Depolymerized konjac glucomannan: preparation and application in health care

Depolymerized konjac glucomannan: preparation and application in health care 解聚型魔芋葡甘露聚糖的制备以及生理活性研究的进展 1 first-author 1 1 2 corresp ‡Corresponding Author ‡Corresponding Author †E-mail:shijs@163.com †E-mail:shijs@163.com shijs@163.com 3 4 School of Pharmaceutical, Jiangnan University, Wuxi 214122, China School of Pharmaceutical, Jiangnan University, Wuxi 214122, China Key Laboratory of Carbohydrate Chemistry and Biotechnology, Ministry of Education, School of Pharmaceutical Science, Jiangnan University, Wuxi 214122, China Key Laboratory of Carbohydrate Chemistry and Biotechnology, Ministry of Education, School of Pharmaceutical Science, Jiangnan University, Wuxi 214122, China National Engineering Laboratory for Cereal Fermentation Technology, Jiangnan University, Wuxi 214122, China National Engineering Laboratory for Cereal Fermentation Technology, Jiangnan University, Wuxi 214122, China School of Biotechnology, Jiangnan University, Wuxi 214122, China School of Biotechnology, Jiangnan University, Wuxi 214122, China Konjac glucomannan (KGM) is a water-soluble polysaccharide obtained from the roots and tubers of konjac plants. Recently, a degraded product of KGM, depolymerized KGM (DKGM), has attracted attention because of its low viscosity, improved hydrophily, and favorable physiological functions. In this review, we describe the preparation of DKGM and its prebiotic effects. Other health benefits of DKGM, covering antioxidant and immune activity, are also discussed, as well as its safety. DKGM could be a candidate for use as a tool for the treatment of various diseases, including intestinal flora imbalance, and oxidative-and immune-related disorders. 魔芋葡甘露聚糖是从魔芋块茎中提取的一种高分子水溶性多糖.近些年研究表明,其解聚产物,除了具有高溶解性和低粘度等良好的理化性质外,还具有调节微生物菌群结构、抗氧化、免疫调节等多种生理活性.本文重点综述了解聚型葡甘露聚糖的制备方法以及菌群调节功能.除此之外,对其抗氧化、免疫调节功能以及安全性评价也进行了全面的总结,为解聚型葡甘露聚糖的研究与应用提供一定的依据与思路. Konjac glucomannan Depolymerized konjac glucomannan Prebiotics Immune response Antioxidant 魔芋葡甘露聚糖 解聚型魔芋葡甘露聚糖 益生功能 免疫活性 抗氧化 Introduction 1 Konjac glucomannan (KGM) is a water-soluble polysaccharide obtained from the roots and tubers of the konjac plant ( Amorphophallus konjac ). It consists of D -mannose and D -glucose units at a molar ratio of about 1.6:1.0. These units are joined by β-(1,4)-glycosidic bonds to form the backbone, and a few branches are formed by β-(1,6)-glycosidic units. Part of the short side branches can be formed via linkage between the C-3 positions of the mannoses and the acetyl groups randomly found at the C-6 position of a sugar unit. These sugar units with acetyl groups comprise about 1/9th to 1/20th of the total sugar units. Owing to its high molecular weight (mostly ranging from 1×10 4 to 2×10 6 Da) (Al-Ghazzewi et al., 2007 2007 ) and non-caloric nature, KGM plays a beneficial role in blood cholesterol and sugar reduction, weight loss, promotion of intestinal activity and immune function. Therefore, KGM has been consumed for centuries in East Asia, but in low-added value food forms, such as KGM tofu, noodles, and jellies. Recently, many researchers have studied the preparation of depolymerized KGM (DKGM) (Behera and Ray, 2016 2016 ; Tester and Al-Ghazzewi, 2017 2017 ). Compared with native KGM, DKGM showed significantly reduced viscosity, which is more favorable for food processing. Besides, DKGM displays many unique biological characteristics, especially its prebiotic function. Because of these advantages, DKGM has attracted increasing attention. In this review, we discuss its preparation strategies, safety concerns, and biological function assessments, including its prebiotic, antioxidant, and immune-regulator effects. Factors affecting the extent of its physiological benefits are also discussed. Preparation techniques 1 Recently, a number of studies have been devoted to exploring efficient methods