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Antioxidant and acetylcholinesterase-inhibitory properties of long-term stored medicinal plants

  • Stephen O Amoo1,
  • Adeyemi O Aremu1,
  • Mack Moyo1 and
  • Johannes Van Staden1Email author
BMC Complementary and Alternative MedicineThe official journal of the International Society for Complementary Medicine Research (ISCMR)201212:87

DOI: 10.1186/1472-6882-12-87

Received: 3 April 2012

Accepted: 7 July 2012

Published: 7 July 2012

Abstract

Background

Medicinal plants are possible sources for future novel antioxidant compounds in food and pharmaceutical formulations. Recent attention on medicinal plants emanates from their long historical utilisation in folk medicine as well as their prophylactic properties. However, there is a dearth of scientific data on the efficacy and stability of the bioactive chemical constituents in medicinal plants after prolonged storage. This is a frequent problem in African Traditional Medicine.

Methods

The phytochemical, antioxidant and acetylcholinesterase-inhibitory properties of 21 medicinal plants were evaluated after long-term storage of 12 or 16 years using standard in vitro methods in comparison to freshly harvested materials.

Results

The total phenolic content of Artemisia afra, Clausena anisata, Cussonia spicata, Leonotis intermedia and Spirostachys africana were significantly higher in stored compared to fresh materials. The flavonoid content were also significantly higher in stored A. afra, C. anisata, C. spicata, L. intermedia, Olea europea and Tetradenia riparia materials. With the exception of Ekebergia capensis and L. intermedia, there were no significant differences between the antioxidant activities of stored and fresh plant materials as measured in the β- carotene-linoleic acid model system. Similarly, the EC50 values based on the 2,2-diphenyl-1-picrylhydrazyl (DPPH) free radical scavenging assay were generally lower for stored than fresh material. Percentage inhibition of acetylcholinesterase was generally similar for both stored and fresh plant material. Stored plant material of Tetradenia riparia and Trichilia dregeana exhibited significantly higher AChE inhibition than the fresh material.

Conclusions

The current study presents evidence that medicinal plants can retain their biological activity after prolonged storage under dark conditions at room temperature. The high antioxidant activities of stable bioactive compounds in these medicinal plants offer interesting prospects for the identification of novel principles for application in food and pharmaceutical formulations.

Keywords

Antioxidants Acetylcholinesterase inhibition Long-term storage Medicinal plants Radical scavenging activity

Background

The detrimental effects of oxidative stress to human tissues and cells caused by reactive oxygen species (ROS) arising from aging and disease pathogenesis is well documented. Though the human body has inherent antioxidative mechanisms to counteract the damaging effects of free radicals, there is often a need to use dietary and/or medicinal antioxidant supplements, particularly during instances of disease attack. An imbalance between ROS such as singlet oxygen, superoxide anion radical, hydroxyl radical and hydrogen peroxide, and the natural detoxification capacity of the body in favour of the oxidant molecules causes oxidative stress leading to cellular and DNA damage as well as oxidation of low-density lipoproteins [1, 2]. Oxidative stress disorders caused by the actions of ROS are associated with many acute and chronic diseases such as inflammation and neurodegenerative conditions including Alzheimer’s disease (AD) [3]. Alzheimer’s disease, an age-related neurological disorder, is characterised by progressive loss of cognitive ability primarily memory loss, leading to dementia. The main strategy in the clinical treatment of AD involves the maintenance of adequate levels of acetylcholine (ACh) at neurotransmission sites [4]. Thus, the inhibition of acetylcholinesterase (AChE) prevents the hydrolysis of ACh thereby maintaining normal memory function. The consumption of antioxidants is highly correlated with lower incidences of AD [5, 6]. As a result, the use of natural compounds with high levels of antioxidants has been proposed as an effective therapeutic approach for AD [5].

Against a background of growing concerns about the toxicity and side effects of many synthetic therapeutic agents, there has been a renewed interest globally, in the search for antioxidants and AChE inhibitory compounds from natural sources, particularly medicinal plants [1, 2, 714]. Medicinal plants have long been used to treat cognitive memory dysfunction symptoms [4, 5, 1519]. The growing relevance of medicinal plants as possible sources for the discovery of novel antioxidant molecules is often based on their long historical utilisation in folk medicine, especially in developing countries. In addition, the recognised health benefits of medicinal plants emanate from their prophylactic properties [6]. Most notably, traditional practices in the Ayurvedic, Chinese and African medicinal systems are strongly based on prevention and the promotion of good health; hence plant extracts and herbal preparations are regularly consumed as rejuvenators, tonics and/or nutritional supplements [8]. Traditional medicine practitioners and gatherers often store plants before they are eventually consumed. However, there is a dearth of scientific data on the stability and efficacy of the bioactive compounds in medicinal plants after prolonged storage. In the present study, 21 commonly used South African medicinal plants (Table 1) were investigated for their phytochemical, antioxidant and AChE-inhibitory properties after 12 or 16 years storage in comparison to freshly harvested material. These plants are used in traditional medicine to prevent and/or treat pain-related ailments and infections [2023]. Fresh materials were harvested from the same locations and season as the stored materials [21, 23] to minimise any differences due to geographical and seasonal effects [24].
Table 1

Effect of long-term storage on the total iridoid, phenolic and flavonoid contents of 21 South African medicinal plants

Plant name

Family

Voucher number

Plant part(s)

