Bioprospecting for bioactive polysaccharides from marine algae endemic to South Africa
Fucoidan is a marine-derived sulphated polysaccharide with bioactive properties ideal for the food, chemical and pharmaceutical industries. The polysaccharide consists largely of L-fucose, has a highly heterogeneous structure and is of diverse origin. Fucoidan was extracted from Ecklonia maxima, Laminaria pallida and Splachnidium rugosum and the effect of different extraction methods on fucoidan heterogeneity was assessed. Extraction methods employed hot water, hydrochloric acid or calcium chloride salt. Fucoidan yield and purity were determined by various colorimetric assays. Highest fucoidan yield was obtained with the hot water extraction method as seen by highest L-fucose content. Splachnidium rugosum extracts contained ~5 times more L-fucose than Ecklonia maxima and Laminaria pallida extracts. The salt extraction method yielded extracts free of contaminants, however L-fucose content in all extracts was >20 times lower. Acid extraction yielded highest levels of uronic acid contamination and liberated sulphate from the fucoidan polysaccharide. The fucose-to-sulphate ratio for Ecklonia maxima was approximately 1:5, whilst the ratios for Splachnidium rugosum and Laminaria pallida were approximately 1:1 and 1:2, respectively. The acid and salt extraction methods removed all traces of protein contaminants, while the hot water method retained very low levels of protein. The extraction method used to isolate fucoidan was a determining factor in yield and purity. Chemical compositional analyses of hot water extracts were assessed by gas chromatography mass spectroscopy. Splachnidium rugosum and Laminaria pallida extracts consisted largely of L-fucose, while Ecklonia maxima fucoidan was characterized with high glucose abundance. Crude hot water and acid extracts from Splachnidium rugosum tissue were fractionated and purified by (anionic) ion exchange chromatography as bioactivity has been correlated to lower molecular weight forms. In water extracts, ion exchange chromatography resulted in close to 90% decrease in L-fucose, sulphate and uronic acid, while protein content increased by 57%. Similar results were reported for acid extracts; however protein content did not change significantly. These results show that method of extraction may affect the composition of fucoidan post-purification. Hot water extraction is recommended due to higher fucoidan yield, as reflected by L-fucose content, and higher sulphate-to-fucose ratio. High protein content after ion exchange chromatography was however of concern. Since mucilage in Splachnidium rugosum thallus was free of protein, fucoidan was precipitated from mucilage with ethanol. Fucoidan yield of mucilage was >15-fold higher than content in purified hot water extracts with a sulphate-to-fucose ratio of ~1:1. The average molecular weight of native fucoidan in mucilage was estimated at 2367 kDa. The polysaccharide was hydrolysed by gamma-irradiation levels of 10-50 kGy to fractions ranging between 60 and 15.5 kDa. Hot water crude fucoidan extracts from Ecklonia maxima, Laminaria pallida, and Splachnidium rugosum were assessed for anti-oxidant activity by measuring the ability to scavenge free radicals and the capacity to reduce copper ions with 2,2-Diphenyl-1-picrylhydrazyl and Cupric Reducing Anti-oxidant Capacity assays, respectively. Ecklonia maxima crude fucoidan displayed highest anti-oxidant activity and capacity, having the potential to scavenge reactive oxygen species as well as the capacity to reduce copper to less toxic forms in mammalian systems. Splachnidium rugosum showed weakest anti-oxidant activity and lowest reducing capacity. The anti-cancer activity of crude and purified hot water Splachnidium rugosum extracts, as well as non-irradiated (native) and gamma-irradiated fucoidan, and commercially procured fucoidan were assessed for anti-cancer activity against MCF-7 breast cancer cells. Splachnidium rugosum crude and purified fucoidan displayed a half maximal inhibitory concentration of 0.7 mg/mL and 0.029 mg/mL, respectively. Low cytotoxicity of crude and purified Splachnidium rugosum fucoidan against non-cancerous breast epithelial cell line MCF-12A was observed, as seen by half maximal inhibitory concentration values of 2 mg/mL and 0.663 mg/mL, respectively. The cancer specific selectivity of purified Splachnidium rugosum fucoidan was therefore much higher as reflected by 10-fold higher selectivity index than that of crude fucoidan. Native and low molecular weight gamma-irradiated fucoidan also showed bioactive properties including anti-cancer activity as seen by the reduction of cell proliferation in vitro, whereas crude fucoidan showed the ability to scavenge free radicals, and the capacity to reduce copper ions.