Author’s Accepted Manuscript
Antiplasmodial And Cytotoxic Activities Of The
Constituents Of Turraea Robusta And Turraea
Nilotica
Beatrice N. Irungu, Nicholas Adipo, Jennifer A.
Orwa, Francis Kimani, Matthias Heydenreich,
Jacob O. Midiwo, Per Martin Björemark, Mikael
Håkansson, Abiy Yenesew, Máté Erdélyi
PII:
DOI:
Reference:
www.elsevier.com
S0378-8741(15)30107-0
http://dx.doi.org/10.1016/j.jep.2015.08.039
JEP9704
To appear in: Journal of Ethnopharmacology
Received date: 2 July 2015
Revised date: 15 August 2015
Accepted date: 26 August 2015
Cite this article as: Beatrice N. Irungu, Nicholas Adipo, Jennifer A. Orwa,
Francis Kimani, Matthias Heydenreich, Jacob O. Midiwo, Per Martin Björemark,
Mikael Håkansson, Abiy Yenesew and Máté Erdélyi, Antiplasmodial And
Cytotoxic Activities Of The Constituents Of Turraea Robusta And Turraea
N i l o t i c a , Journal
of
Ethnopharmacology,
http://dx.doi.org/10.1016/j.jep.2015.08.039
This is a PDF file of an unedited manuscript that has been accepted for
publication. As a service to our customers we are providing this early version of
the manuscript. The manuscript will undergo copyediting, typesetting, and
review of the resulting galley proof before it is published in its final citable form.
Please note that during the production process errors may be discovered which
could affect the content, and all legal disclaimers that apply to the journal pertain.
1
Antiplasmodial and cytotoxic activities of the constituents of Turraea robusta and Turraea
nilotica
Beatrice N. Irungua,b, Nicholas Adipoa, Jennifer A. Orwaa, Francis Kimanic, Matthias
Heydenreichd, Jacob O. Midiwoe, Per Martin Björemarkb, Mikael Håkanssonb, Abiy
Yenesewe*, Máté Erdélyib,f*
a
Centre for Traditional Medicine and Drug Research, Kenya Medical Research Institute,
P.O. Box 54840-00200, Nairobi-Kenya
b
Department of Chemistry and Molecular Biology, University of Gothenburg,
Gothenburg, SE-412 96 Sweden
c
Centre for Biotechnology Research and Development, Kenya Medical Research Institute,
P.O. Box 54840-00200, Nairobi, Kenya
d
e
Institut für Chemie, Universität Potsdam, P.O. Box 601553, D-14415 Potsdam, Germany
Department of Chemistry, University of Nairobi, P.O. Box 30197-00100, Nairobi, Kenya
f
Swedish NMR Centre, University of Gothenburg, Gothenburg, SE-405 30 Sweden
*Corresponding authors
Máté Erdélyi, Department of Chemistry and Molecular Biology, University of Gothenburg,
Gothenburg, SE-412 96 Sweden
Tel: +46766229033
E-mail address: mate@chem.gu.se
Abiy Yenesew, Department of Chemistry, University of Nairobi, P.O Box 30197-00100,
Nairobi, Kenya
Tel: +254 733 832 576
E-mail address: ayenesew@uonbi.ac.ke
Abstract
Ethnopharmacological relevance: Turraea robusta and Turraea nilotica are African medicinal
plants used for the treatment of a wide variety of diseases, including malaria. The genus
Turraea is rich in limonoids and other triterpenoids known to possess various biological
activities.
2
Materials and methods: From the stem bark of Turraea robusta six compounds, and from
various parts of Turraea nilotica eleven compounds were isolated by the use of a combination
of chromatographic techniques. The structures of the isolated compounds were elucidated
using NMR and MS, whilst the relative configuration of one of the isolated compounds,
toonapubesin F, was established by X-ray crystallography. The antiplasmodial activities of
the crude extracts and the isolated constituents against the D6 and W2 strains of Plasmodium
falciparum were determined using the semiautomated micro dilution technique that measures
the ability of the extracts to inhibit the incorporation of (G-3H, where G is guanine)
hypoxanthine into the malaria parasite. The cytotoxicity of the crude extracts and their
isolated constituents was evaluated against the mammalian cell lines African monkey kidney
(vero), mouse breast cancer (4T1) and human larynx carcinoma (HEp2).
Results: The extracts showed good to moderate antiplasmodial activities, where the extract of
the stem bark of T. robusta was also cytotoxic against the 4T1 and the HEp2 cells (IC50 < 10
g/ml). The compounds isolated from these extracts were characterized as limonoids,
protolimonoids and phytosterol glucosides. These compounds showed good to moderate
activities with the most active one being azadironolide, IC50 2.4 ± 0.03 M and 1.1 ± 0.01 M
against the D6 and W2 strains of Plasmodium falciparum, respectively; all other compounds
possessed IC50 14.4 - 40.5
M. None of the compounds showed significant cytotoxicity
against vero cells, yet four of them were toxic against the 4T1 and HEp2 cancer cell lines
with piscidinol A having IC50 8.0 ± 0.03 and 8.4 ±0.01 M against the 4T1 and HEp2 cells,
respectively. Diacetylation of piscidinol A resulted in reduced cytotoxicity.
Conclusion: From the medicinal plants Turraea robusta and Turraea nilotica, twelve
compounds were isolated and characterized; two of the isolated compounds, namely 11-epi-
3
toonacilin and azadironolide showed good antiplasmodial activity with the highest selectivity
indices.
