Vaccine process technology-A decade of progress

In the past decade, new approaches to the discovery and development of vaccines have transformed the field. Advances during the COVID-19 pandemic allowed the production of billions of vaccine doses per year using novel platforms such as messenger RNA and viral vectors. Improvements in the analytical toolbox, equipment, and bioprocess technology have made it possible to achieve both unprecedented speed in vaccine development and scale of vaccine manufacturing. Macromolecular structure-function characterization technologies, combined with improved modeling and data analysis, enable quantitative evaluation of vaccine formulations at single-particle resolution and guided design of vaccine drug substances and drug products. These advances play a major role in precise assessment of critical quality attributes of vaccines delivered by newer platforms. Innovations in label-free and immunoassay technologies aid in the characterization of antigenic sites and the development of robust in vitro potency assays. These methods, along with molecular techniques such as next-generation sequencing, will accelerate characterization and release of vaccines delivered by all platforms. Process analytical technologies for real-time monitoring and optimization of process steps enable the implementation of quality-by-design principles and faster release of vaccine products. In the next decade, the field of vaccine discovery and development will continue to advance, bringing together new technologies, methods, and platforms to improve human health.

Accelerating Global Deletion of the Abnormal Toxicity Test for vaccines and biologicals. Planning common next steps. A workshop Report

This online workshop Accelerating Global Deletion of the Abnormal Toxicity Test for vaccines and biologicals. Planning common next steps was organized on October 14th, 2021, by the Animal Free Safety Assessment Collaboration (AFSA), the Humane Society International (HSI), the European Federation of Pharmaceutical Industries and Associations (EFPIA), in collaboration with the International Alliance of Biological Standardization (IABS). The workshop saw a participation of over a hundred representatives from international organizations, pharmaceutical industries and associations, and regulatory authorities of 28 countries. Participants reported on country- and region-specific regulatory requirements and, where present, on the perspectives on the waiving and elimination of the Abnormal Toxicity Test. With AFSA, HSI, EFPIA and IABS representatives as facilitators, the participants also discussed specific country/global actions to further secure the deletion of ATT from all regulatory requirements worldwide.

Development of functionally relevant potency assays for monovalent and multivalent vaccines delivered by evolving technologies

A potency or potency-indicating assay is a regulatory requirement for the release of every lot of a vaccine. Potency is a critical quality attribute that is also monitored as a stability indicator of a vaccine product. In essence, a potency measurement is a test of the functional integrity of the antigen and is intended to ensure that the antigen retains immunocompetence, i.e., the ability to stimulate the desired immune response, in its final formulation. Despite its central importance, there is incomplete clarity about the definition and expectation of a potency assay. This article provides a perspective on the purpose, value, and challenges associated with potency testing for vaccines produced by new technologies. The focus is on messenger RNA vaccines in the light of experience gained with recombinant protein-based vaccines, which offer the opportunity to directly correlate in vitro antigenicity with in vivo immunogenicity. The challenges with developing immunologically relevant in vitro assays are discussed especially for multivalent vaccine products, the importance of which has been reinforced by the ongoing emergence of SARS-CoV-2 variants of concern. Immunoassay-based release of multivalent vaccine products, such as those containing multiple antigens from different variants or serotypes of the same virus, require antibodies that are selective for each antigen and do not significantly cross-react with the others. In the absence of such exclusively specific antibodies, alternative functional assays with demonstrable correlation to immunogenicity may be acceptable. Initiatives for geographically distributed vaccine technology facilities should include establishing these assay capabilities to enable rapid delivery of vaccines globally.

Animal testing for vaccines. Implementing replacement, reduction and refinement: challenges and priorities

Transition to in vitro alternative methods from in vivo in vaccine release testing and characterization, the implementation of the consistency approach, and a drive towards international harmonization of regulatory requirements are most pressing needs in the field of vaccines. It is critical for global vaccine community to work together to secure effective progress towards animal welfare and to ensure that vaccines of ever higher quality can reach the populations in need in the shortest possible timeframe. Advancements in the field, case studies, and experiences from Low and Middle Income Countries (LMIC) were the topics discussed by an international gathering of experts during a recent conference titled "Animal Testing for Vaccines - Implementing Replacement, Reduction and Refinement: Challenges and Priorities". This conference was organized by the International Alliance for Biological Standardization (IABS), and held in Bangkok, Thailand on December 3 and 4 2019. Participants comprised stakeholders from many parts of the world, including vaccine developers, manufacturers and regulators from Asia, Europe, North America, Australia and New Zealand. In interactive workshops and vibrant panel discussions, the attendees worked together to identify the remaining barriers to validation, acceptance and implementation of alternative methods, and how harmonization could be promoted, especially for LMICs.

