Abstract
The aim of the present study is to investigate the chemical composition and biological activity of polysaccharides (PSs) and secondary metabolites, isolated from poorly explored cyanobacterial species, in search of immunomodulating and antioxidant compounds. Polysaccharide complexes (PSCs) were obtained by boiling water extraction from biomasses of Anabaena laxa (AL-PSC), Oscillatoria limosa (OL-PSC) and Phormidesmis molle (PM-PSC), pretreated with 80% (v/v) ethanol and pure acetone. The total carbohydrate content in the complexes varies between 53.0% and 63.5% (w/w). The complexes contain PSs composed of neutral monosaccharides as main building blocks and the uronic acids (GalA and GlcA) are in smaller amounts (4.2–9.5%). AL-PSC and OL-PSC are rich in Glc (33.1% and 38.6%), followed by Man, Gal and Xyl as other dominant monomers. PM-PSC has the highest amount of Gal (45%), followed by Glc (21%). Additionally, a neutral exopolysaccharide (EPS) (PM-EPSC), from a (hetero)xylan type (Xyl content 70.5%), was isolated from P. molle. The weight-average molecular weights (Mw) of AL-PSC, OL-PSC and PM-PSC are estimated to be 67.0, 71.8 and 74.7 × 104 g/mol. AL-PSC (200 μg/mL) expresses the most potent ex vivo stimulating activity on human monocytes and granulocytes. AL-PSC and OL-PSC (50–200 μg/mL) manifest better in vitro growth-inhibitory effect against human colorectal adenocarcinoma (HT-29) and colon adenocarcinoma (LS-180) cells than PM-PSC, affecting their lysosomal activity. The organic extracts, obtained from the biomasses extracted with 80% ethanol and acetone, are rich in carbohydrates, followed by fatty acids and other triacylglycerol metabolism compounds, terpenes, Krebs cycle metabolites, chlorophylls and phenolics. Among the three species, the extract from A. laxa (AL-E) contains the highest phenolic content, as well as exhibiting the strongest antioxidant activity by the oxygen radical absorbance capacity (ORAC) assay. A. laxa and the other cyanobacteria should be considered valuable for isolation of immunomodulators and antioxidants with possible application in dietetics and medicine.
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