Can Arginine Inhibit Insulin Aggregation? A Combined Protein Crystallography, Capillary Electrophoresis, and Molecular Simulation Study

Insulin fibrillation under physicochemical parameters of bioprocessing and intervention by peptides and surface-active agents

Even after the centenary celebration of insulin discovery, there prevail challenges concerning insulin aggregation, not only after repeated administration but also during industrial production, storage, transport, and delivery, significantly impacting protein quality, efficacy, and effectiveness. The aggregation reduces insulin bioavailability, increasing the risk of heightened immunogenicity, posing a threat to patient health, and creating a dent in the golden success story of insulin therapy. Insulin experiences various physicochemical and mechanical stresses due to modulations in pH, temperature, ionic strength, agitation, shear, and surface chemistry, during the upstream and downstream bioprocessing, resulting in insulin unfolding and subsequent fibrillation. This has fueled research in the pharmaceutical industry and academia to unveil the mechanistic insights of insulin aggregation in an attempt to devise rational strategies to regulate this unwanted phenomenon. The present review briefly describes the impacts of environmental factors of bioprocessing on the stability of insulin and correlates with various intermolecular interactions, particularly hydrophobic and electrostatic forces. The aggregation-prone regions of insulin are identified and interrelated with biophysical changes during stress conditions. The quest for novel additives, surface-active agents, and bioderived peptides in decelerating insulin aggregation, which results in overall structural stability, is described. We hope this review will help tackle the real-world challenges of insulin aggregation encountered during bioprocessing, ensuring safer, stable, and globally accessible insulin for efficient management of diabetes.

Insulin Stabilization Designs for Enhanced Therapeutic Efficacy and Accessibility

Insulin has remained indispensable in the treatment of diabetes since it was first discovered in 1921. Unlike small molecular drugs, insulin and other protein drugs are prone to degradation when exposed to elevated temperatures, mechanical agitation during transportation, and prolonged storage periods. Therefore, strict cold-chain management is crucial for the insulin supply, requiring significant resources, which can limit the access to insulin, particularly in low-income areas. Moreover, although insulin formulations have advanced tremendously in the last century, insulin treatment still imposes a challenging regimen and provides suboptimal outcomes for the majority of patients. There is an increasing focus on pursuing improved pharmacology, specifically on safer, more user-friendly insulin therapies that minimize the self-management burden. These challenges underscore the need for developing novel insulin formulations with improved stability that are compatible with advanced insulin therapy. Insulin stabilization can be achieved through either chemical modification of insulin or formulation component design. Inspired by insulin-like peptides from invertebrates, we have developed novel stable insulin analogs based on a fundamental understanding of the insulin receptor engagement for insulin bioactivity. We created a novel four-disulfide insulin analog with high aggregation stability and potency by introducing a fourth disulfide bond between a C-terminal extended insulin A-chain and residues near the C-terminus of the B-chain. In an effort to stabilize insulin in its monomeric state to develop ultrafast-acting insulin with rapid absorption upon injection, we have developed a series of structurally miniaturized yet fully active insulin analogs that do not form dimers due to the lack of the canonical B-chain C-terminal octapeptide. Additionally, our study provided strategies for expanding the scope of cucurbit[7]uril (CB[7])-assisted insulin stabilization by engineering safe and biodegradable CB[7]-zwitterionic polypeptide excipients. We also explored insulin N-terminal substitution methods to achieve pH-dependent insulin stabilization without prolonging the duration of action. This Account describes our exploration of engineering stable insulin analogs and formulation design strategies for stabilizing insulin in aqueous solutions. Beyond conventional stabilization strategies for insulin injections, the unmet challenges and recent innovations in insulin stabilization are discussed, addressing the growing demand for alternative, less invasive routes of insulin administration. Additionally, we aim to provide a thorough overview of insulin stabilization from the perspective of commercially available insulin drugs and common pharmaceutical engineering practices in the industry. We also highlight unresolved insulin stabilization challenges and ongoing research strategies. We anticipate that further emphasis on collective efforts of protein engineering, pharmaceutical formulation design, and drug delivery will inform the development of stable and advanced insulin therapy.

