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Abstract
Australian native plants have a long history of being used for nutritional and medicinal purposes; however, the scientific foundation of such uses is not well established. In this thesis, three native Australian plants, namely samphire (Sarcocornia quinqueflora), saltbush (Atriplex nummularia) and sea parsley (Apium prostratum), were investigated for their phenolic compositions, antioxidant capacities and inhibitory activities on four enzymes: α-glucosidase, α-amylase, pancreatic lipase and hyaluronidase, for the first time. These enzymes are closely related to disorders such diabetes, overweight, obesity and inflammation. Furthermore, the phenolic compounds in sea parsley were identified and quantified by a combination of high-performance liquid chromatography (HPLC) and liquid chromatography-high resolution mass spectrometry (LC-HDMS) analyses. Phenolic compounds in the plants were extracted with methanol 80% (v/v) and purified with XAD-7 Amberlite® resin. The three plants contained relatively high levels of phenolic compounds and antioxidant capacities as well as enzyme-inhibition activities that are comparable with other Australian native plants. Of the three plants, sea parsley had the highest total phenolic content, exhibited the largest ABTS and DPPH free-radical scavenging capacities, and was the most potent inhibitor of α-glucosidase, α-amylase and pancreatic lipase. Samphire had the highest ferric reducing antioxidant power (FRAP) and was the most potent inhibitor of hyaluronidase while saltbush had the lowest phenolic content, displayed the lowest antioxidant capacity and was the least potent inhibitor of all the enzymes. Purification of the extracts resulted in a significant concentration of the phenolic compounds (1.43-2.67 times) with corresponding increases in the bioactivities. Seven phenolic compounds were identified in sea parsley, with the main ones being apiin (48.2%), apigenin (24.8%), caffeic acid (6%) and ferulic acid (2%), while the minor compounds were ρ-coumaric acid, luteolin and catechin which were present at trace levels (<1%). Significantly, catechin was identified for the first time in the Apium genus. The high levels of health-related bioactivities and the presence of several phenolic compounds known to have disease-preventing effects indicate that the consumption of these native Australian plants could bring significant health benefits to the consumer.