Article
Acetylcholinesterase Inhibitory, Antioxidant and Phytochemical Properties of Selected Medicinal Plants of the Lamiaceae Family
1
Department of Pharmacognosy, Faculty of Pharmacy and Biochemistry, University of Zagreb, Marulićev trg 20, Zagreb 10000, Croatia; E-Mails:svladimir@pharma.hr (S.V.-K.); mkindl@pharma.hr (M.K.)
2
Department of Biotechnology and Pharmaceutical Engineering, Faculty of Technology, University of Novi Sad, Bulevar cara Lazara 1, Novi Sad 21000, Serbia; E-Mail: vladicjelena@gmail.com
3
Department of Pharmacognosy, Institute of Pharmaceutical Sciences, Karl-Franzens-University Graz, Universitaetsplatz 4, Graz 8010, Austria; E-Mails:agnieszka.ln86@gmail.com (A.D.L.-N.); adelheid.brantner@uni-graz.at (A.H.B.)
*
Author to whom correspondence should be addressed; E-Mail:bblazekovic@pharma.hr; Tel.: +385-1-6394-793; Fax: +385-1-6394-400.
Received: 6 November 2013; in revised form: 31 December 2013 / Accepted: 2 January 2014 /
Published: 9 January 2014
Published: 9 January 2014
Abstract
: The present study aimed to evaluate acetylcholinesterase (AChE) inhibitory and antioxidant activities of Lamiaceae medicinal plants growing wild in Croatia. Using Ellman’s colorimetric assay all tested ethanolic extracts and their hydroxycinnamic acid constituents demonstrated in vitro AChE inhibitory properties in a dose dependent manner. The extracts of Mentha x piperita, M. longifolia, Salvia officinalis, Satureja montana, Teucrium arduini, T. chamaedrys, T. montanum, T. polium and Thymus vulgaris at 1 mg/mL showed strong inhibitory activity against AChE. The antioxidant potential of the investigated Lamiaceae species was assessed by DPPH• scavenging activity and total antioxidant capacity assays, in comparison with hydroxycinnamic acids and trolox. The extracts differed greatly in their total hydroxycinnamic derivatives content, determined spectrophotometrically. Rosmarinic acid was found to be the predominant constituent in most of the investigated medicinal plants (by RP-HPLC) and had a substantial influence on their AChE inhibitory and antioxidant properties, with the exception of Teucrium species. These findings indicate that Lamiaceae species are a rich source of various natural AChE inhibitors and antioxidants that could be useful in the prevention and treatment of Alzheimer’s and other related diseases.
Keywords:
Croatian medicinal plants; Lamiaceae; acetylcholinesterase inhibition; antioxidant; hydroxycinnamic derivatives; rosmarinic acid1. Introduction
Alzheimer’s disease (AD), the most common cause of dementia, is a progressive age-related neurodegenerative disorder which is becoming a serious public health issue and a massive economic burden. AD patients lose their memory, their cognitive abilities and even their personalities may change dramatically. These changes are due to the progressive dysfunction and death of nerve cells responsible for the storage and processing of information [1]. AD arises as a result of malfunction of different biochemical pathways. Multiple pathogenic factors, including aggregated amyloid-β-peptide and tau protein, excessive transition metals, oxidative stress and reduced acetylcholine (ACh) levels have been implicated in AD pathology [2]. Acetylcholinesterase (AChE), the predominant cholinesterase in the brain, hydrolyzes ACh to choline and acetate, thereby terminating the effect of this neurotransmitter at cholinergic synapses. Therefore, AChE is the target of cholinesterase inhibitors used for addressing the cholinergic deficit in AD patients. Despite decades of research and advances in our understanding of its aetiology and pathogenesis, current pharmacotherapeutic options for AD are still very limited and represent an area of need that is currently unmet. The leading AD therapeutics involve AChE inhibitors, resulting in increased acetylcholine concentrations in the synaptic cleft and enhanced cholinergic transmission [3,4]. Compounds showing an AChE inhibitory effect are also used for the treatment of senile dementia, myastenia gravis, Parkinson’s disease and ataxia [5]. Taking into account that the inhibition of AChE has been one of the most used strategies for treating AD and that existing drugs are effective only against mild to moderate type of disease while presenting considerable side effects, the search for new sources of effective and selective antiacetylcholinesterase agents with fewer side effects is imperative. The free radical and oxidative stress theories of aging suggest that oxidative damage is a major player in the degeneration of cells. In this respect, recent findings clearly indicate that oxidative damage is one of the earliest events in the pathogenesis of AD, so targeting oxidative stress could be considered beneficial in both its prevention and treatment [6].
