Extraction optimization, structure modification and immunomodulatory activity in vitro for arabinoxylans from cereals

Zhang, Zhengxiao (2015) Extraction optimization, structure modification and immunomodulatory activity in vitro for arabinoxylans from cereals. Doctoral thesis (PhD), Manchester Metropolitan University.

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Abstract

BACKGROUD: The industrial cereal brans produced as by-products of milling have been confirmed to be rich sources of arabinoxylans (AXs). The immunomodulatory properties of the extracted AXs from cereals have been reported, which potentially lead to corresponding health benefit in immune enhancement. However, this requires a clearer understanding of the relationship between the molecular structures of AXs and their immune-modulating activities (the structure-activity relationship). It is also considered essential to gain an understanding of the mechanisms of the immune-modulating properties of AXs. AIMS & DESIGN: The aims of this study were to develop and improve the AXs extraction process from pentosan fraction of wheat flour and corn bran and to examine the molecular structures of extracted and modified AXs and investigate the molecular structure-immunomodulatory activity relationship in vitro for AXs. The effects of the different enzymatic extraction conditions (endoxylanase dosage, extraction temperature and time) on AXs extraction yield from the pentosan fraction of wheat flour were investigated by using a Box-Behnken experimental design and response surface methodology. For the AXs extraction process from corn bran, various extraction methods (alkaline, aqueous and enzyme extraction methods) were studied and compared. In addition, the treatments of three varieties of endoxylanases for AX molecular structure modification were studied. Furthermore, the extracted and modified AXs with different molecular structures were investigated with respect to their ability to modulate nitric oxide (NO) production and inducible NO synthase (iNOS) expression in vitro as an indication of their immunomodulatory potential. RESULTS: From the pentosan fraction, the maximum recovery rate of AXs reached 86% of total pentosan AXs (dry matter basis) under the optimum extraction conditions. In contrast to the AXs obtained by aqueous extraction, the molecular weight (Mw) distribution of enzymatically extracted AXs was significantly different being more concentrated in the low Mw range (1KDa to 10KDa). The degree of branching (A/X ratio) increased from 0.48 to 0.83 as the concentration of enzyme increased. From the corn bran, using alkaline treatment, the recovery rate of AX was up to 80% (dry matter basis) of total corn bran AX and the Mw distribution of extracted AXs was in the high Mw range (100KDa to 794KDa). Following enzymatic modification, more than 30% of AXs extracted were reduced to the lower Mw range (0.1KDa to 10KDa). In vitro studies showed that the extracted and modified AXs from these two cereal sources significantly elevated the level of NO synthesis and iNOS expression by U937 cells (p<0.05), but modified AXs with higher portion of low Mw showed stronger activity than extracted AXs with higher portion of high Mw (p<0.05). It was also observed that the stimulatory effect of AXs on NO production by U937 cells was associated with their concentrations and sources. In addition, the investigation on the immune-modulatory activity of AXs extracted from 10 cereal sources showed that the stimulatory effect of AXs on NO production by U937 cells seems to be associated with average molecular weight. More interestingly, it was noted that extracted and modified AXs had a significantly different effect on iNOS expression in U937 cells (p<0.05), suggesting that NO synthesis stimulated by AXs in vitro is closely mirrored by iNOS expression. CONCLUSIONS: As results of this study, the extraction process of AXs from corn bran and pentosan fraction of wheat flour was optimised. The conditions for modifying the molecular features of AXs were standardised. The experimental conditions for controlling the Mw distributions produced during the extraction and modification in order to enhance the immune-modulating activities of the AXs. The results of in vitro assessments should be useful in further understanding the mechanisms of the structure-activity relationship of AXs.