Acid stabilization of insulin

Abstract

The effect of pH on the conformational stability of insulin was studied. Surprisingly, the Gibbs free energy of unfolding increased approximately 30% by acidification. pH titration of insulin's conformational stability is described by a transition involving a single proton with an apparent pK(a) of 7.0. The acid stabilization of insulin's conformation was attributed to the protonation of histidine at position 5 on the B-chain (HB5) as determined by 1H-NMR of the histidines, selective amino acid alteration, and enthalpies of ionization. Further acidification (at least to pH 2) does not decrease the free energy of unfolding. A conformational change in the tertiary structure, as indicated by the near-UV circular dichroism spectrum, accompanies this change in stability. We propose that this acid stabilization of insulin is physiologically important in maintaining insulin stability in the acid environment of the secretory/storage granules of the beta-cell of the pancreatic islets of Langerhans.

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

“…In addition, since zinc-free insulin has been proven to mainly exist as dimers under physiological conditions [33,34], the molar ratio of EGCG and insulin dimer ([EGCG]/[insulin dimer]) in the precipitates were calculated according to the data listed in Tables 1, 4 and 5, and the results are given in Table 6. Comparisons of Table 1 with Table 6 suggest that the A600 values are relatively large (about 1.6-2.4, bold data in Table 1) at ratios of [EGCG]/[insulin dimer] ≈ 1-9 and very small (smaller than 0.2, italic and underlined data in Table 1) at ratios of [EGCG]/[insulin dimer] lower than 0.7 or higher than 50.…”

Suppression of Insulin Aggregation by Heparin

“…Because aggregation mechanisms vary with the experimental conditions, studies at extremes of pH or temperature might not fully explain the aggregation of insulin at physiological conditions, nor more generally at room temperature, low ionic strength (I < 100 mM NaCl), at pH near pI. The purpose of the present study was to investigate this last form of aggregation, the so-called "isoelectric precipitation" of insulin 19 at low I and the effect on it of heparin, a highly charged biopolymer whose biological functions are intimately connected with its ability to bind biofunctionally numerous proteins. 20,21 A member of the glycosaminoglycan family, heparin is composed of variable disaccharide units of uronic acid and glucosamine.…”

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

“…At the same time, with the increase of pH, insulin gradually formed polymers and will hinder the processes of electrochemical oxidation. 24 The influence of temperature. Temperature has a complicated influence on the anodic peaks (Fig.…”