Keywords: Turraea robusta; Turraea nilotica; antiplasmodial activity; cytotoxicity; limonoid;
toonapubesins F; toonacilin; azadironolide.
1.0
Introduction
The genus Turraea (family Meliaceae) consists of circa 70 species, mainly shrubs and small
trees, and is widely distributed in Eastern Africa. Several of its species are used in indigenous
medicine for the treatment of gastrointestinal disorders (Kokwaro, 2009). For example T.
floribunda is used as an emetic and purgative, T. holstii is used to treat diarrhoea and
constipation, and T. mombassana against excess bile, malaria and other fevers. T. robusta is
utilized to treat malaria, stomach pain, diarrhoea and gastrointestinal discomfort (Gathirwa et
al., 2008; Kokwaro, 2009). Its leaves are used as an antidote for poisoning. The roots of
Turraea nilotica are also used for the treatment of stomach disorders in traditional medicine
(Kokwaro, 2009).
The methanolic root bark extract of T. robusta was previously reported to possess
antiplasmodial and antimalarial activities, with negligible toxicity (mice oral LD50 > 5000
mg/kg body weight) (Irungu et al., 2007; Gathirwa et al., 2008). Previous phytochemical
investigations of the root bark of T. robusta led to the isolation of triterpenoids which
included five limonoids (Rajab et al., 1988; Bentley et al., 1992). There are no phytochemical
and antiplasmodial reports on the stem bark of T. robusta. The root and stem bark of T.
nilotica led to the identification of a limonoid and three protolimonoids (Mulholland and
Taylor, 1988; Bentley et al., 1995). These compounds were not evaluated for their
antiplasmodial and cytotoxic properties. In our search for plant secondary metabolites with
4
potential cytotoxic and/or antiplasmodial activities, the crude extracts and the isolated
secondary metabolites of T. nilotica Kotschy and Peyr, and of T. robusta Gürke were tested
against chloroquine resistant (W2) and chloroquine sensitive (D6) strains of Plasmodium
falciparum. In addition, their cytotoxicity against three mammalian cell lines, African green
monkey kidney (vero), mouse breast cancer (4T1) and human larynx carcinoma (HEp2) was
evaluated. Here, the isolation and the characterization of the constituents of the crude extracts
as well as their antiplasmodial and cytotoxic activities are reported.
2.0
Materials and Methods
The stem bark of Turraea robusta Gürke (BN/2011/1) was collected from Chiromo Campus
(01o16’31.34’’S; 036o38’64’’E) of the University of Nairobi in July 2011. The leaves, stem and
root bark of Turraea nilotica Kotschy & Peyr. (BN/2012/1) were collected from the Mombasa
Diani area (039o38’17.06’’E; 04o1λ’04.72’’S) in February 2012. The plants were
authenticated by Mr. Patrick Mutiso of the Herbarium, School of Biological Science,
University of Nairobi where voucher specimens were deposited.
2.1.
Antiplasmodial assay
Continuous in vitro cultures of asexual erythrocytic stages of P. falciparum strains (W2 and
D6) were maintained following previously described procedures (Kigondu et al., 2009). A
drug assay was carried out following a modification of the semiautomated micro dilution
technique that measures the ability of the extracts to inhibit the incorporation of (G-3H)
hypoxanthine into the malaria parasite (Gathirwa et al., 2008). Plates were harvested onto
glass fibre filters and hypoxanthine (G-3H) uptake determined using a micro-beta trilux
liquid scintillation and luminescence counter (Wallac, MicroBeta TriLux) and results
recorded as counts per minute (cpm) per well at each drug concentration. Data was
5
transferred into Microsoft Excel 2007, and expressed as percentage of the untreated controls.
Results were expressed as the drug concentration required for 50% inhibition of (G-3H)
hypoxanthine incorporation into parasite nucleic acid, using a non-linear regression analysis
of the dose-response curve. The criterion for scoring activity of compounds described by
Batista et al., (2009) was adopted: IC50 < 1 M, highly active; IC50 ≥ 1 and <20 M, active;
IC50 ≥ 20-100 M, moderate activity; IC50 > 100 inactive.
2.2.
Rapid
Cytotoxicity assay
colorimetric
assay
was
carried
out
using
3-(4,5-dimethylthiazol-2-yl)-2,5-
diphenyltetra-zolium bromide (MTT) (Mosmann, 1983). This assay is based on the ability of
a mitochondrial dehydrogenase enzyme of viable cells to cleave the tetrazolium rings of the
pale yellow MTT and thereby form dark blue formazan crystals, which are largely
impermeable to cell membranes, resulting in their accumulation within healthy cells. The
amount of generated formazan is directly proportional to the number of cells (Mosmann,
1983). In this assay, the mammalian cell lines African monkey kidney (vero), mouse breast
cancer (4T1) and human larynx carcinoma (HEp2) were used. Cells were maintained in
Eagle’s Minimum Essential Medium (MEM) containing 10% fetal bovine serum (FBS). A
cell density of 20 000 cells per well in 100 L serum were seeded on λ6-well plates and
incubated for 12 hours at 37 °C and 5% CO2 to attach to the surface. Following 12 hours, the
medium was replaced with maintenance medium containing the appropriate drug
concentration, 0.14 to 100
g/mL, or vehicle control (≤ 1.0% v/v DMSO). After an
incubation of 48 hours, cell viability was measured by addition of 10 µL of MTT reagent (5
mg MTT in 1ml of PBS). The plates were incubated for an additional 4 hours at the same
conditions. Next, all media was removed from the plates and 100 µL DMSO was added to
dissolve the formazan crystals. The plates were read on a Multiskan EX Labsystems scanning
multi-well spectrophotometer at 562 nm, and 620 nm as reference. The results were recorded
6
as optical density (OD) per well at each drug concentration, the data was transferred into the
software Microsoft Excel 2007 and expressed as percentage of the untreated controls.