Global harmonization of vaccine testing requirements: Making elimination of the ATT and TABST a concrete global achievement

This one-day symposium organized by Humane Society International (HSI) brought together 18 international experts from Argentina, Brazil, China, Europe, India, Russia, South Africa and the United States to discuss the elimination of the abnormal toxicity test (ATT) from the testing requirements for human vaccines as well as the target animal batch safety test (TABST) and the laboratory animal batch safety test (LABST) for veterinary vaccines. Participants reported on country-specific regulatory requirements and, where present, the perspectives on waiver and elimination of those tests. In addition, the attendees, with HSI in the role of facilitator, moved to define the barriers to the complete elimination or waiving of these tests. This report expounds the outcomes of the symposium, and introduces a proposed roadmap - populated with country specific activities - for the elimination of these tests.

Effect of 2 Emulsion-Based Adjuvants on the Structure and Thermal Stability of Staphylococcus aureus Alpha-Toxin

The effects of 2 squalene-based emulsion adjuvant systems (MedImmune emulsion 0 [ME.0] and Stable Emulsion [SE]) on the structure and stability of the recombinant protein antigen alpha-toxin (AT), a potential vaccine candidate for Staphylococcus aureus infection, were investigated using Fourier-transform infrared spectroscopy and both steady-state and time-resolved intrinsic fluorescence spectroscopy as well as differential scanning calorimetry (DSC). A component study, performed to identify the effects of the individual emulsion's components, showed negligible interactions between AT and ME.0. DSC analysis showed the ME.0 emulsion thermally destabilized AT, probably because of changes in the buffer composition of AT upon mixing. The SE emulsion caused increased alpha-helix and decreased beta-sheet content in AT, and a significant blue shift in the fluorescence spectra relative to that of AT in solution. DSC analysis showed SE exerted a dramatic thermal stabilization effect on AT, probably attributable to an interaction between AT and SE. Size exclusion chromatography showed a complete loss in the recovery of AT when mixed with SE, but not ME.0, indicating a high degree of interaction with SE. This work successfully characterized the biophysical properties of AT in the presence of 2 emulsion adjuvants including a component study to rationalize how emulsion components affect protein antigen stability.

Monoclonal antibody based in vitro potency assay as a predictor of antigenic integrity and in vivo immunogenicity of a Respiratory Syncytial Virus post-fusion F-protein based vaccine

The post-fusion form of Respiratory Syncytial Virus (RSV) fusion (F) protein has been used recently in clinical trials as a potential vaccine antigen with the objective of eliciting protective immune response against RSV. In this paper, in vitro antigenicity and in vivo immunogenicity of recombinant, soluble F protein of RSV (RSVsF) were evaluated by several assays. In Vitro Relative Potency (IVRP) of RSVsF was measured in a sandwich ELISA using two antibodies, each specific for epitope site A or C. Therefore, IVRP reflected the integrity of the antigen in terms of changes in antibody binding affinity of either or both of these sites. RSVsF samples with a wide range of IVRP values were generated by applying UV irradiation (photo) and high temperature (heat) induced stress for varying lengths of time. These samples were characterized in terms of stress induced modifications in primary and secondary structures as well as aggregation of RSVsF. Immunogenicity, also referred to as In vivo potency, was measured by induction of total F-protein specific IgG and RSV-neutralizing antibodies in mice dosed with these RSVsF samples. Comparison of results between IVRP and these immunogenicity assays revealed that IVRP provided a sensitive read-out of the integrity of epitope sites A and C, and a conservative and reliable evaluation of the potency of RSVsF as a vaccine antigen. This high throughput and fast turn-around assay allowed us to efficiently screen many different RSVsF antigen lots, thereby acting as an effective filter for ensuring high quality antigen that delivered in vivo potency. In vitro and in vivo potencies were further probed at the level of individual epitope sites, A and C. Results of these experiments indicated that site A was relatively resistant to stress induced loss of potency, in vitro or in vivo, compared to site C.

Analytical Characterization of an Oil-in-Water Adjuvant Emulsion

Adjuvants are typically used in subunit vaccine formulations to enhance immune responses elicited by individual antigens. Physical chemical characterization of novel adjuvants is an important step in ensuring their effective use in vaccine formulations. This paper reports application of a panel of quantitative assays developed to analyze and characterize an oil-in-water adjuvant emulsion, which contains glucopyranosyl lipid A (GLA) and is a squalene-based emulsion. GLA is a fully synthetic analogue of monophosphoryl lipid A, which is a Toll-like receptor type 4 agonist and an FDA-approved adjuvant. The GLA-stable emulsion (GLA-SE) is currently being used for a respiratory syncytial virus vaccine in a phase 2 clinical trial. GLA was quantitated using reverse-phased high-performance liquid chromatography (RP-HPLC) coupled to a mass spectrometric detector, achieving higher assay sensitivity than the charged aerosol detection routinely used. Quantitation of the excipients of GLA-SE, including squalene, egg phosphatidyl choline, and Poloxamer 188, was achieved using a simple and rapid RP-HPLC method with evaporative light scattering detection, eliminating chemical derivatization typically required for these chromophore-lacking compounds. DL-α-tocopherol, the antioxidant of the GLA-SE, was quantitated using a RP-HPLC method with conventional UV detection. The experimental results compared well with values expected for these compounds based on targeted composition of the adjuvant. The assays were applied to identify degradation of individual components in a GLA-SE sample that degraded into distinct aqueous and oil phases. The methods developed and reported here are effective tools in monitoring physicochemical integrity of the adjuvant, as well as in formulation studies.

