Characterization, prebiotic and immune-enhancing activities of rhamnogalacturonan-I-rich polysaccharide fraction from molokhia leaves
Introduction
The human gut microbiota consists of more than 400 species of bacteria, with Bifidobacteria and Lactobacilli being the predominant members of the intestinal microbiota that have beneficial effects on health [1]. Prebiotics are defined as substrates that are selectively utilized by the intestinal microbiota and confer health benefits to the host [2]. Prebiotics are important for maintaining the gut microflora balance but only a few carbohydrate compounds, including fructooligosaccharide, inulin, and galactooligosaccharide, can be classified as prebiotics [3]. Recently, many studies have focused on evaluating the prebiotic activities of polysaccharides extracted from plant sources, including bamboo shoots [4], Platycodon grandiflorus [5], and Codonopsis pilosula roots [6].
Polysaccharides, which are common in plants, are important biological macromolecules with relatively low toxicity and can be used as therapeutics [7,8]. In addition, many studies have reported that plant-extracted polysaccharides exert strong immune-enhancing activities [9,10]. Immunoglobulin A (IgA), an immunomodulatory indicator, is induced in Peyer's patches (PP) against specific mucosal antigens and is modulated by transforming growth factor (TGF)-β, interleukin (IL)-6, and IL-10 [11].
Molokhia (Corchorus olitorius L.) is a member of the family Malvaceae and is mostly cultivated in Egypt and Southeast Asian countries as a fibre crop [12]. Molokhia leaves are well-known for being rich in nutrients such as phenolic compounds, and especially are high in water-soluble mucilage polysaccharide [13]. Many studies have shown that molokhia leaves have anti-inflammatory, hepatoprotective, and antioxidant effects [14,15]. Furthermore, we have recently shown that water extracts of molokhia leaves exert beneficial effects on gut health [16]. We hypothesized that the effects on gut microbiota and intestinal health could be greater in polysaccharide extracts than in water extracts. However, the intestinal immune-enhancing and prebiotic activities of polysaccharides extracted from molokhia leaves have not been evaluated. Therefore, we investigated and characterized the prebiotic activity and immune-enhancing effects of molokhia leaf polysaccharides.
Section snippets
Preparation of molokhia leaf polysaccharide fraction
Molokhia leaves were cultivated in Hongcheon-gun, Republic of Korea. The leaf powder (100 g) was extracted in 2 L of distilled water for 3 h at 80 °C. After filtration and concentration using a vacuum rotary evaporator (R-114; Buchi Labortechnik, Flawil, Switzerland), the leaf powder was precipitated with four volumes of ethanol for 16 h at 4 °C. After centrifugation (3000 ×g, 30 min), the precipitates were dialyzed for two days at 4 °C using a dialysis membrane (molecular weight cut-off:
Chemical composition of MPF
The chemical composition of MPF is summarized in Table 1. The neutral sugar and uronic acid contents were 47.1% and 44.7%, respectively. MPF mainly consisted of sugars, such as rhamnose (22.4%), galactose (22.1%), galacturonic acid (22.1%), glucuronic acid (17.9%), and glucose (5.7%). Trace amounts of arabinose (2.6%), xylose (0.5%), and mannose (0.4%) were detected. The high rhamnose and galacturonic acid contents suggested that MPF is a pectic polysaccharide containing the rhamnogalacturonan
Conclusion
MPF was obtained from molokhia leaves, and structural analysis of its chemical composition, GPC, and methylation revealed that it is a polysaccharide of rhamnogalacturonan-I structure composed of several side chains, such as galactans and linear glucans. MPF demonstrated prebiotic activity on probiotic strains, such as Lactobacillus and Bifidobacterium by increasing its prebiotic scores and concentrations of total SCFAs. In addition, MPF demonstrated intestinal immune-enhancing activity in the
CRediT authorship contribution statement
Hye-Bin Lee: Investigation, Data curation, Writing – original draft. Seung-U Son: Investigation. Jang-Eun Lee: Methodology. Sang-Hoon Lee: Software, Validation. Chang-Ho Kang: Investigation. Young-Soo Kim: Conceptualization. Kwang-Soon Shin: Writing – review & editing. Ho-Young Park: Conceptualization, Methodology, Writing – original draft.
Declaration of competing interest
Authors declare no conflict of interest.
Acknowledgements
This research was supported by the Main Research Program (E021600-01) of the Korea Food Research Institute (KFRI) funded by the Ministry of Science and ICT. This work was also supported by Kyonggi University‘s Graduate Research Assistantship 2020.
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