br Introduction Humans have consumed mushrooms for centuries

Introduction Humans have consumed mushrooms for centuries because of their high nutrient content and low lipid content, as well as their excellent flavor and texture [1]. Edible mushrooms possess notable medicinal properties and bioactivities, including antitumor, antiviral, antidiabetic, and antioxidant activities [2]. Polysaccharides, the most abundant biopolymers, are major active constituents in edible mushrooms [3]. Many studies have demonstrated that nefiracetam isolated from various kinds of mushrooms have wide-ranging bioactivities, especially strong antioxidant activities [4]. Therefore, there is increasing interest in characterizing the polysaccharides from various mushroom species and exploring their antioxidant activities for use as natural antioxidants. The bioactivities of polysaccharides can be related to structural characteristics such as monosaccharide composition, molecular weight distributions and degree of branching, as well as degree of sulfation [5,6]. Sun, Wang and Zhou [7] demonstrated that the molecular weight (Mw) of polysaccharides had a notable effect on their biological activities, with low-Mw polysaccharides having better immunomodulatory effects than those of higher Mw. The antioxidant properties of polysaccharides may depend on the ratios of the different monosaccharide components; for example, rhamnose extracted from Lentinula edodes was the most significant determinant [8]. Recently, Ren et al. [9] also showed that polysaccharides with a low Mw or a beta configuration in the pyranose form had higher antioxidant activity. To date, although there are some studies on the structure and function of polysaccharides, the specificity and pertinence of these studies are strong, and there are lacking a variety of systematic comparisons and comprehensive studies. Mushrooms of the genus Boletus, among the most delicious and widely consumed mushrooms, are distributed in many regions of China, but primarily in Sichuan and Yunnan [10]. Our preliminary study showed that five species of boletus mushrooms, among the most popular species in local markets, had high total carbohydrate contents and low crude fat contents. It also demonstrated that most of the mushrooms had antioxidant activities, and that samples of Suillellus luridus were remarkably superior among those tested [11]. However, more comprehensive studies of the polysaccharides of the boletus mushrooms from Southwest China are needed. Most of the available studies have investigated the crude or purified polysaccharides extracted from fruiting bodies and the mycelium [12,34]. Studies that characterize and investigate the antioxidant activities of polysaccharides isolated from caps and stipes of boletus mushrooms would be particularly useful. In this study, water-soluble polysaccharides were extracted from the caps and stipes of 13 boletus mushrooms collected in Southwest China. These mushroom samples represent five different species: Boletus aereus, Suillus bovinus, Suillellus luridus [formerly Boletus luridus], Boletus edulis and Boletus violaceo-fuscus Chiu. The structures of the polysaccharides were characterized using gas chromatography (GC), high performance liquid chromatography (HPLC), and Fourier-transform infrared spectroscopy (FT-IR). Their antioxidant properties were investigated using in-vitro assays of 1, 1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging activity, reducing power, and metal chelating activity. These data were used to investigate the relationship between polysaccharide structure and antioxidant activity.
Materials and methods
Results and discussion
Conclusions The results of this study demonstrated that the monosaccharide compositions and Mws of the water-extracted crude polysaccharides from the caps and stipes of the same boletus mushrooms, collected in different regions, were significantly different. Simultaneously there was also a significant regional difference in the properties of the polysaccharides from boletus mushrooms. The in-vitro antioxidant activities of the polysaccharides were related to their structural characteristics. The EC50 of DPPH radical scavenging activity showed significant correlations with arabinose and galactose content, and the galactose contents also showed a significant correlation with ferrous ion reducing power. Xylose content had a significant influence on metal chelating activity. Furthermore, polysaccharides with a beta configuration in the pyranose form have higher antioxidant activity. Finally, Suillellus luridus collected in Pingwu, Mianyang, Sichuan, China had remarkably superior antioxidant activity and shows potential for development as a natural antioxidant.