Water-Soluble Saccharina latissima Polysaccharides and Relation of Their Structural Characteristics with In Vitro Immunostimulatory and Hypocholesterolemic Activities

Abstract

Brown macroalgae are an important source of polysaccharides, mainly fucose-containing sulphated polysaccharides (FCSPs), associated with several biological activities. However, the structural diversity and structure-function relationships for their bioactivities are still undisclosed. Thus, the aim of this work was to characterize the chemical structure of water-soluble Saccharina latissima polysaccharides and evaluate their immunostimulatory and hypocholesterolemic activities, helping to pinpoint a structure-activity relationship. Alginate, laminarans (F1, neutral glucose-rich polysaccharides), and two fractions (F2 and F3) of FCSPs (negatively charged) were studied. Whereas F2 is rich in uronic acids (45 mol%) and fucose (29 mol%), F3 is rich in fucose (59 mol%) and galactose (21 mol%). These two fractions of FCSPs showed immunostimulatory activity on B lymphocytes, which could be associated with the presence of sulphate groups. Only F2 exhibited a significant effect in reductions in in vitro cholesterol's bioaccessibility attributed to the sequestration of bile salts. Therefore, S. latissima FCSPs were shown to have potential as immunostimulatory and hypocholesterolemic functional ingredients, where their content in uronic acids and sulphation seem to be relevant for the bioactive and healthy properties.

Keywords: Laminariales; NMR; flow cytometry; fucans; fucoidans; hypocholesterolemic effect; lymphocytes; mice; seaweed; sulphated polysaccharides.

Figure 1

Procedure used for extraction and fractionation of S. latissima polysaccharides. Values in parenthesis indicate the yield from 100 g of freeze-dried biomass.

Figure 2

Content (%, w/w) of sugars (neutral and uronic acids), sulphates (calculated as -SO₃⁻), and proteins found in polysaccharide-enriched fractions obtained from S. latissima (Sn_CaCl₂, F1, F2, and F3). Mean values are represented. Mean ± SD are presented in Table 1 (for sugars) and Supplementary Table S1 (for sulphates and proteins).

Figure 3

Anion-exchange chromatography on DEAE-Trisacryl M of Sn_CaCl₂ fraction, with stepwise elution using 0.05 M HCl (F1), 1 M NaCl (F2), and 2 M NaCl (F3). Elution profile was monitored by phenol-sulfuric acid method (490 nm). No signal was observed with 4 M NaCl used for column cleaning.

Figure 4

Percentage of B cells activated by incubation with polysaccharide-enriched fractions obtained from S. latissima (Sn_CaCl₂, Ppt_CaCl₂, F1, F2, and F3) at concentrations of 25, 100, and 250 µg/mL, in the absence and presence of polymyxin B (PB). Culture medium alone (RPMI) and incubation with lipopolysaccharide (LPS) was used as negative and positive control, respectively. Mean (±SD) values are represented. Different letters above the bars indicate statistically significant differences between compared groups (p < 0.05).

Figure 5

Concentration of (a) bile salts (GDCA) that remained in solution and (b) cholesterol solubilized in the presence of polysaccharide-enriched fractions obtained from S. latissima (Sn_CaCl₂, F1, F2, and F3). Model with GDGA and cholesterol was used as negative control and cationic resin was used as positive control. Mean (±SD) values are represented. Different letters above the bars indicate statistically significant differences between compared groups (p < 0.05).