Understanding the Specific Implications of Amino Acids in the Antibody Development

As the demand for immunotherapy to treat and manage cancers, infectious diseases and other disorders grows, a comprehensive understanding of amino acids and their intricate role in antibody engineering has become a prime requirement. Naturally produced antibodies may not have the most suitable amino acids at the complementarity determining regions (CDR) and framework regions, for therapeutic purposes. Therefore, to enhance the binding affinity and therapeutic properties of an antibody, the specific impact of certain amino acids on the antibody's architecture must be thoroughly studied. In antibody engineering, it is crucial to identify the key amino acid residues that significantly contribute to improving antibody properties. Therapeutic antibodies with higher binding affinity and improved functionality can be achieved through modifications or substitutions with highly suitable amino acid residues. Here, we have indicated the frequency of amino acids and their association with the binding free energy in CDRs. The review also analyzes the experimental outcome of two studies that reveal the frequency of amino acids in CDRs and provides their significant correlation between the outcomes. Additionally, it discusses the various bond interactions within the antibody structure and antigen binding. A detailed understanding of these amino acid properties should assist in the analysis of antibody sequences and structures needed for designing and enhancing the overall performance of therapeutic antibodies.

Mechanism of Protein Aggregation Inhibition by Arginine: Blockage of Anionic Side Chains Favors Unproductive Encounter Complexes

Aggregation refers to the assembly of proteins into nonphysiological higher order structures. While amyloid has been studied extensively, much less is known about amorphous aggregation, a process that interferes with protein expression and storage. Free arginine (Arg+) is a widely used aggregation inhibitor, but its mechanism remains elusive. Focusing on myoglobin (Mb), we recently applied atomistic molecular dynamics (MD) simulations for gaining detailed insights into amorphous aggregation (Ng J. Phys. Chem. B 2021, 125, 13099). Building on that approach, the current work for the first time demonstrates that MD simulations can directly elucidate aggregation inhibition mechanisms. Comparative simulations with and without Arg+ reproduced the experimental finding that Arg+ significantly decreased the Mb aggregation propensity. Our data reveal that, without Arg+, protein-protein encounter complexes readily form salt bridges and hydrophobic contacts, culminating in firmly linked dimeric aggregation nuclei. Arg+ promotes the dissociation of encounter complexes. These "unproductive" encounter complexes are favored because Arg+ binding to D- and E- lowers the tendency of these anionic residues to form interprotein salt bridges. Side chain blockage is mediated largely by the guanidinium group of Arg+, which binds carboxylates through H-bond-reinforced ionic contacts. Our MD data revealed Arg+ self-association into a dynamic quasi-infinite network, but we found no evidence that this self-association is important for protein aggregation inhibition. Instead, aggregation inhibition by Arg+ is similar to that mediated by free guanidinium ions. The computational strategy used here should be suitable for the rational design of aggregation inhibitors with enhanced potency.

Structural, kinetic, and thermodynamic aspects of insulin aggregation

Given the significance of protein aggregation in proteinopathies and the development of therapeutic protein pharmaceuticals, revamped interest in assessing and modelling the aggregation kinetics has been observed. Quantitative analysis of aggregation includes data of gradual monomeric depletion followed by the formation of subvisible particles. Kinetic and thermodynamic studies are essential to gain key insights into the aggregation process. Despite being the medical marvel in the world of diabetes, insulin suffers from the challenge of aggregation. Physicochemical stresses are experienced by insulin during industrial formulation, storage, delivery, and transport, considerably impacting product quality, efficacy, and effectiveness. The present review briefly describes the pathways, mathematical kinetic models, and thermodynamics of protein misfolding and aggregation. With a specific focus on insulin, further discussions include the structural heterogeneity and modifications of the intermediates incurred during insulin fibrillation. Finally, different model equations to fit the kinetic data of insulin fibrillation are discussed. We believe that this review will shed light on the conditions that induce structural changes in insulin during the lag phase of fibrillation and will motivate scientists to devise strategies to block the initialization of the aggregation cascade. Subsequent abrogation of insulin fibrillation during bioprocessing will ensure stable and globally accessible insulin for efficient management of diabetes.

The Effect of Arginine on the Phase Stability of Aqueous Hen Egg-White Lysozyme Solutions

The effect of arginine on the phase stability of the hen egg-white lysozyme (HEWL) has been studied via molecular dynamics computer simulations, as well as experimentally via cloud-point temperature determination. The experiments show that the addition of arginine increases the stability of the HEWL solutions. The computer simulation results indicate that arginine molecules tend to self-associate. If arginine residues are located on the protein surface, the free arginine molecules stay in their vicinity and prevent the way protein molecules "connect" through them to form clusters. The results are not sensitive to a particular force field and suggest a possible microscopic mechanism of the stabilizing role of arginine as an excipient.

