptide at pH 2 is, however, similar to that at pH 7 as judged by CD spectroscopy. The reduced form of rTAP at acidic pH thus resembles a molten globule-like state.

Deamidation of polyanion-stabilized acidic fibroblast growth factor

The deamidation of polyanion-stabilized acidic fibroblast growth factor (aFGF; FGF-1) can be induced by prolonged storage under accelerated conditions of elevated pH and temperature. A urea-isoelectric focusing (urea-IEF) method has been developed to monitor aFGF deamidation in the presence of highly negatively charged polyanions which are required to maintain the conformational stability of the protein. The kinetics of aFGF deamidation have been established by a combination of urea-IEF and an enzymatic ammonia assay. Native, non-deamidated aFGF (complexed with heparin) has a half-life of 16 weeks at pH 7, 30 degrees C, and 4 weeks at pH 8, 40 degrees C. The mitogenic activity and biophysical properties of deamidated aFGF were compared to the non-deamidated protein. These initial deamidation events have no significant effect on the protein's overall conformation, thermal stability, interaction with heparin, or bioactivity. At longer times, however, limited aggregation of the protein was observed after prolonged storage under some conditions. N-terminal protein sequencing of the protein's first 21 amino acid residues have identified one of the deamidation sites in a flexible, peptide-like region of the protein (Asn8-Tyr9).

Simultaneous monitoring of the environment of tryptophan, tyrosine, and phenylalanine residues in proteins by near-ultraviolet second-derivative spectroscopy

A method for deconvolution of the near-uv second-derivative spectra of proteins into their component tryptophan, tyrosine, and phenylalanine spectra is described. In this approach, the second-derivative spectra of tryptophan and tyrosine model compounds are numerically shifted to create a set of reference spectra corresponding to anticipated peak positions in protein environments of different polarity. The relative contributions of these individual standard spectra are varied until the best fit to the experimental protein spectrum is obtained. Separate addition of tryptophan and tyrosine standard spectra, weighted by their contributions as determined in the fitting procedure, yields an accurate representation of the spectra of these residues in proteins. The position of the intersection of these spectra with the wavelength axis is used as a measure of spectral position in ethylene glycol perturbation experiments in which the average solvent accessibility is assessed by relating the observed shifts in the tryptophan and tyrosine spectra to the shifts observed for corresponding model compounds. The phenylalanine peak positions in the set of 16 proteins studied are determined as described previously [H. Mach et al. (1991) Arch. Biochem. Biophys. 287, 33-40]. For all three aromatic residues in proteins, no consistent correlation between absolute spectral band positions and average solvent accessibility is observed, suggesting a significant influence of other local (e.g., electrostatic) effects on near-uv spectra of proteins. The maximum spectral shift observed between solvent-exposed model compounds and side chains entirely buried in apolar protein core was found to be approximately 5 nm for tyrosine, 4 nm for tryptophan, and 2 nm for phenylalanine residues.

Interaction of nucleotides with acidic fibroblast growth factor (FGF-1)

A wide variety of nucleotides are shown to bind to acidic fibroblast growth factor (aFGF) as demonstrated by their ability to (1) inhibit the heat-induced aggregation of the protein, (2) enhance the thermal stability of aFGF as monitored by both intrinsic fluorescence and CD, (3) interact with fluorescent nucleotides and displace a bound polysulfated naphthylurea compound, suramin, (4) reduce the size of heparin-aFGF complexes, and (5) protect a reactive aFGF thiol group. The binding of mononucleotides, diadenosine compounds (ApnA), and inorganic polyphosphates to aFGF is enhanced as the degree of phosphorylation of these anions is increased with the presence of the base reducing the apparent binding affinity. The nature of the base appears to have much less effect. Photoactivatable nucleotides (8N3-ATP, 2N3-ATP, 8N3-GTP, and 8N3-Ap4A) were employed to covalently label the aFGF nucleotide binding site. In general, Kd's in the low micromolar range are observed. Protection against 90% displacement is observed at several hundred micromolar nucleotide concentration. Using 8N3-ATP as a prototypic reagent, photolabeled aFGF was proteolyzed with trypsin and chymotrypsin and labeled peptides were isolated and sequenced resulting in the identification of 10 possible labeled amino acids (Y8, G20, H21, T61, K112, K113, S116, R119, R122, H124). On the basis of the crystal structure of bovine aFGF, eight of the prospective labeled sites appear to be dispersed around the perimeter of the growth factor's presumptive polyanion binding site. On residue (T61) is more distally located but still proximate to several positively charged residues, and another (Y8) is not locatable in crystal structures. Using heparin affinity chromatography, at least three distinct photolabeled aFGF species were resolved. These labeled complexes display diminished affinity for heparin and a reduced ability to stimulate mitogenesis even in the presence of polyanions such as heparin. In conclusion, nucleotides bind apparently nonspecifically to the polyanion binding site of aFGF but nevertheless are capable of modulating the protein's activity. Evidence for the presence of a second or more extended polyanion binding site and the potential biological significance of these results in terms of potential natural ligands of aFGF are also discussed but not resolved.