for degrading KGM. In general, these methods can be divided into two classes: physicochemical degradation and biodegradation Physicochemical degradation 2 Various physicochemical degradation methods, such as acid-treatment (Suzuki et al., 2010 2010 ), thermal treatment (Jin et al., 2014b 2014b ), oxidative degradation (Zheng et al., 2015 2015 ; Zhang et al., 2016 2016 ), ultrasound treatment (Huang et al., 2006 2006 ; Li et al., 2017 2017 ), and irradiation treatment (Xu et al., 2007 2007 ; Jian et al., 2013 2013 , 2017 2017 ; Pan et al., 2013 2013 ; Li et al., 2018 2018 ), have been used to degrade polysaccharides. Although strong acid is powerful in breaking down the glycosidic chains of KGM, the process is not very controllable and carries a high risk of environment pollution. The use of γ-rays is a simple and manageable way to degrade polysaccharides in any physical form (e.g. solid, suspension, paste, or solution) without heating. The ionizing energy can rapidly penetrate the granule and randomly rupture the backbone of KGM. However, the molecular mass distribution of the final product is broad. Degradation of KGM by irradiation is believed to be dose-dependent (Xu et al., 2007 2007 ; Jian et al., 2013 2013 , 2017 2017 ). While there is agreement among international experts that irradiation dosages up to 10 kGy are safe (Jian et al., 2013 2013 , 2017 2017 ; Jin et al., 2014a 2014a ), carbonyl groups and double bonds might be formed at relatively high irradiation doses, leading to browning of the final product (Xu et al., 2007 2007 ). Therefore, irradiation dosage is the priority consideration during the process. Besides irradiation dosage, the solvent used to dissolve KGM is important. A higher efficiency of KGM degradation by irradiation is observed in some solvents than that found by irradiation alone. For example, the hydrogen peroxide solution alone cannot degrade KGM efficiently, but produces hydroxyl radicals, and the concentration of hydroxyl radicals increases after irradiation. The hydroxyl radicals are powerful oxidative agents, and could directly break the glucosidic linkages of KGM by absorbing carbon-bound hydrogen atoms. Therefore, the molecular weights of products with irradiation alone (6.58×10 4 Da) and synergetic degradation (6.48×10 4 Da) are much lower than that of the untreated sample (4.80×10 5 Da) (Pan et al., 2013 2013 ). Nevertheless, many factors are involved in the process: (1) free radicals, generated by the solvent, probably play either a positive or negative role in radiation; (2) molecular conformation varies with the solvent type and concentration. For instance, KGM molecular conformation and hydrogen atoms played a restrained effect during KGM irradiation in ethanol. At low irradiation doses, stability predominated, leading to the viscosity-average molecular weights of pre-immersed samples being higher than those of irradiation controls. However, at high doses, the inhibition effect was relatively less significant, and the irradiation effect played a leading role in the hydrolysis process. For this reason there was no significant difference between ethanol-treated and untreated groups (Jin et al., 2014a 2014a ). So far, there have been only a few studies of the use of ultrasound degradation of KGM. It was thought that ultrasound treatment could weaken the intermolecular interactions between KGM molecules in an aqueous solution as well as the intramolecular interactions between the KGM polymer chains, leading to their degradation. The changes in the structure and degradation of KGM depended on the ultrasound power and other treatment parameters (Huang et al., 2006 2006 ; Chen and Qian, 2008 2008 ). Moreover, the degradation process of KGM by ultrasound was deemed to fit first-order polymer degradation kinetics (Li et al., 2017 2017 ). Enzyme hydrolysis 2 Compared with physicochemical