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DOI: 10.1055/s-2005-873200
Antiplasmodial Flavonoids from Erythrina sacleuxii
Abiy Yenesew
Department of Chemistry
University of Nairobi
P.O. Box 30197
Nairobi
Kenya
Phone: +254-20-444-9004 ext 2170
Fax: +254-20-4446138
Email: ayenesew@uonbi.ac.ke
Publication History
Received: April 25, 2005
Accepted: July 6, 2005
Publication Date:
05 December 2005 (online)
Abstract
The acetone extracts of the root bark and stem bark of Erythrina sacleuxii showed antiplasmodial activities against the chloroquine-sensitive (D6) and chloroquine-resistant (W2) strains of Plasmodium falciparum. Chromatographic separation of the acetone extract of the root bark afforded a new isoflavone, 7-hydroxy-4′-methoxy-3′-prenylisoflavone (trivial name 5-deoxy-3′-prenylbiochanin A) along with known isoflavonoids as the antiplasmodial principles. Flavonoids and isoflavonoids isolated from the stem bark of E. sacleuxii were also tested and showed antiplasmodial activities. The structures were determined on the basis of spectroscopic evidence.
Some Erythrina species of Kenya, including E. sacleuxii are used traditionally for the treatment of microbial infections and malaria [1], [2]. It is established that flavonoids and isoflavonoids are responsible for the traditional antimicrobial uses of Erythrina species [3], [4]. Recently we reported antiplasmodial flavonoids and isoflavonoids isolated from the root and stem bark of E. abyssinica [5], [6]. Here we report the identification and antiplasmodial activity of a new isoflavone (1) along with other flavonoids from E. sacleuxii. []
HR-MS analysis of compound 1 showed a molecular ion peak at m/z = 336.1347 corresponding to the molecular formula C21H20O4. The UV (λmax = 263 nm), 1H- (δ = 8.14 for H-2) and 13C-NMR (δ = 154.0 for C-2, 125.9 for C-3 and 176.4 for C-4) spectra indicated an isoflavone skeleton. The presence of a prenyl, a hydroxy and a methoxy substituent was evident from the mass, 1H- and 13C-NMR spectra. For the A-ring, the 1H-NMR spectrum showed an AXY spin system at δ = 8.06 (d, J = 8.7 Hz for H-5), 6.98 (1H, dd, J = 2.1, 8.7 Hz, for H-6) and 6.90 (d, J = 2.1 Hz, for H-8) indicating that C-7 is oxygenated as expected biogenetically. In the MS, the fragment ion at m/z = 137 resulting from retro-Diels-Alder fragmentation of the C-ring suggested that the A-ring contains one hydroxy group, viz at C-7, and hence the methoxy and the prenyl groups should be located in the B-ring. For this ring, the 1H- and 13C-NMR spectra are similar with those of 3′-prenylbiochanin A, an isoflavone earlier reported from the stem bark of this plant [7], and suggesting identical substitution pattern. Thus in this ring, an ABX spin system at δ = 7.40 (d, J = 2.1 Hz for H-2′), 6.99 (d, J = 8.4 Hz, for H-5′) and 7.43 (dd, J = 2.1, 8.4 Hz, for H-6′) is in agreement with the placement of the methoxy at C-4′ and the prenyl at C-3′. The HMBC spectrum showed correlations of the methylene protons (δ = 3.34) of the prenyl group with C-4′ (δ = 158.7) and C-6′ (δ = 129.3), confirming the substitution pattern in the B-ring. Thus this compound was characterized as 7-hydroxy-4′-methoxy-3′-prenylisoflavone for which the trivial name 5-deoxy-3′-prenylbiochanin A is suggested. This compound has been reported as a synthetic derivative [8]. However this is the first report on its occurrence in nature.