Bacteriophage T7 RNA polymerase and its active-site mutants. Kinetic, spectroscopic and calorimetric characterization

It has been demonstrated that the amino acids Asp537, Asp812, Lys631, His811 and Tyr639 are involved in bacteriophage T7 RNA polymerase catalysis. In the present paper, we report kinetic, spectroscopic and calorimetric characterization of the wild-type and mutant T7 RNA polymerases generated at these five loci (D537N, E; K631M, R; Y639F, S, A, W; H811Q, A; D812N, E). The wild-type enzyme has a substantial amount of secondary structure as determined by CD analysis (alpha-helix, 43%; beta-sheet, 14%; beta-turn, 25%; unordered, 18%). The CD spectra of 12 mutants at five loci are very similar to that of the wild-type, except for the mutant Y639W. Within experimental error, the thermal transition temperatures measured by CD and DSC as well as the lambda max values of the fluorescence spectra were the same for the wild-type and all of the mutants. Therefore, the overall folding and stability of the mutant enzymes are very similar to those of the wild-type enzyme, although small local conformational changes cannot be excluded. For the synthesis of the pentamer pppGGACU, the mutants D537E and D812E showed an approximately two- to threefold decrease in (kcat)app and an approximately two- to threefold increase in (Km)app, relative to the wild-type, in contrast to the mutants D537N and D812N which exhibited no detectable activity. The mutant K631R showed a sevenfold reduction in (kcat)app and a two- to threefold increase in (Km)app, supporting our earlier observation with the mutant K631M that Lys631 may be involved in phosphodiester bond formation. The mutant Y639S can synthesize the trimer GGA with an approximately 50-fold decrease in (kcat)app and a tenfold increase in (Km)app, relative to the wild-type, underlining the importance of the phenyl ring of Tyr639. The mutant H811A, in which the side-chain at position 811 is incapable of forming a hydrogen bond, can synthesize the trimer GGA with an approximately tenfold decrease in (kcat)app and an approximately 35-fold increase in (Km)app. Thus, either the hydrogen-bonding capacity of this residue is non-essential or some other group can functionally substitute for the His811 side-chain. The wild-type enzyme showed significant effects of the base position in the sequence on the apparent binding constants for the NTPs. The kinetics of GpG-primed trimer, tetramer and pentamer synthesis on three 22 bp templates were investigated for the wild-type and mutant enzymes with measurable activity.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence for an equilibrium intermediate in the folding-unfolding pathway of a transforming growth factor-alpha-Pseudomonas exotoxin hybrid protein

TP40 is a chimeric protein containing transforming growth factor-alpha (TGF-alpha) at the N-terminus and a Cys-->Ala mutant (PE40 delta Cys) of a 40,000-dalton segment (PE40) of Pseudomonas exotoxin (PE). The guanidine hydrochloride (Gdn-HCl)-induced unfolding of TP40 and PE40 delta Cys has been studied by tryptophan fluorescence, circular dichroism (CD), and high-performance size exclusion chromatography (HPSEC). The equilibrium unfolding of both proteins involves at least one intermediate (I). In the I state(s), which may be induced by 1.3-2.0 M Gdn-HCl, the tertiary structure is fully or partially collapsed as detected by tryptophan fluorescence and near-UV CD, but the protein largely retains the native secondary structure and a semicompact shape as judged by far-UV CD and HPSEC, respectively. Soluble aggregates of TP40 and PE40 delta Cys are observed in addition to monomers at these intermediate (but not at higher) Gdn-HCl concentrations, suggesting that self-association is possibly mediated by thermodynamically stable, partially unfolded I states. The kinetics of refolding of TP40 upon dilution of Gdn-HCl involve two or more phases. Re-formation of secondary structure occurs rapidly (t 1/2 < 10 s) as determined by CD and is followed by a biphasic refolding of the native tertiary structure as detected by changes in tryptophan fluorescence. The midpoint (Tm) of the thermal unfolding transition occurs at a lower temperature when measured by tryptophan fluorescence than when detected by DSC and CD. These data suggest that Gdn-HCl and temperature can induce conformation(s) of TP40 that are distinct from native (N) and unfolded (U) states.(ABSTRACT TRUNCATED AT 250 WORDS)

