Acid stabilization of insulin

Surface-catalyzed liquid-liquid phase separation and amyloid-like assembly in microscale compartments

Liquid-liquid phase separation is a key phenomenon in the formation of membrane-less structures within the cell, appearing as liquid biomolecular condensates. Protein condensates are the most studied for their biological relevance, and their tendency to evolve, resulting in the formation of aggregates with a high level of order called amyloid. In this study, it is demonstrated that Human Insulin forms micrometric, round amyloid-like structures at room temperature within sub-microliter scale aqueous compartments. These distinctive particles feature a solid core enveloped by a fluid-like corona and form at the interface between the aqueous compartment and the glass coverslip upon which they are cast. Quantitative fluorescence microscopy is used to study in real-time the formation of amyloid-like superstructures. Their formation results driven by liquid-liquid phase separation process that arises from spatially heterogeneous distribution of nuclei at the glass-water interface. The proposed experimental setup allows modifying the surface-to-volume ratio of the aqueous compartments, which affects the aggregation rate and particle size, while also inducing fine alterations in the molecular structures of the final assemblies. These findings enhance the understanding of the factors governing amyloid structure formation, shedding light on the catalytic role of surfaces in this process.

Elucidating the Degradation Pathways of Human Insulin in the Solid State

While there have been significant advances in the development of peptide oral dosage forms in recent years, highlighted by the clinical and commercial success of approved peptides such as Rybelsus®, there remain several barriers in the way of broad range applicability of this approach to peptide delivery. One such barrier includes the poor physical and chemical stability inherent to their structures, which persists in the solid state although degradation typically occurs at different rates and via different pathways in comparison to the solution state. Using insulin as a model peptide, this work sought to contribute to the development of analytical techniques for investigating common insulin degradation pathways. Chemically denatured, deamidated and aggregated samples were prepared and used to benchmark circular dichroism spectroscopy, reverse phase HPLC and size exclusion chromatography methods for the investigation of unfolding, chemical modifications and covalent aggregation of the insulin molecule respectively. Solid state degraded samples were prepared by heating insulin powder at 60 °C and 75% relative humidity for 1, 3, 5 and 7 d, and the degradation profiles of the samples were evaluated and compared with those observed in solution. While no unfolding was observed to occur, significant deamidation and covalent aggregation were detected. Reductive disulfide bond cleavage using dithiothreitol allowed for separation of the insulin A- and B-chains, offering a facile yet novel means of assessing the mechanisms of deamidation and covalent aggregation occurring in the solid state.

Supplementary Information

The online version contains supplementary material available at 10.1007/s41664-024-00302-5.

Biochemical Toxicological Study of Insulin Overdose in Rats: A Forensic Perspective

Due to nonspecific pathological changes and the rapid degradation of insulin in postmortem blood samples, the identification of the cause of death during insulin overdose has always been a difficulty in forensic medicine. At present, there is a lack of studies on the toxicological changes and related mechanisms of an insulin overdose, and the specific molecular markers of insulin overdose are still unclear. In this study, an animal model of insulin overdose was established, and 24 SD rats were randomly divided into a control group, insulin overdose group, and a recovery group (n = 8). We detected the biochemical changes and analyzed the toxicological mechanism of an insulin overdose. The results showed that after insulin overdose, the rats developed irregular convulsions, Eclampsia, Opisthotonos, and other symptoms. The levels of glucose, glycogen, and C-peptide in the body decreased significantly, while the levels of lactate, insulin, and glucagon increased significantly. The decrease in plasma K+ was accompanied by the increase in skeletal muscle K+. The PI3K-AKT signaling pathway was significantly activated in skeletal muscle, and the translocation of GLUT4/Na+-K+-ATPase to sarcolemma was significantly increased. Rare glycogenic hepatopathy occurred in the recovery group after insulin overdose. Our study showed that insulin overdose also plays a role in skeletal muscle cells, mainly through the PI3K-Akt signaling pathway. Therefore, the detection of signaling pathway proteins of the skeletal muscle cell membrane GLUT4 and Na+-K+-ATPase has a certain auxiliary diagnostic value for forensic insulin overdose identification. Glycogen detection in the liver and skeletal muscle is important for the diagnosis of insulin overdose, but it still needs to be differentiated from other causes of death. Skeletal muscle has great potential for insulin detection, and the ratio of insulin to the C-peptide (I:C) can determine whether an exogenous insulin overdose is present.

Oral delivery of biomacromolecules by overcoming biological barriers in the gastrointestinal tract: an update

INTRODUCTION

Biomacromolecules have proven to be an attractive choice for treating diseases due to their properties of strong specificity, high efficiency, and low toxicity. Besides greatly improving the patient's complaint, oral delivery of macromolecules also complies with hormone physiological secretion, which has become one of the most innovative fields of research in recent years.

AREAS COVERED

Oral delivery biological barriers for biomacromolecule, transport mechanisms, and various administration strategies were discussed in this review, including absorption enhancers, targeting nanoparticles, mucoadhesion nanoparticles, mucus penetration nanoparticles, and intelligent bionic drug delivery systems.

EXPERT OPINION

The oral delivery of biomacromolecules has important clinical implications; however, these are still facing the challenges of low bioavailability due to certain barriers. Various promising technologies have been developed to overcome the barriers and improve the therapeutic effect of oral biomacromolecules. By considering safety and efficacy comprehensively, the development of intelligent nanoparticles based on the GIT environment has demonstrated some promise in overcoming these barriers; however, a more comprehensive understanding of the oral fate of oral biomacromolecules is still required.

Inhibitory effects of fluorinated benzenesulfonamides on insulin fibrillation

Amyloid fibrils are protein aggregates formed by protein assembly through cross β structures. Inhibition of amyloid fibril formation may contribute to therapy against amyloid-related disorders like Parkinson's, Alzheimer's, and type 2 diabetes. Here we report that several fluorinated sulfonamide compounds, previously shown to inhibit human carbonic anhydrase, also inhibit the fibrillation of different proteins. Using a range of spectroscopic, microscopic and chromatographic techniques, we found that the two fluorinated sulfonamide compounds completely inhibit insulin fibrillation over a period of 16 h and moderately suppress α-synuclein and Aβ fibrillation. In addition, these compounds decreased cell toxicity of insulin incubated under fibrillation-inducing conditions. We ascribe these effects to their ability to maintain insulin in the native monomeric state. Molecular dynamic simulations suggest that these compounds inhibit insulin self-association by interacting with residues at the dimer interface. This highlights the general anti-aggregative properties of aromatic sulfonamides and suggests that sulfonamide compounds which inhibit carbonic anhydrase activity may have potential as therapeutic agents against amyloid-related disorders.

