Highly amyloidogenic two-chain peptide fragments are released upon partial digestion of insulin with pepsin

Proteases play a well recognized role in the emergence of highly aggregation-prone protein fragments in vivo, whereas in vitro limited proteolysis is often employed to probe different phases of amyloidogenic pathways. Here, we show that addition of moderate amounts of pepsin to acidified bovine insulin at close to physiological temperature results in an abrupt self-assembly of amyloid-like fibrils from partially digested insulin fragments. Biochemical analysis of the pepsin-induced fibrils implicates peptide fragments (named H) consisting of the 13 or 15 N-terminal residues of the A-chain and 11 or 13 N-terminal residues of the B-chain linked by the disulfide bond between Cys-7A-Cys-7B as the main constituents. There are up to eight pepsin-cleavage sites remaining within the double chain peptide, which become protected upon fast fibrillation unless concentration of the enzyme is increased resulting in complete digestion of insulin. Controlled re-association of H-peptides leads to "explosive" fibrillation only under nonreducing conditions implying the key role of the disulfide bond in their amyloidogenicity. Such re-assembled amyloid is similar in terms of morphology and infrared features to typical bovine insulin fibrils, although it lacks the ability to seed the intact protein.