Comparative Characterisation and In Vitro Metabolic Bioactivities of Fucoidan-Rich Extracts from Three South African Brown Macroalgae

Fucoidans are sulphated polysaccharides from brown macroalgae that have attracted considerable interest due to their diverse biological activities and potential applications in metabolic health. In this study, fucoidan-rich extracts were obtained from three South African brown macroalgal species, Ecklonia maxima, Ecklonia radiata, and Sargassum incisifolium, and comparatively characterised using biochemical and physicochemical analyses. The extracts were evaluated for carbohydrate, sulphate, L-fucose, protein, polyphenol, and uronic acid contents, while molecular weight distribution, Fourier-transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and proton Nuclear Magnetic Resonance (1H NMR) spectroscopy were used to investigate structural characteristics. All extracts displayed features characteristic of fucoidans, including sulphated fucose-containing polysaccharides. Notable species-dependent differences were observed, with S. incisifolium exhibiting the highest sulphate (~24%) and polyphenol (~6.76%) contents, stronger sulphate-associated FTIR signals, greater molecular weight heterogeneity, and enhanced thermal stability. The fucoidan-rich extracts exhibited concentration-dependent pancreatic lipase and α-glucosidase inhibitory activities, together with DPPH radical-scavenging activity. At 1.0 mg/mL, S. incisifolium demonstrated approximately 66% DPPH radical scavenging and 49% pancreatic lipase inhibition. S. incisifolium and E. radiata exhibited strong α-glucosidase inhibition, with IC50 values of ~29 and ~46 µg/mL, respectively, compared to ~354 µg/mL for acarbose. The findings highlight South African brown macroalgae as promising sources of fucoidans and contribute to understanding the relationships among fucoidan composition, structure, and bioactivity.