Total iridoids (μg HE/g DW)

Total phenolics (mg GAE/g DW)

Total flavonoids (mg CE/g DW)

Stored

Fresh

Stored

Fresh

Stored

Fresh

Acokanthera oppositifolia (Lam.) Coddδ

Apocynaceae

A. Aremu 1 NU

Roots

264.6 ± 4.82 **

134.5 ± 5.51

7.5 ± 0.37 *

9.3 ± 0.44

4.8 ± 0.12 *

5.4 ± 0.17

Artemisia afra Jacq. ex Willd#

Asteraceae

S. Amoo 15 NU

Aerial parts

356.9 ± 22.72 ns

341.7 ± 19.97

28.5 ± 1.15 ns

25.8 ± 0.03

18.3 ± 0.65 ns

16.7 ± 0.34

Artemisia afra Jacq. ex Willdδ

Asteraceae

S. Amoo 15 NU

Aerial parts

195.1 ± 63.35 ns

341.7 ± 19.97

34.7 ± 1.79 **

25.8 ± 0.03

19.7 ± 0.87 *

16.7 ± 0.34

Buddleja salviifolia (L.) Lam#

Buddlejaceae

S. Amoo 16 NU

Leaves

60.8 ± 15.84 **

409.9 ± 13.77

9.0 ± 0.36 ***

20.0 ± 0.81

6.6 ± 0.28 ***

14.6 ± 0.32

Buddleja salviifolia (L.) Lam#

Buddlejaceae

S. Amoo 16 NU

Twigs

111.1 ± 9.64 **

400.3 ± 27.54

8.3 ± 0.25 ***

11.1 ± 0.24

5.0 ± 0.25 *

5.9 ± 0.11

Clausena anisata (Willd.) Hook. F. ex Benth#

Rutaceae

S. Amoo 18 NU

Leaves & Twigs

3019.6 ± 63.35 ns

3264.7 ± 96.40

31.3 ± 0.05 *

28.1 ± 0.99

11.7 ± 0.17 ***

7.6 ± 0.20

Cussonia spicata Thunb. #

Araliaceae

S. Amoo 09 NU

Leaves

82.8 ± 39.25 ns

38.8 ± 11.71

11.4 ± 0.16 **

7.6 ± 0.69

9.1 ± 0.53 ***

3.4 ± 0.27

Dombeya rotundifolia Hochst. #

Malvaceae

S. Amoo 11 NU

Leaves

7076.6 ± 177.64 **

9499.6 ± 117.75

45.3 ± 0.89 ns

47.3 ± 1.94

29.7 ± 3.05 ns

35.4 ± 0.87

Ekebergia capensis Sparrmδ

Meliaceae

S. Amoo 23 NU

Leaves & Twigs

547.6 ± 22.03 ***

2221.5 ± 53.02

31.7 ± 1.29 ***

44.9 ± 0.78

22.8 ± 1.25 ns

26.0 ± 0.29

Leonotis intermedia Lindl.δ

Lamiaceae

S. Amoo 08 NU

Leaves

56.0 ± 1.38 *

72.5 ± 2.75

15.1 ± 0.57 **

11.6 ± 0.23

12.1 ± 0.38 ***

6.8 ± 0.10

Leonotis leonurus (L.) R.Br. δ

Lamiaceae

S. Amoo 12 NU

Leaves

51.8 ± 1.38 ns

171.0 ± 30.99

10.5 ± 0.22 ***

18.2 ± 0.76

6.6 ± 0.23 ***

10.3 ± 0.01

Merwilla plumbea (Lindl.) Septaδ

Hyacinthaceae

S. Amoo 21 NU

Bulbs

64.2 ± 8.26 ns

207.5 ± 75.74

7.8 ± 0.29 **

9.8 ± 0.25

1.4 ± 0.09 ns

1.7 ± 0.37

Ocotea bullata (Burch.) Baill.δ

Lauraceae

S. Amoo 13 NU

Bark

3060.9 ± 121.19 **

6112.6 ± 207.95

32.7 ± 0.82 **

46.4 ± 2.00

18.4 ± 0.62 ***

26.8 ± 0.50

Olea europaea L.#

Oleaceae

S. Amoo 14 NU

Leaves

0 ns

283.2 ± 79.87

17.2 ± 0.41 *

18.7 ± 0.06

13.1 ± 0.31 ***

9.7 ± 0.28

Pittosporum viridiflorum Sims#

Pittosporaceae

S. Amoo 24 NU

Leaves & Twigs

63.6 ± 8.95 ns

194.4 ± 65.41

10.6 ± 0.20 ***

26.0 ± 0.91

5.3 ± 0.12 ***

15.6 ± 0.22

Plumbago auriculata Lam.δ

Plumbaginaceae

S. Amoo 06 NU

Leaves

9.8 ± 7.57 **

521.4 ± 50.95

7.6 ± 0.66 ***

15.0 ± 0.46

1.3 ± 0.15 **

5.5 ± 0.64

Protorhus longifolia (Bernh.) Engl.δ

Anacardiaceae

S. Amoo 19 NU

Leaves

1034.4 ± 47.51 **

7787.2 ± 290.57

51.8 ± 1.27 ***

114.4 ± 7.83

10.1 ± 0.65 ***

18.3 ± 0.10

Solanum mauritianum S cop.δ

Solanaceae

S. Amoo 07 NU

Leaves

71.1 ± 6.89 *

14.0 ± 11.71

8.0 ± 0.11 ***

13.9 ± 0.24

2.0 ± 0.21 ns

1.5 ± 0.05

Spirostachys africana Sond.#

Euphorbiaceae

S. Amoo 26 NU

Leaves & Twigs

553.8 ± 3.44 ns

527.6 ± 11.71

86.2 ± 1.91 **

69.1 ± 2.13

8.5 ± 0.09 ***

26.7 ± 0.57

Synadenium copulare (Boiss.) L.C. Wheeler δ

Euphorbiaceae

S. Amoo 25 NU

Leaves

11.9 ± 11.02 ns

273.6 ± 71.61

8.5 ± 0.37 ***

15.2 ± 0.33

4.2 ± 0.15 ns

4.3 ± 0.15

Tetradenia riparia (Hochst.) Coddδ

Lamiaceae

S. Amoo 20 NU

Leaves

46.3 ± 9.64 *

0

6.1 ± 0.20 ns

7.2 ± 0.38

2.7 ± 0.08 ***

1.5 ± 0.02

Trichilia dregeana Sond.δ

Meliaceae

S. Amoo 22 NU

Leaves & Twigs

431.9 ± 16.53 ns

412.0 ± 50.27

34.4 ± 10.26 ns

32.6 ± 1.17

8.7 ± 0.61 ***

20.2 ± 0.19

Ziziphus mucronata Willd.#

Rhamnaceae

S. Amoo 17 NU

Leaves

314.2 ± 37.87 ns

412.7 ± 22.03

23.6 ± 1.61 **

33.4 ± 0.62

7.1 ± 0.10 ***

9.0 ± 0.09