Bacterial DNA replication enzymes as targets for antibacterial drug discovery

INTRODUCTION

The bacterial replisome is composed of a large number of enzymes, which work in exquisite coordination to accomplish chromosomal replication. Effective inhibition inside the bacterial cell of any of the 'essential' enzymes of the DNA replication pathway should be detrimental to cell survival.

AREAS COVERED

This review covers DNA replication enzymes that have been shown to have a potential for delivering antibacterial compounds or drug candidates including: type II topoisomerases, a clinically validated target family, and DNA ligase, which has yielded inhibitors with in vivo efficacy. A few of the 'replisome' enzymes that are structurally and functionally well characterized and have been subjects of antibacterial discovery efforts are also discussed.

EXPERT OPINION

Identification of several essential genes in the bacterial replication pathway raised hopes that targeting these gene products would lead to novel antibacterials. However, none of these novel, single gene targets have delivered antibacterial drug candidates into clinical trials. This lack of productivity may be due to the target properties and inhibitor identification approaches employed. For DNA primase, DNA helicase and other replisome targets, with the exception of DNA ligase, the exploitation of structure for lead generation has not been tested to the same extent that it has for DNA gyrase. Utilization of structural information should be considered to augment HTS efforts and initiate fragment-based lead generation. The complex protein-protein interactions involved in regulation of replication may explain why biochemical approaches have been less productive for some replisome targets than more independently functioning targets such as DNA ligase or DNA gyrase.

A review of multiple approaches towards an improved hepatitis B vaccine

BACKGROUND

Hepatitis B is a DNA virus that can cause liver inflammation, cirrhosis, and cancer in chronically infected and symptomatic carriers. Antiviral treatments are usually limited in their effectiveness in treating the disease states. Vaccination against hepatitis B in pediatric and adolescent populations has proven to be a generally effective means for preventing diseases that could be potentially caused by this virus. Some 5 - 10% of the vaccinees do not develop protective immunity against the virus. Therefore, a significant amount of effort has been made in many research laboratories across the world to increase the potency of the vaccine by various innovative means, e.g., increasing the immunogenicity of the antigen or through introduction of novel adjuvants that elicit strong humoral and cell-mediated immune responses.

OBJECTIVES/METHODS

The objective of this review is to highlight publications of significant developments that have been made over the past decade and efforts that are continuing towards producing an improved vaccine. A number of patents that protect novel hepatitis B vaccine formulations, including those claiming novel hepatitis B core antigen formulations and combinations of a vaccine with small molecule therapeutics, are discussed.

CONCLUSION

There have been promising developments in the area of new adjuvants and delivery systems. The practical need for reducing the total number of childhood vaccinations has driven development of, and patent filings on, multivalent and combination vaccine formulations in which the hepatitis B vaccine is included as one component. Efforts and some advances have also been made in the critical area of therapeutic application of the vaccine. The existence of a large population of already infected patients and the inadequacy of most of the current antiviral drugs against hepatitis B diseases have also inspired efforts to produce a vaccine that would be efficacious in clearing an exiting infection.

Applications of mass spectrometry in early stages of target based drug discovery

Mass spectrometry (MS) has been applied to drug discovery for many years. With the advent of new ionization techniques, MS has emerged as an important analytical tool in identification and characterization of protein targets, structure elucidation of synthetic compounds, and early drug metabolism and pharmacokinetics studies. Two MS-based strategies, function-based and affinity-based, have been employed in recent years for screening and evaluation of compounds. In the function-based approach, the effects of compounds on the biological activity of a target molecule are measured. In the affinity-based approach, compounds are screened based on their binding affinities to target molecules. The interaction between targets and compounds can be directly evaluated by monitoring the formation of non-covalent target-ligand complexes (direct detection) or indirectly evaluated by detecting the compounds after separating bound compounds from unbound (indirect detection). Various techniques including high performance liquid chromatography (HPLC)-MS, size exclusion chromatography (SEC)-MS, frontal affinity chromatography (FAC)-MS and desorption/ionization on silicon (DIOS)-MS can be applied. The recent advances, relative advantages, and limitations of each MS-based method as a tool in compound screening and compound evaluation in the early stages of drug discovery are discussed in this review.

