Fibrillation of human insulin A and B chains.
Human insulin, which consists of disulfide cross-linked A and B polypeptide chains, readily forms amyloid fibrils under slightly destabilizing conditions. We examined whether the isolated A and B chain peptides of human insulin would form fibrils at neutral and acidic pH. Although insulin exhibits a pH-dependent lag phase in fibrillation, the A chain formed fibrils without a lag at both pHs. In contrast, the B chain exhibited complex concentration-dependent fibrillation behavior at acidic pH. At higher concentrations, e.g., >0.2 mg/mL, the B chains preferentially and rapidly formed stable protofilaments rather than mature fibrils upon incubation at 37 degrees C. Surprisingly, these protofilaments did not convert into mature fibrils. At lower B chain concentrations, however, mature fibrils were formed. The explanation for the concentration dependence of B chain fibrillation is as follows. The B chains exist as soluble oligomers at acidic pH, have a beta-sheet rich conformation as determined by CD, and bind ANS strongly, and these oligomers rapidly form dead-end protofilaments. However, under conditions in which the B chain monomer is present, such as low B chain concentration (<0.2 mg/mL) or in the presence of low concentrations of GuHCl, which dissociates the soluble oligomers, mature fibrils were formed. Thus, both A and B chain peptides can form amyloid fibrils, and both are likely to be involved in the interactions leading to the fibrillation of intact insulin.
Hong
Dong-Pyo
DP
Department of Chemistry and Biochemistry, University of California, Santa Cruz, California 95064, USA.
Ahmad
Atta
A
Fink
Anthony L
AL
D000595:Amino Acid Sequence; D000682:Amyloid / Q000378:metabolism; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007328:Insulin / Q000737:chemistry* / Q000235:genetics; D007700:Kinetics; D008969:Molecular Sequence Data; D010446:Peptide Fragments / Q000737:chemistry* / Q000235:genetics / Q000648:ultrastructure; D017510:Protein Folding; D021122:Protein Subunits / Q000737:chemistry* / Q000378:metabolism
D000682:Amyloid; D007328:Insulin; D010446:Peptide Fragments; D021122:Protein Subunits; C060675:insulin B-chain tetrapeptide amide B22-B25