Percentage cytotoxicity (PC) as compared to the untreated controls was calculated using the
following equation:
[
]
(1)
where A is the mean OD of the untreated cells and B is the mean OD at each drug
concentration. The drug concentration required for 50% inhibition of cell growth, using
nonlinear regression analysis of the dose-response curve, was calculated. Cytotoxicity was
defined as toxic if IC50 < 100 ug/ml.
2.3.
Extraction and Isolation
2.3.1. Turraea robusta
The air dried and ground stem bark of T. robusta (1.4 kg) was extracted with MeOH/CH2Cl2
(1:1) at room temperature (2 x 3 L, 48 h each). The filtrate was dried in vacuo using a rotary
evaporator to yield dark red oil (144 g). A 79 g portion of the extract was fractionated using
silica gel column chromatography (600 g, 5 cm × 30 cm) with gradient elution of petroleum
ether (40-60 oC) and increasing proportions of ethyl acetate. A total of 41 eluents, ca. 250 mL
each, were collected and combined into 11 fractions, labelled TR1-TR11 on the basis of their
TLC profiles. Fraction TR4 (1.2 g) was re-chromatographed over silica gel (10 g, 2 cm × 30
cm) using petroleum ether/acetone (95:5) to yield azadirone (1, 32.4 mg). Fraction TR7 (4.0
g) was fractionated on SephadexR LH-20 (40 g, 2 cm × 30 cm) eluting with methanol and
was further purified by PTLC eluting with petroleum ether/chloroform/methanol (16:2:1) to
yield 12α-acetoxy-7-deacetylazadirone (2, 7.0 mg) and mzikonone (3, 6.3 mg). Fraction TR8
(3.0 g) was re-chromatographed on silica gel column (30 g, 2 cm × 30 cm) eluting with
7
petroleum ether/ethyl acetate (9:1) and a subfraction containing one major compound was
purified on PTLC, eluting with petroleum ether/acetone (7:4) to yield azadironolide (5, 16.5
mg). Fractions TR9-10 were combined (145.2 mg) and were separated on PTLC eluting with
petroleum ether/chloroform/methanol (10:2:1) yielding 11-epi-toonacilin (4, 3.0 mg). The
purification
of
fraction
TR11
(331.9
mg)
on
PTLC
using
petroleum
ether/chloroform/methanol (12:2:1) eluent gave turranolide (6, 17.7 mg).
2.3.2. Turraea nilotica
The air dried and grounded stem bark of T. nilotica (1.2 kg) was extracted and dried
following the procedure described in section 2.3.1. to yield a dark gum (59 g). A 58 g portion
of the extract was fractionated by silica gel column chromatography (400 g, 5 cm × 30 cm)
using petroleum ether (40-60 °C) and increasing proportions of ethyl acetate as eluent. A total
of 50 fractions, ca. 250 mL each, were collected and combined into 20 major fractions,
labelled TN1-20, upon TLC analyses. Repeated silica gel column chromatography (10 g, 2
cm × 30 cm) of fraction TN12 (1.35 g) eluting with 9:1 petroleum ether/acetone yielded
niloticin (8, 9.3 mg). Fractions TN13-15 (7g) were combined and purified by silica gel
column chromatography (100 g, 2 × 30 cm) using petroleum ether and increasing portions of
acetone; crystallization of the residue from an acetone/dichloromethane mixture yielded
toonapubesin F (11, 20.1 mg). A repeated column chromatographic (30 g, 2 cm × 30 cm)
separation of fraction TN16 (3 g) in silica gel followed by crystallization from a
methanol/dichloromethane mixture yielded piscidinol A (10, 775.6 mg). Fraction TN18 was
crystallized from methanol/dichloromethane (1:9) to yield hispidol B (9, 84.2 mg).
The root bark of Turraea nilotica (837 g) was extracted and dried as described in section 2.3.1
above yielding 13 g of a yellowish gum. A portion of the extract (11 g) was fractionated by
8
silica gel column chromatography (88 g, 2 cm × 30 cm) using a petroleum ether:acetone
gradient, with increasing polarity. Forty six fractions, ca. 100 mL each, were collected and
combined into 20 major fractions, labelled TN21-40, based on their TLC profile. Fraction
TN27 (140.7 mg) was purified on PTLC eluting with petroleum ether/acetone (9:1) to yield
azadirone (1, 8.3 mg). Fraction TN32 (366 mg) was subjected to RP-HPLC (CH3OH/water)
yielding 12α-acetoxy-7-deacetylazadirone (2, 18.5 mg) and mzikonone (3, 4.4 mg). Repeated
column chromatographic purification of fraction TN36 (358 mg) followed by PTLC with a
petroleum ether/chloroform/methanol (10:2:1) mixture as eluent yielded 1α,3α-diacetyl-7αtigloyvilasinin (7, 11.6 mg).
