Insulin solubility transitions by pH‐dependent interactions with proinsulin C‐peptide

Abstract

Proinsulin processing into insulin and C-peptide in the secretory granules of the pancreatic b-cells occurs under mildly acidic conditions and at high peptide concentrations (> 10 mM). Mature insulin has reduced solubility and a propensity to adopt an amyloid-like structure, but is physiologically released as a mixture of a zinc-containing core and a zinc-free, C-peptiderich fluid phase. C-peptide is known to function in the insulin secretion, but its exact mode of interaction is not established. We now demonstrate that C-peptide in sub-stoichiometric amount versus insulin coprecipitates with insulin at the pH found in secretory vesicles. Precipitation is reversible and the precipitate is dissolved by elevation of the pH. This effect was found to be dependent on relatively conserved glutamate residues in the otherwise poorly conserved C-peptide. Together, the data show that C-peptide has the ability to influence insulin solubility. The physiological pH changes between insulin processing and release sites may therefore affect the quaternary structure of insulin, as well as the phase transitions during insulin sorting and secretion. Structured digital abstractInsulin and C-peptide bind by molecular sieving (View Interaction: 1, 2) C-peptide and Insulin bind by dynamic light scattering (View Interaction: 1, 2) C-peptide and Insulin bind by fluorescence technology (View Interaction: 1,2,3,4)

C‐peptide evolution: Generation from few structural restrictions of bioactivities not necessarily functional

“…The importance of the C-peptide negative charge is supported by reports that the position of Glu-3 has co-evolved with hydrophobic and acidic residues in the insulin A-and B-chains [28]. Similarly, all the four most conserved Glu residues seem to correlate with insulin charge interactions and with a 2:1 optimal stoichiometry in insulin/Cpeptide complexes [29]. Further, once the insulin moiety is folded, the C-peptide segment has to adopt a protruding, loop-like structure [30].…”

PLGA–HPMC nanoparticles prepared by a modified supercritical anti-solvent technique for the controlled release of insulin

“…The release profile of a sustained release nano-system is significantly influenced by the chemical nature of the release liquid [29][30][31][32]. The oral dosage is exposed to different pH values and ionic strengths along the GI path.…”

“…Both graphs show a gradual prolonged release with similar trends. It has been reported that the combination of HPMC with water-insoluble polymers retards the release profile in buffer liquids [30,31]. This retardation is attributed to the formation of a strong gel layer with slow erosion and diffusion rates.…”

“…Though the erosion rate of both the PLGA and HPMC must be faster in low pH solutions [30,31], this retardation arises from the particular influence of the ionic composition on the erosion and degradation of HPMC, which is in agreement with the work performed by Nokhodchi et al [7]. Increasing the ionic strength results in the reduction of the HPMC erosion rate, leading to prolonged matrix degradation and retarded release [31]. In fact, in ionic media, HPMC is the release-controlling polymer.…”

Transthyretin microheterogeneity and molecular interactions: implications for amyloid formation

“…The ligand binding properties may also differ depending on the localization of the protein and stabilize TTR specifically under aggregation-promoting conditions. A similar example is the regulation of insulin solubility by proinsulin C-peptide, which has a monomerizing effect on insulin at granular pH (71). In the light of this, the molecular interaction networks of TTR in intracellular and extracellular environments may hold valuable information with regard to its roles in health and disease.…”