Structural Ensemble of the Insulin Monomer

Experimental evidence suggests that monomeric insulin exhibits significant conformational heterogeneity, and modifications of apparently disordered regions affect both biological activity and the longevity of pharmaceutical formulations, presumably through receptor binding and fibrillation/degradation, respectively. However, a microscopic understanding of conformational heterogeneity has been lacking. Here, we integrate all-atom molecular dynamics simulations with an analysis pipeline to investigate the structural ensemble of human insulin monomers. We find that 60% of the structures present at least one of the following elements of disorder: melting of the A-chain N-terminal helix, detachment of the B-chain N-terminus, and detachment of the B-chain C-terminus. We also observe partial melting and extension of the B-chain helix and significant conformational heterogeneity in the region containing the B-chain β-turn. We then estimate hydrogen-exchange protection factors for the sampled ensemble and find them in line with experimental results for KP-insulin, although the simulations underestimate the importance of unfolded states. Our results help explain the ready exchange of specific amide sites that appear to be protected in crystal structures. Finally, we discuss the implications for insulin function and stability.

Zinc oxide nanoparticle interface moderation with tyrosine and tryptophan reverses the pro-amyloidogenic property of the particle

Zinc oxide nanoparticle with negative surface potential (ZnONP) enhances bovine insulin fibrillation. Here, we are exploring ZnONP with positive surface potential (ZnONP_Unc) and surface functionalized with tyrosine and tryptophan amino acids to observe the effects of surface potential and surface functional groups on the fibrillation. ZnONP_Unc, despite of inversed surface potential, enhances the insulin fibrillation with increase in the interface concentration at physiological pH. Whereas, the interface moderation with the amino acids mitigates the surface-mediated insulin fibrillation propensity. Additionally, the study indicates that the change in interfacial functional groups at ZnONP_Unc significantly reverses the interface-mediated destabilization of insulin conformation. The functional groups from the amino acids, like CO, N-H and aromatic functional groups, are anticipated to further stabilize the insulin conformation by forming hydrogen bond and van der Waals interactions with the key amyloidogenic sequences of insulin, A13-A20 from A-chain and B9-B20 from B-chain. Hence, the altered interaction profile, with change in interfacial functional groups, mitigates the interface-mediated insulin fibrillation and the ZnONP_Unc-/fibril-mediated cytotoxicity.

Detection of insulins in postmortem tissues: an optimized workflow based on immunopurification and LC-MS/HRMS detection

Diabetes is a worldwide disease in perpetual expansion. Type 1 and sometimes type 2 diabetic patients require daily human insulin (HI) or analog administration. Easy access to insulins for insulin-treated diabetics, their relatives, and medical professionals can enable abuse for suicidal or homicidal purpose. However, demonstrating insulin overdose in postmortem blood is challenging. Tissue analyses are contributive, as insulins can accumulate before death or undergo only limited degradation. The present study describes an assay for HI and synthetic analogs (lispro, aspart, glulisine, detemir and degludec, glargine and its main metabolite (M1)) in liver, kidney, muscle, and injection site samples. It is based on a 5-step sample preparation (reduction of tissue sample size, homogenization, extraction, concentration, and immunopurification) associated with liquid chromatography coupled to high-resolution mass spectrometry (LC-MS/HRMS). Selectivity and limit of detection (LOD) for all target analogs were assessed in the above matrices. LOD was determined at 25 ng/g for HI and for analogs except detemir and degludec, where LOD was 50 ng/g in kidney and injection site samples and 80 ng/g in the liver and muscle. The method was applied to13 forensic cases in which insulin use was suspected.

Is insulin intoxication still the perfect crime? Analysis and interpretation of postmortem insulin: review and perspectives in forensic toxicology

Insulin is an anabolic hormone essential to glucose homeostasis. Insulin therapy, comprising human insulin (HI) or biosynthetic analogs, is critical for the management of type-1 diabetes and many of type-2 diabetes. However, medication error including non-adapted dose and confusion of insulin type, and misuse, such as massive self-administration or with criminal intent, can have lethal consequences. The aim of this paper is to review the state of knowledge of insulin analysis in biological samples and of the interpretation of insulin concentrations in the situation of insulin-related death investigations. Analytic aspects are considered, as quantification can be strongly impacted by methodology. Immunoanalysis, the historical technique, has a prominent role due to its sensitivity and ease of implementation. Recently, liquid chromatography coupled to mass spectrometry has provided indispensable selectivity in forensic contexts, distinguishing HI, analogs, and degradation products. We review the numerous antemortem (dose, associated pathology, injection-to-death interval, etc.) and postmortem parameters (in corpore degradation, in vitro degradation related to hemolysis, etc.) involved in the interpretation of insulin concentration. The interest and limitations of various alternative matrices providing a valuable complement to blood analysis are discussed. Vitreous humor is one of the most interesting, but the low diffusion of insulin in this matrix entails very low concentrations. Injection site analysis is relevant for identifying which type of insulin was administered. Muscle and renal cortex are matrices of particular interest, although additional studies are required. A table containing most case reports of fatal insulin poisoning published, with analytical data, completes this review. A logic diagram is proposed to highlight analytical issues and the main parameters to be considered for the interpretation of blood concentrations. Finally, it remains a challenge to provide reliable biological data and solid interpretation in the context of death related to insulin overdose. However, the progress of analytical tools is making the "perfect crime" ever more difficult to commit.

A Nanocomposite Vehicle Based on Metal-Organic Framework Nanoparticle Incorporated Biodegradable Microspheres for Enhanced Oral Insulin Delivery

Oral insulin delivery has revolutionized diabetes treatment, but challenges including degradation in the gastrointestinal environment and low permeation across the intestinal epithelium remain. Herein, to overcome these barriers, we developed a novel biodegradable nanocomposite microsphere embedded with metal-organic framework (MOF) nanoparticles. An iron-based MOF nanoparticle (NP) (MIL-100) was first synthesized as a carrier with an insulin loading capacity of 35%. The insulin-loaded MIL-100 nanoparticles modified with sodium dodecyl sulfate (Ins@MIL100/SDS) promoted insulin permeation across Caco-2 monolayer models in vitro. To improve resistance to the gastric acid environment, Ins@MIL100/SDS nanoparticles were embedded into a biodegradable microsphere to construct the nanocomposite delivery system (Ins@MIL100/SDS@MS). The microspheres effectively protected the MOF NPs from rapid degradation under acidic conditions and could release insulin-loaded MOF NPs in the simulated intestinal fluid. After the oral administration of Ins@MIL100/SDS@MS into BALB/c nude mice, increased intestinal absorption of the insulin was detected compared to the oral administration of free insulin or Ins@MIL100/SDS. Furthermore, significantly enhanced plasma insulin levels were obtained for over 6 h after oral administration of Ins@MIL100/SDS@MS into diabetic rats, leading to a remarkably enhanced effect in lowering blood glucose level with a relative pharmacological availability of 7.8%. Thus, the MOF-nanoparticle-incorporated microsphere may provide a new strategy for effective oral protein delivery.