Stabilization of human papillomavirus virus-like particles by non-ionic surfactants

Human papillomavirus (HPV) virus-like-particles (VLPs) produced by recombinant expression systems are promising vaccine candidates for prevention of cervical cancers as well as genital warts. At high protein concentrations, HPV VLPs, comprised of the viral capsid protein L1 and expressed and purified from yeast, are protected against detectable aggregation during preparation and storage by high concentrations of NaCl. At low protein concentrations, however, high salt concentration alone does not fully protect HPV VLPs from aggregation. Moreover, the analytical analysis of HPV VLPs proved to be a challenge due to surface adsorption of HPV VLPs to storage containers and cuvettes. The introduction of non-ionic surfactants into HPV VLP aqueous solutions provides significantly enhanced stabilization of HPV VLPs against aggregation upon exposure to low salt and protein concentration, as well as protection against surface adsorption and aggregation due to heat stress and physical agitation. The mechanism of non-ionic surfactant stabilization of HPV VLPs was extensively studied using polysorbate 80 (PS80) as a representative non-ionic surfactant. The results suggest that PS80 stabilizes HPV VLPs mainly by competing with the VLPs for various container surfaces and air/water interfaces. No appreciable binding of PS80 to intact HPV VLPs was observed although PS80 does bind to the denatured HPV L1 protein. Even in the presence of stabilizing level of PS80, however, an ionic strength dependence of HPV VLP stabilization against aggregation is observed indicating optimization of both salt and non-ionic surfactant levels is required for effective stabilization of HPV VLPs in solution.

Development of an LC–MS based enzyme activity assay for MurC: application to evaluation of inhibitors and kinetic analysis

An enzyme activity assay, based on mass spectrometric (MS) detection of specific reaction product following HPLC separation, has been developed to evaluate pharmaceutical hits identified from primary high throughput screening (HTS) against target enzyme Escherichia coli UDP-N-acetyl-muramyl-L-alanine ligase (MurC), an essential enzyme in the bacterial peptidoglycan biosynthetic pathway, and to study the kinetics of the enzyme. A comparative analysis of this new liquid chromatographic-MS (LC-MS) based assay with a conventional spectrophotometric Malachite Green (MG) assay, which detects phosphate produced in the reaction, was performed. The results demonstrated that the LC-MS assay, which determines specific ligase activity of MurC, offers several advantages including a lower background (0.2% versus 26%), higher sensitivity (> or = 10 fold), lower limit of quantitation (LOQ) (0.02 microM versus 1 microM) and wider linear dynamic range (> or = 4 fold) than the MG assay. Good precision for the LC-MS assay was demonstrated by the low intraday and interday coefficient of variation (CV) values (3 and 6%, respectively). The LC-MS assay, free of the artifacts often seen in the Malachite Green assay, offers a valuable secondary assay for hit evaluation in which the false positives from the primary high throughput screening can be eliminated. In addition, the applicability of this assay to the study of enzyme kinetics has also been demonstrated.

Pharmaceutical and immunological evaluation of a single-shot hepatitis B vaccine formulated with PLGA microspheres

A single-shot Hepatitis B vaccine formulation using poly(d,l)-lactide-co-glycolide acid (PLGA) microspheres as a delivery system was examined using a variety of biophysical and biochemical techniques as well as immunological evaluation in C3H mice. PLGA microsphere encapsulation of the Hepatitis B surface antigen (HBsAg), a lipoprotein particle, resulted in good recoveries of protein mass, protein particle conformational integrity, and in vitro antigenicity. Some partial delipidation of the HBsAg, however, was observed. The loading and encapsulation efficiency of HBsAg into the PLGA microspheres were measured along with the morphology and size distribution of the vaccine-loaded PLGA microspheres. The in vitro release kinetics of HBsAg from the PLGA microspheres was evaluated and found to be affected by experimental conditions such as stirring rate. HBsAg showed enhanced storage stability at 37 degrees C in the slightly acidic pH range reported to be found inside PLGA microspheres; thus, the antigen is relatively stable under conditions of temperature and pH that may mimic in vivo conditions. The immunogenicity of the microsphere formulations of HBsAg was compared with conventional aluminum adjuvant formulated HBsAg vaccine in C3H mice. Comparisons were made between aluminum formulations (one and two injections), PLGA microsphere formulations (single injection), and a mixture of aluminum and PLGA microsphere formulations (single injection). The nine-month serum antibody titers indicate that a single injection of a mixture of aluminum and PLGA-formulated HBsAg results in equal or better immune responses than two injections of aluminum-formulated HBsAg vaccine. Based on these in vitro and in vivo studies, it is concluded that HBsAg can be successfully encapsulated and recovered from the PLGA microspheres and a mixture of aluminum-adjuvanted and PLGA-formulated HBsAg can auto-boost an immune response in manner comparable to multiple injections of an aluminum-formulated vaccine.

