Purification and identification of a novel heteropolysaccharide RBPS2a with anti-complementary activity from defatted rice bran

Abstract

A novel heteropolysaccharide RBPS2a with anti-complementary activity was obtained from defatted rice bran by hot water extraction, ethanol precipitation, and purified by gel chromatography after anion-exchange chromatography. This fraction exhibited more potent anti-complementary activity than other polysaccharide fractions. RBPS2a was eluted as a single symmetrical narrow peak on high-performance gel-permeation chromatography (HPGPC) and the average molecular weight was 90,000 Da. We found RBPS2a contained 86.7% polysaccharide and 8.7% protein. The amino acid pattern showed that RBPS2a contained large amount of glutamic acid, arginine, aspartic acid, lysine, and alanine. The molar content of the above five amino acids constituted 59.31% of the total amino acids. Gas chromatography of absolute acid hydrolysate of RBPS2a suggested that it was composed of arabinose, xylose, glucose and galactose with a molar ratio of 4:2:1:4. The Fourier-transform infrared spectra (FT-IR) and ¹H, ¹³C NMR spectroscopy analysis revealed that RBPS2a had a backbone consisting of β-(1→3)-linked d-galacopyranosyl residues substituted at O-2 with glycosyl residues composed of α-d-xylose-(1→4)-α-d-arabinose-(1→ and α-d-glucose-(1→4)-α-d-arabinose-(1→ linked residues. Furthermore, some of the fractions extracted and purified from defatted rice bran exhibited strong anti-complementary activity. Among these fractions, the purified polysaccharide RBPS2a had the highest activity.

Introduction

Rice is consumed in its polished form as a staple food in many countries. Rice bran is the outer layer of brown rice, obtained as a by-product of the rice milling industry. It has been under-utilized as a human food and has traditionally been used primarily in feeds (Spears, Grieshop, & Fahey, 2004). Rice bran is a good source of protein, fat and carbohydrates, in addition to several valuable phytonutrients, antioxidants, vitamins and minerals (Asaf et al., 1997, Gurpreet and Sogi, 2007, Orthoefer, 1996, Qureshi et al., 2000). Studies have repeatedly shown that rice bran gives interesting health benefits (Azizah and Yu, 2000, Juan et al., 2006).

Rice bran polysaccharides have drawn the attention of chemists and immunologists in recent years because of the immune-modulation and anti-tumour properties. Many biologically active polysaccharides extracted from rice bran appeared to elicit excellent physiological properties in maintaining health and preventing diseases. For example, an arabinogalactan isolated from rice bran has anti-tumour properties in gastrointestinal carcinoma and colon cancer (Cummings et al., 1992, Takeshita et al., 1992). Rice bran hemicelluloses have a significant effect in increasing the peripheral blood lymphocytes (Takenaka & Itoyama, 1993), enhancing the immune function (Tzianabos, 2000), reducing thymus atrophy in rats (Takenaka, 1992), and improving the peripheral utilization of insulin (Hikino, Takahashi, Oshima, & Konno, 1988). MGN-3, a modified arabinoxylane from rice bran is a potent biological response modifier that is able to enhance natural killer (NK) cell activity in cancer patients, to increase T and B cell mitogen response upon ingestion, and has a strong anti-HIV activity (Ghoneum, 1998, Ghoneum and Gollapudi, 2003).

Anti-complementary activity is an immune-modulating activity, which is responsible to the immunological defense system. The complement system plays an important role in the host defense against foreign invasive organisms such as bacteria, fungi, and viruses (Yamagishi, Tsuboi, & Kikuchi, 2003). However, some smaller molecules involved complementary cascade could induce the release of mediators from mast cells and lymphocytes (Na, Byung, Hyeong, Jong, & Jae, 2005). Aberrant activation of these molecules may cause a variety of inflammatory diseases (Vogt, 1985). Therefore, the ability to modulate complement activity would clearly be beneficial in the therapy of inflammatory diseases. Data published revealed that many kinds of polysaccharides have shown strong capability on anti-complementary (Samuelsen et al., 1998, Zhang et al., 1997). It has been reported that polysaccharides from different resource have different anti-complementary activities in vitro, depending on their structural features. However, little attempt has been made to study the polysaccharides isolated from the defatted rice bran and their anti-complementary activity.

The aim of our study was to investigate the separation, bioactivity-directed fractionation, purification and identification of heteropolysaccharide RBPS2a. The anti-complementary activities of these major polysaccharide fractions isolated from defatted rice bran were also evaluated.