Enhanced Intestinal Absorption of Insulin by Capryol 90, a Novel Absorption Enhancer in Rats: Implications in Oral Insulin Delivery

Labrasol^® is a self-emulsifying excipient that contains saturated polyglycolysed C₆-C₁₄ glycerides and this additive is known to improve the intestinal absorption of poorly absorbed drugs after oral administration. However, the effects of formulations similar to Labrasol^® on the intestinal absorption of poorly absorbed drugs have not been characterized. In this study, we used insulin as a model peptide drug and examined the absorption-enhancing effects of Labrasol^® and its related formulations for insulin absorption in rats. The co-administration of Labrasol-related formulations with insulin reduced the blood glucose levels. Among these formulations, Capryol 90 was the most effective additive. Notably, the effect of Capryol 90 was greater at pH 3.0 than at pH 7.0. Additionally, almost no mucosal damage was observed in the presence of these formulations, as these formulations did not affect the activity of lactate dehydrogenase (LDH) and the amount of protein released from the small intestine. In mechanistic studies, Capryol 90 improved the stability of insulin and suppressed the association with insulin under acidic conditions. The loosening of the tight junctions (TJs) could be the underlying mechanism by which Capryol 90 improved intestinal insulin absorption via a paracellular route. These findings suggest that Capryol 90 is an effective absorption enhancer for improving the intestinal absorption of insulin, without inducing serious damage to the intestinal epithelium.

Resveratrol as a nontoxic excipient stabilizes insulin in a bioactive hexameric form

Insulin aggregation is the leading cause of considerable reduction in the amount of active drug molecules in liquid formulations manufactured for diabetes management. Phenolic compounds, such as phenol and m-cresol, are routinely used to stabilize insulin in a hexameric form during its commercial preparation. However, long term usage of commercial insulin results in various adverse secondary responses, for which toxicity of the phenolic excipients is primarily responsible. In this study we aimed to find out a nontoxic insulin stabilizer. To that end, we have selected resveratrol, a natural polyphenol, as a prospective nontoxic insulin stabilizer because of its structural similarity with commercially used phenolic compounds. Atomic force microscopy visualization of resveratrol-treated human insulin revealed that resveratrol has a unique ability to arrest hINS in a soluble oligomeric form having discrete spherical morphology. Most importantly, resveratrol-treated insulin is nontoxic for HepG2 cells and it effectively maintains low blood glucose in a mouse model. Cryo-electron microscopy revealed 3D morphology of resveratrol-stabilized insulin that strikingly resembles crystal structures of insulin hexamer formulated with m-cresol. Significantly, we found that, in a condition inductive to amyloid fibrillation at physiological pH, resveratrol is capable of stabilizing insulin more efficiently than m-cresol. Thus, this study describes resveratrol as an effective nontoxic natural molecule that can be used for stabilizing insulin in a bioactive oligomeric form during its commercial formulation.

Insulin fibrillation: toward strategies for attenuating the process

The environmental factors affecting the rate of insulin fibrillation. The factors are representative.

Propylene Glycol Caprylate as a Novel Potential Absorption Enhancer for Improving the Intestinal Absorption of Insulin: Efficacy, Safety, and Absorption-Enhancing Mechanisms

Sodium caprate (C10) acts as an absorption enhancer. However, the absorption-enhancing effects of compounds with structures similar to C10 have not been characterized. In the present study, insulin was used as a model drug. We examined the effects of C10 and its related compounds on intestinal absorption of insulin using an in situ closed loop in rats. Insulin absorption was significantly enhanced by propylene glycol caprylate (Sefsol-218), a C10-related compound, after large intestinal administration. In addition, activity of lactate dehydrogenase did not increase in the intestinal epithelium in the presence of Sefsol-218 at concentrations equivalent to or lower than 1% (v/v). However, a significant increase in lactate dehydrogenase activity was observed in response to C10. These findings suggested that Sefsol-218 was safer than C10. Furthermore, mechanistic studies showed that increased membrane fluidity and loosening of tight junctions (TJs) might be underlying mechanisms by which this compound improved intestinal absorption of insulin. Furthermore, Sefsol-218 opened TJs by reducing the expression of claudin-4, which is a major TJ protein. These findings suggested that Sefsol-218 effectively enhanced intestinal insulin absorption without causing serious damage to the intestinal epithelium.

Enhanced intranasal insulin delivery by formulations and tumor protein-derived protein transduction domain as an absorption enhancer

One of the key factors for successful development of an intranasal insulin formulation is an absorption enhancer that would deliver insulin efficiently across nasal membranes without causing damage to mucosa or inducing protein aggregation under physiological conditions. In the present study, a protein transduction domain (PTD1) and its L-form with the double substitution A6L and I8A (PTD4), derived from human translationally controlled tumor protein, were used as absorption enhancers. PTD4 exhibited higher compatibility with insulin in terms of biophysical properties analyzed using μDSC, DLS, and CD. In addition, thermodynamic properties indicated stable complex formation but higher propensity of protein aggregation. Arginine hydrochloride (ArgHCl) was used to suppress protein aggregation and carbohydrates (i.e., mannitol, sucrose, and glycerin) were used as osmolytes in the formulation. The relative bioavailability of insulin co-administered intranasally using PTD4, 16 mg/mL glycerin and 100 mM ArgHCl was 58% and that using PTD4, 1 w/v% sucrose, and 25 mM ArgHCl was 53% of the bioavailability obtained via the subcutaneous route. These values represented a remarkable increase in bioavailability of intranasal insulin, causing a significant decrease in blood glucose levels within one hour. The pharmacokinetic properties of intranasal absorption were dependent on the concentration of carbohydrates used. These results suggest that the newly designed formulations with PTD represent a useful platform for intranasal delivery of insulin and other biomolecules.

