Antidiabetic potentials of crude and purified sulphated polysaccharides isolated from Gracilaria gracilis, a seaweed from South Africa

Over 90 % of all cases of diabetes that have been diagnosed are type 2 diabetes (T2D), a disease exacerbated by an increase in sedentary behaviour, bad eating habits, and obesity. This study investigated the antidiabetic properties of Gracilaria gracilis, using in vitro and ex vivo experimental models. The sulphated polysaccharides (SPs) from crude extracts of the seaweed powder was prepared via hot (100°C) and cold (25°C) aqueous extraction procedures before purification via an anion exchange chromatographic technique. Both the crude and purified extracts were characterised by Fourier-transform infrared spectroscopy (FT-IR), LC-MS analysis, and Nuclear Magnetic Resonance (NMR) spectroscopy. The crude cold-aqueous and purified hot-aqueous SPs from G. gracilis had the strongest α-glucosidase inhibitory effect with IC₅₀ value of 0.15 and 0.07 mg/ml, respectively. The purified cold-aqueous SP was the most potent inhibitor of α-glucosidase with an IC₅₀ value of 0.17 mg/ml. The crude and purified SP-rich extracts inhibited pancreatic lipase (hot aqueous SP = 0.03 mg/ml) activity and effectively stimulated glucose uptake in yeast cells. Moreover, they showed significantly (p < 0.05) better intestinal glucose absorption inhibitory properties at the highest concentration (1 mg/ml) and displayed significantly (p < 0.05) better muscle glucose uptake compared to the commercial antidiabetic drug, metformin, at the same concentration. Overall, the current findings indicate that G. gracilis SPs may inhibit carbohydrate-hydrolysing enzymes, limit the release of simple sugars from the gut whilst effectively stimulating the use of glucose by peripheral tissue thus may be suitable to develop antidiabetic food supplements after further animal and clinical trials.