Retardation of human insulin amyloid fibrillation by vitamin B1

Water-soluble B-complex vitamins have been proven to undergo significant interactions with proteins both in vivo and in vitro. Understanding the impact of major vitamin B-interacting residues (VBIRs), like Gly, His, Asn, Ser, Thr, Trp and Tyr, is relevant to designing experiments related to the beneficial effects of protein-vitamin interactions. The maintenance of native folds and the monomeric form is necessary for the functioning of therapeutic proteins like insulin. Some fatal cases of amyloid generation before (during storage and in the delivery pump) and/or after insulin application (at the injection site) to patients with diabetes mellitus have been reported, which are a matter of concern. Amyloid fibrils of insulin are irreversible aggregates of insulin, rich in beta-sheet content instead of the native alpha-helical conformation. Inspired by the protein-vitamin interactions reported in the literature, the present work explored three vitamin B complexes, viz., vitamins B1, B6 and B12, and separately investigated their amyloid inhibitory activities on insulin. The thermally generated amyloid fibrils of recombinant human insulin were compared with those of vitamin pre-incubated insulin under identical heating conditions by employing different fluorimetric, spectroscopic, light-scattering, and imaging studies. The results showed the considerable amyloid inhibitory activity of thiamine on insulin aggregation in vitro. The possible mechanism includes the shielding effect of Vit. B1 through several non-covalent interactions with insulin. Vit. B6 did not exhibit sufficient insulin amyloid inhibitory activity, even though it maintained the alpha-helical form to some extent during thermal aggregation, unlike Vit. B1. Vit. B12, having a bulkier structure, could not interact with insulin in a stable manner and consequently disrupted the solvated state of insulin.