Physico-chemical properties of co-formulated fast-acting insulin with pramlintide

Since the discovery of amylin its use has been discouraged by the inadequacy of the protocol involving multiple injections in addition to insulin. We aimed here to develop a combined fixed-dose formulation of pramlintide with fast-acting insulin. We have investigated the compatibility of regular and fast-acting insulin analogues (Aspart, Asp^B28, and LisPro, Lys^B28Pro^B29) with the amylin analogue pramlintide by using electrospray ionization - ion mobility spectrometry-mass spectrometry (ESI-IMS-MS), kinetic aggregation assays monitored by thioflavin T, and transmission electron microscopy (TEM) in the evaluation of the aggregation product. Insulin interacts with pramlintide, forming heterodimers as probed by ESI-IMS-MS. While their interaction is likely to delay the amyloid aggregation of pramlintide in phosphate-buffered solution pH 7.0, they do not prevent aggregation at this condition. At acidic sodium acetate solution pH 5.0, combination of pramlintide and the fast-acting insulin analogues become stable against amyloid aggregation. The co-formulated product at high concentration of both pramlintide (600 μg/mL,150 μM) and LisPro insulin (50 IU/mL, 300 μM) showed also stability against amyloid aggregation. These data indicate the physico-chemical short-term stability of the co-formulated preparation of LisPro insulin with pramlintide, which could bring benefits for the combined therapy.

Self-inhibition of insulin amyloid-like aggregation

NaCl induces formation of insulin tetramers leading to inhibition of amyloid formation.

A Comprehensive Survey of the Roles of Highly Disordered Proteins in Type 2 Diabetes

Type 2 diabetes mellitus (T2DM) is a chronic and progressive disease that is strongly associated with hyperglycemia (high blood sugar) related to either insulin resistance or insufficient insulin production. Among the various molecular events and players implicated in the manifestation and development of diabetes mellitus, proteins play several important roles. The Kyoto Encyclopedia of Genes and Genomes (KEGG) database has information on 34 human proteins experimentally shown to be related to the T2DM pathogenesis. It is known that many proteins associated with different human maladies are intrinsically disordered as a whole, or contain intrinsically disordered regions. The presented study shows that T2DM is not an exception to this rule, and many proteins known to be associated with pathogenesis of this malady are intrinsically disordered. The multiparametric bioinformatics analysis utilizing several computational tools for the intrinsic disorder characterization revealed that IRS1, IRS2, IRS4, MAFA, PDX1, ADIPO, PIK3R2, PIK3R5, SoCS1, and SoCS3 are expected to be highly disordered, whereas VDCC, SoCS2, SoCS4, JNK9, PRKCZ, PRKCE, insulin, GCK, JNK8, JNK10, PYK, INSR, TNF-α, MAPK3, and Kir6.2 are classified as moderately disordered proteins, and GLUT2, GLUT4, mTOR, SUR1, MAPK1, IKKA, PRKCD, PIK3CB, and PIK3CA are predicted as mostly ordered. More focused computational analyses and intensive literature mining were conducted for a set of highly disordered proteins related to T2DM. The resulting work represents a comprehensive survey describing the major biological functions of these proteins and functional roles of their intrinsically disordered regions, which are frequently engaged in protein–protein interactions, and contain sites of various posttranslational modifications (PTMs). It is also shown that intrinsic disorder-associated PTMs may play important roles in controlling the functions of these proteins. Consideration of the T2DM proteins from the perspective of intrinsic disorder provides useful information that can potentially lead to future experimental studies that may uncover latent and novel pathways associated with the disease.

Equilibrium Ensembles for Insulin Folding from Bias-Exchange Metadynamics

Earliest events in the aggregation process, such as single molecule reconfiguration, are extremely important and the most difficult to characterize in experiments. To this end, we have used well-tempered bias exchange metadynamics simulations to determine the equilibrium ensembles of an insulin molecule under amyloidogenic conditions of low pH and high temperature. A bin-based clustering method that uses statistics accumulated in bias exchange metadynamics trajectories was employed to construct a detailed thermodynamic and kinetic model of insulin folding. The highest lifetime, lowest free-energy ensemble identified consisted of native conformations adopted by a folded insulin monomer in solution, namely, the R-, the R^f-, and the T-states of insulin. The lowest free-energy structure had a root mean square deviation of only 0.15 nm from native x-ray structure. The second longest-lived metastable state was an unfolded, compact monomer with little similarity to the native structure. We have identified three additional long-lived, metastable states from the bin-based model. We then carried out an exhaustive structural characterization of metastable states on the basis of tertiary contact maps and per-residue accessible surface areas. We have also determined the lowest free-energy path between two longest-lived metastable states and confirm earlier findings of non-two-state folding for insulin through a folding intermediate. The ensemble containing the monomeric intermediate retained 58% of native hydrophobic contacts, however, accompanied by a complete loss of native secondary structure. We have discussed the relative importance of nativelike versus nonnative tertiary contacts for the folding transition. We also provide a simple measure to determine the importance of an individual residue for folding transition. Finally, we have compared and contrasted this intermediate with experimental data obtained in spectroscopic, crystallographic, and calorimetric measurements during early stages of insulin aggregation. We have also determined stability of monomeric insulin by incubation at a very low concentration to isolate protein-protein interaction effects.

Destabilization of Human Insulin Fibrils by Peptides of Fruit Bromelain Derived From Ananas comosus (Pineapple)

Deposition of insulin aggregates in human body leads to dysfunctioning of several organs. Effectiveness of fruit bromelain from pineapple in prevention of insulin aggregate was investigated. Proteolyses of bromelain was done as par human digestive system and the pool of small peptides was separated from larger peptides and proteins. Under conditions of growth of insulin aggregates from its monomers, this pool of peptides restricted the reaction upto formation of oligomers of limited size. These peptides also destabilized preformed insulin aggregates to oligomers. These processes were followed fluorimetrically using Thioflavin T and 1-ANS, size-exclusion HPLC, dynamic light scattering, atomic force microscopy, and transmission electron microscopy. Sequences of insulin (A and B chains) and bromelain were aligned using Clustal W software to predict most probable sites of interactions. Synthetic tripeptides corresponding to the hydrophobic interactive sites of bromelain showed disaggregation of insulin suggesting specificity of interactions. The peptides GG and AAA serving as negative controls showed no potency in destabilization of aggregates. Disaggregation potency of the peptides was also observed when insulin was deposited on HepG2 liver cells where no formation of toxic oligomers occurred. Amyloidogenic des-octapeptide (B23-B30 of insulin) incapable of cell signaling showed cytotoxicity similar to insulin. This toxicity could be neutralized by bromelain derived peptides. FT-IR and far-UV circular dichroism analysis indicated that disaggregated insulin had structure distinctly different from that of its hexameric (native) or monomeric states. Based on the stoichiometry of interaction and irreversibility of disaggregation, the mechanism/s of the peptides and insulin interactions has been proposed. J. Cell. Biochem. 118: 4881-4896, 2017. © 2017 Wiley Periodicals, Inc.

