pH dependent conformational changes in the T- and R-states of insulin in solution: circular dichroic studies in the pH range of 6 to 10

Common Motif at the Red Luminophore in Bovine Serum Albumin-, Ovalbumin-, Trypsin-, and Insulin-Gold Complexes

We examined the static and dynamic characters of the red luminescence in the protein-Au(III) compounds, directly comparing multiple proteins: BSA, OVA, trypsin, and insulin. These four protein-Au(III) complexes showed a nearly identical excitation-emission pattern, not only the wavelength of luminescence (λ_em ∼ 640 nm). Lifetimes of the red luminescence shared a common value of ∼300 ns. Kinetics of the luminophore formation was consistently described by a Langmuir-type chemisorption of Au(III) for these proteins, coinciding with the protein conformation change at pH ∼ 10. These observations and the protein structural analyses support that the red luminophore formation involves Au(III) coordination to a common motif within these proteins.

Stabilization of Deformable Nanovesicles Based on Insulin-Phospholipid Complex by Freeze-Drying

Deformable nanovesicles have been extensively investigated due to their excellent ability to penetrate biological barriers. However, suffering from serious physical and chemical instabilities, the wide use of deformable nanovesicles in medical applications is still limited. Moreover, far less work has been done to pursue the lyophilization of deformable nanovesicles. Here, we aimed to obtain stable deformable nanovesicles via freeze-drying technology and to uncover the underlying protection mechanisms. Firstly, the density of nanovesicles before freeze-drying, the effect of different kinds of cryoprotectants, and the types of different reconstituted solvents after lyophilization were investigated in detail to obtain stable deformable nanovesicles based on insulin-phospholipid complex (IPC-DNVs). To further investigate the underlying protection mechanisms, we performed a variety of analyses. We found that deformable nanovesicles at a low density containing 8% lactose and trehalose in a ratio of 1:4 (8%-L-T) have a spherical shape, smooth surface morphology in the lyophilized state, a whorl-like structure, high entrapment efficiency, and deformability after reconstitution. Importantly, the integrity of IPC, as well as the secondary structure of insulin, were well protected. Accelerated stability studies demonstrated that 8%-L-T remained highly stable during storage for 6 months at 25 °C. Based on in vivo results, lyophilized IPC-DNVs retained their bioactivity and had good efficacy. Given the convenience of preparation and long term stability, the use of combined cryoprotectants in a proper ratio to protect stable nanovesicles indicates strong potential for industrial production.

Mechanisms of deformable nanovesicles based on insulin-phospholipid complex for enhancing buccal delivery of insulin

BACKGROUND

Non-injectable delivery of peptides and proteins are not feasible due to its large molecular, high hydrophilic and gastrointestinal degradation. Therefore, proposing a new method to solve this problem is a burning issue.

PURPOSE

The objective of this study was to propose a novel protein delivery strategy to vanquish the poor efficacy of buccal mucosa delivery systems for protein delivery and then investigate the detailed mechanisms of the enhanced buccal delivery of protein, using insulin as a model drug.

MATERIALS AND METHODS

Insulin-phospholipid complex combined with deformable nanovesicles (IPC-DNVs) were prepared, using deformable nanovesicles based on insulin (INS-DNVs) and conventional nanovesicles based on insulin-phospholipid complex (IPC-NVs) as references. Besides, their physicochemical characterization, in vitro transport behavior, in vivo bioactivity and hypoglycemic effect were systematically characterized and compared. Finally, we evaluated the in vivo safety of IPC-DNVs.

RESULTS

First, IPC-DNVs increased insulin permeability through deposition of the IPC and deformability of the DNVs, which was revealed by an in vitro mucosal permeation study. Second, DNVs could act as a drug carrier and penetrate the mucosa to reach the receiver medium as intact nanovesicles, which was supported by the observation of intact nanovesicles in the receiver medium through transmission electron microscopy (TEM). Third, IPC-DNVs exhibited both transcellular and paracellular transport in the form of IPC and DNVs, respectively, which was proved by confocal laser scanning microscopy (CLSM). Unlike the other two formulations, IPC-DNVs exhibited a sustained mild hypoglycemic effect, with a relative bioavailability (Fp) of 15.53% (3.09% and 1.96% for INS-DNVs and IPC-NVs, respectively). Furthermore, buccal administration of IPC-DNVs resulted in no visible mucosal irritation to the buccal mucosa.

CONCLUSION

Our work reveals the mechanisms underlying the enhanced buccal delivery of IPC-DNVs: the DNVs facilitate penetration through the main barrier, and the deposition of IPC enhances buccal absorption. Our results and proposed mechanisms could be an important reference to understand other nanocarriers based on protein (peptide)-phospholipid complexes that penetrate the mucosa and provide a theoretical basis for the future development of buccal delivery systems for insulin.

Spectroscopic and molecular dynamics simulation studies of the interaction of insulin with glucose

The interaction between monomeric insulin and monosaccharides has been investigated through circular dichroism, fluorescence spectroscopy and two dimensional nuclear magnetic resonance. CD spectra indicate that D-glucose interacts with monomeric insulin whereas D-galactose, D-mannose and 2-deoxy-D-glucose have a lower effect. Fluorescence emission was quenched at sugar concentrations of 5-10 mM. Titration with the different sugars produces a quenching of the tyrosine spectrum from which a binding free energy value for the insulin-sugar complexes has been evaluated. Transfer nuclear Overhauser enhancement NMR experiments indicate the existence of dipolar interactions at short interatomic distances between C-1 proton of D-glucose in the beta form and the monomeric insulin. Further, NMR total correlation spectra experiments revealed that the hormone is in the monomeric form and that upon addition of glucose no aggregation occurs. The interaction does not involve relevant changes in the secondary structure of insulin suggesting that the interaction occur at the side chain level. Molecular dynamics simulations and modeling studies, based on the dynamic fluctuations of potential binding moiety sidechains, argued from results of NMR spectroscopy, provide additional informations to locate the putative binding sites of D-glucose to insulin.

Intracellular retention of newly synthesized insulin in yeast is caused by endoproteolytic processing in the Golgi complex

An insulin-containing fusion protein (ICFP, encoding the yeast prepro-alpha factor leader peptide fused via a lysine-arginine cleavage site to a single chain insulin) has been expressed in Saccharomyces cerevisiae where it is inefficiently secreted. Single gene disruptions have been identified that cause enhanced immunoreactive insulin secretion (eis). Five out of six eis mutants prove to be vacuolar protein sorting (vps)8, vps35, vps13, vps4, and vps36, which affect Golgi<-->endosome trafficking. Indeed, in wild-type yeast insulin is ultimately delivered to the vacuole, whereas vps mutants secrete primarily unprocessed ICFP. Disruption of KEX2, which blocks intracellular processing to insulin, quantitatively reroutes ICFP to the cell surface, whereas loss of the Vps10p sorting receptor is without effect. Secretion of unprocessed ICFP is not based on a dominant secretion signal in the alpha-leader peptide. Although insulin sorting mediated by Kex2p is saturable, Kex2p functions not as a sorting receptor but as a protease: replacement of Kex2p by truncated secretory Kex2p (which travels from Golgi to cell surface) still causes endoproteolytic processing and intracellular insulin retention. Endoproteolysis promotes a change in insulin's biophysical properties. B5His residues normally participate in multimeric insulin packing; a point mutation at this position permits ICFP processing but causes the majority of processed insulin to be secreted. The data argue that multimeric assembly consequent to endoproteolytic maturation regulates insulin sorting in the secretory pathway.