Identification and characterization of antioxidant and immune-stimulatory polysaccharides in flaxseed hull

Highlights

• Flaxseed hull polysaccharides were isolated, fractionated, and identified.

• The polysaccharide fractions possessed high antioxidant capacity.

• The fractions had good immune-enhancing activities partly via the MAPK pathway.

• These polysaccharide fractions may be useful as functional food ingredients.

Abstract

Flaxseeds are widely consumed for their desirable sensory attributes and health benefits. We focused on enhancing the sustainability and economic potential of flaxseeds by characterizing functional attributes of polysaccharides isolated from flaxseed hull residues. In particular, antioxidant and immune-stimulatory polysaccharides were isolated and purified from flaxseed hull. Infrared spectroscopy was used to identify the key functional groups. The polysaccharides were composed of mannose, rhamnose, galactose, glucose, galactose, xylose, arabinose, and fucose. In vitro studies showed certain flaxseed hull polysaccharide fractions exhibited strong antioxidant activities, increased nitric oxide levels, and enhanced the production of cytokines (TNF-α and IL-6). In the presence of 200 μg/mL of one of these fractions, the levels of p-ERK, p-JNK, and p-p38 increased significantly by 1.8-, 9.0-, and 6.7-fold. These polysaccharide fractions may exhibit their immune-regulatory properties partly by modulating the MAPK pathway. The flaxseed hull polysaccharides identified have potential application as natural antioxidants and immune-enhancing nutraceuticals.

Introduction

Flaxseed (Linum usitatissimum L.) is one of the oldest cultivated crops and is still widely grown in Europe, Asia, and North America. This plant contains considerable quantities of bioactive components, such as soluble (9–10%) fibers, insoluble (20–30%) fibers, omega-3 fatty acids, and phytochemicals (Moczkowska et al., 2019, Shim et al., 2014). These bioactive constituents can be isolated and used as health-promoting ingredients that can be incorporated into functional food products (Soto et al., 2012, Wang et al., 2017). For instance, flaxseed hull is a rich source of both lignin- and polysaccharide-based dietary fibers, which may promote digestive health. Flaxseed hull is, however, currently regarded as an industrial residue and most of it is treated as waste (Ray et al., 2013). More detailed studies of the functional properties of flaxseed hull may lead to the development of novel nutraceutical ingredients to improve human health and wellbeing through dietary interventions. Moreover, they may stimulate economic activity and improve environmental sustainability by converting a waste material into a valuable functional ingredient.

Polysaccharides exhibit a broad range of molecular characteristics, such as composition, molecular weight, branching, charge and polarity, depending on their biological origin, such as specific plants, animals, or microbial cells (Khan et al., 2019, Liu et al., 2015). These macromolecules have attracted considerable attention from researchers in the food and pharmacological areas due to their diverse range of functional attributes, low toxicity, and high biodegradability (Khan et al., 2019). Recently, many types of polysaccharides, such as purple sweet potato and Angelica sinensis polysaccharides, have been shown to possess immune regulatory activities in vitro and in vivo (Tang et al., 2019, Wang et al., 2016). Macrophages are a critical component in the non-specific immune system, playing an important role in regulating the immune response via the production of cytokines (Olefsky & Glass, 2010). When the body is influenced by some pathological stimuli or injury, macrophages regulate the immune response by releasing inflammatory molecules such as nitric oxide. Cell culture models, such as the RAW264.7 model, are commonly used for the in vitro evaluation of the immune regulatory activity of bioactive compounds (Sun et al., 2015, Yang et al., 2018).

In the present study, water-soluble polysaccharides were isolated from flaxseed hull and then their composition and structure were characterized using a range of chemical and biochemical methods. In addition, the antioxidant and immune regulatory activities of these polysaccharides were determined using the RAW264.7 model to establish structure–activity relationships. This study also aimed to investigate the potential mechanism of flaxseed hull polysaccharides on the MAPK immune pathway.