Computational and structural evidence for neurotransmitter-mediated modulation of the oligomeric states of human insulin in storage granules

Human insulin is a pivotal protein hormone controlling metabolism, growth, and aging and whose malfunctioning underlies diabetes, some cancers, and neurodegeneration. Despite its central position in human physiology, the in vivo oligomeric state and conformation of insulin in its storage granules in the pancreas are not known. In contrast, many in vitro structures of hexamers of this hormone are available and fall into three conformational states: T6, T3Rf3, and R6 As there is strong evidence for accumulation of neurotransmitters, such as serotonin and dopamine, in insulin storage granules in pancreatic β-cells, we probed by molecular dynamics (MD) and protein crystallography (PC) if these endogenous ligands affect and stabilize insulin oligomers. Parallel studies independently converged on the observation that serotonin binds well within the insulin hexamer (site I), stabilizing it in the T3R3 conformation. Both methods indicated serotonin binding on the hexamer surface (site III) as well. MD, but not PC, indicated that dopamine was also a good site III ligand. Some of the PC studies also included arginine, which may be abundant in insulin granules upon processing of pro-insulin, and stable T3R3 hexamers loaded with both serotonin and arginine were obtained. The MD and PC results were supported further by in solution spectroscopic studies with R-state-specific chromophore. Our results indicate that the T3R3 oligomer is a plausible insulin pancreatic storage form, resulting from its complex interplay with neurotransmitters, and pro-insulin processing products. These findings may have implications for clinical insulin formulations.

Drastic acceleration of fibrillation of insulin by transient cavitation bubble

Amyloid-fibril formation of proteins can be accelerated by ultrasonic irradiation to the peptide solutions. Although this phenomenon contributes to understanding pathogenic behavior of amyloidosis, its physical mechanism has not been clarified, because several factors (cavitation, temperature increase, stirring effect, and so on) related to ultrasonic irradiation can participate in the fibrillation reaction. Here, we independently study contributions of the possible factors, using insulin, which is extremely stable and then suitable for the mechanism clarification. We find that the optimized ultrasonic irradiation can drastically accelerate the fibrillation reaction; the time for completing the reaction is shortened compared with the high-speed (1200rpm) stirring agitation by a factor of 430. The fibrillation reaction proceeds only when the subharmonic-mode intensity exceeds a threshold, indicating generation of the transient cavitation bubbles. Our results reveal that not the temperature increase but the transient cavitation bubbles work as the dominant accelerator of the fibrillation reaction.

pH-Driven Polymorphism of Insulin Amyloid-Like Fibrils

Prions are infective proteins, which can self-assemble into different strain conformations, leading to different disease phenotypes. An increasing number of studies suggest that prion-like self-propagation may be a common feature of amyloid-like structures. Thus it is important to unravel every possible factor leading to the formation of different amyloid strains. Here we report on the formation of two types of insulin amyloid-like fibrils with distinct infrared spectroscopic features grown under slightly different pH conditions. Similar to prion strains, both insulin fibril types are able to self-propagate their conformational template under conditions, favoring spontaneous formation of different type fibrils. The low-pH-induced insulin amyloid strain is structurally very similar to previously reported strains formed either in the presence of 20% ethanol, or by modification of the amino acid sequence of insulin. A deeper analysis of literature data in the context of our current findings suggests a shift of the monomer-dimer equilibrium of insulin as a possible factor controlling the formation of different strains.

Amyloid fibrillation of insulin under water-limited conditions

Amyloid fibrillation in water-organic mixtures has been widely studied to understand the effect of protein-solvent interactions on the fibrillation process. In this study, we monitored insulin fibrillation in formamide and its methyl derivatives (formamide, N-methyl formamide, N,N-dimethyl formamide) in the presence and absence of water. These model solvent systems mimic the cellular environment by providing denaturing conditions and a hydrophobic environment with limited water content. Thioflavin T (ThT) assay revealed that binary mixtures of water with formamide and its methyl derivatives enhanced fibrillation rates and ?-sheet abundance, whereas organic solvents suppressed insulin fibrillation. We utilized solution small-angle x-ray scattering (SAXS) and differential scanning calorimetry (DSC) to investigate the correlation between protein-solvent interactions and insulin fibrillation. SAXS experiments combined with simulated annealing of the protein indicated that the degree of denaturation of the hydrophobic core region at residues B11-B17 determines the fibrillation rate. In addition, DSC experiments suggested a crucial role of hydrophobic interactions in the fibrillation process. These results imply that an environment with limited water, which imitates a lipid membrane system, accelerates protein denaturation and the formation of intermolecular hydrophobic interactions during amyloid fibrillation.

Light scattering coupled with reversed phase chromatography to study protein self-association under separating conditions

An on-line method, coupling reversed phase chromatography with static light scattering, was developed to determine the association state of freshly eluted proteins. Under downstream process conditions, human insulin desB30 and human insulin AspB28 were tested at concentrations up to 8.5mg/mL. The refractive index increment (dn/dc) for insulin was found to depend strongly on the solvent used. A refractive index increment of 0.184±0.003mL/g was found in an aqueous buffer, pH 7.4, whereas the value was 0.155±0.003mL/g in 30%, w/w ethanol. The methodology combines on-line SLS and UV measurements with the pre-determined refractive index increment values. The developed on-line method was verified by standard off-line measurements establishing the association state at concentrations between 0.2 and 6.0mg/mL. The equipment was calibrated utilizing insulin under conditions reported to ensure either monomer or hexamer forms. The self-association of human insulin desB30 was found to be strongly suppressed in 30%, w/w ethanol at pH 7.4 in which the monomer predominates. When stabilized by zinc ions in 30%, w/w ethanol at pH 7.4, an average association number of 3.7 was found. These data demonstrate the effect of ethanol to lower strongly the energy advantage by protein self-association. Potassium chloride and/or calcium chloride in the eluents were found to be of no consequence to the association state.

Structural meta-analysis of regular human insulin in pharmaceutical formulations

We have studied regular acting, wild-type human insulin at potency of 100 U/mL from four different pharmaceutical products directly from their final finished formulation by the combined use of mass spectrometry (MS), dynamic light scattering (DLS), small-angle X-ray scattering (SAXS), nuclear magnetic resonance (NMR), and single-crystal protein crystallography (PX). All products showed similar oligomeric assembly in solution as judged by DLS and SAXS measurements. The NMR spectra were compatible with well folded proteins, showing close conformational identity for the human insulin in the four products. Crystallographic assays conducted with the final formulated products resulted in all insulin crystals belonging to the R3 space group with two a dimer in the asymmetric unit, both with the B-chain in the T configuration. Meta-analysis of the 24 crystal structures solved from the four distinct insulin products revealed close similarity between them regardless of variables such as biological origin, product batch, country origin of the product, and analytical approach, revealing a low conformational variability for the converging insulin structural ensemble. We propose the use of MS, SAXS, NMR fingerprint, and PX as a precise chemical and structural proof of folding identity of regular insulin in the final, formulated product.