Delta-sleep-inducing peptide: solution conformational studies of a membrane-permeable peptide

Peptides and peptide-like molecules as a class have very poor permeability through biological membranes, which severely compromises their potential effectiveness as therapeutic agents. In order to gain insight into the problem of delivering peptide and protein drugs and to establish a model in which the effects of systematic structural variations on transport can be explored, an investigation of the solution conformation of a membrane-permeable peptide was undertaken. Delta-sleep-inducing peptide (DSIP, MW 849) was used in this investigation. DSIP is a charged, hydrophilic peptide that possesses the unusual ability to diffuse passively across the blood-brain barrier (BBB) in vivo [Kastin, A. J., Banks, W. A., Castellanos, P. F., Nissen, C., & Coy, D. H. (1982) Pharmacol. Biochem. Behav. 17, 1187-1191] and across monolayers of brain microvessel endothelial cells in vitro, a model of the BBB [Raeissi, S., & Audus, K. L. (1989) J. Pharm. Pharmacol. 41, 848-852]. This nonapeptide was studied in solution using one- and two-dimensional nuclear magnetic resonance (NMR), circular dichroism (CD), Fourier transform infrared (FT-IR), and fluorescence spectroscopies in conjunction with molecular modeling. Our spectroscopic findings suggest that DSIP exists in a dynamic equilibrium between unordered and folded structures. Residues 2-5 and 6-9 tend to form type I beta-turns in aqueous solution and a similar, but more ordered, helix-like structure inducible in 40% trifluoroethanol (TFE). NMR, FT-IR, and CD studies in aqueous solution support the dynamic equilibrium hypothesis with the IR data, suggesting that the beta-turn population is approximately 40%.(ABSTRACT TRUNCATED AT 250 WORDS)

Probing the affinity of polyanions for acidic fibroblast growth factor by unfolding kinetics

The relationship between ligand-protein affinity and the extent of protein stabilization induced by such interactions has been investigated using the binding of polyanions to acidic fibroblast growth factor (aFGF) as a model system. It was found that the experimentally observed unfolding rate constant of aFGF consists of two components: one equal to the unfolding rate constant of the aFGF-ligand complex and the other the product of the unfolding rate constant of free aFGF, the aFGF-ligand dissociation constant (Kd), and the reciprocal of the molar ligand concentration. This reflects the presence of two possible unfolding pathways: at high ligand excess dissociation is suppressed and slow unfolding of the aFGF-ligand complex itself prevails. When lower concentrations of ligand allows equilibrium-driven appearance of free aFGF, a more rapid unfolding of dissociated protein predominates. Existence of a steady state of dissociated aFGF undergoing unfolding was demonstrated by computer simulation of the elementary events, using experimentally determined rate constants. The potential applications of such simulations are outlined. An equation allowing estimation of dissociation constants from equilibrium denaturation curves obtained in the presence of a varying amount of ligand is also proposed. In addition, determination of initial unfolding rates in the presence of excess protein permits the the stoichiometry of the interaction to be determined.

The analysis of protein pharmaceuticals: near future advances

The analysis of protein pharmaceuticals currently involves a complex series of chromatographic, electrophoretic, spectroscopic, immunological and biological measurements to unequivocally establish their identity, purity and integrity. In this review, I briefly consider the possibility that at least the functional identity and integrity of a protein drug might be established by either a single analysis involving X-ray diffraction, NMR or mass spectrometry, or by a chromatographically based multi-detector system in which a number of critical parameters are essentially simultaneously determined. The use of a protein standard to obtain comparative measurements and new advances in the technology of each of these methods is emphasized. A current major obstacle to the implementation of these approaches is the frequent microheterogeneity of protein preparations. The evolution of biological assays into measurements examining more defined intracellular signal transduction events or based on novel biosensors as well as the analysis of vaccines is also briefly discussed.

