Composition of native Australian herbs polyphenolic-rich fractions and in vitro inhibitory activities against key enzymes relevant to metabolic syndrome
Highlights
► Native Australian herbs identified as new sources of ellagic acid and derivatives. ► Identified significant inhibitory effect of Australian herbs against α-glucosidase. ► Tasmannia pepper leaf exhibited pronounced inhibitory activity towards lipase. ► All herbs showed some inhibitory effect towards angiotensin converting enzyme. ► The herbs have a potential to be utilised in functional food/nutraceuticals.
Introduction
Among the native Australian plants there are many with a history of medicinal use. Barr. A. et al. (1993) described over 300 medicinal plant sources from the Northern Territory, encompassing knowledge previously disseminated only by word of mouth. More recently, Lassak and McCarthy (2001) published a compendium of medicinal plants from throughout Australia, which included information of their traditional uses, methods of preparation, and any identified pharmacological constituents.
In recent years in Australia native edible plants are being re-discovered and are being used in sweet and savoury products, as well as herbal infusions. Although a number of native herbs and spices have already been commercialised, information on their potential health benefits is limited and unsubstantiated.
The use of herbs and spices in food is steadily increasing and attention has been given to indigenous plants, which are also suggested as possibly being a source of pharmaceutically active ingredients (Yu, Rafi, & Ho, 2006). Earlier studies on common culinary and medicinal herbs reported their superior antioxidant capacities to berries, fruits, vegetables, and nuts, which originated from presence of polyphenolic compounds (Zheng & Wang, 2001). Polyphenolic-rich extracts obtained from herbs were reported to modulate the activity of selected digestive enzymes, such as α-glucosidase (Matsui et al., 2001), lipase (Grove, Sae-tan, Kennett, & Lambert, 2011) and angiotensin I-converting enzyme (ACE) (Balasuriya & Rupasinghe, 2011).
Metabolic syndrome, characterised by glycemic index imbalance, glucose intolerance, hypertension, dyslipidemia and/or obesity, is an early sign of potential future development of chronic conditions, such as type 2 diabetes, which is characterised by postprandial hyperglycemia (a rapid increase of blood glucose level after food consumption). The rapid increase of blood glucose can be reduced through inhibition of enzymes involved in the release of glucose from foods and this approach is used in the management of type 2 diabetes, with the main target being α-glucosidase enzyme. α-Glucosidase is a membrane-bound enzyme located in the epithelium of the small intestine, that catalyses the cleavage of glucose from disaccharides and oligosaccharides, which facilitates an uptake of glucose into the blood stream. Hence, inhibition of α-glucosidase activity reduces glucose release and subsequently the uptake. Consumption of α-glucosidase inhibitors naturally occurring in food is an important supporting factor in the management of postprandial hyperglycemia. Ishikawa et al. (2007) reported that ingestion of leaves from an Indian plant Nerium indicum Mill., used as a folk remedy for type II diabetes, reduced postprandial blood glucose level in humans. An inhibitory effect of an endemic Sri Lankan plant Cassia auriculata (Leguminosae) against α-glucosidase was reported to be comparable to that of a therapeutic drug acarbose (Abesundara, Matsui, & Matsumoto, 2004). The inhibitory activities of plant extracts vary among the plants depending on their phytochemical composition. Significant differences in α-glucosidase inhibitory activity were reported due to the molecular structure and substitution pattern of anthocyanin molecules (Matsui et al., 2001).
Two other enzymes that have an impact on metabolic syndrome are pancreatic lipase (Grove et al., 2011) and angiotensin I-converting enzyme (ACE) (Balasuriya & Rupasinghe, 2011). Lipase, primarily produced in the pancreas, hydrolyses lipids to form fatty acids so they can be absorbed in the human digestive system. Pancreatic lipase is the key enzyme which hydrolyses triglyceride into glycerol and fatty acids, facilitating an uptake of fat (triglycerides). Grove et al. (2011) suggested that inhibition of pancreatic lipase was the possible mechanism of modulatory activity of epigallocatechin-3-gallate isolated from tea on fat absorption in male C57bl/6 mice on a high fat diet, which resulted in increased faecal lipid content (by 29.4%) and reduced final body weight.
