Acetylcholinesterase and butyrylcholinesterase inhibitory activity of some selected Nigerian medicinal plants

Atividade inibitória da acetilcolinesterase e butirilcolinesterase de algumas plantas medicinais da Nigéria

ARTIGO

Acetylcholinesterase and butyrylcholinesterase inhibitory activity of some selected Nigerian medicinal plants

Atividade inibitória da acetilcolinesterase e butirilcolinesterase de algumas plantas medicinais da Nigéria

Taiwo O. Elufioye^I; Efere M. Obuotor^II; Afolake T. Sennuga^II; Joseph M. Agbedahunsi^III,*; Saburi A. Adesanya^I

^IDepartment of Pharmacognosy, Faculty of Pharmacy Obafemi Awolowo University, Ile-Ife, Nigeria

^IIDepartment of Biochemistry, Obafemi Awolowo University, Ile-Ife, Nigeria

^IIIDrug Research and Production Unit, Faculty of Pharmacy Obafemi Awolowo University, Ile-Ife, Nigeria

ABSTRACT

Plants have been found to be useful as memory enhansers as well as antiaging. Twenty two of such plants from sixteen families were investigated for their acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE) inhibitory activities using the in vitro Ellman's spectrophotometric and in situ bioautographic methods with physostigmine as standard. At least three morphological parts were examined for each of the plants investigated and the test concentration was 42.5 µg/ mL. Some plants were active on both enzymes though with some morphological parts being more active than others. The root bark of Spondias mombin showed the highest activity to the two enzymes; 64.77% and 83.94% on AChE and BuChE respectively. Other plant parts of the selected plants exhibited some remarkable selectivity in their actions. Those selectively active against AChE were Alchornia laxiflora stem bark (41.12%) and root bark, Callophyllum inophyllurn root bark (56.52%). The leaves of C. jagus (74.25%), Morinda lucida leaves (40.15%), Peltophorum pterocarpum leaves and stem bark (49.5% and 68.85%, respectively), physiostigmine gave 90.31% inhibition. Generally higher activities were found against BuChE. Bombax bromoposenze leaves, root bark and stem bark were particularly active. The inhibition was over 80%. Other selective plant parts are the leaves Antiaris africana, Cissampelos owarensis aerial parts (78.96%), Combretum molle leaves and stem bark (90.42% and 88.13%, respectively), Dioscorea dumentorum root bark and tuber (over 87%), G. kola leaves, Markhamia tomentosa root bark, Pycnanthus angolensis stem bark and Tetrapleura tetraptera leaves. Most of these plants are taken as food or are food ingredients in Nigeria and may account for the low incidence of Alzheimer's disease in the country and may play certain roles in the mediation of the disease.

Keywords: Ethnomedicinal plants, acetylcholinesterase, butrylcholinesterase, memory loss.

RESUMO

As plantas podem ser úteis para estimular a memória, bem como serem usadas para combater o envelhecimento. Vinte e duas plantas, de dezesseis famílias, foram investigadas in vitro oara verificar sua atividade inibidora das enzimas acetilcolinesterase (AChE) e butirilcolinesterase (BuChE) pelo método espectrofotométrico de Ellman in situ e métodos de bioautografia utilizando fisostigmina como padrão. Pelo menos três partes morfológicas de cada planta foram analisadas e a concentração de ensaio foi de 42,5 µg/mL. Algumas plantas foram ativas em ambas as enzimas, embora com algumas partes mais ativas que outras. A casca da raiz de Spondias mombin apresentou a maior atividade as duas enzimas, 64,77% para AChE e 83,94% para BuChE. Outras partes das plantas selecionadas apresentaram boa seletividade em suas ações. As plantas seletivamente ativos contra AChE foram as casca do caule e casca da raiz de Alchornia laxiflora (41,12%), e casca da raiz de Callophyllum inophyllurn (56,52%). As folhas de C. jagus (74,25%), folhas de Morinda lucida (40,15%), folhas e casca do caule de Peltophorum pterocarpum (49,5% e 68,85%, respectivamente), physiostigmine inibiu 90,31%. Em geral, atividades melhoras foram apresentadas contra BuChE. Folhas, casca da raiz e casca do caule Bombax bromoposenze foram particularmente ativos. A inibição foi acima de 80%. Outras partes de algumas espécies também foram seletivas, como as partes aéreas de Antiaris africana, Cissampelos owarensis (78,96%), folhas e casca do caule de Combretum molle (90,42% e 88,13%, respectivamente), casca da raiz e de tubérculos de Dioscorea dumentorum (mais de 87%), folhas de G cola, cascas de raiz de Markhamia tomentosa, casca do caule de Pycnanthus angolensis e folhas de Tetrapleura tetraptera. A maioria destas plantas são utilizadas como alimentos ou ingredientes alimentares na Nigéria e podem ser responsáveis pela baixa incidência da doença de Alzheimer no país e desempenhar determinadas funções na mediação da doença.