Ziziphus mucronata Willd.δ

Rhamnaceae

S. Amoo 17 NU

Leaves

90.4 ± 1.38 **

412.7 ± 22.03

19.7 ± 0.42 ***

33.4 ± 0.62

6.9 ± 0.34 **

9.0 ± 0.09

ns = not significant; P = 0.05 (*); P = 0.01 (**); P = 0.001 (***).

HE = harpagoside equivalents; GAE = gallic acid equivalents; CE = catechin equivalents.

δ = Voucher number of plant material stored for 16 years was as described by Jäger et al. (1996); # = Voucher number of plant material stored for 12 years was as described by McGaw et al. (2000).

Merwilla plumbea (Lindl.) Speta was formerly known as Scilla natalensis Planch.

Methods

Chemicals and reagents

Acetylcholine iodide, AChE from electric eel (type VI-S lyophilized powder), β-carotene, 2,2-diphenyl-1-picrylhydrazyl (DPPH), 5,5-dithiobis-2-nitrobenzoic acid (DTNB), galanthamine, gallic acid, catechin and linoleic acid were obtained from Sigma-Aldrich (Steinheim, Germany); butylated hydroxytoluene (BHT) from BDH Chemicals Ltd. (Poole, England); and harpagoside from Extrasynthèse (France). All chemicals and reagents used were of analytical grade.

Plant material and preparation of extracts

Table 1 shows the scientific names, and voucher specimen numbers of the evaluated plant materials. Following oven-drying at 50 °C, plant materials were stored at room temperature (25 °C) in brown paper bags in the dark for 12 or 16 years. Fresh plant materials collected from the same locations and season as the stored ones were similarly oven-dried at 50 °C. The plants were identified by Dr C. Potgieter and voucher specimens deposited in The Bews Herbarium, University of KwaZulu-Natal, Pietermaritzburg, South Africa.

Dried plant materials were ground to fine powders and extracted with 50% methanol at 20 ml/g in a sonication bath containing ice-cold water for 1 h for antioxidant and AChE assays. Extracts were then filtered through Whatman No. 1 filter paper, concentrated in vacuo at 40 °C and completely air-dried at room temperature in glass vials.

The extraction method described by Makkar [25] was used for phytochemical analysis. Dried plant materials, ground to fine powders (0.2 g), were extracted with 50% aqueous methanol (10 ml) in a sonication bath containing ice-cold water for 20 min. The extracts were then centrifuged at approximately 3000 U/min for 5 min using a Hettich Universal 1200 01 Centrifuge. The supernatants were collected and kept on ice for phytochemical analysis.

Phytochemical analysis

Total iridoid content of the plant material was quantified using the method described by Levieille and Wilson [26]. The calibration curve was plotted using harpagoside as the standard. Total iridoid content for each plant material was expressed in μg harpagoside equivalents (HE) per g dry weight (DW).

For the determination of total phenolic content, the Folin & Ciocalteu [27] method was used with slight modifications [28]. Gallic acid was used as the standard for plotting the calibration curve. Total phenolic content was expressed in mg gallic acid equivalents (GAE) per g DW.

The flavonoid content of the plant materials were quantified using the aluminium chloride colorimetric method [29]. Catechin was used as a standard for the calibration curve. Flavonoid content was expressed in mg catechin equivalents (CE) per g DW.

The butanol-HCl method [25] was used to quantify condensed tannin (proanthocyanidin) content of the plant materials. Condensed tannins (% in dry matter) were expressed as leucocyanidin equivalents were calculated using the formula:
C o n d e n s e d t a n n i n s % d r y m a t t e r = A 550 n m × 78.26 × D i l u t i o n f a c t o r % d r y m a t t e r × 100
(1)

where A 550nm is the absorbance of the sample at 550 nm. The formula assumes that the effective E 550 1 % of leucocyanidin is 460 [30].