The dried and grounded leaves of T. nilotica (500 g) were extracted as described in section
2.3.1, yielding 30 g of a dark green gum. A 20 g portion of the crude extract was fractionated
using silica gel column chromatography (200 g, 5 cm × 30 cm) with petroleum ether/acetone
gradient with increasing polarity. A total of fifty eight fractions, ca 100 mL each, were
collected and combined into the 12 major fractions and labelled TN41-52, based on TLC
profiling. Fraction TN43 was crystallized from acetone giving β-sitosterol (15) and
stigmasterol (16) as a mixture (4.7 mg). Fraction TN50 was crystallized from acetone
yielding sitosterol 3-O-β-D-glucopyranoside acetate (12) and stigmasterol-3-O-β-Dglucopyranoside acetate (13) as a mixture (9.8 mg). Fraction TN52 was crystallized in
acetone to yield sitosterol-3-O-β-D-glucopyranoside (14, 4.7 mg).
2.3.3. Structure Elucidation
For structure elucidation 1H,
13
C, gCOSY, gNOESY, gHSQC and gHMBC NMR spectra
were acquired on an Bruker Avance III HD 800 MHz, on a Varian VNMR-S 500 or a Varian
400 MR spectrometer. The spectra were processed using the MestReNova (v9.0.0) software.
Chemical shifts were referenced indirectly to tetramethylsilane via the residual solvent signal
9
(CHCl3, 1H at 7.26 ppm and
13
C at 77.16 ppm). LC(ESI)MS spectra was acquired on a PE
SCIEX API 150EX instrument (Perkin Elmer, Waltham, MA, USA) equipped with a
Turbolon spray ion source (30 eV ionization energy) and a Gemini 5 mm C-18 110 Å HPLC
column, using water:acetonitrile gradient (80:20 to 20:80).
2.4.
X-ray diffraction
Crystals of compound 11 was selected and mounted under a stereo microscope on to a glass
fibre and transferred to a Rigaku R-AXIS IIc image plate system. Diffracted intensities were
measured using graphite-monochromated Mo Kα ( = 0.710 73 Å) radiation from a RU-H3R
rotating anode operated at 50 kV and 40 mA. Using the R-AXIS IIc detector, 90 oscillation
photos with a rotation angle of 2° were collected and processed using the CrystalClear
software package. An empirical absorption correction was applied using the REQAB
program under CrystalClear. All structures were solved by direct methods (SIR 97)
(Altomare et al., 1999) and refined using full-matrix least-squares calculations on F2
(SHELXL-97) (Sheldrick, 2007) operating in the WinGX program package. Anisotropic
thermal displacement parameters were refined for all the non-hydrogen atoms. Hydrogen
atoms were included in calculated positions and refined using a riding model. Displacement
ellipsoids are drawn with ORTEP-3 for Windows under WinGX.
2.5.
Acetylation
A 20 mg portion of compound 8 or 10 was dissolved in 1 mL pyridine and then 1 mL acetic
anhydride was added. The mixture was stirred overnight, and subsequently methanol was
added and the solvent was removed under vacuum. The acetate precipitated by addition of
water while stirring briskly, and it was filtered and dried. The procedure yielded niloticin
acetate (17, 5 mg) and piscidinol A diacetate (18, 7 mg), respectively.
10
3.0
Results and Discussion
3.1.
Antiplasmodial and cytotoxic activities
The crude extracts of T. nilotica and T. robusta were evaluated for antiplasmodial activity
against the chloroquine-resistant (W2) and chloroquine-sensitive (D6) Plasmodium
falciparum strains (Table 1). The stem bark of T. robusta showed high antiplasmodial activity
with IC50 2.8 ± 0.02
g/mL and 2.3 ± 0.05
g/mL against the W2 and D6 strains,
respectively. The stem and root barks of T. nilotica also displayed considerable
antiplasmodial activities (IC50 < 10 g/ml) while its leaves showed only moderate activity.
Cytotoxicity of these extracts was evaluated against three mammalian cell lines, namely
African green monkey kidney (vero), mouse breast cancer (4T1) and human larynx
carcinoma (HEp2) (Table 1). The stem bark extract of T. robusta was cytotoxic against the
4T1 and the HEp2 cells (IC50 < 10 g/ml), whereas the extract of T. nilotica possessed
moderate cytotoxicity against the three cell lines studied. Notably, all the extracts had low
selectivity index (< 10), defined as the ratio of IC50 vero cells to IC50 P. falciparum (D6). This
may indicate that the good to moderate antiplasmodial activity observed for the above
extracts may be due to their general cytotoxicity. The fractions obtained from the crude
extracts were evaluated for their cytotoxicity against vero cells. Generally, they had lower
cytotoxicity than the crude extracts. Except for fraction TN12 (IC50 = 4.34 ± 0.04 g/mL),
most fractions showed moderate cytotoxicity.
11
Table 1. Antiplasmodial and cytotoxic activities of selected plant parts
IC50 (µg/mL)
Plant (part)
W2
D6
Vero
4T1
Hep2
SI
T. robusta (SB)
2.8±0.0
2.3±0.1
21.9±2.0
5.3±0.6
4.2±1.0
9.5
T. nilotica (SB)
7.3±0.1
6.9±0.0
17.7±1.3
ND
22.4±4.4
2.6
T. nilotica (RB)
9.5±1.0
7.9±1.0
13.7±2.0
18.6±1.2 27.2±3.6
1.7
T. nilotica (L)
59.0±4.0
47.4±3.0
21.5±2.0
39.1±4.0 37.4±1.3
0.5
Chloroquine
a
108.0±0.1
a
7.7.0±0.0
43.9±0.5
ND
ND
5701
Legend: SB: stem bark, RB: root bark, L: leaves, SI: selectivity index [IC50 vero / IC50 (D6)]; Positive control:
podophyllum resin, IC50 (4T1) = 0.47 ± 0.05 µg/mL melarsoprol IC50 (Vero) = 0.76 ± 0.01 µg/mL; a IC50: half
maximal inhibitory concentration given in nM for chloroquine. ND: not determined, due to the small sample
amount available.