Angiotensin converting enzyme (ACE) plays an important part in the regulation of blood pressure and normal cardiovascular function. It catalyses the conversion of angiotensin I to angiotensin II, which increases blood pressure, therefore inhibition of ACE may help to reduce hypertension (Shalaby, Zakora, & Otte, 2006). Intake of pycnogenol, a proanthocyanin oligomer, isolated from French maritime pine (Pinus maritime L.) was reported as an effective mediator of blood pressure regulation in humans, possibly due to the inhibition of ACE (Zibadi, Rohdewald, Park, & Watson, 2008). Suzuki et al. (2002) demonstrated ACE inhibitory activity of pure chlorogenic acid in hypertensive rats. Kozuma, Tsuchiya, Kohori, Hase, and Tomokitsu (2005) demonstrated blood pressure lowering effect in humans with mild hypertension of aqueous extracts of green coffee beans, containing chlorogenic acid as the main compound.
The objective of the present study was to evaluate the efficacy of three commercially grown native Australian herbs; anise myrtle, lemon myrtle and Tasmannia pepper leaf against the principal components of metabolic syndrome; hyperglycemia, dislipidemia and hypertension. These herbs were selected based on a preliminary screening of their antioxidant properties (Konczak, Zabaras, Dunstan, & Aguas, 2010). Anise myrtle and lemon myrtle represent the Myrtaceae family, distributed in South America, Southern Asia, Africa and Australia. They superficially resemble bay leaf and are used in a similar manner: in seasoning and in preparation of main meals. Additionally, due to their unique flavour in Australia they are included in herbal infusions. Tasmannia pepper is endemic to Australia, and grows in Tasmania and the south-eastern part of Victoria. Due to its pungency it is used in cooking and as an additive in processed foods, e.g. cheese. During food preparation, food consumption and the digestive process, food constituents are released and may interact with a number of digestive enzymes. Thus, the aim of our study was to analyse the composition of phenolic-rich fractions obtained from anise myrtle, lemon myrtle and Tasmannia pepper leaf and to evaluate their inhibitory effects on the key enzymes relevant to metabolic syndrome: α-glucosidase, pancreatic lipase and angiotensin I-converting enzyme (ACE). It needs to be mentioned that the results obtained in this study originate from a single lot of samples produced during one vegetative season using plant sources selected by the industry. Commercially available bay leaf (Laurus nobilis L.) was used as a reference sample.
Section snippets
Plant material
Dry sample of Tasmannia pepper leaf (Tasmannia lanceolata R. Br., Winteracea; TPL) was supplied by the company Diemen Pepper (Birchs Bay, Hobart, Tasmania, Australia); anise myrtle (Syzygium anisatum Vickery, Craven, & Biffen, Myrtaceae; AM) and lemon myrtle (Backhousia citriodora F. Muell, Myrtaceae; LM) were obtained from the Australian Rainforest Products (Lismore, NSW, Australia). Commercially available bay leaf (Laurus nobilis L., Lauraceae) (Hoyts Food Industries Pty., Ltd., Moorabbin,
Yield, antioxidant capacity and composition of polyphenolic-rich fractions
The extraction yield of polyphenolic-rich fractions obtained from anise myrtle (AM), lemon myrtle (LM), Tasmannia pepper leaf (TPL) and the reference sample bay leaf (BL) are presented in Table 1. TPL produced the largest polyphenolic-rich fraction (162.6% that of BL). The polyphenolic extraction yields of AM and LM were comparable to that of BL. The fractions were characterised with regards to total phenolics, total flavonoids and their antioxidant capacities using reagent-based assays (Table 1
Conclusions
Polyphenolic-rich fractions obtained from Tasmannia pepper leaf, anise myrtle and lemon myrtle contained higher levels of total phenolics and exhibited significantly higher antioxidant capacities than bay leaf used as a reference sample. Ellagic acid, ellagitannins and ellagic acid glycosides were tentatively identified as the major constituents of anise myrtle and lemon myrtle, and were accompanied by flavonoids (catechin, myricetin, quercetin and hesperetin). Tasmannia pepper leaf comprised
Acknowledgement
This project was supported by the Rural Industries Research and Development Corporation (RIRDC), Australia.
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