Free gallic acid and gallotannin contents were evaluated using the rhodanine assay [25, 31]. The calibration curves were plotted using gallic acid as a standard. Free gallic acid and gallotannin contents were expressed in μg GAE per g DW.

Antioxidant activity

DPPH free radical scavenging activity

The DPPH assay [32] was used to evaluate the free radical scavenging activity of the plant extracts. Methanol was used as a negative control while ascorbic acid and BHT were used as positive controls. Any absorbance due to extract colour was removed by including a background solution with methanol in place of DPPH solution for each extract. Each sample was evaluated in triplicate. The radical scavenging activity (RSA) was calculated using the equation:
R S A % = 1 A extract - A background A control × 100
(2)
where А extract A background and A control are the absorbance readings of the extract, background solution and negative control, respectively at 517 nm. The EC50, which is the extract concentration required to scavenge 50% of DPPH free radical, was determined for each extract. Antioxidant activity index (AAI) for each extract was calculated using the equation [33]:
A A I = F i n a l D P P H c o n c e n t r a t i o n E C 50
(3)

β-Carotene-linoleic acid model system

The assay was done following the method described by Moyo et al. [34]. Methanol and BHT were used as negative and positive controls, respectively. Each sample was prepared in triplicate. The plant extracts and BHT were evaluated at a final assay concentration of 200 μg/ml. Antioxidant activity (%), measured at t = 120 min, was calculated using the following equations:
R a t e o f β c a r o t e n e b l e a c h i n g = I n A t = 0 A t = t × 1 t
(4)
A n t i o x i d a n t a c t i v i t y % = R c o n t o r l - R s a m p l e R c o n t r o l × 100
(5)

where At = 0 is the initial absorbance at t = 0 min, At = t is the absorbance at time t = 120 min, t = 120 min and R is the rate of β-carotene bleaching.

Acetylcholinesterase inhibitory activity

The AChE assay was performed using the colorimetric method [35]. Each extract was evaluated in triplicate at a final assay concentration of 1.0 mg/ml. Galanthamine at a final assay concentration of 20 μM was used as a positive control. The rate of reaction was calculated for each of the plant extracts, the blank (methanol) and positive control (galanthamine). The percentage inhibition by each plant extract was calculated using the formula:
A C h E i n h i b i t i o n % = 1 S a m p l e r e a c t i o n r a t e B l a n k r e a c t i o n r a t e × 100
(6)

Data analysis

The levels of significant difference between the mean values of stored and fresh plant materials were determined using the t-test (SigmaPlot version 8.0). Regression analysis and the determination of EC50 values were done using GraphPad Prism software (version 4.03).

Results and discussion

Phytochemical analysis

The effects of long-term storage on the total iridoid, phenolic and flavonoid content of the plant materials evaluated are presented in Table 1. Of the 21 fresh and stored plant materials evaluated, the levels of total iridoid present in nine plants were significantly higher in fresh compared to the stored plant materials. The total iridoid contents of stored materials in Acokanthera oppositifolia, Solanum mauritanum and Tetradenia riparia were significantly higher than those of fresh ones. There was no significant difference between the iridoid content of fresh and stored plant materials in approximately 50% of the evaluated plants.

The total phenolic contents of Artemisia afra Clausena anisata Cussonia spicata Leonotis intermedia and Spirostachys africana stored materials were significantly higher than in freshly collected material. With the exceptions of A. afra D. rotundifolia T. riparia and T. dregeana (where there was no significant difference between the stored and fresh materials), the phenolic contents of the remaining 15 fresh plant materials were significantly higher than in the stored material. Similarly, a comparison of fresh material and herbarium specimens of three Quillaja species revealed non-significant differences in their phenolic constituents [36]. Remarkably, one of the tested herbarium specimens in the Bate-Smith [36] study was 100 years old.

The flavonoid content was significantly higher in stored A. afra C. anisata C. spicata L. intermedia T. riparia and Olea europea materials when compared to their corresponding fresh materials. It is noteworthy that the stored materials of the former four species had higher total phenolic contents than their fresh materials perhaps due to their higher flavonoid content compared to the fresh materials. Higher flavonoid contents were observed in 12 fresh plant materials when compared to their respective stored materials. Previous studies comparing the phenolic constituents of some Dillenia species showed differences in the flavonoid profiles of fresh and herbarium materials as some flavonoids were not detected in the latter [37]. The results suggested that some flavonoids are easily oxidised during the drying process [37].