Characterization of insulin crystalline form in isolated β-cell secretory granules

Insulin is stored in vivo inside the pancreatic β-cell insulin secretory granules. In vitro studies have led to an assumption that high insulin and Zn²⁺ concentrations inside the pancreatic β-cell insulin secretory granules should promote insulin crystalline state in the form of Zn²⁺-stabilized hexamers. Electron microscopic images of thin sections of the pancreatic β-cells often show a dense, regular pattern core, suggesting the presence of insulin crystals. However, the structural features of the storage forms of insulin in native preparations of secretory granules are unknown, because of their small size, fragile character and difficult handling. We isolated and investigated the secretory granules from MIN6 cells under near-native conditions, using cryo-electron microscopic (Cryo-EM) techniques. The analysis of these data from multiple intra-granular crystals revealed two different rhomboidal crystal lattices. The minor lattice has unit cell parameters (a ≃ b ≃ 84.0 Å, c ≃ 35.2 Å), similar to in vitro crystallized human 4Zn²⁺-insulin hexamer, whereas the largely prevalent unit cell has more than double c-axis (a ≃ b ≃ c ≃ 96.5 Å) that probably corresponds to two or three insulin hexamers in the asymmetric unit. Our experimental data show that insulin can be present in pancreatic MIN6 cell granules in a microcrystalline form, probably consisting of 4Zn²⁺-hexamers of this hormone.

Formation of Protamine and Zn-Insulin Assembly: Exploring Biophysical Consequences

The insulin-protamine interaction is at the core of the mode of action in many insulin formulations (Zn + insulin + protamine) and to treat diabetes, in which protamine is added to the stable form of hexameric insulin (Zn-insulin). However, due to the unavailability of quantitative data and a high-resolution structure, the binding mechanism of the insulin-protamine complex remains unknown. In this study, it was observed that Zn-insulin experiences destabilization as observed by the loss of secondary structure in circular dichroism (CD), and reduction in thermal stability in melting study, upon protamine binding. In isothermal titration calorimetry (ITC), it was found that the interactions were mostly enthalpically driven. This is in line with the positive ΔC _m value (+880 cal mol^-1), indicating the role of hydrophilic interactions in the complex formation, with the exposure of hydrophobic residues to the solvent, which was firmly supported by the 8-anilino-1-naphthalene sulfonate (ANS) binding study. The stoichiometry (N) value in ITC suggests the multiple insulin molecules binding to the protamine chain, which is consistent with the picture of the condensation of insulin in the presence of protamine. Atomic force microscopy (AFM) suggested the formation of a heterogeneous Zn-insulin-protamine complex. In fluorescence, Zn-insulin experiences strong Tyr quenching, suggesting that the location of the protamine-binding site is near Tyr, which is also supported by the molecular docking study. Since Tyr is critical in the stabilization of insulin self-assembly, its interaction with protamine may impair insulin's self-association ability and thermodynamic stability while at the same time promoting its flexible conformation desired for better biological activity.

Progress in Simulation Studies of Insulin Structure and Function

Insulin is a peptide hormone known for chiefly regulating glucose level in blood among several other metabolic processes. Insulin remains the most effective drug for treating diabetes mellitus. Insulin is synthesized in the pancreatic β-cells where it exists in a compact hexameric architecture although its biologically active form is monomeric. Insulin exhibits a sequence of conformational variations during the transition from the hexamer state to its biologically-active monomer state. The structural transitions and the mechanism of action of insulin have been investigated using several experimental and computational methods. This review primarily highlights the contributions of molecular dynamics (MD) simulations in elucidating the atomic-level details of conformational dynamics in insulin, where the structure of the hormone has been probed as a monomer, dimer, and hexamer. The effect of solvent, pH, temperature, and pressure have been probed at the microscopic scale. Given the focus of this review on the structure of the hormone, simulation studies involving interactions between the hormone and its receptor are only briefly highlighted, and studies on other related peptides (e.g., insulin-like growth factors) are not discussed. However, the review highlights conformational dynamics underlying the activities of reported insulin analogs and mimetics. The future prospects for computational methods in developing promising synthetic insulin analogs are also briefly highlighted.