Quantitative determination of saccharide surfactants in protein samples by liquid chromatography coupled to electrospray ionization mass spectrometry

A direct and highly selective method, combining liquid chromatography (LC) with electrospray ionization mass spectrometry (ESI-MS), has been developed for quantifying saccharide surfactants. Saccharide surfactants, such as n-octyl-beta-d-glucopyranoside (NOG), are widely used to solubilize or refold membrane-bound or lipophilic proteins. In the present study, we have developed an LC-MS method to quantify NOG in protein samples. Protein-bound NOG was completely dissociated from proteins by reversed-phase LC, allowing the total amount of saccharide surfactant in protein samples to be quantified by MS. A chemical analog of NOG was used as an internal standard for improving the reproducibility of the method. Linearity was found in the range of 10 microg/mL-1.0 mg/mL NOG concentrations. Seven major surfactant oligomeric ions were detected under the ionization conditions applied and their relative abundance was essentially unchanged over the range of 0.05-1.0 mg/mL NOG concentrations. Consequently, ions with characteristic mass-to-charge ratios could be used for quantification of NOG. Analytical accuracy of the method was examined by determining the amounts of NOG recovered from apolipoprotein A-I and myoglobin samples spiked with NOG.

Direct biophysical characterization of human apolipoprotein A-1 in ISCOMs

Human apolipoprotein A-1 was formulated in "Immune Stimulating Complexes" (ISCOMs). The structure of the protein in ISCOMs was examined directly using several biophysical techniques including Fourier transform infrared (FTIR) spectroscopy, near UV circular dichroism (CD), and fluorescence spectroscopy. Amide I FTIR data indicate that human apolipoprotein A-1 displays a slightly increased alpha-helical content after its incorporation into ISCOMs. Near UV CD and tryptophan fluorescence data suggest that association with ISCOMs results in the tryptophan residues of the protein experiencing a relatively hydrophobic environment, motional restriction, and local electrostatic interactions. These observations are consistent with an increased order in the protein structure upon incorporation in ISCOMs. In addition, biomolecular interaction analysis (BIA), based on surface plasmon resonance (SPR) measurements, suggests that the binding affinity of human apolipoprotein A-1 to a monoclonal anti-human apolipoprotein A-1 antibody is moderately decreased (by 20%) after its incorporation into ISCOMs. This study demonstrates that these biophysical techniques can be used to noninvasively monitor integrity of or changes in secondary and tertiary structure of proteins within the ISCOM particles without the need for protein extraction.

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.

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.

Human cytoplasmic 3-hydroxy-3-methylglutaryl coenzyme A synthase: expression, purification, and characterization of recombinant wild-type and Cys129 mutant enzymes

A cDNA for the human cytoplasmic 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) synthase (EC 4.1.3.5) was subcloned and expressed from a T7-based vector in Escherichia coli. The over-produced enzyme was purified using a three-step protocol that generated 20 to 30 mg protein/liter cell culture. The physical and catalytic properties of the recombinant synthase are similar to those reported for the nonrecombinant enzymes from chicken liver [Clinkenbeard et al. (1975a) J. Biol. Chem. 250, 3124-3135] and rat liver [Mehrabian et al. (1986) J. Biol. Chem. 261, 16249-16255]. Mutation of Cys129 to serine or alanine destroys HMG-CoA synthase activity by disrupting the first catalytic step in HMG-CoA synthesis, enzyme acetylation by acetyl coenzyme A. Furthermore, unlike the wild-type enzyme, neither mutant was capable of covalent modification by the beta-lactone inhibitor, L-659,699 [Greenspan et al. (1987) Proc. Natl. Acad. Sci. USA 84, 7488-7492]. Kinetic analysis of the inhibition by L-659,699 revealed that this compound is a potent inhibitor of the recombinant human synthase, with an inhibition constant of 53.7 nM and an inactivation rate constant of 1.06 min-1.