The root bark of E. sacleuxii also afforded the ferulate ester erythrinasinate A [9], the pterocarpan shinpterocarpin [10], the isoflav-3-ene 7,4′-dihydroxy-2′,5-dimethoxyisoflav-3-ene [5], the isoflavanone prostratol C [11], the isoflavones corylin [12] and erysubin F [13]. This is the first report on the occurrence of prostratol C in the genus Erythrina.
The acetone extract of the stem bark and the root bark of E. sacleuxii showed antiplasmodial activity against the D6 (IC50 = 3.8 ± 0.9 μg/mL for stem bark extract and 2.2 ± 0.6 μg/mL for the root bark extract) and W2 (IC50 = 6.3 ± 1.4 μg/mL for stem bark and 1.34 ± 0.3 μg/mL for the root bark) strains of P. falciparum. This supports the traditional use of this plant to treat malaria in East Africa [2]. The flavonoids and isoflavonoids previously reported from the stem bark of this plant [7], [14], as well as those isolated in this investigation from the root bark were tested for antiplasmodial activities (Table [1]). This plant mainly elaborates isoflavones and all the eight isoflavones tested showed activities with 5′-prenylpratensein being the most active. Prior to this work very little had been reported on the antiplasmodial activities of isoflavones [15]. Activities have also been observed in other subclasses of flavonoids against both strains (Table [1]). The flavanone abyssinone V and the pterocarpan shinpterocarpin are among the most active.
| Flavonoids | IC50 (μM)* | |
| D6 | W2 | |
| Flavanones | ||
| Abyssinone V [14] | 4.9 ± 0.8 | 6.1 ± 1.3 |
| Abyssinone V 4′-methyl ether [14] | 11.3 ± 2.4 | 11.1 ± 2.1 |
| Sigmoidin B 4′-methyl ether [14] | 13.0 ± 2.0 | 12.7 ± 2.9 |
| Isoflavones | ||
| 5-Deoxy-3′-prenylbiochanin A (1) | 17.6 ± 1.7 | 22.5 ± 2.1 |
| Corylin [12] | 16.6 ± 3.8 | 19.7 ± 4.3 |
| Erysubin F [13] | 12.0 ± 0.5 | 12.8 ± 0.7 |
| 3′-Prenylbiochanin A [7] | 23.7 ± 4.3 | 28.4 ± 4.8 |
| 7-Demethylrobustigenin [7] | 27.2 ± 3.3 | 31.7 ± 5.7 |
| 5′-Prenylpratensein [7] | 6.3 ± 0.3 | 8.7 ± 1.5 |
| 5′-Formylpratensein [7] | 21.7 ± 8.6 | 27.9 ± 6.2 |
| 2,3-Dehydrokeivetone [14] | 15.1 ± 3.4 | 12.7 ± 2.3 |
| Isoflavanones | ||
| Prostratol C [11] | 17.6 ± 1.7 | 19.8 ± 3.5 |
| Saclenone [14] | 24.2 ± 3.5 | 22.6 ± 1.4 |
| 2,3-Dihydro-7-demethylrobustigenin [14] | 28.0 ± 5.3 | 31.8 ± 6.1 |
| Pterocarpan | ||
| Shinpterocarpin [10] [14] | 6.6 ± 1.2 | 8.3 ± 1.1 |
| Isoflav-3-ene | ||
| 7,4′-Dihydroxy-2′,5′-dimethoxyisoflav-3-ene [5] | 22.0 ± 2.4 | 24.9 ± 3.0 |
| Reference drugs | ||
| Chloroquine | 0.008 ± 0.002 | 0.075 ± 0.002 |
| Quinine | 0.050 ± 0.02 | 0.28 ± 0.02 |
| * Values are expressed as mean ± SD, with n = 3. | ||
Material and Methods
The root bark of Erythrina sacleuxii was collected from the South Coast of Kenya, in July 2003. The plant was identified at the herbarium, Botany Department, University of Nairobi, where a voucher specimen (AY-SGM-2003 - 02) is deposited.