Influence of Aqueous Arginine Solution on Regulating Conformational Stability and Hydration Properties of the Secondary Structural Segments of a Protein at Elevated Temperatures: A Molecular Dynamics Study

The effects of aqueous arginine solution on the conformational stability of the secondary structural segments of a globular protein, ubiquitin, and the structure and dynamics of the surrounding water and arginine were examined by performing atomistic molecular dynamics (MD) simulations. Attempts have been made to identify the osmolytic efficacy of arginine solution, and its influence in guiding the hydration properties of the protein at an elevated temperature of 450 K. The similar properties of the protein in pure water at elevated temperatures were computed and compared. Replica exchange MD simulation was performed to explore the arginine solution's sensitivity in stabilizing the protein conformations for a wide range of temperatures (300-450 K). It was observed that although all the helices and strands of the protein undergo unfolding at elevated temperature in pure water, they exhibited native-like conformational dynamics in the presence of arginine at both ambient and elevated temperatures. We find that the higher free energy barrier between the folded native and unfolded states of the protein primarily arises from the structural transformation of α-helix, relative to the strands. Our study revealed that the water structure around the secondary segments depends on the nature of amino acid compositions of the helices and strands. The reorientation of water dipoles around the helices and strands was found hindered due to the presence of arginine in the solution; such hindrance reduces the possibility of exchange of hydrogen bonds that formed between the secondary segments of protein and water (PW), and as a result, PW hydrogen bonds take longer time to relax than in pure water. On the other hand, the origin of slow relaxation of protein-arginine (PA) hydrogen bonds was identified to be due to the presence of different types of protein-bound arginine molecules, where arginine interacts with the secondary structural segments of the protein through multiple/bifurcated hydrogen bonds. These protein-bound arginine formed different kinds of bridged PA hydrogen bonds between amino acid residues of the same secondary segments or among multiple bonds and helped protein to conserve its native folded form firmly.

Rapid-acting insulin analogues: Theory and best clinical practice in type 1 and type 2 diabetes

Since the discovery of insulin 100 years ago, insulin preparations have improved significantly. Starting from purified animal insulins, evolving to human insulins produced by genetically modified organisms, and ultimately to insulin analogues, all in an attempt to mimic physiological insulin action profiles seen in individuals without diabetes. Achieving strict glucose control without hypoglycaemia and preventing chronic complications of diabetes while preserving quality of life remains a challenging goal, but the advent of newer ultra-rapid-acting insulin analogues may enable intensive insulin therapy without being too disruptive to daily life. Ultra-rapid-acting insulin analogues can be administered shortly before meals and give better coverage of mealtime-induced glucose excursions than conventional insulin preparations. They also increase convenience with timing of bolus dosing. In this review, we focus on the progress that has been made in rapid-acting insulins. We summarize pharmacokinetic and pharmacodynamic data, clinical trial data supporting the use of these new formulations as part of a basal-bolus regimen and continuous subcutaneous insulin infusion, and provide a clinical perspective to help guide healthcare professionals when and for whom to use ultra-fast-acting insulins.

Arginine and Arginine-Rich Peptides as Modulators of Protein Aggregation and Cytotoxicity Associated With Alzheimer's Disease

A substantial body of evidence indicates cationic, arginine-rich peptides (CARPs) are effective therapeutic compounds for a range of neurodegenerative pathologies, with beneficial effects including the reduction of excitotoxic cell death and mitochondrial dysfunction. CARPs, therefore, represent an emergent class of promising neurotherapeutics with multimodal mechanisms of action. Arginine itself is a known chaotrope, able to prevent misfolding and aggregation of proteins. The putative role of proteopathies in chronic neurodegenerative diseases such as Alzheimer's disease (AD) warrants investigation into whether CARPs could also prevent the aggregation and cytotoxicity of amyloidogenic proteins, particularly amyloid-beta and tau. While monomeric arginine is well-established as an inhibitor of protein aggregation in solution, no studies have comprehensively discussed the anti-aggregatory properties of arginine and CARPs on proteins associated with neurodegenerative disease. Here, we review the structural, physicochemical, and self-associative properties of arginine and the guanidinium moiety, to explore the mechanisms underlying the modulation of protein aggregation by monomeric and multimeric arginine molecules. Arginine-rich peptide-based inhibitors of amyloid-beta and tau aggregation are discussed, as well as further modulatory roles which could reduce proteopathic cytotoxicity, in the context of therapeutic development for AD.