Investigation into the mechanism of (-)-epigallocatechin-3-gallate-induced precipitation of insulin

The molecular interactions between EGCG and insulin were investigated to probe the mechanism of EGCG-induced insulin precipitation. The results indicated that 1-5mM EGCG induced insulin into reversible globular precipitates of 185-365 nm. The formation of precipitates was facilitated at high salt concentration and pH values close to insulin's isoelectric point, indicating that hydrophobic interaction was the main driving force. The precipitation was positively related to insulin concentration, but for EGCG, there was a suitable concentration (2 mM at 2 mg/mL of insulin) at which the precipitate content reached maximum. Mass spectroscopy analysis indicated that EGCG formed clusters in the aqueous solution and the clusters correlate with the insulin precipitation. Based on extensive investigation, a physical model was proposed to explain the molecular interactions between EGCG and insulin. Namely, EGCG monomers and clusters first bound to insulin dimers via hydrophobic interaction, leading to the reduction of the thickness of the hydration layer and the partial denaturation of insulin. Then, EGCG clusters acted as bridges to induce the aggregation and precipitation of insulin.

Multimerization and aggregation of native-state insulin: effect of zinc

The aggregation of insulin is complicated by the coexistence of various multimers, especially in the presence of Zn(2+). Most investigations of insulin multimerization tend to overlook aggregation kinetics, while studies of insulin aggregation generally pay little attention to multimerization. A clear understanding of the starting multimer state of insulin is necessary for the elucidation of its aggregation mechanism. In this work, the native-state aggregation of insulin as either the Zn-insulin hexamer or the Zn-free dimer was studied by turbidimetry and dynamic light scattering, at low ionic strength and pH near pI. The two states were achieved by varying the Zn(2+) content of insulin at low concentrations, in accordance with size-exclusion chromatography results and literature findings (Tantipolphan, R.; Romeijn, S.; Engelsman, J. d.; Torosantucci, R.; Rasmussen, T.; Jiskoot, W. J. Pharm. Biomed. 2010, 52, 195). The much greater aggregation rate and limiting turbidity (τ(∞)) for the Zn-insulin hexamer relative to the Zn-free dimer was explained by their different aggregation mechanisms. Sequential first-order kinetic regimes and the concentration dependence of τ(∞) for the Zn-insulin hexamer indicate a nucleation and growth mechanism, as proposed by Wang and Kurganov (Wang, K.; Kurganov, B. I. Biophys. Chem. 2003, 106, 97). The pure second-order process for the Zn-free dimer suggests isodesmic aggregation, consistent with the literature. The aggregation behavior at an intermediate Zn(2+) concentration appears to be the sum of the two processes.

Conformational dynamics of insulin

We have exploited a prandial insulin analog to elucidate the underlying structure and dynamics of insulin as a monomer in solution. A model was provided by insulin lispro (the active component of Humalog(®); Eli Lilly and Co.). Whereas NMR-based modeling recapitulated structural relationships of insulin crystals (T-state protomers), dynamic anomalies were revealed by amide-proton exchange kinetics in D(2)O. Surprisingly, the majority of hydrogen bonds observed in crystal structures are only transiently maintained in solution, including key T-state-specific inter-chain contacts. Long-lived hydrogen bonds (as defined by global exchange kinetics) exist only at a subset of four α-helical sites (two per chain) flanking an internal disulfide bridge (cystine A20-B19); these sites map within the proposed folding nucleus of proinsulin. The anomalous flexibility of insulin otherwise spans its active surface and may facilitate receptor binding. Because conformational fluctuations promote the degradation of pharmaceutical formulations, we envisage that "dynamic re-engineering" of insulin may enable design of ultra-stable formulations for humanitarian use in the developing world.

Cyclic voltammetry: a new strategy for the evaluation of oxidative damage to bovine insulin

Research on protein oxidative damage may give insight into the nature of protein functions and pathological conditions. In this work, the oxidative damage of bovine insulin on Au electrode was investigated by cyclic voltammetry (CV). The experimental results show that there are two anodic peaks for the oxidative damage of bovine insulin, which arise from the oxidation of the exposed disulfide bond S--S(CYS7A,CYS7B), forming sulfenic acid RSOH (1.20 V, vs. SCE), sulfinic acid RSO(2)H and sulfonic acid RSO(3)H (1.35 V, vs. SCE). These in vitro findings not only demonstrate the applicability of CV in simulating/evaluating the oxidative damage of nonredox proteins but also find two promising candidates (two anodic peaks) for measuring insulin.

Tyrosine autofluorescence as a measure of bovine insulin fibrillation

The traditional approach to investigating the partial unfolding and fibrillation of insulin, and proteins at large, has involved use of the dyes 1-anilinonaphthalene-8-sulphonic acid (ANS) and Thioflavin T (ThT), respectively. We compare the kinetic profiles of ThT, ANS, light scattering, and intrinsic Tyr fluorescence during insulin fibrillation. The data reveal that the sequence of structural changes (dimers --> monomers --> partially unfolded monomers --> oligomeric aggregates --> fibrils) accompanying insulin fibrillation can be detected directly using intrinsic Tyr fluorescence. The results indicate that at least two distinguishable structural intermediates precede fibril development. There is no evidence of tyrosinate or dityrosine during insulin aggregation. Obtaining such critical information from the protein itself is complementary to existing aggregation probes and affords the advantage of directly examining structural changes that occur at the molecular level, providing concrete details of the early events preceding fibrillation.

Molecular modeling of the misfolded insulin subunit and amyloid fibril

Insulin, a small hormone protein comprising 51 residues in two disulfide-linked polypeptide chains, adopts a predominantly alpha-helical conformation in its native state. It readily undergoes protein misfolding and aggregates into amyloid fibrils under a variety of conditions. Insulin is a unique model system in which to study protein fibrillization, since its three disulfide bridges are retained in the fibrillar state and thus limit the conformational space available to the polypeptide chains during misfolding and fibrillization. Taking into account this unique conformational restriction, we modeled possible monomeric subunits of the insulin amyloid fibrils using beta-solenoid folds, namely, the beta-helix and beta-roll. Both models agreed with currently available biophysical data. We performed molecular dynamics simulations, which allowed some limited insights into the relative structural stability, suggesting that the beta-roll subunit model may be more stable than the beta-helix subunit model. We also constructed beta-solenoid-based insulin fibril models and conducted fiber diffraction simulation to identify plausible fibril architectures of insulin amyloid. A comparison of simulated fiber diffraction patterns of the fibril models to the experimental insulin x-ray fiber diffraction data suggests that the model fibers composed of six twisted beta-roll protofilaments provide the most reasonable fit to available experimental diffraction patterns and previous biophysical studies.