Air-dried and powdered root bark of E. sacleuxii (524 g) was extracted with acetone (2.5 L × 3) by percolation at 25 °C to yield 52 g of a brown sticky extract after concentration under vacuum. A portion of the extract (50 g) was chromatographed on oxalic acid-impregnated silica gel (280 g, 5 × 50 cm) eluting with hexane containing increasing amounts of acetone (1, 2, 3, 5 and 7 % acetone in hexane). Five major fractions, each ca. 1 L, were collected and labelled A to E. Erythrinasinate A (40 mg, Rf = 0.52, hexane/CH2Cl2, 3 : 7) precipitated from fraction A (eluted with 1 % acetone in hexane). Fraction B (2 % acetone in hexane) was purified on Sephadex LH-20 (5 × 50 cm, eluted with CH2Cl2/MeOH, 1 : 1) and preparative TLC (silica gel, hexane/CH2Cl2, 3 : 7, multiple development) to give shinpterocarpin (980 mg, Rf = 0.31, hexane/CH2Cl2, 3 : 7). Fraction C (3 % acetone in hexane) was further purified by CC (oxalic acid-impregnated silica gel, 2 × 50 cm, elution with CH2Cl2/EtOAc, 1 : 1) to give prostratol C (120 mg, Rf = 0.45, CH2Cl2). Fraction D (5 % acetone in hexane) was separated by CC on Sephadex LH-20 (2 × 50 cm, elution with CH2Cl2/MeOH; 1 : 1), and preparative TLC (silica gel, 1 % MeOH in CH2Cl2, multiple development) to give 7,4′-dihydroxy-2′,5′-dimethoxyisoflav-3-ene (8 mg, Rf = 0.35, 1 % MeOH in CH2Cl2) and 1 (6 mg, Rf = 0.28, 1 % MeOH in CH2Cl2). Fraction E (eluted with 7 % acetone in hexane) was treated as above and afforded corylin (6 mg, Rf = 0.25, 1 % MeOH in CH2Cl2) and erysubin F (91 mg, Rf = 0.18, 1 % MeOH in CH2Cl2).
5-Deoxy-3′-prenylbiochanin A (1): Needles (CH2Cl2), m. p. 190 - 192 °C; UV (MeOH): λmax (log ε) = 263 (4.3) nm; 1H NMR (acetone-d 6, 500 MHz): δ = 8.14 (1H, s, H-2), 8.06 (1H, d, J = 8.7 Hz, H-5), 6.98 (1H, dd, J = 2.1, 8.7 Hz, H-6), 6.90 (1H, d, J = 2.1 Hz, H-8), 7.40 (1H, d, J = 2.1 Hz, H-2′), 6.99 (1H, d, J = 8.4 Hz, H-5′), 7.43 (dd, J = 2.1, 8.4 Hz, H-6′), 3.34 (2H, d, J = 7.5 Hz, H-1′′), 5.31 (1H, m, H-2′′), 1.70 (3H, s, Me-4′′), 1.73 (3H, s, Me-5′′), 3.88 (1H, s, 4′-OMe); 13C-NMR (acetone-d 6, 125 MHz): δ = 154.0 (C-2), 125.9 (C-3), 176.4 (C-4), 130.9 (C-4a), 129.1 (C-5), 111.6 (C-6), 159.5 (C-7), 103.8 (C-8), 164.2 (C-8a), 126.0 (C-1′), 131.6 (C-2′), 130.9 (C-3′), 158.7 (C-4′), 119.1 (C-5′), 129.3 (C-6′), 31.8 (C-1′′), 124.3 (C-2′′), 133.2 (C-3′′), 18.5 (4′′-Me), 26.6 (5′′-Me), 56.5 (OMe); EI-MS: m/z (rel. int.) = 336 (100) [M+], 137 (76); HR-MS: m/z = 336.1347 [M]+; calcd. for C21H20O4 : 336.1362.
