Sargassum horneri Fucoidan Suppresses the Early-Stage Amyloid Fibrillation of Insulin

Amyloid fibril formation is implicated in numerous protein misfolding disorders. Fucoidan, a sulfated polysaccharide primarily derived from brown algae, exhibits diverse biological activities. In this study, we investigated the inhibitory effect of Sargassum horneri fucoidan (S-fucoidan) on insulin amyloid fibrillation in vitro. Initial screening of fucoidans derived from Cladosiphon okamuranus, Kjellmaniella crassifolia, and S. horneri identified S-fucoidan as the most effective inhibitor of insulin fibrillation. Therefore, we characterized the inhibitory effect of S-fucoidan using Thioflavin T fluorescence, transmission electron microscopy, circular dichroism spectroscopy, size-exclusion chromatography, and cell viability assays. Both S-fucoidan and low-molecular-weight S-fucoidan inhibited fibrillation in a concentration-dependent manner. When present from the onset of the reaction, S-fucoidan prevented detectable fibril formation, preserved the initial secondary structure of insulin, and maintained insulin in a monomeric or near-monomeric state. In contrast, addition during nucleation or elongation did not effectively halt ongoing fibrillation. These findings indicate that S-fucoidan predominantly suppresses the early stage of insulin amyloid fibrillation and provide a basis for investigating the molecular mechanisms underlying its anti-fibrillation activity.