Primary steps of pH-dependent insulin aggregation kinetics are governed by conformational flexibility

Insulin aggregation critically depends on pH. The underlying energetic and structural determinants are, however, unknown. Here, we measure the kinetics of the primary aggregation steps of the insulin monomer in vitro and relate it to its conformational flexibility. To assess these primary steps the monomer concentration was monitored by mass spectrometry at various pH values and aggregation products were imaged by atomic force microscopy. Lowering the pH from 3 to 1.6 markedly accelerated the observed aggregation kinetics. The influence of pH on the monomer structure and dynamics in solution was studied by molecular dynamics simulations, with the protonation states of the titrable groups obtained from electrostatic calculations. Reduced flexibility was observed for low pH values, mainly in the C terminus and in the helix of the B chain; these corresponded to an estimated entropy loss of 150 J mol(-1) K(-1). The striking correlation between entropy loss and pH value is consistent with the observed kinetic traces. In analogy to the well-known Phi value analysis, this result allows the extraction of structural information about the rate determining transition state of the primary aggregation steps. In particular, we suggest that the residues in the helix of the B chain are involved in this transition state.

Electrochemical study of insulin at the polarized liquid-liquid interface

This paper reports on the electrochemical behavior of bovine insulin at the interface between two immiscible electrolyte solutions (ITIES). The voltammetric ion-transfer response obtained in the presence of insulin was dependent on the aqueous phase pH conditions and on the nature of the organic phase electrolyte employed in experiments. Optimal detection was obtained at acidic pH below the isoelectric point of insulin where it was positively charged. A shift in transfer potentials to lower potential values was observed with decreasing hydrophobicity of the anion of the organic phase electrolyte. No ion-transfer response was observed at pH values of the aqueous phase above the isoelectric point, where insulin was negatively charged. These results suggest that the voltammetric response is due to ion-pairing interactions at the ITIES between positively charged insulin and the hydrophobic anion of the organic phase electrolyte, together with adsorption of the ion-pair at the interface. The voltammetric response was obtained for insulin at concentrations down to 1 muM. These results show that electrochemistry is useful in studying the behavior of this important protein molecule at the polarized water-1,2-DCE interface and provides an alternative detection mode for bioanalytical applications.

Guiding protein aggregation with macromolecular crowding

Macromolecular crowding is expected to have a significant effect on protein aggregation. In the present study we analyzed the effect of macromolecular crowding on fibrillation of four proteins, bovine S-carboxymethyl-alpha-lactalbumin (a disordered form of the protein with reduced three out of four disulfide bridges), human insulin, bovine core histones, and human alpha-synuclein. These proteins are structurally different, varying from natively unfolded (alpha-synuclein and core histones) to folded proteins with rigid tertiary and quaternary structures (monomeric and hexameric forms of insulin). All these proteins are known to fibrillate in diluted solutions, however their aggregation mechanisms are very divers and some of them are able to form different aggregates in addition to fibrils. We studied how macromolecular crowding guides protein between different aggregation pathways by analyzing the effect of crowding agents on the aggregation patterns under the variety of conditions favoring different aggregated end products in diluted solutions.

Role of a novel excipient poly(ethylene glycol)-b-poly(L-histidine) in retention of physical stability of insulin at aqueous/organic interface

The aim of this study was to investigate whether a cationic polyelectrolyte, poly(ethylene glycol)-b-poly(L-histidine) diblock copolymer (PEG-polyHis), can stabilize insulin, at the aqueous/methylene chloride interface formed during the microencapsulation process. Insulin aggregation at this interface was monitored spectrophotometrically at 276 nm. The effects of protein concentration, pH of the aqueous medium, and the presence of poly(lactic-co-glycolic acid) (PLGA) in methylene chloride (MC) on insulin aggregation were observed. For the 2.0 mg/mL insulin solutions in phosphate buffer (PB), the effect of addition of Pluronic F-127 as a positive control and addition of PEG-polyHis as a novel excipient in PB was also evaluated at various insulin/polymeric excipient weight ratios. The conformation of insulin protected by PEG-polyHis and recovered after interfacial exposure was evaluated via circular dichroism (CD) spectroscopy. Greater loss in soluble insulin was observed with increasing insulin concentrations. pH 6.0 was selected for optimal ionic interactions between insulin and PEG-polyHis based on zeta potential and particle size studies. pH 4.5 and 7.4 (no ionic complexation between insulin and PEG-polyHis) were selected as controls to compare the stabilization effect of PEG-polyHis with that at pH 6.0. Incubation of PEG-polyHis with insulin at pH 6.0 drastically reduced protein aggregation, even in the presence of PLGA. PEG-polyHis and F-127 reduced insulin aggregation in noncomplexing pH conditions pointing to the role played by PEG in modulation of insulin adsorption at the interface. Far-UV (205-250 nm) CD study revealed negligible qualitative effects on soluble insulin's secondary structure after interfacial exposure. RP-HPLC and size-exclusion HPLC showed no deamidation of insulin or formation of soluble high molecular weight transformation products respectively. MALDI-TOF mass spectrometry confirmed the results from chromatographic procedures. Radioimmunoassay carried out on select samples showed that recovered soluble insulin had retained its immunoreactivity. An experimental method to simulate interfacial denaturation of proteins was designed for assessment of protein stability at the interface and screening for novel protein stabilizers. Understanding and manipulation of such polyelectrolyte-insulin complexation will likely play a role in insulin controlled delivery via microsphere formulation(s).

The component polypeptide chains of bovine insulin nucleate or inhibit aggregation of the parent protein in a conformation-dependent manner

Amyloid fibrils are typically rigid, unbranched structures with diameters of approximately 10 nm and lengths up to several micrometres, and are associated with more than 20 diseases including Alzheimer's disease and type II diabetes. Insulin is a small, predominantly alpha-helical protein consisting of 51 residues in two disulfide-linked polypeptide chains that readily assembles into amyloid fibrils under conditions of low pH and elevated temperature. We demonstrate here that both the A-chain and the B-chain of insulin are capable of forming amyloid fibrils in isolation under similar conditions, with fibrillar morphologies that differ from those composed of intact insulin. Both the A-chain and B-chain fibrils were found to be able to cross-seed the fibrillization of the parent protein, although these reactions were substantially less efficient than self-seeding with fibrils composed of full-length insulin. In both cases, the cross-seeded fibrils were morphologically distinct from the seeding material, but shared common characteristics with typical insulin fibrils, including a very similar helical repeat. The broader distribution of heights of the cross-seeded fibrils compared to typical insulin fibrils, however, indicates that their underlying protofilament hierarchy may be subtly different. In addition, and remarkably in view of this seeding behavior, the soluble forms of the A-chain and B-chain peptides were found to be capable of inhibiting insulin fibril formation. Studies using mass spectrometry suggest that this behavior might be attributable to complex formation between insulin and the A-chain and B-chain peptides. The finding that the same chemical form of a polypeptide chain in different physical states can either stimulate or inhibit the conversion of a protein into amyloid fibrils sheds new light on the mechanisms underlying fibril formation, fibril strain propagation and amyloid disease initiation and progression.