Unitermos: Plantas medicinais, acetilcolinesterase, butrilcolinesterase, perda da memória.

INTRODUCTION

In Nigeria ethnomedicine, Alzheimer's disease (AD) is not well defined. However, senile conditions are observed amomg the aged which could be due to neurodegeneration. Some plants used in Indian and Chinese traditional medical practice have been justified as having potentials in the management of CNS related disorder. Bacopa monniera and Gingko biloba from Indian and Chinese traditional medicine respectively are well documented cognitive enhancers (Das et al, 2002). Phytochemical analysis of such plants led to the characterization of cyanotroside from Cynanchum atratum and zeathin from Fiatoua villosa as potent AChE inhibitors (Lee et al., 2004, Heo et al., 2002). Houghton et al 2004 has reported the cholinesterase inhibitory activity of two Nigerian Crinum species i.e. Crinum jagus and Crinum glaucum, which could justify their use as memory enhancers. It is also known that physiostigmine, a potent AChE inhibitor, was isolated from Physiostigma venenosu (Calabar bean), a medicinal plant native to Nigeria.

Acetylcholine is critical for an adequately functioning memory, and it is the subject of the majority of research looking for treatments for memory defects, like those found in Alzheimer's disease. Any mental health issue that involves memory or lack thereof, directly or indirectly relates to acetylcholine.

Loss of this neurotransmitter plays an instrumental role in the pathogenesis of AD. Postmorterm studies of AD patients consistently have demonstrated the loss of basal forebrain and cortical cholinergic neurons and the depletion of choline acetyltranferase, the enzymes responsible for acetylcholine synthesis. It is also known that the central cholinergic system plays a key role in the retrieval and storage of memory items in the central nervous system (CNS) of mammals (Taylor, 1991, Costa et al., 1999, Okello et al., 2004). There is increasing evidence that cognitive dysfunction due to loss or decline of cholinergic activity in the key areas of the brain, involved in cognition is a normal biological process associated with aging as well as some forms of progressive neurodegenerative disorders such as Alzheimer's disease (AD) (Perry et al., 1996). The decline of cholinergic activity can be ameliorated by agents that restore or enhance cholinergic transmission in the synaptic cleft (Grantham & Geerts, 2002). Current therapeutic strategies for the symptomatic treatment of Alzheimer's disease and other related disorders such as vascular dementia and dementia with Lewy bodies are aimed at enhancing the associated cholinergic deficit by inhibiting acetylcholinesterase (AChE) (Rosler et al., 1999) resulting in a boost in endogenous level of acetylcholine (ACh) in the brain and an improvement of the cognitive function (Costa et al, l991, Nordberg et al., 2001: Giacobini et al., 2002).

Although the precise role of BuChE in the pathogenesis of most dementia disorders have not been unequivocally delineated, it has however been observed that BuChE increases in the brain of AD patients. It has been suggested that inhibition of both enzymes may be important in the management of cognitive deficits associated with the AD (Greig et al., 2001, Ghayur et al., 2008) from our survey (unpublished work) some plants are used as memory enhansers and anti aging. Some of these plant are compiled and investigated for their AChE and BuChE inhibitory activities in order to ascertain their role in the traditional management of memory loss.