Table 2 presents the condensed tannin, free gallic acid and gallotannin contents of both the stored and fresh materials of plant species evaluated in this study. No condensed tannins were detected in both fresh and stored materials of A. oppositifolia Pittosporum viridiflorum and Merwilla plumbea. With the exceptions of Buddleja salviifolia (leaves), Plumbago auriculata and Ziziphus mucronata, the condensed tannin content in the stored plant materials was either significantly higher or not different when compared to the fresh materials. Unlike the stored materials, no condensed tannins were detected in fresh material of A. afra C. spicata L. intermedia Leonotis leonurus and O. europea. Among the 21 species evaluated, free gallic acid was detected in 15 fresh and/or stored plant materials. In most cases, there was no significant difference in the free gallic acid contents of the fresh materials when compared to the stored ones. With the exceptions of A. oppositifolia A. afra and Ekebergia capensis, the gallotannin content of the stored plant materials was either higher or not significantly different when compared to the fresh ones. It has been shown that phytochemical constituents of medicinal plants, such as alkaloids, flavonoids, volatile oils and amino acids are sufficiently stable to even be detected in herbarium specimens [38]. However, based on the results of the present study, the degree of stability of phenolic compounds seems to be species dependent.
Table 2

Effect of long-term storage on the condensed tannin, free gallic acid and gallotannin contents of 21 South African medicinal plants

Plant name

Plant part(s)

Condensed tannins (% in dry matter)

Free gallic acid (μg GAE/g DW)

Gallotannins (μg GAE/g DW)

Stored

Fresh

Stored

Fresh

Stored

Fresh

Acokanthera oppositifolia δ

Roots

0

0

2.996 ± 2.9963 ns

1.284 ± 1.2841

32.960 ± 0.4281 *

60.355 ± 6.4207

Artemisia afra #

Aerial parts

0.078 ± 0.0005 ***

0

0

0

76.621 ± 6.4207 ns

97.167 ± 10.7012

Artemisia afra δ

Aerial parts

0.004 ± 0.0002 *

0

0

0

27.823 ± 11.5573 *

97.167 ± 10.7012

Buddleja salviifolia #

Leaves

0.011 ± 0.0002 *

0.056 ± 0.0073

29.535 ± 20.1183 ns

0

80.720 ± 17.0557 ns

41.949 ± 1.7122

Buddleja salviifolia #

Twigs

0.017 ± 0.0047 ns

0.005 ± 0.0050

17.122 ± 3.4244 ns

8.133 ± 5.5646

14.982 ± 8.1329 ns

38.096 ± 17.5500

Clausena anisata #

Leaves & Twigs

1.394 ± 0.0318 ns

1.329 ± 0.0159

0

0

68.488 ± 5.1366 **

0

Cussonia spicata #

Leaves

0.012 ± 0.0016 *

0

138.260 ± 41.5208 ns

12.842 ± 12.8415

397.377 ± 55.8931 ns

468.758 ± 81.3346

Dombeya rotundifolia #

Leaves

1.804 ± 0.0116 **

0.973 ± 0.0529

0

0

41.949 ± 19.6903 ns

0

Ekebergia capensis δ

Leaves & Twigs

0.654 ± 0.0040 ns

0.523 ± 0.0706

0

0

0 **

19.690 ± 2.5683

Leonotis intermedia δ

Leaves

0.008 ± 0.0007 **

0

0 ns

3.424 ± 3.4244

17.550 ± 5.5646 ns

11.129 ± 4.2805

Leonotis leonurus δ

Leaves

0.011 ± 0.0002 ***

0

0 ***

47.085 ± 1.7122

24.827 ± 0.8561 *

5.565 ± 3.8524

Merwilla plumbea δ

Bulbs

0

0

8.133 ± 8.1329 ns

23.971 ± 7.7049

167.367 ± 13.2695

ND

Ocotea bullata δ

Bark

1.154 ± 0.0162 **

0.699 ± 0.0354

0

0

68.060 ± 8.9890 *

14.982 ± 7.2768

Olea europaea #

Leaves

0.010 ± 0.0019 *

0

0

0

127.559 ± 4.2805 ns

121.566 ± 1.7122

Pittosporum viridiflorum #

Leaves & Twigs

0

0

5.565 ± 5.5646 ns

0

75.337 ± 0.8561 ns

66.776 ± 6.8488

Plumbago auriculata δ

Leaves

0.003 ± 0.0011 **

0.024 ± 0.0013

3.852 ± 3.8524 ns

0

20.118 ± 5.5646 ns

4.7085 ± 4.7085

Protorhus longifolia δ

Leaves

0.400 ± 0.0127 ns

0.724 ± 0.0885

2398.787 ± 112.1485 ns

1901.394 ± 137.8318

2726.245 ± 615.9627 ns

4039.926 ± 1368.0443

Solanum mauritianum δ

Leaves

0.013 ± 0.0013 ns

0.005 ± 0.005

32.103 ± 4.7085 ns

23.971 ± 0.8561

183.047 ± 75.6858

ND

Spirostachys africana #

Leaves & Twigs

0.348 ± 0.0083 ns

0.365 ± 0.0311

1107.363 ± 228.1501 *

0

2445.016 ± 118.1414 **

16.266 ± 16.2659

Synadenium cupulare #

Leaves

0.010 ± 0.0002 **

0.004 ± 0.0004

0 ns

8.561 ± 8.5610

20.546 ± 4.2805 ns

54.362 ± 21.8305

Tetradenia riparia δ

Leaves

0.002 ± 0.0004 ns

0.005 ± 0.0022

0 ***

14.982 ± 0.4280

11.985 ± 1.7122 ns

22.259 ± 13.6976

Trichilia dregeana δ

Leaves & Twigs

0.198 ± 0.0099 ns

0.138 ± 0.0148

118.998 ± 5.1366 **

0

442.603 ± 65.0634 *

13.270 ± 1.2841

Ziziphus mucronata #

Leaves

0.008 ± 0.0000 ***

0.046 ± 0.0013

0

0

38.953 ± 12.4134 ns

28.679 ± 1.2842

Ziziphus mucronata δ

Leaves

0.077 ± 0.0003 **

0.046 ± 0.0013

0

0

47.085 ± 17.1220 ns

28.679 ± 1.2842

ns = not significant; P = 0.05 (*); P = 0.01 (**); P = 0.001 (***); ND = not determined; GAE = Gallic acid equivalents.