degradation, enzymes have been widely used in polysaccharide hydrolysis because of their advantages such as high extraction yield, reproducibility, environmental-friendliness, energy efficiency, and simple protocols (van Zyl et al., 2010 2010 ; Bhotmange et al., 2017 2017 ; Jiao et al., 2017 2017 ). The molecular structure of KGM can provide multiple cutting sites. Thus, it can be degraded by different enzymes (Chen J et al., 2013 2013 ; Jian et al., 2013 2013 ; Mikkelson et al., 2013 2013 ; Liu HX et al., 2015 2015 ; Chen CY et al., 2016 2016 ; Yang et al., 2017 2017 ). Cellulase and hemicellulase derived from different strains have been most well studied so far (Table 1 1 ). The commercial cellulase product (a mixture of endoglucanase, exoglucanase, and glucosidase) has been the enzyme most commonly used for KGM hydrolysis. In the mixed product, the glucanase can break the β-(1,4)-glycosidic bond between glucose and mannose, the exoglucanase can remove the 1,4-glucopyranose units at the non-reducing end, and the β-glucosidase can cleave glucobiose into glucose units (Zhang et al., 2003 2003 ). As for the hemicellulase, β-mannanase has been the most commonly used enzyme. It is an endo-acting hydrolase which acts by randomly catalyzing the β-(1,4)-mannosidic linkages, resulting in the production of DKGM (Dhawan and Kaur, 2007 2007 ; van Zyl et al., 2010 2010 ; Liu HX et al., 2015 2015 ). Therefore, the choice of enzyme determines the structural characteristics of the final product. For example, M(mannan)G(glucose), MM, MMG, and MMM are abundant when KGM is hydrolyzed by endoglucanase, whereas plenty of GM, MM, GGM, and GMM are obtained by using β-mannanase (Cescutti et al., 2002 2002 ). Consistent with this, 99% of DKGM obtained from catalysis by β-mannanase had M as the reducing end pyranosyl unit, while KGM hydrolyzed by endoglucanase had both M and G as the reducing end pyranosyl unit (Mikkelson et al., 2013 2013 ). E/S: the ratio of enzyme to substance; DP: degree of polymerization Category Origin Reaction condition Product information Reference 17 Hemicellulase pH 6.5, 20% konjac, 45 °C, E/S 250 U/g, 0.83 h KGM oligosaccharide yield 35%, mainly consists of triose and tetrose units He et al., 2013 2013 Aspergillus niger pH 4.2, 2% konjac, 65 °C, E/S 108 U/g, 4 h KGM oligosaccharide yield 32.3% Xu et al., 2005 2005 A . niger pH 7.0, 15% konjac, 50 °C, E/S 50 U/g, 6 h KGM oligosaccharide yield 100% after yeast fermentation Li et al., 2007 2007 A . niger pH 7.0, 24% konjac, 50 °C, E/S 120 U/g, 8 h The average DP of DKGM was 1.8–1.9 Xu et al., 2008 2008 A . niger pH 5.0, 1% konjac, 30 °C, E/S 46.2 U/g, 24 h Oligosaccharide covering the whole range of DP 3–9 Albrecht et al., 2009 2009 pH 5.5, 1% konjac, 45 °C, E/S 100 U/g, 1.5 h KGM oligosaccharide yield 100% after yeast fermentation Wu et al., 2010 2010 A . niger pH 3.5–8.5, 0.05%–3.05% konjac, 14–62 °C; 20% konjac, 0.04–1.00 h The molecular weight and molecular weight distribution depended on the enzymatic parameters Yao et al., 2011 2011 A . niger pH 3.5, 18% konjac, 65 °C, E/S 30 U/g, 4 h KGM oligosaccharide yield 35.73% after yeast fermentation Xu SC et al., 2011 2011 Bacillus subtilis pH 6.0, 12.5% konjac, 50 °C, E/S 200 U/g, 3 h Xu LP et al., 2011 2011 B . subtilis pH 7.5, 5% konjac, 45 °C, E/S 50 U/g, 24 h Oligosaccharide covering the whole range of DP 2–9 Kang et al., 2012 2012 pH 6.0, 61.3% konjac, 55 °C, E/S 1500 U/g, 4.2 h DKGM yield 76.34% after ethanol precipitate, galactopyranose type DKGM covering the whole range of DP 2–10 Qin et al., 2013 2013 pH 6.0, 61.3% konjac, 55 °C, E/S 1500 U/g, 4.2 h DKGM yield 52.67% Deng et al., 2013 2013 Trichoderma reesei pH 5.0, 1% konjac, 45 °C, 48 h 17% DKGM (DP 2–6) Mikkelson et al., 2013 2013 pH 7.1, 20% konjac, 41 °C, E/S 0.49, 3.4 h Chen et al., 2013 2013 Thermobifida fusca 50 °C, 24 h The weight-average molecular weight 3089 Da Cheng