Secondary Metabolites from Sida rhombifolia L. (Malvaceae) and the Vasorelaxant Activity of Cryptolepinone

The phytochemical study of Sida rhombifolia L. (Malvaceae) led to the isolation through chromatographic techniques of eleven secondary metabolites: sitosterol (1a) and stigmasterol (1b), sitosterol-3-O-β-d-glucopyranoside (2a) and stigmasterol-3-O-β-d-glucopyranoside (2b), phaeophytin A (3), 173-ethoxypheophorbide A (4), 132-hydroxy phaeophytin B (5), 173-ethoxypheophorbide B (6), 5,7-dihydroxy-4'-methoxyflavone (7), cryptolepinone (8) and a salt of cryptolepine (9). Their structures were identified by 1H- and 13C-NMR using one- and two-dimensional techniques. In addition, the vasorelaxant activity of cryptolepinone in rat mesenteric artery rings is reported herein for the first time.


Introduction
The genus Sida has wide neotropical distribution, with several species well represented in the Americas. In Brazil this genus has many representatives in the Northeast and South and, to a lesser extent, in the North, Midwest and Southeast [1]. Sida rhombifolia (Malvaceae) is popularly known in Brazil as "matapasto", "guanxuma" and "relógio". It is used in Indian folklore medicine against hypertension, diabetes [2], as well as a traditional medicinal plant for the treatment of gout in Indonesia [3].

OPEN ACCESS
A variety of chemical constituents has been isolated from this species, including ecdysteroids [4] quinazoline alkaloids, besides β-phenylethylamines and carboxylated tryptamines [5]. The present study aimed to isolate and identify other classes of secondary metabolites from S. rhombifolia in order to associate them to pharmacological properties. We report the isolation and identification of two mixtures of steroids 1a, 1b and 2a and 2b, four porphyrins 3, 4, 5 and 6, a flavone 7, and two indoquinoline alkaloids 8 and 9, besides showing for the first time the vasorelaxant activity of 8 in rat mesenteric artery with and without functional endothelium.

Results and Discussion
Compound 8 appeared as yellow crystals with melting point >300 °C. The IR spectrum indicated the presence of NH (3431 cm −1 ) and carbonyl (1622 cm −1 ) groups. The 1 H-NMR spectra of compound 8 (DMSO-d 6 [6,7]. Furthermore, the spectra showed the presence of two singlets [δ H 11.89 (1H, s), 4.36 (3H, s)] due to an indole NH and a methyl group bonded to nitrogen, respectively [6]. The 13 C BB-NMR spectrum of 8 (DMSO-d 6 ) showed 16 signals, consistent with an unsubstituted indoquinoline nucleus, being 15 aromatic carbons and one sp 3 -hybridized carbon. The presence of a methyl group bonded to nitrogen [δ C 35.41] and a carbonyl [δ C 166.45] could be highlighted. Based on the 2D NMR data of HMQC experiment the hydrogenated carbons found in 13 C BB-NMR spectrum were established. The 2D NMR data of HMBC experiment showed the position of the nitrogen methyl in the quinoline nucleus due to the 3 J correlation signal of the proton at δ H 4.36 (s) to C-4a (δ C 139.73) and C-5a (δ C 129.89). The identity of hydrogen of the indole nucleus at δ H 11.89 (s) was further confirmed by its 2 J correlations to C-9a (δ C 138.25) and C-10a (δ C 123.03) and 3 J correlations to C-6a (δ C 115.70) and C-10a  Table 1) ( Figure 1). Spectral data compilation and comparison of data in the literature [5] showed that 8 is cryptolepinone, a compound already isolated in the genus Sida [8], but isolated now for the first time from the species Sida rhombifolia.    The vasorelaxant activity of 8 was evaluated by increasing cumulative addition of 10 −12 to 10 −3 M in rat cranial mesenteric artery rings pre-contracted with phenylephrine (PHE) at the concentration of 1 μM. It was observed that 8 caused a vasorelaxation in rings with functional endothelium (E max = 91.6 ± 4.0%, n = 6) and after removal of endothelium, the vasorelaxant effect of 8 was significantly changed (61.3 ± 6.9%, n = 7) ( Figure 4).