Molecular computations of preferential interactions of proline, arginine.HCl, and NaCl with IgG1 antibodies and their impact on aggregation and viscosity

Preferential interactions of excipients with the antibody surface govern their effect on the stability of antibodies in solution. We probed the preferential interactions of proline, arginine.HCl (Arg.HCl), and NaCl with three therapeutically relevant IgG1 antibodies via experiment and simulation. With simulations, we examined how excipients interacted with different types of surface patches in the variable region (Fv). For example, proline interacted most strongly with aromatic surfaces, Arg.HCl was included near negative residues, and NaCl was excluded from negative residues and certain hydrophobic regions. The differences in interaction of different excipients with the same surface patch on an antibody may be responsible for variations in the antibody's aggregation, viscosity, and self-association behaviors in each excipient. Proline reduced self-association for all three antibodies and reduced aggregation for the antibody with an association-limited aggregation mechanism. The effects of Arg.HCl and NaCl on aggregation and viscosity were highly dependent on the surface charge distribution and the extent of exclusion from highly hydrophobic patches. At pH 5.5, both tended to increase the aggregation of an antibody with a strongly positive charge on the Fv, while only NaCl reduced the aggregation of the antibody with a large negative charge patch on the Fv. Arg.HCl reduced the viscosities of antibodies with either a hydrophobicity-driven mechanism or a charge-driven mechanism. Analysis of this data presents a framework for understanding how amino acid and ionic excipients interact with different protein surfaces, and how these interactions translate to the observed stability behavior.

A tale of two tails: Self-assembling properties of A- and B-chain parts of insulin's highly amyloidogenic H-fragment

Due to the spontaneous transition of native insulin into therapeutically inactive amyloid, prolonged storage decreases effectiveness of the hormone in treatment of diabetes. Various regions of the amino acid sequence have been implicated in insulin aggregation. Here, we focus on smaller fragments of the highly amyloidogenic H-peptide comprising disulfide-bonded N-terminal sections of insulin's A-chain (13 residues) and B-chain (11 residues). Aggregation patterns of N-terminal fragments of A-chain (ACC_1-13, ACC_1-11, ACC_6-13, ACC_6-11, all retaining Cys6A-Cys11A disulfide bond) and B-chain (B_1-11(7A)) are examined at acidic and neutral pH. ACC_1-11 is the smallest fragment found to be amyloidogenic at either pH; removal of the N-terminal GIVEQ section renders this fragment entirely non-amyloidogenic. The self-assembling properties of ACC_1-11 contrast with aggregation-resistant behavior of B_1-11(7A) and its disulfide-linked homodimer, (B_1-11)₂ aggregating only at neutral pH. Fibrillar ACC_1-11 is similar to insulin amyloid in terms of morphology and infrared features. Secondary nucleation is likely to account for the detected shortening of insulin aggregation lag phase at neutral pH upon cross-seeding with pre-formed fibrils of ACC_1-11 or (B_1-11)₂. An aggregation-enhancing effect of monomeric ACC_1-11 on co-dissolved native insulin is also observed. Our findings are discussed in the context of mechanisms of insulin aggregation.

A radioligand receptor binding assay for measuring of insulin secreted by MIN6 cells after stimulation with glucose, arginine, ornithine, dopamine, and serotonin

We adapted a radioligand receptor binding assay for measuring insulin levels in unknown samples. The assay enables rapid and accurate determination of insulin concentrations in experimental samples, such as from insulin-secreting cells. The principle of the method is based on the binding competition of insulin in a measured sample with a radiolabeled insulin for insulin receptor (IR) in IM-9 cells. Both key components, radiolabeled insulin and IM-9 cells, are commercially available. The IR binding assay was used to determine unknown amounts of insulin secreted by MIN6 β cell line after stimulation with glucose, arginine, ornithine, dopamine, and serotonin. The experimental data obtained by the IR binding assay were compared to the results determined by RIA kits and both methods showed a very good agreement of results. We observed the stimulation of glucose-induced insulin secretion from MIN6 cells by arginine, weaker stimulation by ornithine, but inhibitory effects of dopamine. Serotonin effects were either stimulatory or inhibitory, depending on the concentration of serotonin used. The results will require further investigation. The study also clearly revealed advantages of the IR binding assay that allows the measuring of a higher throughput of measured samples, with a broader range of concentrations than in the case of RIA kits. The IR binding assay can provide an alternative to standard RIA and ELISA assays for the determination of insulin levels in experimental samples and can be especially useful in scientific laboratories studying insulin production and secretion by β cells and searching for new modulators of insulin secretion.