et al., 2016 2016 pH 4.0, 0.2% konjac, 50 °C, E/S 360 kU/g, 24 h The molecular mass lower than 2200 Da after combination of γ-irradiation and enzymatic hydrolysis Jian et al., 2013 2013 B . subtilis pH 7.0, 0.33% konjac, 60 °C, E/S 6 U/g, 1 h DKGM yield 35.96%, most DKGM with DP 2–6 Cheng et al., 2016 2016 5 6 Cellulase Trichoderma viride pH 5.0, 10% konjac, 40 °C, E/S 50 U/g, 2 h Weight-average molecular weight 7160 Da, number-average molecular weight 5100 Da Zhang et al., 2003 2003 pH 4.5, 10% konjac, 60 °C, E/S 450 U/g, 1 h Elamir et al., 2008 2008 T . viride pH 5.0, 1% konjac, 30 °C, 24 h Albrecht et al., 2009 2009 pH 3.5, 0.5% konjac, 30 °C, E/S 58 U/g, 24 h Oligosaccharide covering the whole range of DP 3–14 Albrecht et al., 2009 2009 10% konjac, 50 °C, 1.3 h Dang et al., 2015 2015 T . reesei pH 5.0, 1% konjac, 45 °C, 48 h 18.0%, 4.7% DKGM (DP 2–6) for Cel7A and Cel7B, respectively Mikkelson et al., 2013 2013 Compared with enzymatic hydrolysis alone, chemical and physical pretreatments enhance the KGM enzymatic degradation process. Jian et al. ( 2013 2013 ) found that the molecular weight of KGM (1.68×10 6 Da) following γ-irradiation pre-treatment could be further reduced to 2200 Da with enzymes. Similarly, the molecular weight distribution of KGM enzymatic hydrolysis with or without mechanical shear pre-treatment was investigated. Compared with non-treated controls, the average molecular weights of the supernatant and pellet of DKGM following mechanical shear pre-treatment were further reduced by nearly 5% and 35%, respectively (Yang et al., 2017 2017 ). Although enzymatic degradation has been widely studied, most of these studies aimed mainly to reduce the molecular weight of KGM, and there is lack of information about its structure and purity. We believe that the health benefits of DKGM are closely associated with its molecular characteristics (e.g. its molecular structure and degree of polymerization (DP)). Therefore, controllable and limited enzymatic degradation as well as product purification is still the most challenging problems in its application. Safety 1 It is widely regarded that overconsumption of oligosaccharides might induce flatulence and even cause an imbalance of essential nutrients, which would be counterproductive (Liu et al., 2016 2016 ). Although the intrinsic safety of DKGM is supported by its natural occurrence in konjac, the safety of DKGM is still worthy of investigation. So far, only a few studies of DKGM safety have been reported. A previous study by our research group demonstrated that DKGM (DP=2–7) of up to 7.5 g/kg daily was safe for rats for 90 d. The results of genotoxicity studies in vitro (using the Ames assay and erythrocyte micronucleus assay) and in vivo (using a sperm malformation test in mice) indicated that administration of DKGM up to 10.0 g/kg had no mutagenic potential. In addition, DKGM up to 2.5 g/kg showed neither maternal toxicity nor teratogenicity in pregnant rats (Jiang et al., 2016 2016 ). Biological activity 1 More recently, DKGM, especially low molecular weight DKGM, has been widely studied because of its health benefits. In this part, its prebiotic effects and relevant factors are summarized. Potential applications of DKGM in the fields of anti-oxidation and immune function are also discussed. Prebiotic activity 2 The prebiotic activity of DKGM has been mentioned briefly in some reviews which focused mainly on native KGM (Bateni et al., 2013 2013 ; Behera and Ray, 2016 2016 ; Tester and Al-Ghazzewi, 2017 2017 ). In general, recent studies (Table 2 2 ) revealed that DKGM is valuable as a prebiotic via a number of mechanisms: (1) It can selectively stimulate Lactobacillus and Bifidobacterium in the colon (Feng et al., 2015 2015 ; Liu et al., 2016 2016 ), cecum (Wang et al., 2016a 2016a ), vagina and skin surface (Tester et al., 2012 2012 ). (2) By blocking their adhesion