Three-dimensional structure of echistatin and dynamics of the active site

The snake venom protein echistatin contains the cell recognition sequence Arg-Gly-Asp and is a potent inhibitor of platelet aggregation. The three-dimensional structure of echistatin and the dynamics of the active RGD site are presented. A set of structures was determined using the Distance Geometry method and subsequently refined by Molecular Dynamics and energy minimization. Disulfide pairings are suggested, based on violations of experimental constraints. The structures satisfy 230 interresidue distance constraints, derived from nuclear Overhauser effect measurements, five hydrogen-bonding constraints, and 21 torsional constraints from vicinal spin-spin coupling constants. The segment from Gly5 to Cys20 and from Asp30 to Asn42 has a well-defined conformation and the Arg-Gly-Asp sequence, which adopts a turn-like structure, is located at the apex of a nine-residue loop connecting the two strands of a distorted beta-sheet. The mobility of the Arg-Gly-Asp site has been quantitatively characterized by 15N relaxation measurements. The overall correlation time of echistatin was determined from fluorescence measurements, and was used in a model-free analysis to determine internal motional parameters. The active site has order parameters of 0.3-0.5, i.e., among the smallest values ever observed at the active site of a protein. Correlation of the flexible region of the protein as characterized by relaxation experiments and the NMR solution structures was made by calculating generalized order parameters from the ensemble of three-dimensional structures. The motion of the RGD site detected experimentally is more extensive than a simple RGD loop 'wagging' motional model, suggested by an examination of superposed solution structures.

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)

In vivo evidence that theophylline is metabolized principally by CYP1A in rats

The role of various subfamilies of rat hepatic cytochrome P-450 in the oxidation of theophylline was evaluated by comparing theophylline clearance in control rats and those pretreated with relatively selective inducers and inhibitors of the cytochromes P-450. Pretreatment with the CYP1A inducer, beta-naphthoflavone (BNF), increased theophylline clearance 4.5-fold (p < 0.001), and the CYP1A inhibitor, alpha-naphthoflavone, significantly attenuated the BNF effect. Pretreatment with phenobarbital, an inducer of CYP2B/C in rats, had a far more modest effect, increasing theophylline clearance only 1.6-fold (p < 0.005). The phenobarbital-mediated increase in theophylline clearance was attenuated by orphenadrine, a CYP2B/C inhibitor. The CYP2E inducer, isoniazid and the CYP2E inhibitor, diallyl sulfide were virtually without effect, as was the CYP4A inducer, clofibrate, and the CYP4A inhibitor, 10-undecynoic acid. Ajmaline, and inhibitor of CYP2D, was also without any effect on theophylline clearance. While the powerful CYP3A inducer clotrimazole did not increase theophylline clearance, troleandomycin, an inhibitor of CYP3A, did slow theophylline clearance by about 25% (p < 0.002). Together, these findings suggest that CYP1A is principally responsible for the overall oxidation of theophylline in rats, and that CYP2B/C probably also mediates some theophylline oxidation. The involvement of CYP2D, CYP2E, CYP4A, and CYP3A is relatively trivial.

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)

Conformational changes in chicken thyroid hormone receptor alpha 1 induced by binding to ligand or to DNA

A classic model of steroid/thyroid hormone receptor activation postulates that a conformational change or "transformation" occurs upon ligand binding as a first step toward regulation of gene transcription. In order to test this model, physical studies have been carried out using purified full-length chicken thyroid hormone receptor alpha 1 (cT3R-alpha 1) expressed in Escherichia coli. Circular dichroism spectroscopic studies reveal that cT3R-alpha 1 adopts a different conformation upon specific binding to a cognate ligand triiodothyroacetic acid as well as to a thyroid hormone response element, an idealized inverted repeat AGGTCA TGACCT. These results suggest that cT3R-alpha 1 may adopt distinct conformations whether free or bound to ligand or to DNA. These states may reflect the changes in the conformation of steroid/thyroid hormone receptors in the signal transduction pathway.

Structural implications of spectroscopic characterization of a putative zinc finger peptide from HIV-1 integrase

The N-terminal domain of human immunodeficiency virus (HIV-1) integrase (IN) contains the sequence motif His-Xaa3-His-Xaa23-Cys-Xaa2-Cys, which is strongly conserved in all retroviral and retrotransposon IN proteins. This structural motif constitutes a putative zinc finger in which a metal ion may be coordinately bound by the His and Cys residues. A recombinant peptide, IN(1-55), composed of the N-terminal 55 amino acids of HIV-1 IN was expressed in Escherichia coli and purified. Utilizing a combination of techniques including UV-visible absorption, circular dichroism, Fourier transform infrared, and fluorescence spectroscopies, we have demonstrated that metal ions (Zn2+, Co2+, and Cd2+) are bound with equimolar stoichiometry by IN(1-55). The liganded peptide assumes a highly ordered structure with increased alpha-helical content and exhibits remarkable thermal stability. UV-visible difference spectra of the peptide-Co2+ complexes directly implicate thiols in metal coordination, and Co2+ d-d transitions in the visible range indicate that Co2+ is tetrahedrally coordinated. Mutant peptides containing conservative substitutions of one of the conserved His or either of the Cys residues displayed no significant Zn(2+)-induced conformational changes as monitored by CD and fluorescence spectra. We conclude that the N terminus of HIV-1 IN contains a metal-binding domain whose structure is stabilized by tetrahedral coordination of metal by histidines 12 and 16 and cysteines 40 and 43. A preliminary structural model for this zinc finger is presented.