MATERIALS AND METHODS

Plant material

The various plant parts were collected from Obafemi Awolowo University (O.A.U), Ile-Ife, Nigeria Campus and authenticated by Dr. H. Iloh of the Botany Department, Obafemi Awolowo University, Ile-Ife. Herberia specimen of all the plants were prepared and voucher specimens deposited at the Herbarium located at the Department of Botany, O.A.U. Other details are as presented on Table1.

Chemicals

Acetylcholinesterase (EC.3.1.1.7) from electrical eel and butrylcholinesterase (EC 3.1.1.8) from horse serum were products of Fluka Co., Germany. Acetylthiocholine iodide (ATChI), butyrylthiocholine chloride (BTChCl), 5:5-dithiobis-2-nitrobenzoic acid (DTNB), eserine and sodium bicarbonate were purchased from Sigma Co. UK. Buffers and other chemicals were of analytical grade.

Extraction of samples

Fresh samples of the various plant parts were oven dried (the leaves and flower at 40 ºC, while root bark and stem bark were at 60 ºC). The different plant parts were separetely milled into powder. The powdered plant parts were soaked with 80% methanol for 72 h; the extracts were filtered and concentrated in vacuo using rotary evaporator at 40 ºC. The extracts were reconstituted in methanol to obtain a 1 mg/mL stock solution.

Cholinesterase assay

Acetycholinesterase and butyrylcholinesterase were respectively carried out using the colorimetric method of Ellman et al. (1961). The reaction assay mixture consisted 2000 mL 100mM phosphate butter pH8.0, 100ml of test sample stock solution in methanol (a final concentration of 42.5 µg/mL), 100 mL, of enzyme AChE or BuCHE solution at a final concentration of 0.03 U/mL and 0.01 µ/mL respectively, 100 µL of DTNB (0.3 mM) prepared in 100 M phosphate buffer pH 7.0 containing 120 mM sodium bicarbonate. The reaction mixture was vortexed and then pre-incubated in a water bath at 37 ºC for 30 min. The reaction was then initiated by the addition of 100 µL of ATCI or BTCI at a final concentration of 0.5 mM as a negative control. The inhibitor solution was replaced with methanol. The change in absorbance at λ_max 412 nm was then measured for a period of 5 min at ambient temperature. All assays were carried out in triplicate. The final concentration of the sample was 42.5 µg/mL. Eserine (-) physiotigmine) was used as positive control at same concentration. The % inhibition was calculated as follows:

% = x 100

Where: a = ΔA/min of control; b = ΔA/min of test sample; ΔA = Change in absorbance.

RESULTS AND DISCUSSION

The selective cholinergic neurodegeneration forms the basis for the so-called cholinergic hypothesis of cognitive hypofunction (Perry et al., 1996; Perry et al., 1998; Bartus et al., 1982), that postulates that many of the cognitive, functional and behavioral symptoms experienced by patients with cognitive dysfunction results from a deficiency in neurotransmitter ACh, and thus in cholinergic neurotransmission. Numerous pharmacological and lesion studies in animals have been shown to support the involvement of central cholinergic systems in learning and memory (Camps & Munoz-Torrero, 2002).

The cholinergic hypothesis for cognitive hypofunction has provided the rationale for the current major therapeutic approach to cognitive dysfunction: which holds that the enhancement or restoration of central cholinergic function may significantly improve the cognitive impairments present in cognitive disorders (Francis et al., 1999). Currently, the only approved therapies for cognitive dysfunction are a group of indirect cholinomimetics which enhance function by inhibiting ACh degradation.

Cholinesterase inhibitors (ChEI), most especially of AChE, however constitute, to date, the most effective approach to treat the cognitive symptoms of AD (Giacobini, 2002). They have shown clear therapeutic utility on both cognitive performances, as well as on the quality of life in these patients (Francis et al, 1999). Indeed, the only drugs currently approved for the treatment of cognitive disorders are AChEI (e.g. tacrine, donepezilm rivastigmine and galanthamine). A large amount of AChEI has been developed, which differ among themselves in selectivity for AChE and BuChE, mechanism of inhibition and reversibility.

