Biological activities and structural characterisation of polysaccharides from three Australian sweet lupin species

Abstract

Lupin is a major rotational crop produced abundantly in Australia. Three species of sweet lupins are grown in Australia which are blue lupin (Lupinus angustifolius), white lupin (Lupinus albus) and yellow lupin (Lupinus luteus). In this study, preliminary screening of crude polysaccharides from the three lupin species have been carried out with a view to make an informed decision about further detailed study on individual sweet lupin species (Chapter 3). Polysaccharides from all the three species exhibited significant antioxidant, immunostimulatory and prebiotic activities and hence have been selected for further detailed study. Blue lupin polysaccharides (BLPs) have been isolated and their biological activities investigated (Chapter 4). The antioxidant, Immunostimulatory and prebiotic activities of these fractions were measured. Three of the blue lupin polysaccharides (BLP-1, BLP-2 and BLP-5) displayed significant biological activities. FT-IR and 13C NMR studies have revealed that the lupin galactans are characterised by I²-Dgalactopyranosyl units linked through (1, 4) glycosidic bond. Lupin galactomannans have galactose to mannose ratio of 1:2 and mannopyranosyl units that are linked through I²-D-(1, 4) glycosidic bond and alternate mannopyranosyl units have a galactose unit branching out at ""OH6. The observed antioxidant and immunostimulatory activities of BLP-2 and BLP-5 have shown strong correlation to the quantities of galactans and galactomannans. Lupinus albus L. (white lupin) seeds have good nutritional profile that has not been fully exploited. In this study, water-soluble polysaccharides were isolated from the whole seeds of white lupin (Chapter 5) and purified and a spectrum of their biological activities investigated. Two isolated polysaccharide fractions WLP-1 and WLP-2 have displayed highly significant radical scavenging, iron chelating, immunostimulatory and prebiotic activities. These activities of WLP-1 and WLP-2 showed good correlation to their galactose contents. Structural characterisation revealed that the WLPs contain galactans and galactomannans with similar structure to that of blue lupin polysaccharides. In this research, polysaccharides from Lupinus luteus L (yellow lupin) were isolated from the whole seeds of yellow lupin and their bioactivities were investigated (Chapter 6). Yellow lupin polysaccharides (YLPs) exhibited good antioxidant, immunostimulatory and prebiotic activities. YLP-2 with largest galactose content displayed best activity amongst the three isolated polysaccharides. Spectroscopic methods were used to determine the structures of YLPs that are similar to blue and white lupin polysaccharides. The results presented in this thesis strongly suggest that the yellow lupin polysaccharides possess antioxidant, immune enhancing and prebiotic activities. The MRI contrast potentials of Gd (III) complexes (Chapter 7) with lupin polysaccharides have been investigated. Low molecular weight lupin polysaccharides were T1 agents and high molecular weight lupin polysaccharides were T2 agents. Excellent biological activities displayed by the polysaccharides from the three lupin species studied showed important biological activities suitable for nutraceutical and functional agents. Findings of this research strongly point to the fact that galactan is the major polysaccharide that determines the activities of sweet lupin polysaccharides.

Date of Award	2017
Original language	English