on mucosal surfaces of animals, DKGM can restrain the growth of pathogenic bacteria such as Coliforms , Enterococci , Staphylococcus aureus (Al-Ghazzewi et al., 2012 2012 ; Liu et al., 2016 2016 ), Propionibacterium acnes (Al-Ghazzewi and Tester, 2010 2010 ), Clostridium (Chen et al., 2005 2005 ), and Salmonella typhi (Al-Ghazzewi et al., 2012 2012 ). (3) During the fermentation of DKGM, short chain fatty acids (SCFAs) (mainly acetic, propionic, and butyric acid) are generated, leading to a lowering of the gut pH. These SCFAs are a source of nutrition for gut microbes, and show positive effects for the treatment of inflammation and carcinogenesis in the gut and other organs (Tester et al., 2012 2012 ; Bateni et al., 2013 2013 ; Wan et al., 2015 2015 ; Sivaprakasam et al., 2016 2016 ; Tao et al., 2016 2016 ; Unger et al., 2016 2016 ; Primec et al., 2017 2017 ). DP: degree of polymerization; IBD: inflammatory bowel disease; SCFA: short chain fatty acid Test Substance Subject Prebiotic effect Reference 3 9 In vitro DP 10–70 Single microculture Enhanced all Lactobacillus and Bifidobacterium strains examined Al-Ghazzewi et al., 2007 2007 Reduced Propionibacterium Al-Ghazzewi and Tester, 2010 2010 Molecular weight 1000–6000 Da Microculture with human faeces Selective stimulation of beneficial gut microbiota and a favourable SCFA profile Connolly et al., 2010 2010 Containing less than 10% by weight degree of DP 3 Single microculture Enhanced lactic acid bacteria Al-Ghazzewi and Tester, 2012 2012 Molecular weight 1000–6000 Da Co-microculture Reduced Staphylococcus aureus and Salmonella typhimurium Al-Ghazzewi et al., 2012 2012 2000 u, 5000 u, 10000 u Single microculture Enhanced the proliferation of Lactobacillus acidophilus Wang et al., 2015 2015 DP 2–6 Single microculture Enhanced four different lactic acid bacteria Wang et al., 2016b 2016b Co-microculture Lactobacillus plantarum was the dominant strain in the co-culture with Enterococcus Guo et al., 2017 2017 Microculture with human faeces Improved the composition of fermentation broth and the prebiotic effect can be enhanced by γ-irradiation Li et al., 2017 2017 7 3 10 In vivo Mice Enhanced anaerobes and Lactobacillus ; reduced Clostridium perfringens and Escherichia coli Elamir et al., 2008 2008 Female patients suffering from vaginal infection Decreased the clue cells and yeast-like fungi Tester et al., 2012 2012 Female patients suffering from acne vulgaris Improved skin health Bateni et al., 2013 2013 Molecular weight <10 000 Da and >10 000 Da Patients suffering IBD Improve the patients’ life style Suwannaporn et al., 2013 2013 DP 2–10 Mice Enhanced Bifidobacterium and Lactobacillus Qin et al., 2014 2014 DP 2–10 ≥85%, DP 2–6 ≥50% Ulcerative colitis mice Decreased the ulcer areas and rates of the colon Feng et al., 2015 2015 DP 2–4 Mice Improved the gut environment; enhanced the probiotics Wan et al., 2015 2015 DP 2–4 Rat Improved the intestinal environment; regular intestinal flora Wang et al., 2016a 2016a DP 2–10 ≥85%, DP 2–6≥50% Ulcerative colitis rat Improved the gut environment; improved the gut microbiota structure; anti-inflammatory Liu et al., 2016 2016 DP 2–10 ≥85% Ulcerative colitis rat Improved the gut environment; improved the gut microbiota structure Liu et al., 2017 2017 Generally, the fermentation process of DKGM is a time-and dose-dependent procedure (Tester et al., 2012 2012 ; Behera and Ray, 2016 2016 ). The DP, structural characteristics, and constituents of DKGM change with the preparation methods, often leading to different fermentation behaviors (Wang et al., 2008 2008 ; Albrecht et al., 2009 2009 ). Four substrates (a monomer-free DKGM with a cellulose digest, a monomer-free DKGM with an endo-β-(1,4)-glucanase digest, a monomer-containing DKGM with a cellulose digest, and native KGM) were fermented in a fecal slurry consisting of a mix of feces from three human