Characterization, antioxidant activity and hepatoprotective effect of purple sweetpotato polysaccharides

https://doi.org/10.1016/j.ijbiomac.2018.04.033Get rights and content

Highlights

  • Three polysaccharides were successively isolated from purple sweetpotato tubers.

  • All the polysaccharides exhibited moderate antioxidant activity in vitro.

  • Polysaccharides had protective effect on tetrachloromethane-induced liver damage.

  • Water soluble polysaccharide showed higher bioactivities than alkali soluble ones.

Abstract

In this study, three kinds of polysaccharides (named PSWP, PSAP-1 and PSAP-2) were successively isolated from purple sweetpotato tubers by hot water, 0.5 M and 2 M sodium hydroxide solutions. The characterization, in vitro antioxidant activity and in vivo hepatoprotective effect of these polysaccharides were investigated. Results indicated that PSWP, PSAP-1 and PSAP-2 were all β-type polysaccharides containing different contents of uronic acid, protein and polyphenol. Both PSWP and PSAP-1 were composed by arabinose, glucose and galactose, whereas PSAP-2 was consisted of arabinose, rhamnose and glucose. All the polysaccharides exhibited moderate 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging activity and reducing power. As compared with tetrachloromethane (CCl4) treatment group, mice administrated with PSWP, PSAP-1 and PSAP-2 exhibited decreased levels of serum enzymes (alkaline phosphatase, alanine transaminase and aspartate transaminase) and hepatic lipid peroxidation, whereas increased levels of hepatic antioxidant enzymes (superoxide dismutase, catalase and glutathione peroxidase) and non-enzymatic antioxidant (glutathione). Notably, PSWP displayed stronger antioxidant activity and hepatoprotective effect than PSAP-1 and PSAP-2. The hepatoprotective effect of PSWP was comparable to positive standard of silymarin. Our results suggested polysaccharides from purple sweetpotato possessed potential antioxidant activity and protective effect against CCl4-induced acute liver damage.

Introduction

Xenobiotics such as viruses, drugs, alcohol and chemicals can cause hepatic injury through declining the metabolic functions of liver. Tetrachloromethane (CCl4) induced hepatic injury is a classic model to investigate free radical-mediated hepatotoxicity [1]. CCl4 is metabolized in liver cells by cytochrome P450 system to produce trichloromethyl radical (CCl3radical dot), which can be rapidly converted into trichloromethyl peroxyl radical (CCl3OOradical dot) in the presence of oxygen [2]. Thereafter, reactive oxygen species (ROS) are generated and oxidative stress occurs, which finally results in the damage of liver. Till now, several polysaccharides from herbs have been demonstrated to possess hepatoprotective effect against CCl4-induced injury [[3], [4], [5]].

Sweetpotato (Ipomoea batatas L.) is an annual or perennial herb that belongs to the family Convolvulaceae. It is now the seventh most important crop in the world, following wheat, rice, maize, potato, barley and cassava [6]. According to the Food and Agriculture Organization (FAO), the world production and cultivated area of sweetpotato is around 105 million tons and 9 million ha in 2016, respectively. Nowadays, China is the largest sweetpotato producer, accounting for 67% of global production [7]. In China, this root-derived food plant is usually used for human food, animal feed and industrial raw materials. Sweetpotato is also regarded as a nutritionally rich food due to its nutritional values and biological activities [8]. In recent years, polysaccharides have been recognized as one kind of the most important bioactive ingredients in sweetpotato. Zhao et al. [9] isolated (1  6)-α-d-glucan from sweetpotato and demonstrated the polysaccharide had immunostimulatory activity. Recently, Yuan et al. [10] reported the structural characterization and the antioxidant, antitumor, and antidiabetic activities of selenylation of polysaccharide from the sweetpotato.

Purple sweetpotato is purple-fleshed one containing abundant anthocyanins with numerous health benefits [[11], [12], [13], [14]]. Till now, a variety of phytochemicals (especially anthocyanins and phenolic acids) have been isolated from purple sweetpotatoes and their diverse biological activities have been revealed [[15], [16], [17], [18], [19], [20]]. However, polysaccharide, one of main bioactive components in purple sweetpotato, is often ignored. One recent study suggested that polysaccharides from purple sweetpotato could inhibit the proliferation and induce apoptosis in human gastric carcinoma SGC7901 and human colon cancer SW620 cells [21]. However, little attention has been paid to the structural characterization and other bioactivities of polysaccharides from purple sweetpotato.

In general, aqueous and alkaline extractions are common and convenient methods to obtain polysaccharides. As reported, different extraction methods have considerable impacts on the structural characterization and bioactivities of polysaccharides [22,23]. Compared with aqueous extraction, alkaline extraction may break alkali-sensitive chemical bonds between polysaccharides and plant cell walls, resulting in the release of polysaccharides with different chemical structures [22,24]. Till now, the sequential extraction of polysaccharides from purple sweetpotato with hot water, dilute and concentrated alkali has not been reported.

In this study, the characterization, in vitro antioxidant activity and in vivo hepatoprotective effect of polysaccharides from purple sweetpotatoes were investigated. Firstly, three polysaccharide fractions were sequentially extracted from purple sweetpotatoes and characterized by chemical analysis, gas chromatography (GC), ultraviolet-visible (UV–vis) and Fourier transform-infrared (FT-IR) spectroscopy. Then, the 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging activities and reducing power of polysaccharides were measured. Finally, the in vivo hepatoprotective effect of polysaccharides against CCl4-induced hepatotoxicity in mice was evaluated.

Section snippets

Plant materials

Tuberous roots of purple sweetpotato (variety of Xushu No. 3) were obtained from National Sweetpotato Improvement Center, Xuzhou, China. The tuberous roots were washed by running tap water, cut into small pieces (cube size of 5 cm3), treated with liquid nitrogen and stored at −80 °C until further analysis.

Chemicals and reagents

Folin-Ciocalteu reagent, potassium ferricyanide and CCl4 were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Arabinose, rhamnose, xylose, mannose, galactose, glucose,

Chemical composition

Chemical compositions of three purple sweetpotato polysaccharides are listed in Table 1. The total carbohydrate contents of PSWP, PSAP-1 and PSAP-2 were 91.28%, 83.28%, and 95.19%, respectively. All polysaccharide fractions also contained small amounts of protein and uronic acid, which was similar to the results of Wu et al. [21]. In addition, a small amount of polyphenols were found in all fractions. Among three polysaccharide fractions, PSWP had the highest contents of uronic acid (10.23%)

Conclusions

Three polysaccharides including PSWP, PSAP-1 and PSAP-2 were isolated from purple sweetpotato tubers through continuous extraction. Monosaccharide analysis showed these polysaccharides were consisted of arabinose, rhamnose, glucose and galactose in different molar ratios. PSWP, PSAP-1 and PSAP-2 were demonstrated to possess antioxidant activity in vitro and hepatoprotective effect against CCl4-induced hepatic injury in vivo. Notably, the antioxidant activity and hepatoprotective effect of PSWP

Conflict of interest

The authors declare that there are no conflicts of interest.

Acknowledgments

The authors gratefully acknowledge the funds from Jiangsu Self-innovation Fund for Agricultural Science and Technology (No. CX [17] 2023) and China Agriculture Research System (CARS-10-B20), National Natural Science Foundation of China (No. 31571788 and 31101216), Natural Science Foundation of Jiangsu Province (No. BK20151310), Qing Lan Project of Jiangsu Province, and High Level Talent Support Program of Yangzhou University.

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