In vitro antiplasmodial activity: The crude extracts and pure compounds were evaluated for antiplasmodial activity against the chloroquine-sensitive (D6) and chloroquine-resistant (W2) strains of P. falciparum using a [3 H]hypoxanthine uptake assay as described in [5].
#Acknowledgements
We acknowledge the financial support by the Deutsche Forschungsgemeinschaft, and the Bundesministerium für Zusammenarbeit, Germany, Grant No. Pe 264/14 - 5 and -6. Mr. S. G. Mathenge is thanked for identification of the plant material. A.W.A. thanks the Department of Chemistry, University of Nairobi for hosting him at the department’s research laboratories. A.Y. is grateful to the German Academic Exchange Service (DAAD) for a research visit to the University of Potsdam.
#References
- 1 Kokwaro J O. Medicinal Plants of East Africa, 2nd edition. Nairobi; Kenya Literature Bureau 1993: p 158
- 2 Gessler M C, Nkunya M HH, Mwasumbi L B, Heinrich M, Tanner M. Screening Tanzanian medicinal plants for antimalarial activity. Acta Trop. 1994; 56 65-77
- 3 Mitscher L A, Simon K, Gollapudi S R, Okwute S K. A modern look at folkloric use of anti-infective agents. J Nat Prod. 1987; 50 1025-40
- 4 Kamat V S, Chuo F Y, Kubo I, Nakanishi K. Antimicrobial agents from an East African plant Erythrina abyssinica . Heterocycles. 1981; 15 1163-70
- 5 Yenesew A, Derese S, Irung B, Midiwo J O, Waters N C, Liyala P. et al . Flavonoids and isoflavonoids with anti-plasmodial activities from the root bark of Erythrina abyssinica . Planta Med. 2003; 69 658-61
- 6 Yenesew A, Induli M, Derese S, Midiwo J O, Heydenreich M, Peter M G. et al . Anti-plasmodial flavonoids from the stem bark of Erythrina abyssinica . Phytochemistry. 2004; 65 3029-32
- 7 Yenesew A, Midiwo J O, Heydenreich M, Peter M G. Four isoflavones from the stem bark of Erythrina sacleuxii . Phytochemistry. 1998; 49 247-54
- 8 Saxena V K, Bhadoria B K. 3’-Prenyl-4-methoxy-isoflavone-7-O-β-D-(2″-O-p-coumaroyl)glucopyranoside, a novel phytoestrogen from Sopubia delphinifolia . J Nat Prod. 1990; 55 62-5
- 9 Yenesew A, Midiwo J O, Miessner M, Heydenreich M, Peter M G. Two prenylated flavanones from the stem bark of Erythrina burttii . Phytochemistry. 1998; 48 1439-43
- 10 Kitagawa I, Che W Z, Hori K, Harada E, Yasuda N, Yoshikawa M. et al . Chemical studies of Chinese licorice roots of Glycyrihiza glabra L. collected in Xinjiang. Chem Pharm Bull. 1994; 42 056-62
- 11 Iinuma M, Ohyama M, Tanaka T. Three isoflavonoids from the roots of Sophora prostrata . Phytochemistry. 1994; 37 1713-6
- 12 Jain A C, Anand S M, Dhar M L, Gupta G L, Rao P R. Isolation and constitution of corylin, a new isoflavone from the fruits of Psoralea corylifolia . Indian J Chem. 1974; 12 659-60
- 13 Tanaka H, Etoh H, Watanabe N, Shimizu H, Ahmad M, Rizwani G H. Erysubins C - F, four isoflavonoids from Erythrina suberosa var. glabrescences . Phytochemistry. 2001; 56 769-73