Owing to their richness in secondary metabolites that exhibit a remarkable diversity of both chemical structures and biological activities, medicinal plants are being recognized as promising sources of lead compounds for new drugs targeting neurodegenerative diseases [7,8,9]. Croatia is one of Europe’s most biodiversity-rich countries due to its unique geographical position. As an integral part of the Mediterranean area, its flora is represented by a high number of species and subspecies, in total 5,516 taxa belonging to 184 families, among which Lamiaceae could be considered one of the richest in medicinal plants [10]. Lamiaceae species have been reported to possess a wide range of biological activity, and a wide diversity of phytochemicals. Essential oils, hydroxycinnamic acids and flavonoids were found as the major bioactive constituents of the most common Lamiaceae species, such as thyme, rosemary, peppermint, sage, lemon balm and oregano [11]. The essential oil composition of Thymus vulgaris can vary greatly and various chemotypes have been recorded, particularly regarding thymol and carvacrol. Other constituents of thyme include rosmarinic acid and flavonoids (luteolin, eriodictyol, apigenin and some methylated flavones) [12,13]. Rosemary leaves contain an essential oil whose composition may vary according to the plant chemotype (eucalyptol, camphor-borneol and α-pinene-verbenone types are distinguished). Rosmarinic acid and flavonoids as well as diterpenes, which are structural derivatives of carnosic acid, belong to the phenolic fraction [14,15]. Menthol and menthone are the major components of peppermint essential oil. The phenolic constituents of peppermint include rosmarinic acid and several flavonoids, primarily eriocitrin, luteolin and hesperidin [16]. The essential oil of Salvia officinalis is characterized by a high amount of α- and β-thujone. The non-volatile fraction of sage is mainly composed of various diterpenes, phenolic acids and flavonoids [17,18]. Lemon balm yields only a small quantity of essential oil, with citral and citronellal as the principal components. It also contains hydroxycinnamic acids (rosmarinic, p-coumaric and caffeic acids) and flavonoids, e.g., luteolin, quercetin, apigenin and kaempferol [19]. The composition of Origanum vulgare essential oil from different geographical origins is most commonly characterised by carvacrol and thymol as the major components, though the proportions vary widely. The phenolic compounds including flavonoids and phenolic acids are another kind of abundant constituent in oregano [20]. Investigation of the species belonging to the genus Teucrium revealed the presence of diterpenes, triterpenes, phytosterols, iridoids, flavonoids and essential oils [21,22]. It was found that Lavandula species contain essential oil, triterpenes, coumarins, hydroxycinnamic acids and flavonoids [23]. Most of the chemical studies on Micromeria andCalamintha species were carried out for investigation of their volatile constituents and flavonoids [24,25,26].
A large number of species belonging to the genus Calamintha, Lavandula, Mentha,Melissa, Origanum, Rosmarinus, Salvia, Teucrium and Thymus has traditionally been used in Croatia and neighbouring countries to treat respiratory diseases, gastrointestinal problems and various nervous system disorders [27,28,29]. Some of them have been investigated for their antioxidant and neuroprotective effects using various in vitro and in vivo methods. These activities have mostly been attributed to the presence of polyphenols, particularly rosmarinic acid [30,31,32,33]. Along with other more common hydroxycinnamates, such as caffeic, ferulic, p-coumaric and chlorogenic acid, rosmarinic acid is widely distributed in the plant kingdom, occurring particularly in species of the Lamiaceae and Boraginaceae families. Recent biological and pharmacological studies have shown that this phytochemical possesses many beneficial effects, including antiviral, antibacterial, antioxidant, antiinflammatory, antiangiogenic, antidepressive, anticancer and antihepatotoxic activities [11,34,35].
Aiming to discover new and promising sources of potential anti-AD drugs, the present study was undertaken to evaluate the antiacetylcholinesterase and antioxidant properties of selected Croatian medicinal plants belonging to the Lamiaceae family. Additionally, their hydroxycinnamic acid profiles were investigated.