Identifying hydrophobic protein patches to inform protein interaction interfaces

Interactions between proteins lie at the heart of numerous biological processes and are essential for the proper functioning of the cell. Although the importance of hydrophobic residues in driving protein interactions is universally accepted, a characterization of protein hydrophobicity, which informs its interactions, has remained elusive. The challenge lies in capturing the collective response of the protein hydration waters to the nanoscale chemical and topographical protein patterns, which determine protein hydrophobicity. To address this challenge, here, we employ specialized molecular simulations wherein water molecules are systematically displaced from the protein hydration shell; by identifying protein regions that relinquish their waters more readily than others, we are then able to uncover the most hydrophobic protein patches. Surprisingly, such patches contain a large fraction of polar/charged atoms and have chemical compositions that are similar to the more hydrophilic protein patches. Importantly, we also find a striking correspondence between the most hydrophobic protein patches and regions that mediate protein interactions. Our work thus establishes a computational framework for characterizing the emergent hydrophobicity of amphiphilic solutes, such as proteins, which display nanoscale heterogeneity, and for uncovering their interaction interfaces.

Fast-Acting Insulin Aspart: A Review of its Pharmacokinetic and Pharmacodynamic Properties and the Clinical Consequences

Fast-acting insulin aspart (faster aspart) is insulin aspart (IAsp) with two added excipients, l-arginine and niacinamide, to ensure formulation stability with accelerated initial absorption after subcutaneous administration compared with previously developed rapid-acting insulins. The pharmacokinetic/pharmacodynamic properties of faster aspart have been characterised in clinical pharmacology trials with comparable overall methodology. In subjects with type 1 (T1D) or type 2 (T2D) diabetes, the serum IAsp concentration–time and glucose-lowering effect profiles are left-shifted for faster aspart compared with IAsp. In addition, faster aspart provides earlier onset, doubling of initial exposure, and an up to 2.5-fold increase in initial glucose-lowering effect within 30 min of subcutaneous injection, as well as earlier offset of exposure and effect. Similar results have been shown using continuous subcutaneous insulin infusion (CSII). The improved pharmacological properties of faster aspart versus IAsp are consistent across populations, i.e. in the elderly, children, adolescents and the Japanese. Thus, the faster aspart pharmacological characteristics more closely resemble the mealtime insulin secretion in healthy individuals, giving faster aspart the potential to further improve postprandial glucose control in subjects with diabetes. Indeed, change from baseline in 1-h postprandial glucose increment is in favour of faster aspart versus IAsp when used as basal-bolus or CSII treatment in phase III trials in subjects with T1D or T2D. This review summarises the currently published results from clinical pharmacology trials with faster aspart and discusses the potential clinical benefits of faster aspart compared with previous rapid-acting insulin products.

Analyzing the driving forces of insulin stability in the basic amino acid solutions: A perspective from hydration dynamics

Amino acids having basic side chains, as additives, are known to increase the stability of native-folded state of proteins, but their relative efficiency and the molecular mechanism are still controversial and obscure as well. In the present work, extensive atomistic molecular dynamics simulations were performed to investigate the hydration properties of aqueous solutions of concentrated arginine, histidine, and lysine and their comparative efficiency on regulating the conformational stability of the insulin monomer. We identified that in the aqueous solutions of the free amino acids, the nonuniform relaxation of amino acid-water hydrogen bonds was due to the entrapment of water molecules within the amino acid clusters formed in solutions. Insulin, when tested with these solutions, was found to show rigid conformations, relative to that in pure water. We observed that while the salt bridges formed by the lysine as an additive contributed more toward the direct interactions with insulin, the cation-π was more prominent for the insulin-arginine interactions. Importantly, it was observed that the preferentially more excluded arginine, compared to histidine and lysine from the insulin surface, enriches the hydration layer of the protein. Our study reveals that the loss of configurational entropy of insulin in arginine solution, as compared to that in pure water, is more as compared to the entropy loss in the other two amino acid solutions, which, moreover, was found to be due to the presence of motionally bound less entropic hydration water of insulin in arginine solution than in histidine or lysine solution.

Enhancing the crystallisation of insulin using amino acids as soft-templates to control nucleation

Amino acid as soft templates in promoting nucleation of insulin.