volunteers for 72 h. Optical density and pH showed that all DKGM were fermented well except native KGM. The monomer-free endo-β-(1,4)-glucanase digest behaved similarly to the cellulose digests. However, the fermentation of the monomer-free cellulose digest showed a different pattern compared to the monomer-containing digest. For the monomer-free digest, the monomers were fermented during the first 24 h, and then most of the dimers and trimers were fermented between 24 and 72 h. No significant change was observed for the larger oligomers. In contrast, for the monomer-containing cellulose digest, no change was observed in the first 24 h. Moreover, DP 6 was completely degraded and DP 5 mostly degraded between 24 and 72 h, whereas DP 4 was more resistant (Albrecht et al., 2009 2009 ). Similarly, DKGM with a different structure and constituent was obtained following treatments with endo-β-(1,4)-glucanase, endo-β-(1,4)-mannanase, and an equivalent mixture (1:1) of cellulose-mannanase. Their utilization by lactic acid bacteria was then evaluated. The Lactobacillus growth profiles showed that DKGM produced with cellulose enzymes was the most effective growth promoter among the three groups (Al-Ghazzewi and Tester, 2012 2012 ). Although lots of research has demonstrated the prebiotic effect of DKGM, its selectivity towards specific microorganisms and the fermentation profile are still unclear. According to Wang et al. ( 2016b 2016b ) and Yang et al. ( 2017 2017 ), the fermentation of DKGM varies among species. DKGM with different molecular weights was prepared by mechanical-pretreated enzymatic hydrolysis, and fermentation by five Lactobacillus and three Bifidobacterium was investigated in vitro. Results showed that all the test bacteria (except Lactobacillus delbrueckii subsp. bulgaricus ) were DKGM fermenters, but Bifidobacterium strains were not as strong as Lactobacillus . Moreover, there was variation among the Lactobacillus strains (Yang et al., 2017 2017 ). Similarly, Wang et al. ( 2016b 2016b ) investigated the regular fermentation patterns of DKGM by four acid bacteria ( Lactobacillus animals , Lactobacillus plantarum , Enterococcus faecalists , and Lactobacillus reuteri ) in vitro. Results showed that L. plantarum and E. faecalists could quickly and completely consume DKGM with DP 2–3, whereas L. animals and L. reuteri utilized only the DKGM with DP2. In vivo studies in healthy animals (Elamir et al., 2008 2008 ; Qin et al., 2014 2014 ; Wan et al., 2015 2015 ; Wang et al., 2016a 2016a ), ulcerative colitis animals (Feng et al., 2015 2015 ; Liu et al., 2016 2016 ), and humans suffering from IBD (Suwannaporn et al., 2013 2013 ) (Table 2 2 ) verified the prebiotic effects of DKGM. Mechanism studies revealed that oral administration of DKGM not only promotes the proliferation of probiotics (mainly Lactobacillus and Bifidobacterium ), which is consistent with the in vitro results, but also influences the intestinal environment (increasing intestinal villi height and SCFAs) and reduces the levels of inflammatory factors (malondialdehyde, inducible nitric oxide synthase, cyclooxygenase-2, tumor necrosis factor-α (TNF-α), and interleukin-1β (IL-1β)). Furthermore, the physiological effects of DKGM in the gut were verified to be effective in the skin and vagina of humans (Tester et al., 2012 2012 ; Bateni et al., 2013 2013 ), implying that the use of DKGM can be expanded to other ecological systems beyond the gut. Anti-oxidative activity 2 In recent years, there has been increasing interest in natural antioxidants because synthetic antioxidants might induce teratogenicity, carcinogenicity or mutagenicity. A few studies have demonstrated that DKGM could have great potential as a natural antioxidant (Wang et al., 2008 2008 ; Liu J et al., 2015 2015 ; Jian et al., 2017 2017 ). In general, its anti-oxidative effects may result from two