Amyloid-like Fibril Formation of Co-chaperonin GroES: Nucleation and Extension Prefer Different Degrees of Molecular Compactness

The molecular chaperone GroES, together with GroEL from Escherichia coli, is the best characterized protein of the molecular chaperone family. Here, we report on the in vitro formation of GroES amyloid-like fibrils and the mechanism of formation. When incubated for several weeks at neutral pH in the presence of the denaturant guanidine hydrochloride, GroES formed a typical amyloid fibril; unbranched, twisted, and extended filaments stainable by thioflavin T and Congo red. GroES fibril formation was accelerated by the addition of preformed fibril seeds, in accordance with a nucleation-extension mechanism. Interestingly, whereas the spontaneous formation of GroES fibrils was favored in the structural transition region of GroES dissociation/unfolding, the extension of fibrils from preformed fibril seeds was favored in the region corresponding to an expanded molecular state. We concluded that the two stages of GroES fibril formation prefer different molecular states of the same protein. The significance of this preference is discussed.

Conformational constraints for amyloid fibrillation: the importance of being unfolded

Recent reports give strong support to the idea that amyloid fibril formation and the subsequent development of protein deposition diseases originate from conformational changes in corresponding amyloidogenic proteins. In this review, recent findings are surveyed to illustrate that protein fibrillogenesis requires a partially folded conformation. This amyloidogenic conformation is relatively unfolded, and shares many structural properties with the pre-molten globule state, a partially folded intermediate frequently observed in the early stages of protein folding and under some equilibrium conditions. The inherent flexibility of such an intermediate is essential in allowing the conformational rearrangements necessary to form the core cross-beta structure of the amyloid fibril.

Rheological characterization of nephila spidroin solution

We report the results of an investigation into the rheology of solutions of natural spider silk dope (spinning solution). We demonstrate that dilute dope solutions showed only shear thinning as the shear rate increased while more concentrated solutions showed an initial shear thinning followed by a shear thickening and a subsequent decline in viscosity. The critical shear rate for shear thickening depended on dope concentration and was very low in concentrated solutions. This helps to explain how spiders are able to spin silk at very low draw rates and why they use a very concentrated dope solution. We also show that the optimum shear rate for shear thickening in moderately concentrated solutions occurred at pH 6.3 close to the observed pH at the distal end of the spider's spinning duct. Finally, we report that the addition of K(+) ions to dilute dope solutions produced a spontaneous formation of nanofibrils that subsequently aggregated and precipitated. This change was not seen after the addition of other common cations. Taken together, these observations support the hypothesis that the secretion of H(+) and K(+) by the spider's duct together with moderate strain rates produced during spinning induce a phase separation in the silk dope in which the silk protein (spidroin) molecules are converted into insoluble nanofibrils.

Conformational stability of adsorbed insulin studied with mass spectrometry and hydrogen exchange

A new method is described for direct monitoring of the conformational stability of proteins that are physically adsorbed from solution onto a solid substrate. The adsorption-induced conformational changes of insulin are studied using a combination of hydrogen/deuterium (H/D) exchange and matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry. The effect of the surface hydrophobicity on the adsorption-induced conformational changes in the insulin structure is probed by adsorbing insulin on a hydrophilic silica and a hydrophobic methylated silica surface before subjecting the insulin molecules to the isotopic exchange process. The present study describes the experimental procedure of this new application of MALDI. Results show that insulin is more highly and more irreversibly adsorbed to a hydrophobic methylated silica surface than to a hydrophilic silica surface. Hydrogen-exchange experiments clearly demonstrate that the strong interaction of insulin with the hydrophobic surface is accompanied by a strong increase in the H/D-exchange rates and thus in a reduction in the insulin native structural stability. In contrast, H/D-exchange rates of insulin are somewhat reduced upon adsorption on silica from solution.

The tetramer model: a new view of class II MHC molecules in antigenic presentation to T cells

Crystallographic studies suggest a plausible divalent interaction between T-cell receptor (TCR) and MHC class II molecules. In addition, biochemical data suggest that these divalent MHC molecules are preformed at the membrane of the antigen-presenting cell. The tetramer model is based on these preformed tetrameric class II molecules that can be loaded with identical or different peptides in their two grooves. This enables divalent class II molecules to deliver two different messages to T cell: 1) a two-peptide message, in which the tetramer with two identical peptides is able to cross-link two TCRs triggering full activation of a T cell. At the thymic level we propose that this message induces negative selection; or 2) a one-peptide message: only one of the peptides loaded in the class II tetramer is able to interact with that TCR. This message would be involved in triggering partial activation phenomena in mature lymphocytes, whereas in thymocytes this message would mediate positive selection. Since high concentrations of a peptide would favor the load of tetramers with identical peptides, the tetramer could therefore be viewed as a quantitative-qualitative transducer that would trigger different responses depending on the concentration of antigenic peptides.

Conformational effects in reversed-phase high-performance liquid chromatography of polypeptides. I. Resolution of insulin variants

In order to further characterise the role of conformation in the retention behaviour of polypeptides and proteins in reversed-phase high-performance liquid chromatography (RP-HPLC), the chromatographic properties of four different insulins have been studied as a function of temperature (over the range 5-85 degrees C) and column residence time (over the range 10-60 min). The role of the ligand structure was also investigated by comparing results obtained with a n-octadecyl (C18) and a n-butyl (C4) ligand immobilised to the same porous silica. Comparative structure-retention-stability relationships were determined from an examination of the influence of temperature on a number of chromatographic parameters including the chromatographic contact area, the affinity constant and the experimental band width. The results demonstrated that variations in temperature can be used to affect significant changes in selectivity between the different insulins despite their very high degree of sequence homology. These observations have permitted specific amino acid residues, and in particular those residues encompassing the region A8-A10, to be proposed to be directly involved in the chromatographic contact area of the insulin molecules. Overall, the analysis of the changes in various chromatographic parameters in response to variation of the amino acid sequence, temperature and other experimental parameters provides a powerful tool to elucidate the structural basis for the interfacial stability and the role of conformation on the retention behaviour of polypeptides and proteins in RP-HPLC.