Composition of native Australian herbs polyphenolic-rich fractions and in vitro inhibitory activities against key enzymes relevant to metabolic syndrome

Polyphenolic-rich fractions obtained from three native Australian herbs: Tasmannia pepper leaf, anise myrtle and lemon myrtle were characterised with regards to their composition, antioxidant capacities and inhibitory activities against α-glucosidase, pancreatic lipase and angiotensin I-converting enzyme, using in vitro models. Ellagic acid and derivatives were the dominant compounds of anise myrtle and lemon myrtle fractions, accompanied by flavonoids (catechin, myricetin, hesperetin, and quercetin). Tasmannia pepper leaf fraction comprised chlorogenic acid and quercetin derivatives, exhibited the highest oxygen radical absorbance capacity and effectively inhibited α-glucosidase (IC50: 0.83 mg/ml) and pancreatic lipase (IC50: 0.60 mg/ml). Anise myrtle and lemon myrtle fractions had pronounced α-glucosidase-inhibitory activities (IC50: 0.30 and 0.13 mg/ml, respectively) and were less effective against lipase. Enzyme-inhibitory activities showed various levels of correlation with the levels of total phenolics and antioxidant capacities, indicating a specificity of individual phenolic compounds present in the isolated fractions to complex with proteins.