- 14 Yenesew A, Midiwo J O, Heydenreich M, Schanzenbach D, Peter M G. Two isoflavanones from the stem bark of Erythrina sacleuxii . Phytochemistry. 2000; 55 457-9
- 15 Schwikkard S, van Heerden F R. Antimalarial activity of plant metabolites. Nat Prod Rep. 2002; 19 675-92
Abiy Yenesew
Department of Chemistry
University of Nairobi
P.O. Box 30197
Nairobi
Kenya
Phone: +254-20-444-9004 ext 2170
Fax: +254-20-4446138
Email: ayenesew@uonbi.ac.ke
References
- 1 Kokwaro J O. Medicinal Plants of East Africa, 2nd edition. Nairobi; Kenya Literature Bureau 1993: p 158
- 2 Gessler M C, Nkunya M HH, Mwasumbi L B, Heinrich M, Tanner M. Screening Tanzanian medicinal plants for antimalarial activity. Acta Trop. 1994; 56 65-77
- 3 Mitscher L A, Simon K, Gollapudi S R, Okwute S K. A modern look at folkloric use of anti-infective agents. J Nat Prod. 1987; 50 1025-40
- 4 Kamat V S, Chuo F Y, Kubo I, Nakanishi K. Antimicrobial agents from an East African plant Erythrina abyssinica . Heterocycles. 1981; 15 1163-70
- 5 Yenesew A, Derese S, Irung B, Midiwo J O, Waters N C, Liyala P. et al . Flavonoids and isoflavonoids with anti-plasmodial activities from the root bark of Erythrina abyssinica . Planta Med. 2003; 69 658-61
- 6 Yenesew A, Induli M, Derese S, Midiwo J O, Heydenreich M, Peter M G. et al . Anti-plasmodial flavonoids from the stem bark of Erythrina abyssinica . Phytochemistry. 2004; 65 3029-32
- 7 Yenesew A, Midiwo J O, Heydenreich M, Peter M G. Four isoflavones from the stem bark of Erythrina sacleuxii . Phytochemistry. 1998; 49 247-54
- 8 Saxena V K, Bhadoria B K. 3’-Prenyl-4-methoxy-isoflavone-7-O-β-D-(2″-O-p-coumaroyl)glucopyranoside, a novel phytoestrogen from Sopubia delphinifolia . J Nat Prod. 1990; 55 62-5
- 9 Yenesew A, Midiwo J O, Miessner M, Heydenreich M, Peter M G. Two prenylated flavanones from the stem bark of Erythrina burttii . Phytochemistry. 1998; 48 1439-43
- 10 Kitagawa I, Che W Z, Hori K, Harada E, Yasuda N, Yoshikawa M. et al . Chemical studies of Chinese licorice roots of Glycyrihiza glabra L. collected in Xinjiang. Chem Pharm Bull. 1994; 42 056-62
- 11 Iinuma M, Ohyama M, Tanaka T. Three isoflavonoids from the roots of Sophora prostrata . Phytochemistry. 1994; 37 1713-6
- 12 Jain A C, Anand S M, Dhar M L, Gupta G L, Rao P R. Isolation and constitution of corylin, a new isoflavone from the fruits of Psoralea corylifolia . Indian J Chem. 1974; 12 659-60
- 13 Tanaka H, Etoh H, Watanabe N, Shimizu H, Ahmad M, Rizwani G H. Erysubins C - F, four isoflavonoids from Erythrina suberosa var. glabrescences . Phytochemistry. 2001; 56 769-73
- 14 Yenesew A, Midiwo J O, Heydenreich M, Schanzenbach D, Peter M G. Two isoflavanones from the stem bark of Erythrina sacleuxii . Phytochemistry. 2000; 55 457-9
- 15 Schwikkard S, van Heerden F R. Antimalarial activity of plant metabolites. Nat Prod Rep. 2002; 19 675-92
Abiy Yenesew
Department of Chemistry
University of Nairobi
P.O. Box 30197
Nairobi
Kenya
Phone: +254-20-444-9004 ext 2170
Fax: +254-20-4446138
Email: ayenesew@uonbi.ac.ke