Purification and characterization of a functionally homogeneous 60-kDa species of the retinoblastoma gene product

The retinoblastoma susceptibility gene (RB) encodes a 928-amino acid protein (pRB) that is hypothesized to function in a pathway that restricts cell proliferation. The immortalizing proteins from three distinct DNA tumor viruses (SV40 large T antigen, adenovirus E1a, and human papilloma virus Type 16 E7) have been shown to interact with RB protein through two noncontiguous regions comprised of amino acids 393-572 (domain A) and 646-772 (domain B). We constructed a truncated form of RB (RB p60) that retains these two domains but eliminates the N-terminal 386 amino acids of RB. RB p60 was expressed in Escherichia coli in inclusion bodies. After solubilization, it was refolded in the presence of magnesium chloride, and the active protein was isolated with an E7 peptide affinity column. The protein that elutes from this column is functionally homogenous in its ability to bind immobilized E7 protein. Thermal denaturation studies provide additional evidence for the conformational homogeneity of the isolated protein. This purification scheme allows the isolation of significant amounts of RB p60 protein that is suitable for structural and functional studies.

Proton NMR assignments and secondary structure of the snake venom protein echistatin

The snake venom protein echistatin is a potent inhibitor of platelet aggregation. The inhibitory properties of echistatin have been attributed to the Arg-Gly-Asp sequence at residues 24-26. In this paper, sequence-specific nuclear magnetic resonance assignments are presented for the proton resonances of echistatin in water. The single-chain protein contains 49 amino acids and 4 cystine bridges. All of the backbone amide, C alpha H, and side-chain resonances, except for the eta-NH of the arginines, have been assigned. The secondary structure of the protein was characterized from the pattern of nuclear Overhauser enhancements, from the identification of slowly exchanging amide protons, from 3JC alpha H-NH coupling constants, and from circular dichroism studies. The data suggest that the secondary structure consists of a type I beta-turn, a short beta-hairpin, and a short, irregular, antiparallel beta-sheet and that the Arg-Gly-Asp sequence is in a flexible loop connecting two strands of the distorted antiparallel beta-sheet.

Effect of polyanions on the refolding of human acidic fibroblast growth factor

Acidic fibroblast growth factor (aFGF) is unstable at physiological temperatures in the absence of polyanions such as heparin. Therefore, the effect of temperature on the kinetics of refolding of aFGF has been examined in the presence and absence of several polyanions. The protein folds into its native state at temperatures up to 30 degrees C without polyanions with an activation energy of approximately 14 kcal/mol, but does not acquire native structure above this temperature. When heparin, inositol hexasulfate, or sulfate ion are present, aFGF refolds below 30 degrees C with a slightly reduced activation energy (10-11 kcal/mol). In addition, the protein now also renatures between 30 and 50 degrees C with activation energies of 1-2 (heparin), 16 (inositol hexasulfate), and 7 (sulfate) kcal/mol. Trace heavy metals appear to inhibit the refolding process, but a molecular chaperone (bovine 70-kDa heat shock cognate protein) and a peptidylprolyl isomerase (the FK506-binding protein) have no effect. It is concluded that the rate of refolding of aFGF at physiological temperatures is probably controlled by the interaction of a native-like state of the protein with an unknown polyanionic species.

Stabilization of the FK506 binding protein by ligand binding

Although the rotamase activity of the FK506 binding protein is inhibited by ligand binding, it is hypothesized that the ligand/protein complex itself may be responsible for the immunosuppressive effects of FK506. We have therefore examined the structure of the FK506 binding protein in the presence of an analog of FK506 (FK520) by a combination of fluorescence, CD, FTIR and calorimetry. While only small changes in the overall structure of the protein may be induced by ligand, a large change in thermal stability of the binding protein is observed.

Static quenching of tryptophan fluorescence by oxidized dithiothreitol

The quenching of fluorescence of 5-methoxyindole, N-acetyl-L-tryptophanamide and two single tryptophan containing peptides, melittin and mastoparan X, by oxidized dithiothreitol was studied. The slopes of the Stern-Volmer plots for steady-state fluorescence quenching were 133 M-1, 71.2 M-1, 75.5 M-1 and 35.0 M-1 at 21 degrees C and pH 7.0 for 5-methyoxyindole, N-acetyl-L-tryptophanamide, melittin and mastoparan X respectively. Fluorescence lifetimes of indole or tryptophan in these compounds, as determined by multifrequency phase fluorometry, were decreased by 15% or less at concentrations that produced 50% or more quenching of steady-state fluorescence. Thus, quenching of fluorescence by oxidized dithiothreitol for these derivatives of indole appears to be largely static in nature, suggesting a ground-state interaction.