From the stem bark of Turraea robusta Guerke six known compounds were isolated: three
ring A-D-intact limonoids, namely azadirone (1), 12α-acetoxy-7-deacetylazadirone (2) and
mzikonone (3), and the ring B seco limonoid 11-epi-toonacilin (4) (Figure 1). In addition, it
gave two other triterpenoids azadironolide (5) and turranolide (6). Compound 5 was isolated
as an epimeric mixture, indicated by doubling of some of its NMR signals. Epimeric mixtures
of limonoids having a hemiacetal functionality in place of a furan ring were previously
reported (Cheplogoi and Mulholland, 2003; McFarland et al., 2004). The secondary
metabolite content of T. robusta stem bark was similar to those reported for other Turraea
species, which have limonoids as the main constituents. Compounds 2 and 4 were reported
from this species for the first time. This is the first report on the occurrence of compound 5 in
the genus Turraea.
12
Figure 1. The structures of the limonoids azadirone (1), 12α-acetoxy-7-deacetylazadirone (2),
and mzikonone (3), the ring B seco limonoid 11-epi-toonacilin (4), the triterpenoids
azadironolide (5), and turranolide (6), isolated from the stem bark of Turraea robusta.
From the root bark of Turraea nilotica, four limonoids azadirone (1), acetoxy-7deacetylazadirone (2), mzikonone (3), (Figure 1) and 1α,3α-diacety-7α-tigloyvilasinin (7)
(Figure 2) were isolated. From its stem bark, four protolimonoids niloticin (8), hispidol B (9),
piscidinol A (10) and toonapubesin F (11) were isolated. A mixture of sitosterol-3-O-β-Dglucopyranoside acetate (12) and stigmasterol-3-O-β-D-glucopyranoside acetate (13),
sitosterol-3-O-β-D-glucopyranoside (14) as well as a mixture of β-sitosterol (15) and
stigmasterol (16) were isolated from the leaves. Compounds 1, 2, 3, 7 and 9 are reported here
from this species for the first time. There is no previous report on compounds 11-14 from
genus Turraea.
13
Figure 2. In addition to 1-3 shown in Figure 1, the limonoid 1α,3α-diacety-7α-tigloyvilasinin
(7), the protolimonoids niloticin (8), hispidol B (9), piscidinol A (10) and toonapubesin F
(11), the phytosterols sitosterol-3-O-β-D-glucopyranoside acetate (12), stigmasterol-3-O-βD-glucopyranoside acetate (13), and sitosterol-3-O-β-D-glucopyranoside (14) were isolated
from the root and stem barks of Turraea nilotica.
The structure of toonapubesin F (11), first isolated from Toona ciliate var. pubesins (Wang et
al., 2011), was further confirmed by an X-ray crystallographic analysis (Figure 4) from a
single crystal obtained by slow crystallization from a mixture of dichloromethane and
acetone. The atomic coordinates are given in the Supplementary Information.
14
Figure 3. ORTEP picture of toonapubesin F (11). All nonpolar hydrogens have been omitted
for clarity. Displacement ellipsoids are drawn at the 50% probability level.
As the stereochemistry of the toonapubesin backbone is known (Wang, 2011), the chirogenic
centre at C4 is (S). Hence, this data allows identification of the oxidized methyl functionality
on C4, i.e. C25 and not C24.
The roots, stem and leaves of Turraea nilotica, were found to contain different secondary
metabolites. Limonoids were isolated from the root bark, and protolimonoids from the stem
bark while phytosterols were isolated from the leaves. Limonoids, protolimnoids and steroids
share a tetracyclic triterpenoid skeleton; however, they differ in the oxidation state of their
side chain. Based on the previously proposed biosynthetic pathway of limonoids,
(Champagne et al., 1992; Tan and Luo, 2011) T. nilotica is likely to follow the phytosterol
(leaves) → protolimonoid (stem bark) → limonoid (root bark) biosynthetic pathway.
The antiplasmodial potency of some limonoids and other triterpenoids has been previously
reported (Maneerat et al., 2008; Mohamad et al., 2009). Therefore, some of the compounds
isolated in this work were tested for antiplasmodial activities against the D6 and W2
15
Plasmodium falciparum strains (Table 2). Their activity was scored according to the
classification of Batista et al., (2009), where IC50 < 1 M is highly active; 1 ≤ IC50 ≤ 20 M is
active, 20 M ≤ IC50 ≤ 100 M is moderately active, and IC50 > 100 is inactive. Of the eight
compounds tested, two showed good activity with the most active substance being the
epimeric mixture azadironolide (5) (IC50 < 2.4
M), and six showed moderate activities
against the W2 and D6 strains (Table 2). The antiplasmodial activity of compound 1 is in
agreement with the previous literature data (Chianese et al., 2010).