Sucralfate and soluble sucrose octasulfate bind and stabilize acidic fibroblast growth factor

The actions of the anti-ulcer drug sucralfate have been proposed to be mediated through interaction with fibroblast growth factors (Folkman, J., Szabo, S., Strovroff, M., McNeil, P., Li, W. and Shing, Y. (1991) Ann. Surg. 214, 414-427). We show here that acidic fibroblast growth factor (aFGF; FGF-1) binds in vitro to both the soluble potassium salt and the insoluble aluminum salt of sucrose octasulfate, as demonstrated by a variety of biophysical techniques. Similar to the well-described interaction and stabilization of aFGF by heparin, soluble sucrose octasulfate (SOS) stabilizes aFGF against thermal, urea and acidic pH-induced unfolding as determined by a combination of circular dichroism, fluorescence spectroscopy and differential scanning calorimetry. In addition, SOS also enhances the mitogenic activity of aFGF and partially protects the protein's three cysteine residues from copper-catalyzed oxidation. SOS competes with heparin and suramin for the aFGF polyanion binding site as measured by both fluorescence and light scattering based competitive binding assays. Front-face fluorescence measurements show that the native, folded form of aFGF binds to the insoluble aluminum salt of sucrose octasulfate (sucralfate). Moreover, sucralfate stabilizes aFGF against thermal and acidic pH-induced unfolding to the same extent as observed with SOS. Thus, due to their high charge density, SOS and sucralfate bind and stabilize aFGF via interaction with the aFGF polyanion binding site.

Origin of the isoelectric heterogeneity of monoclonal immunoglobulin h1B4

The origin of the microheterogeneity of a highly purified antiinflammatory humanized monoclonal antibody prepared in mammalian cell culture has been investigated. This antibody is an IgG directed toward human CD18 (a subunit of leukocyte integrins). When the IgG preparation is subjected to isoelectric focusing, it is found to contain four major species with pI values ranging from 6 to 7. Although the relative amounts of each form differ and some species are present only in small quantities, each has been isolated by a combination of high-resolution anion-exchange chromatography and isoelectric focusing. Comparative studies reveal no detectable differences in overall secondary (far UV circular dichroism) or tertiary (intrinsic fluorescence) structure, molecular weight (laser-desorption mass spectroscopy), or antigen binding activity. When each of the isolated species is incubated under conditions which favor deamidation, it is converted to forms of lower pI which appear to correspond to naturally observed species. While the isolated light chain is relatively homogeneous, the heavy chain exhibits a pattern of isoelectric focusing bands similar to that of the intact immunoglobulin. These results suggest that in this case, charge microheterogeneity is due to the sequential deamidation of the immunoglobulin heavy chain.

Partially structured self-associating states of acidic fibroblast growth factor

A combination of near- and far-UV circular dichroism, Fourier-transform infrared spectroscopy, tryptophan fluorescence, size-exclusion chromatography, and a fluorescent extrinsic hydrophobic probe has been employed to characterize partially structured states of human recombinant acidic fibroblast growth factor (aFGF). At low pH, the addition of specific polyanionic ligands or moderate amounts of salts induces states with high secondary but low tertiary structure content. At neutral pH, intermediate amounts of chaotropic agents impose similar partially structured conformational states which also display noncooperative unfolding transitions. Kinetic evidence indicates that similar forms of the protein exist in the first few hundred milliseconds in the refolding pathway of aFGF. The kinetics of their formation appear to be temperature-independent, implying lack of an energy barrier, which is characteristic for further slow folding into the native state. Unlike the native and fully unfolded states, these partially structured conformations exhibit very low solubility, resulting in irreversible aggregation. Potential physiological implications of the existence of such "molten globule" states with regard to the growth factor's transport and biological activity are considered.

