Structural characterization and evaluation of antioxidant, anticancer and hypoglycemic activity of radiation degraded oat (Avena sativa) β- glucan
Highlights
- •Oat β-d-glucan was effectively degraded by gamma irradiation.
- •Degraded oat β-d-glucan significantly modified the biological activities.
- •Modification in biological activities was dose dependent.
- •Radiation processed beta-glucan was biocompatible with normal cells.
- •XRD analysis revealed no change in diffraction pattern of control and irradiated samples.
Abstract
Oat β-d-glucan after extraction was degraded at doses of 3, 6, 9, 12 and 15 kGy. The average molecular weight decreased to 45 kDa at dose of 15 kGy from an initial value of 200 kDa in native sample. XRD analysis revealed no significant change in diffraction pattern of irradiated samples when compared with control, except a decrease in intensity of x-ray diffraction. The results of the antioxidant activity revealed decrease in EC50 values and corresponding increase in antioxidant activity of radiation degraded oat β-d-glucan. Results of the anticancer studies indicated that cytotoxicity of gamma irradiated oat β-d-glucan in cancer cell lines was highest against colo-205 and MCF7 cancer cells compared to T47D cell and no cytotoxicity was observed in normal cell lines at all concentrations used. Evaluation of hypoglycemic activity showed highest inhibition in α-glucosidase activity compared to α-amylase activity due to gamma irradiation of oat β-d-glucan. Comparison of the EC50 values of known standards and gamma irradiated oat beta-glucan samples indicates that radiation treatment significantly modified the biological activity of the beta-glucan samples. Therefore, it is suggested that gamma irradiation can be used for producing low molecular weight oat β-d-glucan; which can help in modifying the biological activities.
Introduction
Oats (Avena sativa) is a cereal grain and a good source of dietary fiber – both soluble and insoluble, antioxidants, proteins and unsaturated fat. Oats has been regarded as a health promoting food and can help prevent a range of ailments when consumed on a regular basis. Consumption of oatmeal can lower cholesterol levels by reducing the ability of blood cells to stick to the insides of the artery walls (Zhang and Daou, 2012). These beneficial effects of oats are chiefly due to the soluble fiber content of oats. The present interest in soluble oat fiber originated from the reports that showed that dietary oats can help in lowering cholesterol, postprandial blood glucose level as well as modifying immune response and reducing risk of various cancers (Regand et al., 2011).
The principal component of the soluble fiber in whole oats comprises a class of polysaccharides known as beta-d-glucan often referred as beta-glucan. Beta-glucan is composed of glucose units linked together by (1−3) linkage of cellotriosyl and cellotetraosyl units to form a long polymer chain. These β- glucans have immune-enhancing activities, which nutritionally potentiate and modulate an immune response (Borchers et al., 2004). The biological activities and immunemodulatory effects of beta-glucan are influenced by molecular weight, degree of branching, length of branch and higher order structure (Qi et al., 2005). Among these, molecular weight is one of the most important factors determining the biological activities of polysaccharides (Hou-Jin et al., 2016, Jiao et al., 2011, Wang et al., 2010). High molecular weight polysaccharides result in low solubility and processability, therefore limiting their penetration and accessibility into the cell to perform a function. Numerous reports are available in the literature regarding the molecular weight-activity relationship of polysaccharides (Hou-Jin et al., 2016, Jiao et al., 2011). Recently it has been reported that low molecular weight laminarin induce the expression of gene coding for immune response proteins and reduce apoptotic cell death (Kim et al., 2006). In another study, it has been reported that low molecular fucoidan promotes revascularization of hindlimb ischemia in rats, boosts osteoblast proliferation for bone regeneration, enhances human endothelial cell formation and increases cytotoxicity in cancer cells (Choi and Kim, 2013).
Various methods (acidolysis, enzymolysis, and gamma irradiation) are used to produce low molecular weight β-glucan. Gamma irradiation leads to the degradation of polysaccharides by the cleavage of glycosidic bonds. The basic advantage of degradation of polysaccharides by radiation include the ability of the process to promote changes reproducibly and quantitatively without the introduction of chemical reagents and minimizes structural changes and side reaction (Cho et al., 2003). There are several reports indicating that low molecular weight polysaccharides degraded by either chemical treatment or gamma irradiation enhance biological activities. Choi et al. (2011) reported that antioxidant activity of laminarin, a polysaccharide in seaweed increases after gamma irradiation. Similar beneficial biological effect has also been reported for low molecular weight fucoidan produced by gamma irradiation (Choi and Kim, 2013). Literature review also reveals that very few studies have been conducted on low molecular weight β-glucan. Byun et al. (2008) reported that β-glucan from black yeast could be efficiently degraded by gamma irradiation without changing functional groups. Preliminary study conducted by Shah et al. (2015) reported that low molecular weight barley β-glucan produced by gamma irradiation had good antioxidant activities. However, the effects of gamma irradiation on the health promoting effects of oat β-glucan have not been evaluated in detail and no comparison has been made between available standards and the irradiated beta glucan samples with respect to biological activities. Therefore the purpose of the present study was to investigate the effect of radiation processing on the structural changes of oat β-glucan and enhancement in biological activities and subsequent comparison with the standards.
Section snippets
Oats samples
The Oats samples of Sabzar variety were purchased from the Department of Agriculture, Jammu and Kashmir, India. The Oats grains were subjected to milling and sieved through 0.50 mm mesh to obtain flour. The flour samples thus obtained were used for extraction of β-glucan.
Extraction of β-glucan
β-glucan extraction was carried out according to the method of Asif et al. (2010). 1 kg of Oat flour was refluxed with 80% ethanol for 1 h followed by mixing with 1 M NaOH in a ratio of 1:7. The contents were stirred on hot plate
Analysis and molecular weight of oat β-d-glucan
The β-d-glucan enzymatic assay kit revealed that purity of oat β-d-glucan used in this study was 91.2%. The changes in the molecular weight of oat β-d-glucan due to gamma irradiation treatments is shown in Fig. 1(A). It is seen from the graph that average molecular weight decreased with increase in irradiation dose. When the irradiation dose was 3 kGy, the molecular weight of oat β-d-glucan decreased to 144 kDa from 200 kDa; contributing to a decrease of 28% in molecular weight. The molecular
Conclusion
Based on the results, it is concluded that oat β-d-glucan can be efficiently degraded to low molecular weight by gamma irradiation. Gamma irradiation significantly (p ≤ 0.05) enhanced the solubility and water absorption capacity of oat beat-glucan in dose dependent manner. Close comparison of the EC50 values of known standards and gamma irradiated oat beta-glucan samples with respect to DPPH radical scavenging, ferric reducing ability power, anti-cancer and hypoglycemic activities indicates
Acknowledgements
Authors acknowledge the support extended by the Division of Analytical chemistry, BARC, Mumbai for conducting FTIR and XRD studies and Division of Biotechnology, Indian Institute of Integrated Medicine, Jammu for carrying out the cell-line studies.
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