The isolated compounds were also tested for cytotoxicity (Table 2) against the mammalian
cell line African green monkey kidney (vero cells). Most of them showed moderate
cytotoxicity (IC50 > 20 M) and a low selectivity index [SI = IC50(vero)/IC50 (D6) < 10].
Thus, the observed moderate antiplasmodial activity of these compounds is likely due to
general cytotoxicity, rather than due to a specific activity against the Plasmodium parasite. It
should, however, be noted that compounds 4 and 5 that are classified as active, were observed
to have comparably high selectivity index SI > 10.5 and 11.5, respectively, though
significantly lower than chloroquine (SI = 5702).
Although the investigated plants are in traditional medicinal use against malaria, and their
crude extracts showed promising antimalarial activities, their isolated metabolites did not
display considerable activity against P. falciparum. This may be explained by synergetic
effects or by the loss of one or some minor yet highly active metabolites during the
purification process. For an improved understanding of the traditional medicinal applicability
of these plants further studies are necessary.
Some limonoids and protolimonoids isolated from other plants were previously reported to
possess substantial cytotoxicity (Maneerat et al., 2008), which, along with the high to
moderate cytotoxicity of the crude extracts and isolated compounds against ‘normal’ vero
16
cells motivated the evaluation of cytotoxicity of the isolated constituents against the
cancerous cell lines 4T1 and HEp2 (Table 2).
Table 2 Antiplasmodial and cytotoxic activities (IC50 in M) of compounds isolated from
Turraea nilotica and Turraea robusta.
Compound
Azadirone (1)
12α-Acetoxy-7deacetylazadirone
(2)
Mzikonone (3)
11-epi-Toonacilin
(4)
Azadironolide (5)
Niloticin (8)
Hispidol B (9)
Piscidinol A (10)
Niloticin acetate
(15)
Piscidinol A
acetate (16)
Chloroquine
D6
23.4±0.2
W2
29.6±1.0
4T1
14.4±0.0
HEp2
12.8±0.0
Vero
> 229.4
SIb
>9.8
31.0±0.2
30.2±0.5
104.6±7.1
40.3±2.2
134.1±2.9
4.3
36.6±0.8
40.5±3.7
38.8±0.4
59.3±1.0
139.6±4.7
3.8
c
c
17.1±0.2
14.4±0.5
88.6±3.2
68.1±1.3
> 180.5
>10.5
2.4±0.0
48.2±2.3
36.8±2.0
37.6±1.4
1.1±0.0
77.0±5.7
37.2±3.2
36.3±4.4
14.7±0.2
14.5±0.5
21.7±3.2
8.0±0.0
8.5±0.5
6.9±0.6
7.4±0.7
8.4±0.0
27.6±0.6
14.5±0.4
130.0±3.1
41.1±5.8
11.5
0.3
3.5
1.1
68.3±5.3
172.9±4.5
ND
121.9±0.1
>c200.8
ND
ND
ND
15.2±0.8
>c179.2
108.0±0
ND
ND
43.9±0.5
a
7.7±0.02
a
5701
The mean values of at least three independent experiments are reported. ND: not determined, due to the small
amount of sample available. a IC50: half maximal inhibitory concentration given in nM for chloroquine. b SI =
IC50 (vero)/IC50(D6); c values >100ug/ml not cytotoxic, Positive control: podophyllum resin, IC50 (4T1) = 0.47 ±
0.05 µg/mL, melarsoprol IC50 (Vero) = 0.76 ± 0.01 µg/mL
Compounds 1, 5, 8-10 showed high cytotoxicities against the 4T1 and HEp2 cell lines with
IC50 < 20 µM. It should be stressed that these compounds showed moderate cytotoxicity
against ‘normal’ vero cells, yet high cytotoxicity against cancerous cell lines. Hence these
compounds may be promising leads for development of anticancer agents (Diantini et al.,
2012).
For assessment of the importance of the free hydroxyl groups of compound 8 and 10 on their
bioactivities, they were acetylated to give niloticin acetate (17) and piscidinol A diacetate
(18), respectively (Figure 4). The acetate derivatives 17 and 18 showed lower cytotoxicity as
17
compared to the parent, non-acetylated compounds 8 and 10, indicating the importance of
free hydroxyl group(s) in the side chain. A reduction in antiplasmodial activity of compound
17 in comparison to the parent compound 8 was also observed.
O
OA c
OA c
O Ac
H
O
H
OH
H
17
O
18
Figure 4. The structure of niloticin acetate (17) and piscidinol A diacetate (18).
3.2.
The chemotaxonomic significance of Turraea limonoids
Various classes of limonoids were reported from the genus Turraea with each species
synthesizing more than one class of limonoids. Ring intact limonoids were reported from
eight Turraea species, namely T. robusta, T. nilotica, T. cornucopia, T. parvifolia, T.
floribunda, T. holstii, T. wakefieldii and T. pubescens (Bentley et al., 1λλ2; Ndung’u et al.,
2004; Owino et al., 2008; Yuan et al., 2013). So far, ring A seco limonoids have only been
isolated from T. wakefieldii (Ndung’u et al., 2003), whereas ring B seco limonoids were
reported in T. floribunda, T. holstii and T. pubescens (Mulholland et al., 1998; McFarland et
al., 2004; Yuan et al., 2013). Ring A-B seco limonoids were reported from T. mombassana
and T. obtusifolia, while ring C seco limonoids from T. holstii and T. pubescens (Adul et al.,
1993; Sarker et al., 1997; Mulholland et al., 1998; Yuan et al., 2013). Interestingly, so far no
ring D seco limonoid was reported from the genus Turraea although several examples were
isolated from other Meliaceace genera. The presence of compounds 1-4 and 7 in T. robusta
and T. nilotica is in line with the fact that limonoids are common constituents of the genus
Turraea. Except for compound 1 that was reported from other genera of the Meliaceace
18
family (Zhou et al., 1997), the limonoids disclosed here have not been reported from any
other genus. The occurrence of five limonoids in Turraea species is shown in Table 3, among
which 3 was isolated from five species.