δ = Plant material stored for 16 years.

# = Plant material stored for 12 years.

Antioxidant properties

The effect of long-term storage on the radical scavenging activity of 21 plant materials is presented in Table 3. The lower the EC50 value, the higher the antioxidant activity index and the free radical scavenging activity. At 100 μg/ml concentration, the radical scavenging activity of all stored plant materials (with the exception of Protorhus longifolia) was either significantly higher or not different when compared to the freshly harvested materials. A comparison based on the EC50 values and antioxidant activity indices revealed a significantly higher radical scavenging activity in 58% of the stored plant materials. With the exception of A. oppositifolia and B. salviifolia (leaves), the radical scavenging activity of the remaining stored plant materials based on their EC50 values was not significantly different when compared to the fresh materials. The DPPH radical acts as both the probe and oxidant by accepting electrons from antioxidant compounds in the extract. There is a direct correlation between degree of hydroxylation of the bioactive compounds and DPPH radical scavenging activity [11]. Potent DPPH radical scavenging activities of medicinal plants have also been reported in other studies [11, 13, 14]. However, the significance of the present study lies in the observed high DPPH radical scavenging activity of aqueous methanol extracts obtained from medicinal plant material after prolonged storage.
Table 3

Effect of long-term storage on the free radical scavenging activity of 21 South African medicinal plants

Plant species

Plant part

Radical scavenging activity (%) at 100 μg/ml

EC50(μg/ml)