Binding of amphiphilic peptides to a carboxy-terminal tryptic fragment of calmodulin

Calmodulin (CaM) fragments 1-77 (CaM 1-77) and 78-148 (CaM 78-148) were prepared by tryptic cleavage of CaM. CaM 78-148 exhibited Ca2+-dependent binding to mastoparan X, Polistes mastoparan, and melittin with apparent dissociation constants less than 0.2 microM as judged from changes in the fluorescence spectrum and anisotropy of the single tryptophan residue of each of these cationic, amphiphilic peptides. This interaction was accompanied by a large spectral blue shift of the peptide fluorescence spectrum. These findings are consistent with earlier results [Malencik, D.A., & Anderson, S.R. (1984) Biochemistry 23, 2420-2428] on the binding of mastoparan X to CaM fragment 72-148. The binding of the peptide to CaM 78-148 also caused a significant loss of the accessibility of the peptide tryptophan to the fluorescence quencher acrylamide. The CaM 78-148 induced effects on the fluorescence spectra and tryptophan accessibility of the peptides were most pronounced for mastoparan X, a peptide with tryptophan on the apolar face of the putative amphiphilic helix. The data were comparable with results from parallel experiments on the Ca2+-dependent interaction of these peptides with intact CaM. Difference circular dichroic spectra suggested that binding to CaM 78-148 was associated with the induction of considerable degrees of helicity in the amphiphilic peptides, which by themselves have predominantly random coil structures in aqueous solution. This finding is also reminiscent of the interaction of these peptides with intact CaM.(ABSTRACT TRUNCATED AT 250 WORDS)

Tryptophan fluorescence studies of subunit interaction and rotational dynamics of human luteinizing hormone

Human luteinizing hormone (hLH) has a single tryptophan residue occurring in the beta-subunit (beta hLH). This provides an intrinsic fluorescent probe, in native hLH and beta hLH, that is unambiguously assigned. The fluorescence intensities of hLH and beta hLH are, however, significantly different. This difference has been utilized in studying the interaction of fluorescent beta hLH with the nonfluorescent alpha-subunit. The accessibility of the tryptophan residue in native hLH and beta hLH has been assessed by measuring the rate of collisional fluorescence quenching and by solvent perturbation (D2O/H2O) of fluorescence. Fluorescence anisotropy measurements have been used in studying the intramolecular dynamics and segmental tryptophan mobility in hLH and beta hLH. Lifetime-resolved anisotropy, measured by the technique of oxygen quenching of fluorescence, has revealed the presence of segmental tryptophan motion. These data can be satisfactorily explained in terms of fast segmental tryptophan motion and rotational diffusion of the whole protein and do not require that intersubunit motion be invoked for intact hLH as it was suggested earlier on the basis of fluorescence depolarization of fluorescein-labeled hLH [Bishop, W. H., & Ryan, R. J. (1975) Biochem. Biophys. Res. Commun. 65, 1184-1190].

Probable role of amphiphilicity in the binding of mastoparan to calmodulin

Two-dimensional helical wheel diagrams and calculations of mean hydrophobic moments show mastoparan, mastoparan X, and Polistes mastoparan to have all the properties expected for amphiphilic helices. Circular dichroic properties are consistent with a random form for these peptides in dilute aqueous solution, but greater than 50% helix is apparent when the peptides are dissolved in 70% trifluoroethanol/water mixtures (v/v) or when the peptides are bound to calmodulin. Changes in the fluorescence spectra, anisotropy, and accessibility of tryptophan whose indole side chain is on the apolar surface of the amphiphilic helix imply a significant role for the apolar surface in the binding of the mastoparans and another amphiphilic peptide, melittin, to calmodulin. These data provide a useful model for designing high-affinity synthetic peptide inhibitors of calmodulin.

The carbon dioxide hydration activity of the sulfonamide-resistant carbonic anhydrase from the liver of male rat: pH independence of the steady-state kinetics

The steady-state kinetic constants for the catalysis of CO2 hydration by the sulfonamide-resistant and testosterone-induced carbonic anhydrase from the liver of the male rat has been determined by stopped-flow spectrophotometry. The turnover number was 2.6 +/- 0.6 X 10(3) s-1 at 25 degrees C, and was invariant with pH ranging from 6.2 to 8.2 within experimental error. The Km at 25 degrees C was 5 +/- 1 mM, and was also pH independent. These data are in quantitative agreement with earlier findings of pH-independent CO2 hydration activity for the mammalian skeletal muscle carbonic anhydrase isozyme III. The turnover numbers for higher-activity isozymes I and II are strongly pH dependent in this pH range. Thus, the kinetic status of the male rat liver enzyme is that of carbonic anhydrase III. This finding is consistent with preliminary structural and immunologic data from other laboratories.