Table 3. Occurrence of some limonoids in Turraea species
Limonoid
Azadirone (1)
12α-acetoxy-7deacetylazadirone (2)
Mzikonone (3)
11-epi-toonacilin (4)
1α,3α-diacety-7αtigloyvilasinin (7)
4.0
Turraea species
T. nilotica
T. robusta
-
-
-
T. robusta
T. cornucopia
T. pubescens
-
-
T. robusta
T. robusta
T. cornucopia
T. holstii
T. parvifolia
T. pubescens
T. nilotica
T. cornucopia
T. pubescens
-
T. nilotica
T. parvifolia
-
-
-
Conclusion
Six compounds were isolated from the stem bark of Turraea robusta. Four of them were
limonoids and two of them triterpenoids. Of these six compounds, azadironolide (5) is new to
the genus. The secondary metabolites present in the stem bark were also found in the root
bark. Compound 3 is a common limonoid in the genus Turraea, and was previously reported
from five Turraea species (Cheplogoi and Mulholland, 2003; Owino et al., 2008; Yuan et al.,
2013). From the leaves, root and stem bark of Turraea nilotica twelve compounds were
isolated. Toonapubesin F (11), sitosterol 3-O-β-D-glucopyranoside acetate (12), stigmasterol3-O-β-D-glucopyranoside acetate (13) and sitosterol-3-O-β-D-glucopyranoside (14) are new
to the genus. Different secondary metabolites were observed in the leaves, and thus the roots
and the stem were observed to contain limonoids and protolimonoids, whereas the leaves
contained phytosterols.
Out of the isolated compounds, 11-epi-toonacilin (4) and azadironolide (5) showed good
antiplasmodial activity with the highest selectivity indices among the isolated compounds.
Azadironolide (5), niloticin (8), hispidol B (9) and piscidol A (10) were cytotoxic to HEp2
19
and 4T1 cells with IC50 < 20 M. The cytotoxicity of the latter constituents against the
‘normal’ vero cell line was comparably low, indicating some degree of selectivity.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the online version, at
http://dx.doi.org/...
Acknowledgements
This work was supported by IFS (grant nr. F/4575-2), UNICEF/UNDP/WORLD
BANK/WHO Special Programme for Research and Training in Tropical Diseases (SPHQ10RTG-6 ID: A90208) and by the Swedish Research Council (grant nr. 2012–6124).
References
Adul, G.O., Bentley, M.D., Benson, B.W., Huang, F.-Y., Gelbaum, L., Hassanali, A., 1993.
Two new prieurianin-class limonoids from Turraea mombasana. Journal of Natural
Products 56, 1414–1417.
Altomare, A., Burla, M.C., Camalli, M., Cascarano, G.L., Giacovazzo, C., Guagliardi, A.,
Moliterni, A.G.G., Polidori, G., Spagna, R., 1999. SIR97: a new tool for crystal structure
determination and refinement. Journal of Applied Crystallography 32, 115–119.
Batista, R., de Jesus Silva Júnior, A., de Oliveira, A.B., 2009. Plant-derived antimalarial
agents: New leads and efficient phytomedicines. Part II. Non-alkaloidal natural products.
Molecules 14, 3037–3072.
Bentley, M.D., Adul, G.O., Alford, A.R., Huang, F.-Y., Gelbaum, L., Hassanali, A., 1995. An
insect antifeedant limonoid from Turraea nilotica. Journal of Natural Products 58, 748–
750.
Bentley, M.D., Gaul, F., Rajab, M.S., Hassanali, A., 1992. Tetranortriterpenes from Turraea
robusta. Journal of Natural Products 55, 84–87.
Champagne, D.E.; Koul, O.; Isman, M.B.; Scudder, G.G.; Neil Towers, G.H., 1992.
Biological Activity of Limonoids from the Rutales. Phytochemistry. 31, 377–394.
Cheplogoi, P.K., Mulholland, D.A., 2003. Limonoids from Turraea parvifolia (Meliaceae).
Biochemical Systematics and Ecology 31, 799–803.
20
Chianese, G., Yerbanga, S.R., Lucantoni, L., Habluetzel, A., Basilico, N., Taramelli, D.,
Taglialatela-Scafati, O., 2010. Antiplasmodial triterpenoids from the fruits of neem,
Azadirachta indica. Journal of Natural Products 73, 1448–1452.
Diantini, A., Subarnas, A., Lestari, K., Halimah, E., Susilawati, Y., Supriyatna, S., Julaeha,
E., Achmad, T.H., Suradji, E.W., Yamazaki, C., Kobayashi, K., Koyama, H., Abdulah, R.,
2012. Kaempferol-3-O-rhamnoside isolated from the leaves of Schima wallichii Korth.
inhibits MCF-7 breast cancer cell proliferation through activation of the caspase cascade
pathway. Oncology Letters 3, 1069–1072.