Antioxidant activity index

Stored

Fresh

Stored

Fresh

Stored

Fresh

Acokanthera oppositifolia δ

Roots

93.3 ± 0.03 **

92.6 ± 0.10

26.8 ± 2.43 *

18.0 ± 0.34

0.7 ± 0.06 **

1.1 ± 0.02

Artemisia afra #

Aerial parts

93.8 ± 0.11 *

92.7 ± 0.34

9.3 ± 0.07 ***

12.4 ± 0.15

2.1 ± 0.02 ***

1.6 ± 0.02

Artemisia afra δ

Aerial parts

94.0 ± 0.07 *

92.7 ± 0.34

6.8 ± 0.50 ***

12.4 ± 0.15

2.9 ± 0.21 **

1.6 ± 0.02

Buddleja salviifolia #

Leaves

96.2 ± 0.06 ***

93.0 ± 0.40

15.5 ± 0.47 **

10.0 ± 0.61

1.3 ± 0.04 **

2.0 ± 0.12

Buddleja salviifolia #

Twigs

94.2 ± 0.13 ns

94.3 ± 0.15

17.2 ± 0.32 ns

17.5 ± 0.40

1.1 ± 0.02 ns

1.1 ± 0.03

Clausena anisata #

Leaves & Twigs

70.8 ± 0.28 ns

72.6 ± 6.21

33.2 ± 3.89 ns

26.8 ± 2.06

0.6 ± 0.07 ns

0.7 ± 0.06

Cussonia spicata #

Leaves

93.7 ± 0.07 ***

61.6 ± 1.67

14.3 ± 0.22 **

43.6 ± 5.73

1.4 ± 0.02 ***

0.5 ± 0.07

Dombeya rotundifolia #

Leaves

96.5 ± 0.56 **

93.6 ± 0.27

5.9 ± 0.12 ns

6.1 ± 0.32

3.3 ± 0.07 ns

3.2 ± 0.16

Ekebergia capensis δ

Leaves & Twigs

94.2 ± 0.42 *

92.8 ± 0.30

4.7 ± 0.37 **

25.5 ± 4.99

4.3 ± 0.32 ***

0.8 ± 0.14

Leonotis intermedia δ

Leaves

93.3 ± 0.09 *

88.5 ± 1.73

10.6 ± 0.37 ***

51.7 ± 0.32

1.9 ± 0.06 ***

0.4 ± 0.00

Leonotis leonurus δ

Leaves

93.7 ± 0.18 **

91.6 ± 0.43

16.8 ± 0.06 ***

30.3 ± 0.92

1.2 ± 0.00 ***

0.7 ± 0.02

Merwilla plumbea δ

Bulbs

8.2 ± 0.61 **

2.6 ± 0.97

ND

ND

ND

ND

Ocotea bullata δ

Bark

95.0 ± 0.25 **

93.8 ± 0.02

3.2 ± 0.14 **

4.3 ± 0.10

6.3 ± 0.28 **

4.6 ± 0.11

Olea europaea #

Leaves

94.9 ± 0.20 **

93.2 ± 0.09

14.0 ± 0.48 ***

20.0 ± 0.16

1.4 ± 0.05 ***

1.0 ± 0.01

Pittosporum viridiflorum #

Leaves & Twigs

93.6 ± 0.10 ns

93.8 ± 0.29

17.9 ± 0.25 ns

17.5 ± 0.27

1.1 ± 0.02 ns

1.1 ± 0.02

Plumbago auriculata δ

Leaves

50.6 ± 3.97 ns

54.6 ± 1.15

ND

ND

ND

ND

Protorhus longifolia δ

Leaves

95.8 ± 0.24 **

97.3 ± 0.21

2.2 ± 0.16 ns

2.3 ± 0.14

9.1 ± 0.71 ns

8.5 ± 0.49

Solanum mauritianum δ

Leaves

34.4 ± 0.73 ***

19.8 ± 1.53

ND

ND

ND

ND

Spirostachys africana #

Leaves & Twigs

96.6 ± 0.06 ***

91.8 ± 0.34

2.0 ± 0.07 ***

14.4 ± 0.58

10.0 ± 0.35 ***

1.4 ± 0.06

Synadenium cupulare δ

Leaves

90.9 ± 0.70 ***

46.0 ± 5.30

55.7 ± 0.35

ND

0.4 ± 0.02

ND

Tetradenia riparia δ

Leaves

68.5 ± 1.39 ***

23.8 ± 2.44

41.0 ± 5.29

ND

0.5 ± 0.06

ND

Trichilia dregeana #

Leaves & Twigs

95.8 ± 0.46 **

92.3 ± 0.16

5.3 ± 0.02 ***

14.6 ± 0.24

3.7 ± 0.01 ***

1.3 ± 0.02

Ziziphus mucronata #

Leaves

90.7 ± 0.42 ns

89.0 ± 2.20

29.7 ± 1.02 ns

30.9 ± 1.94

0.7 ± 0.02 ns

0.6 ± 0.04

Ziziphus mucronata δ

Leaves

91.1 ± 0.18 ns

89.0 ± 2.20

18.1 ± 0.29 **

30.9 ± 1.94

1.1 ± 0.02 ***

0.6 ± 0.04

Ascorbic acid

96.6 ± 0.04

 

2.1 ± 0.05

 

9.4 ± 0.23

 

Butylated hydroxytoluene

93.2 ± 0.34

 

3.0 ± 0.04

 

6.5 ± 0.09

 

ns = not significant; P = 0.05 (*); P = 0.01 (**); P = 0.001 (***).

ND = not determined.

δ = Plant material stored for 16 years.

# = Plant material stored for 12 years.

Table 4 presents the effect of long-term storage on the antioxidant activity of medicinal plant materials evaluated based on β-carotene bleaching model. The β-carotene bleaching assay simulates the oxidation of membrane lipid components and measures antioxidant activity towards linoleic acid [16]. The antioxidant activity of E. capensis stored plant material was significantly higher (almost two-fold) compared to the fresh material. On the other hand, the antioxidant activity of L. intermedia fresh plant material was significantly higher than that of the stored materials. With the exception of E. capensis and L. intermedia, there were no significant differences between the antioxidant activities recorded in both the stored and fresh plant materials. The retention of antioxidant activity in stored plant material suggests the stability of bioactive chemicals during prolonged storage. The detected bioactivity in the stored plant material provides interesting prospects in the future development of stable food additive compounds. In previous studies, high antioxidant activity from polar extracts of some plants has been attributed to hydrogen-donating phenolic compounds and flavonoids [2, 16]. However, the identification of specific phenolic compounds responsible for the high antioxidant activity of long-term stored plant materials remains a challenge for future research.
Table 4

Effect of long-term storage on antioxidant activity based on β -carotene bleaching model and acetylcholinesterase inhibitory properties of 21 South African medicinal plants

Plant species

Plant part(s)