The efficiency of insulin production and its content in insulin-expressing model β-cells correlate with their Zn2+ levels

Insulin is produced and stored inside the pancreatic β-cell secretory granules, where it is assumed to form Zn2+-stabilized oligomers. However, the actual storage forms of this hormone and the impact of zinc ions on insulin production in vivo are not known. Our initial X-ray fluorescence experiment on granules from native Langerhans islets and insulinoma-derived INS-1E cells revealed a considerable difference in the zinc content. This led our further investigation to evaluate the impact of the intra-granular Zn2+ levels on the production and storage of insulin in different model β-cells. Here, we systematically compared zinc and insulin contents in the permanent INS-1E and BRIN-BD11 β-cells and in the native rat pancreatic islets by flow cytometry, confocal microscopy, immunoblotting, specific messenger RNA (mRNA) and total insulin analysis. These studies revealed an impaired insulin production in the permanent β-cell lines with the diminished intracellular zinc content. The drop in insulin and Zn2+ levels was paralleled by a lower expression of ZnT8 zinc transporter mRNA and hampered proinsulin processing/folding in both permanent cell lines. To summarize, we showed that the disruption of zinc homeostasis in the model β-cells correlated with their impaired insulin and ZnT8 production. This indicates a need for in-depth fundamental research about the role of zinc in insulin production and storage.

Insights into the Sensitivity of Arginine Concentration to Preserve the Folded Form of Insulin Monomer under Thermal Stress

Arginine, although popularly known as aggregation suppressor additive, has been found to quench proteins' structure and function by destabilizing their conformations. Driven by such controversial evidence, in this work we performed a series of atomistic molecular dynamics simulations of insulin monomer, a biologically active hormone protein, in arginine solution of varying concentrations (0.5, 1, and 2 M) at ambient and elevated temperature (400 K) to explore the arginine concentration driven structure-based stability of the protein. Our study reveals that the flexibility of the protein's structure is dependent on the arginine concentration, and among all the used solutions, 2 M arginine, a "neutral crowder" that mimics the cellular environment, can preserve the native folded form of the protein at ambient temperature in an excellent manner. Further, while the protein unfolds at 400 K in pure water, this solution worked satisfactorily to preserve the protein's folded conformation more firmly than the other solutions. The replica-exchange MD of insulin in 2 M arginine solution further supports the fact. In this aspect an important issue in molecular pharmacology is to identify and recognize the physical origin of the stability of a protein, i.e, in this case, how arginine directs the conformational flexibility of the protein and preserves its native folded form. We identified that the exclusion of arginine from the protein surface increases the local structuration of water around the protein, thereby preserving its "biological water" layer, and makes the protein more hydrated at 2 M concentration as compared to the other arginine solutions. Additionally, our microscopic investigation on the interactions of the protein-solvation layer revealed that the structural heterogeneity of the protein surface, arising from the differential physicochemical nature of the amino acid residues, controls the favorable formation of sluggish water-arginine mixed solvation layer at higher arginine concentration that helps the protein to maintain its structural rigidity. Importantly, apart from the protein-solvent hydrogen-bonding interactions, the anion-pi interactions, established between the carboxyl group of arginine and the aromatic amino acid residues of insulin, were recognized to facilitate the protein to maintain its native folded form at the experimental temperatures.

Silica-Coated Nanoparticles with a Core of Zinc, l-Arginine, and a Peptide Designed for Oral Delivery

Nanoparticle constructs for oral peptide delivery at a minimum must protect and present the peptide at the small intestinal epithelium in order to achieve oral bioavailability. In a reproducible, scalable, surfactant-free process, a core was formed with insulin in ratios with two established excipients and stabilizers, zinc chloride and l-arginine. Cross-linking was achieved with silica, which formed an outer shell. The process was reproducible across several batches, and physicochemical characterization of a single batch was confirmed in two independent laboratories. The silica-coated nanoparticles (SiNPs) entrapped insulin with high entrapment efficiency, preserved its structure, and released it at a pH value present in the small intestine. The SiNP delivered insulin to the circulation and reduced plasma glucose in a rat jejunal instillation model. The delivery mechanism required residual l-arginine in the particle to act as a permeation enhancer for SiNP-released insulin in the jejunum. The synthetic process was varied in terms of ratios of zinc chloride and l-arginine in the core to entrap the glucagon-like peptide 1 analogue, exenatide, and bovine serum albumin. SiNP-delivered exenatide was also bioactive in mice to some extent following oral gavage. The process is the basis for a platform for oral peptide and protein delivery.