Extraction, structural characterization, and immunostimulatory activity of soluble non-starch polysaccharides of finger millet

Abstract The structural elucidation and investigation of the immunostimulatory effects of crude polysaccharides (ECPs), non-starch polysaccharides (ECPs-I), and deproteinated non-starch polysaccharides (ECPs-II) derived from finger millet (Eleusine coracana) were performed. The extracted ECPs, ECPs-I, and ECPs-II primarily comprised different levels of carbohydrate (82.9, 41.9 and 62.0%), protein (5.3, 38.4 and 22.0%) and uronic acid (0.5, 2.4 and 0.7%). Monosaccharide composition analysis showed that glucose (88.2 and 40.6) was the main sugar unit present in ECPs and ECPs-I, followed by galactose, arabinose, xylose, rhamnose, and mannose; however, ECPs-II contained galactose (42.2%) as a major unit. The average molecular weights of ECPs, ECPs-I, and ECPs-II were found to be 120, 78 and 43 kDa, respectively. The polysaccharides as well as non-starch polysaccharides did not show any toxic effects on RAW 264.7 cells. ECPs-II treatment significantly enhanced the activation of RAW 264.7 cells by inducing nitric oxide (NO) production, upregulating inducible nitric oxide synthase (iNOS) and various pro-inflammatory cytokines including interleukin (IL)-1β, IL-6, IL-10, and tumor necrosis factor alpha (TNF-α). Furthermore, the RAW 264.7 cells were activated after ECPs-II treatment via the nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB), and mitogen-activated protein kinase signaling (MAPK) pathways induced by a cluster of differentiation 40 (CD40) expression. The structure of ECPs-II mainly consists of (1→3,4)-linked galactopyranosyl and (1→3,5) arabinopyranosyl.