Antioxidant activity (%) at 200 μg/ml

AChE inhibition (%) at 1.0 mg/ml

Stored

Fresh

Stored

Fresh

Acokanthera oppositifolia δ

Roots

54.7 ± 3.4 ns

40.0 ± 7.71

81.0 ± 12.11 ns

80.5 ± 1.99

Artemisia afra #

Aerial parts

45.8 ± 3.34 ns

39.8 ± 4.94

83.2 ± 2.28 ns

89.6 ± 7.44

Artemisia afra δ

Aerial parts

44.4 ± 7.20 ns

39.8 ± 4.94

89.8 ± 0.57 ns

89.6 ± 7.45

Buddleja salviifolia #

Leaves

39.1 ± 7.69 ns

58.3 ± 3.04

64.9 ± 11.42 ns

72.5 ± 10.17

Buddleja salviifolia #

Twigs

58.0 ± 3.92 ns

53.8 ± 8.22

73.0 ± 15.63 ns

63.9 ± 4.05

Clausena anisata #

Leaves & Twigs

23.6 ± 4.06 ns

49.8 ± 11.19

77.0 ± 6.86 ns

82.2 ± 3.74

Cussonia spicata #

Leaves

55.7 ± 6.45 ns

41.8 ± 4.70

72.1 ± 12.6 ns

86.5 ± 5.56

Dombeya rotundifolia #

Leaves

51.8 ± 4.13 ns

58.9 ± 1.40

84.1 ± 5.54 ns

87.6 ± 2.88

Ekebergia capensis δ

Leaves & Twigs

93.5 ± 7.05 **

52.1 ± 4.97

73.8 ± 7.24 ns

89.7 ± 6.08

Leonotis intermedia δ

Leaves

32.6 ± 5.34 *

52.9 ± 4.09

68.8 ± 3.12 *

87.8 ± 3.83

Leonotis leonurus δ

Leaves

40.8 ± 2.32 ns

58.6 ± 7.13

78.1 ± 3.67 ns

73.2 ± 0.43

Merwilla plumbea δ

Bulbs

57.0 ± 6.42 ns

45.1 ± 4.06

58.7 ± 6.52 ns

81.5 ± 2.11

Ocotea bullata δ

Bark

57.8 ± 7.33 ns

62.3 ± 8.83

84.8 ± 3.98 ns

87.1 ± 2.63

Olea europaea #

Leaves

48.8 ± 2.84 ns

48.2 ± 0.59

69.2 ± 5.99 ns

85.4 ± 3.39

Pittosporum viridiflorum #

Leaves & Twigs

62.9 ± 6.65 ns

39.1 ± 6.80

96.2 ± 0.71 ns

70.5 ± 8.36

Plumbago auriculata δ

Leaves

62.2 ± 10.87 ns

52.8 ± 1.99

82.3 ± 5.54 ns

87.3 ± 2.20

Protorhus longifolia δ

Leaves

90.9 ± 8.88 ns

72.9 ± 2.62

51.8 ± 9.07 ns

40.07 ± 2.60

Solanum mauritianum δ

Leaves

38.9 ± 10.07 ns

49.4 ± 4.92

78.5 ± 5.84 ns

85.9 ± 3.94

Spirostachys africana #

Leaves & Twigs

62.1 ± 4.40 ns

58.3 ± 3.24

90.4 ± 5.57 ns

82.4 ± 3.51

Synadenium cupulare δ

Leaves

54.5 ± 5.06 ns

45.3 ± 2.04

75.3 ± 4.07 ns

81.1 ± 2.77

Tetradenia riparia δ

Leaves

67.2 ± 4.89 ns

64.5 ± 8.38

80.8 ± 1.73 *

65.4 ± 4.85

Trichilia dregeana #

Leaves & Twigs

65.2 ± 7.46 ns

50.6 ± 8.81

94.8 ± 2.82 *

81.1 ± 3.99

Ziziphus mucronata #

Leaves

54.5 ± 3.65 ns

42.6 ± 6.62

84.8 ± 6.78 ns

87.2 ± 10.04

Ziziphus mucronata δ

Leaves

24.1 ± 11.13 ns

42.6 ± 6.62

90.4 ± 4.09 ns

87.2 ± 10.04

Galanthamine

   

84.1 ± 1.45

  

Butylated hydroxytoluene

 

94.5 ± 1.71

    

ns = not significant; P = 0.05 (*); P = 0.01 (**); P = 0.001 (***).

δ = Plant material stored for 16 years.

# = Plant material stored for 12 years.

Galanthamine (20 μM) was used as a positive control in acetylcholinesterase assay.

Acetylcholinesterase inhibition activity

Table 4 presents the effect of long-term storage on AChE inhibitory properties of the evaluated plant materials. Stored plant materials of T. riparia and T. dregeana showed a significantly higher AChE inhibition than the fresh ones. There was no significant difference between the percentage AChE inhibition by the stored and fresh materials of the remaining plant species. In general, the evaluated plant species exhibited high AChE inhibitory activity. Interestingly, medicinal plant materials retained AChE inhibitory activity even after prolonged storage (12 or 16 years). The results of the present study confirm the therapeutic value of stored medicinal plants in the pharmacotherapy of AD disease. The AChE inhibitory properties of plant-derived extracts obtained from freshly harvested material have been previously reported [16, 32]. Recent studies have demonstrated a direct association between AD and antioxidant activity [16]. However, this is the first report on the antioxidant and AChE inhibitory properties of long-term stored medicinal plants. The present findings are important for traditional systems which are characterised by an holistic approach to health provision, based on the prophylactic properties of medicinal plants [6].

Conclusions

The current study presents evidence that dried medicinal plants stored under dark conditions at room temperature remain biologically active after long-term storage. Extracts of the stored plant material still exhibited potent antioxidant and AChE-inhibitory properties. These findings are significant as some medicinal plants may be utilised long after their time of harvesting. In addition, the prevention strategies practised in the Ayurvedic, Chinese and African medicinal systems often involve regular intake of medicinal plant extracts and/or herbal preparations, which are responsible for counteracting the oxidative stress effects caused by ROS. The high antioxidant activity and stability of the bioactive compounds in these medicinal plants offer interesting prospects for the identification of novel principles for application in food and pharmaceutical formulations. However, in vitro and in vivo safety evaluation of the stored medicinal plants is required.

Abbreviations

ACh: 

Acetylcholine

AChE: 

Acetylcholinesterase

AD: 

Alzheimer’s disease

AAI: 

Antioxidant activity index

BHT: 

Butylated hydroxytoluene

CE: 

Catechin equivalents

DPPH: 

2,2-diphenyl-1-picrylhydrazyl

DTNB: 

5,5-dithiobis-2-nitrobenzoic acid

DW: 

Dry weight

GAE: 

Gallic acid equivalents

HE: 

Harpagoside equivalents

RSA: 

Radical scavenging activity

ROS: 

Reactive oxygen species.

Declarations

Acknowledgements

We are grateful to Mrs A. Young of University of KwaZulu-Natal Botanical Garden, as well as Dr M.E. Light and Mr B. Ncube of the Research Centre for Plant Growth and Development for assistance in the collection of plant material. The University of KwaZulu-Natal and National Research Foundation provided financial support.

Authors’ Affiliations

(1)
Research Centre for Plant Growth and Development, School of Life Sciences, University of KwaZulu-Natal Pietermaritzburg, Private Bag X01

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