pathways: (1) It could act directly as an antioxidant which not only scavenges oxidants, but also activates intracellular antioxidant enzymes. Antioxidant experiments in vitro showed that DKGM (DP 5.2), obtained by β-mannanase degradation, not only could eliminate hydroxyl radicals (OH) and 1,1-diphenyl-2-picrylhydrazyl radicals (DPPH), but also had great reducing power (Liu J et al., 2015 2015 ). Moreover, cytological tests in human hepatic cells revealed that DKGM (53 kDa) degraded by 100 kGy irradiation not only significantly increased cellular survival and activity of glutathione peroxidase and intracellular activity of catalase, but also reduced the levels of lactate dehydrogenase, malondialdehyde, and intracellular accumulation of reactive oxygen species (ROS), providing a protective effect against oxidative damage by H 2 O 2 (Jian et al., 2017 2017 ). (2) Fermentation products of DKGM produced by bacteria also exert anti-oxidative effects. However, the mechanisms vary depending on its molecular weight and the bacterial strains (Wang et al., 2008 2008 ). Native KGM and DKGM with different DPs within five colonic bacteria were compared. The results suggested that native KGM produces anti-oxidative activity mainly through initiation of ferrous ion-induced peroxidation. Nevertheless, fermentation of DKGM (DP=5) produced anti-oxidative effects by reducing ferrous ion-induced peroxidation and the formation of lipid peroxide products and increasing radical-eliminating ability, whereas fermentation of DKGM (DP=10) produced anti-oxidative effects by increasing radical-scavenging ability and eliminating lipid peroxide formation. Immune activity 2 Administration of DKGM has positive immunomodulatory effects on various animal species, including mammals (Suzuki et al., 2010 2010 ) and fish (Zheng et al., 2015 2015 , 2016 2016 ). Mechanism studies revealed that DKGM at an optimal size (between 10 and 500 kDa) could inhibit the production of immunoglobulin E (IgE) and enhance interferon-γ (IFN-γ) in vitro, thereby easing atopic disease in mice (Suzuki et al., 2010 2010 ). DKGM (4.7×10 5 Da) and low molecular-DKGM (L-KGM; 9.29×10 3 Da), obtained by H 2 O 2 and HCl degradation, were used to evaluate effects on immune response in Schizothorax prenanti . Data suggested that the ideal dosage of L-DKGM for enhancement of the immune system was 8.0 g/kg and that the immunomodulatory mechanism involved a significant enhancement of lysozyme, superoxide dismutase (SOD) activity, and immune factors (1L-1β and TNF-α) and a significant reduction in the malondialdehyde level, leading to better resistance to Aeromonas hydrophila (Zheng et al., 2015 2015 , 2016 2016 ). Conclusions 1 The health benefits of DKGM have been confirmed and accepted. 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Immune responses to Aeromonas hydrophila infection in Schizothorax prenanti fed with oxidized konjac glucomannan and its acidolysis product Compliance with ethics guidelines Min JIANG, Heng LI, Jin-song SHI, and Zheng-hong XU declare that they have no conflict of interest. This article does not contain any studies with human or animal subjects performed by any of the authors. 10.1631/jzus.B1700310 Project supported by the National First-Class Discipline Program of Light Industry Technology and Engineering (Nos. LITE2018-18 and LITE2018-11) of China, the Transformation Project for Major Scientific and Technological Achievements in Jiangsu Province (No. BA2015006), the Industry-Academia Cooperation Innovation Fund Project of Jiangsu Province (No. BY2016022-19), and the National Key Technologies R&D Program of China for the 12th Five-year Plan (No. 2012BAD33B06) 2017-06-15 2017-11-20 2022-01-05 Copyright © Zhejiang University and Springer-Verlag GmbH Germany, part of Springer Nature 2018 2018 Journal of Zhejiang University-SCIENCE B (Biomedicine and Biotechnology) 07 19