Twenty two plants from sixteen diferent families i.e. Amaryllidaceae, Anacardiaceae, Apocynaceae, Bignoniaceae, Bombacaceae, Combretaceae, Convovulaceae, Dioscoreaceae, Euphorbiaceae, Guttiferaceae, Fabaceae, Menispermaceae, Moraceae, Myristicaceae, Rubiaceae, and Solanaceae were investigated for their AChE and BuChE activities in this study.

Alchornia laxiflora, Croton zambesicus, Jatropha curcas and Jatropha tangorensis all belonging to the Euphorbiaceae all showed weak (40-49%) activity towards both AChE and BuChE at 42.5 µg/mL except Croton zambesicus whose activity was moderately high (51.29%) towards AChE.

In the Bombaceae family, Bombax bromoposenze and Ceiba pentadra both showed low activity towards AchE (below 19%). However, Bombax bromoposenze showed very high activity towards BuChE (>83%). All the extracts of the different morphorlogical parts tested showed similarly high BuChE activity. This BuChE selective activity was also noted in the Dioscoreaceae family, >87% inhibition of BuChE was observed in the tubers and roots of Dioscorea dumetorum, leaves of Garcinia kola (86.46%) (Guttiferaceae), Combretum molle (Combretaceae), Cissampelos owarensis (Menispermaceae), rootbark of Markhamia tomentosa (Bignoniaceae) and the leaves of Tetrapleura tetraptera (Leguminoceae). Plants studied in this family gave >78% inhibition of BuChE while activity against AChE was rather low (< 24%).

The leaves of Crinum jagus (Amaryllidaceae), stembark of Peltophorum pterocarpum (Leguminosae), stembark of Pycnanthus angolensis (Myristicaceae), rootbark and stembark of Spondias mombin (Anarcardiaceae) all gave high AChE inhibition activity (>60%). Pycnanthus angolensis and Spondias mombin gave high activity in both AChE and BuChE. Their stem bark and root bark gave BuChE inhibition >80%.

Other plants studied, including Antaiaris africana (Moraceae), Capsicum frutensis (Solanaceae), Hollarhena floribunda (Apocynaceae) and Ipomea involucrata (Convovulaceae) all gave weak activity (<40%) towards the enzymes.

This non-selectivity of some anti-ChE has been adduced to the structural and functional homology between degrade acetylcholine and the continued use of ChEI with the dual ability to inhibit AChE and BuChE have been implicated in the number of side effects associated with some cholinesterase inhibitors presently in use. However, it has been speculated that this duality could lead to improved clinical outcomes, particularly in respect to the management of Alzheimer's disease (Greig et al, 2001). Thus the development of selective AChE inhibitors has been a primary challenge still confronting anticholinesterase pharmacology to date. While AChE is very specific in its substrate recognition and requirement, BuChE reconizes a broad range of substrates. For this reason, it has been suggested that circulating plasma BuChE could possibly be serving a scavenging function thereby protecting AChE from naturally occurring inhibitors. In this regard, BuChE could also scavenge anticholinesterase drugs, thereby elevating the dose necessary to achieve reasonable inhibitions at the target region of the brain for the effective therapeutic relief (Perry et al., 1996).

While most researches have been geared towards identifying AChE selective inhibitors, recent results has suggest that inhibition of both AChE and BuChE should be one of the objectives in the treatment of cognitive dysfunctions associated with cases of AD (Greig et al., 2001). Several researches are now aimed at finding plants that inhibits both enzymes (Okello et al., 2004). Thus, plants that show activity towards either or both enzyme may find some use in the management of memory dysfuntion.

CONCLUSION

This work shows that most of the plant parts tested showed cholinesterase inhibitory activity towards either AChE, BuChE or both enzymes and may be considered for further studies in the management of AD. Plants that selectively inhibits AChE may be more useful in early stages of AD while those that inhibits both AChE and BuChE may be of paricular interest in severes cases of AD.

Received 24 March 2009; Accepted 5 August 2009

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Publication Dates

• Publication in this collection
  10 Sept 2010
• Date of issue
  Sept 2010

History

• Received
  24 Mar 2009
• Accepted
  05 Aug 2009