Effect of polyanions on the unfolding of acidic fibroblast growth factor

The urea-induced unfolding of acidic fibroblast growth factor (aFGF) in the presence and absence of various polyanions has been quantitatively examined by fluorescence spectroscopy. In the absence of a stabilizing polyanion, the apparent free energy of unfolding of aFGF is 6.5 kcal mol-1. The presence of equimolar or greater amounts of heparin stabilizes aFGF from unfolding by more than 2.5 kcal mol-1 and slows the rate of unfolding by greater than 2000-fold. The ability of heparin to stabilize aFGF is critically dependent upon many factors including the number of aFGF molecules bound to the heparin chain, ionic strength, temperature, and the extent of sulfation of the polysaccharide. The presence of similar amounts of other polyanions such as sulfated beta-cyclodextrin or heparan sulfate also stabilizes aFGF to a similar extent as heparin. Additional experiments demonstrate that increasing charge density enhances the ability of polyanions such as sulfated beta-cyclodextrins, phosphorylated inositols, and modified heparins to protect aFGF from urea-induced unfolding.

Structure of synthetic peptide analogues of an eggshell protein of Schistosoma mansoni

The peptide (Gly-L-Tyr-L-Asp-L-Lys-L-Tyr)6, referred to as F4-6, was synthesized as a model for a schistosome eggshell protein in which the Gly-Tyr-Asp-Lys-Tyr consensus sequence is repeated over 40 times. Analysis by CD, Fourier transform infrared spectroscopy, potentiometric and spectrophotomertric titrations, NMR, and molecular modeling suggests that F4-6 forms some type of left-handed structure. A likely possibility appears to be a left-handed alpha-helix stabilized by Lysi-Aspi +4 salt bridges and possibly Aspi-Tyri +4 hydrogen bonding and Tyr-Tyr interactions. Spectroscopic studies of a number of F4-6 analogues support this conclusion. For example, substitution of D-Ala for Gly produces a peptide with enhanced left-handed helical spectral characteristics, whereas an L-Ala substitution results in a peptide with minimal structure. These studies suggest that the F4 protein from Schistosoma mansoni may be the first example of a naturally occurring protein devoid of proline and carbohydrate that forms a left-handed helix composed of L-amino acids, although alternative forms of other left-handed structures have yet to be rigorously excluded.

Nature of the interaction of heparin with acidic fibroblast growth factor

The binding of human acidic fibroblast growth factor (aFGF) to heparin has been analyzed by a variety of different approaches to better elucidate the nature of this protein/sulfated polysaccharide interaction. Static and dynamic light scattering as well as analytical ultracentrifugation analyses indicates that 14-15 molecules of a FGF can bind to a 16-kDa heparin chain, with approximately 10 of these bound relatively uniformly to high-affinity sites. The dissociation constants of these latter sites are estimated to be approximately 50-140 nM on the basis of surface plasmon resonance experiments in which the association and dissociation rates of aFGF interaction with immobilized heparin were measured. The size of the binding site of a FGF on heparin was also determined by heparin lyase digestion of a FGF/heparin complexes followed by isolation and characterization of protected oligosaccharides. The smallest aFGF-protected oligosaccharide comigrated with delta UA2S(1-->4)-alpha-D-GlcNp2S6S(1-->4)-alpha-L-IdoAp-2S( 1-->4)-alpha-D-GlcNp2S6S (where delta UA represents 4-deoxy-alpha-L-threo-hex-4-enopyranosyluronic acid and S is sulfate). Thus, aFGF appears to bind at high density (one molecule every 4-5 polysaccharide units) and with high affinity to heparin. This potentially provides a concentrated, stabilized storage form of the growth factor that can be released for receptor-mediated cellular activation in response to the proper stimuli. It is also possible that close proximity of aFGF molecules on the highly sulfated regions of heparan chains may be involved in the induction of receptor aggregation as suggested by Ornitz et al. [Ornitz, D. M., Yayon, A., Flanagan, J. G., Svahn, C. M., Levi, E., & Leder, P. (1992) Mol. Cell. Biol. 12, 240-247].