Gathirwa, J.W., Rukunga, G.M., Njagi, E.N.M., Omar, S.A., Mwitari, P.G., Guantai, A.N.,
Ndiege, I. O., 2008. The in vitro antiplasmodial and in vivo antimalarial efficacy of
combinations of some medicinal plants used traditionally for treatment of malaria by the
Meru community in Kenya. Journal of Ethnopharmacology 115, 223–231.
Irungu, B.N., Rukunga, G.M., Mungai, G.M., Muthaura, C.N., 2007. In vitro antiplasmodial
and cytotoxicity activities of 14 medicinal plants from Kenya. South African Journal of
Botany 73, 204–207.
Kigondu, E.V., Rukunga, G.M., Keriko, J.M., Tonui, W.K., Gathirwa, J.W., Kirira, P.G.,
Ndiege, I.O., 2009. Antiparasitic activity and cytotoxicity of selected medicinal plants
from Kenya. Journal of Ethnopharmacology 123, 504–509.
Kokwaro, J.O., 2009. Medicinal Plants of East Africa (3rd ed.). Nairobi: University of
Nairobi Press.
Maneerat, W., Laphookhieo, S., Koysomboon, S., Chantrapromma, K., 2008. Antimalarial,
antimycobacterial and cytotoxic limonoids from Chisocheton siamensis. Phytomedicine.
15, 1130–1134.
McFarland, K., Mulholland, D. A., Fraser, L.-A., 2004. Limonoids from Turraea floribunda
(Meliaceae). Phytochemistry. 65, 2031–2037.
Mohamad, K., Hirasawa, Y., Litaudon, M., Awang, K., Hadi, A. H.A., Takeya, K., Morita,
H., 2009. Ceramicines B–D, new antiplasmodial limonoids from Chisocheton ceramicus.
Bioorganic Medicinal Chemistry 17, 727–730.
Mosmann, T., 1983. Rapid colorimetric assay for cellular growth and survival: application to
proliferation and cytotoxicity assays. Journal of Immunolgy Methods 65, 55–63.
Mulholland, D.A., Monkhe, T.V., Coombes, P.H., and Rajab, M.S., 1998. Limonoids from
Turraea holstii and Turraea floribunda. Phytochemistry 49, 2585–2590.
Mulholland, D.A., Taylor, D.A., 1988. Protolimonoids from Turraea nilotica. Phytochemistry
27, 1220–1221.
Ndung’u, M., Hassanali, A., Hooper, A.M., Chhabra, S., Miller, T.A., Paul, R.L., Torto, B.,
2003. Ring A-seco mosquito larvicidal limonoids from Turraea wakefieldii.
Phytochemistry 64, 817–823.
21
Ndung’u, M.W., Kaoneka, B., Hassanali, A., Lwande, W., Hooper, A.M., Tayman, F., Torto,
B., 2004. New Mosquito Larvicidal Tetranortriterpenoids from Turraea wakefieldii and
Turraea floribunda. Journal of Agriculture and Food Chemistry 52, 5027–5031.
Owino, J., Ndung’u, M., Hassanali, A., 2008. Two azadiron limonoids from Turraea
cornucopia. Journal Kenya Chemical Society 5, 43–49.
Rajab, M. S., Bentley, M. D., Hassanali, A., Chapya, A., 1988. A new limonoid from Turraea
robusta. Phytochemistry 27, 2353–2355.
Su, R., Kim, M., Kawaguchi, H., Takehiko, Y., Katsumi, G., Tooru, T., Takahashi, S., 1990.
Triterpenoids from the fruits of Phellodendron chinense SCHNEID.The Stereostructure of
Niloticin. Chemical and Pharmaceutical Bulletin. 38, 1616–1619.
Sarker, S.D., Savchenko, T., Whiting, P., Šik, V., Dinan, L., 1997. Two limonoids from
Turraea obtusifolia (Meliaceae), prieurianin and rohitukin, antagonise 20hydroxyecdysone action in a Drosophila cell line. Archives of Insect Biochemistry and
Physiology 35, 211–217.
Sheldrick, G.M., 2007. A short history of SHELX. Acta Crystallography A. 64, 112–122.
Tan, Q.-G.; Luo, X.-D., 2011. Meliaceous Limonoids: chemistry and biological activities.
Chemical Reviews 111, 7437–7522.
Wang, J.-R., Liu, H.-L., Kurtán, T., Mándi, A., Antus, S., Li, J., Guo, Y.-W., 2011.
Protolimonoids and norlimonoids from the stem bark of Toona ciliata var. pubescens.
Organic & Biomolecular Chemistry 9, 7685–7696.
Yuan, C.-M., Tang, G.-H., Zhang, Y., Wang, X.-Y., Cao, M.-M., Guo, F., Hao, X.-J., 2013.
Bioactive limonoid and triterpenoid constituents of Turraea pubescens. Journal of Natural
Products 76, 1166–1174.
Zhou, J.-B., Minami, Y., Yagi, F., Tadera, K., Nakatani, M., 1997. Ring C-seco-limonoids
from Melia toosendan. Phytochemistry 46, 911–914.
22
Graphical abstract
Azadironolide isolated as an epimeric mixture from Turraea robusta stem bark inhibits growth of Plasmodium falciparum strain (W2) with an
IC50 of 1.1 ± 0.01 M
HO
O
O
% Inhibition
100
80
60
40
20
H
O
H
OAc
0
0.0 0.2 0.3 0.6 1.3 2.5 5.0 10.0
Concentration (μg/ml)