Formulation design of acidic fibroblast growth factor

The design of an aqueous formulation for acidic fibroblast growth factor (aFGF) requires an understanding of the type of compounds that can either directly or indirectly stabilize the protein. To this end, spectrophotometric turbidity measurements were initially employed to screen the ability of polyanionic ligands, less specific compounds, and variations in solution conditions (temperature and pH) to stabilize aFGF against heat-induced aggregation. It was found that in addition to the well-known protection of aFGF by heparin, a surprisingly wide variety of polyanions (including small sulfated and phosphorylated compounds) also stabilizes aFGF. These polyanionic ligands are capable of raising the temperature at which the protein unfolds by 15-30 degrees C. Many commonly used excipients were also observed to stabilize aFGF in both the presence and the absence of heparin. High concentrations of some of these less specific agents are also able to increase the temperature of aFGF thermal unfolding by as much as 6-12 degrees C as shown by circular dichroism and differential scanning calorimetry. Other compounds were found which protect the chemically labile cysteine residues of aFGF from oxidation. Aqueous formulations of aFGF were thus designed to contain both a polyanionic ligand that enhances structural integrity by binding to the protein and chelating agents (e.g., EDTA) to prevent metal ion-catalyzed oxidation of cysteine residues. While room-temperature storage (30 degrees C) leads to rapid inactivation of aFGF in physiological buffer alone, several of these aFGF formulations are stable in vitro for at least 3 months at 30 degrees C. Three aFGF topical formulations were examined in an impaired diabetic mouse model and were found to be equally capable of accelerating wound healing.

A transforming growth factor-alpha-Pseudomonas exotoxin hybrid protein undergoes pH-dependent conformational changes conducive to membrane interaction

TP40 is a chimeric protein containing transforming growth factor alpha (TGF-alpha) at the N-terminus and a derivative of a 40,000-Da segment (PE40 delta cys) of Pseudomonas exotoxin (PE). PE40 delta cys contains domains Ib, II, and III of PE in which the cysteines are mutated to alanines. The rationale for inclusion of TGF-alpha is to provide TP40 with selective targeting toward cells expressing the epidermal growth factor receptor (EGFr) on their surface [Pastan, I., & FitzGerald, D. (1989) J. Biol. Chem. 264, 15157-15160]. Translocation across endosomal membranes is thought to be a required step for cytotoxic activity of PE. This step is presumably facilitated by the low pH in endosomes which induces exposure of a hydrophobic surface of the protein, which in turn becomes available to interact with and translocate across the membrane. We have employed the hydrophobic fluorescence probe 2-p-toludinylnaphthalene-6-sulfonate (TNS) and the intrinsic tryptophan fluorophores of TP40 to investigate pH-induced changes in the tertiary structure of this protein. The pH dependence of TP40 interaction with liposomes also provided a model for studying protein-membrane interactions. TNS fluorescence was markedly enhanced in the presence of TP40 below pH 4 and to a lesser degree between pH 7 and 5. A progressive red shift of tryptophan fluorescence with decreasing pH was also seen with the approximate midpoint for this transition occurring around pH 3. Both observations suggest that acidic pH induces exposure of hydrophobic regions of TP40, making them accessible to solvent and TNS. No major alteration of the secondary structure was manifested in the far-UV CD spectrum of TP40 upon a reduction in pH from 7 to 2. Thus, the low-pH-induced structural change of TP40 appears to involve a subtle exposure of one or more hydrophobic surfaces without an extensive unfolding of the protein's secondary structure. In the presence of anionic liposomes, a low-pH-induced blue shift of the TP40 tryptophan fluorescence was observed, suggesting that interaction with liposomes also required the low-pH conformation of the protein. However, the midpoint of this fluorescence blue shift occurred at approximately pH 5, which is presumably closer to the physiological pH within endosomes. Neutral liposomes failed to induce these spectral changes in TP40, implying a lack of interaction with these lipids. At acidic pH values between 2 and 4, self-association of TP40 in solution was detected by equilibrium sedimentation and quasielastic light scattering measurements. This probably results from intermolecular interaction between exposed hydrophobic surfaces.(ABSTRACT TRUNCATED AT 400 WORDS)

Changes in structure of alpha 2-macroglobulin upon reaction with trypsin as assessed by light scattering and differential scanning calorimetry

Employing a combination of static and dynamic light scattering, as well as differential scanning calorimetry (DSC), the structural changes which appear in alpha 2-macroglobulin (alpha 2M) upon trypsin binding have been further characterized. Light-scattering measurements suggest that a 15% reduction in both the hydrodynamic radius and radius of gyration occurs when two molecules of trypsin complex to alpha 2M. Approx. 85% of this trypsin-induced compaction results from the binding of the first proteinase. A complementary result was obtained from DSC measurements in which the major fraction of the trypsin-induced conversion of alpha 2M to a single more thermally stable form results from interaction with the first proteinase molecule. These observations support a functionally asymmetric model of trypsin binding to alpha 2M in which the significant reduction in size of the complex is primarily due to the initial interaction of alpha 2M with a single proteinase molecule.