Collection, Extraction and Isolation
Sida rhombifolia was collected in the municipality of Santa Rita-PB in September 2008 and identified by Dr. Maria de Fátima Agra. A voucher specimen is kept at the herbarium Professor Lauro Pires Xavier (CCEN/UFPB) under the code Agra 7045. The dried and powdered aerial parts of the plant (5.5 kg) were extracted with 95% EtOH (10 L, room temperature) for 72 h. The EtOH extract was concentrated under reduced pressure at 40 °C, providing 570 g of crude ethanol extract (CEE). Part of the CEE (200.0 g) was subjected to filtration under reduced pressure using silica gel 60 as stationary phase and eluted with hexane (Hex.), ethyl acetate (EtOAc) and methanol (MeOH) alone or in binary mixtures following an increasing gradient polarity. The fractions obtained were concentrated under reduced pressure. Fraction Hex.-AcOEt (9:1) (6.0 g) was subjected to silica gel 60 column chromatography (CC) eluted with Hex., dichloromethane (CH 2 Cl 2 ) and MeOH alone or in binary mixtures following an increasing gradient polarity, yielding 35 mg of compound 1. Fraction Hex.-AcOEt (7:3) (6.11 g) was subjected to the same methodology, but using as eluents Hex., EtOAc and MeOH. A fraction of this column was subjected to preparative TLC in Hex.-EtOAc (9:1) to give 35 mg of 3. Then, the initial TLC band was rechromatographed in Hex.-EtOAc (8:2) to give 5.0 mg of compound 5. Compound 2 (33.0 mg) was obtained from the fraction EtOAc (1.23 g) using the previous technique for column chromatography. Fraction EtOAc-MeOH (9:1) yielded a precipitate and a supernatant (1.24 g). The latter was subjected to CC on Sephadex LH-20 using methanol as eluent. This procedure yielded 10.0 mg of compound 7. Another part of the CEE (200.0 g) was dissolved in EtOH-H 2 O (9:1) and subjected to liquid-liquid chromatography using Hex., CH 2 Cl 2 , EtOAc and n-butanol, obtaining the respective phases, besides the hydroalcoholic phase. The CH 2 Cl 2 phase (26.0 g) was subjected to filtration under reduced pressure with the solvents Hex., CH 2 Cl 2 and MeOH. The fractions Hex.-CH 2 Cl 2 (3:7) (17.4 mg) and CH 2 Cl 2 (200.0 mg) were combined and subjected to CC using flash silica as adsorbent and the eluents Hex., EtOAc and MeOH alone or in binary mixtures following an increasing gradient polarity. Compounds 4 (25.0 mg) and 6 (15.0 mg) were thus obtained. The CEE of Sida rhombifolia reacted positively with Dragendorff, Bouchadart and Mayer reagents, as expected according to literature. The classic alkaloid extraction was performed with 100 g of the CEE. The total alkaloid content (800 mg) was subjected to CC on neutral alumina and the elution system was the same of the previous methodology. Compounds 8 (28.0 mg) and 9 (18.0 mg) were isolated.     Table 1.

Bioactivity Assay
Experimental procedures were performed as described by França-Silva et al. [14] and were approved by the Federal University of Paraíba Ethical Committee for Animal Use under the protocol CEPA#305/09. After euthanasia by decaptation using a guilliotine, the cranial mesenteric artery was isolated, placed in Tyrode's solution and dissected in order to make it free of adhering tissue. In endothelium-denuded experiments, endothelium was removed by rubbing the intimal surface of the vessels. Rings with 1-2 mm were obtained and placed in physiological Tyrode's solution, maintained to 37 °C, gassed with carbogenic mixture (95% O 2 and 5% CO 2 ) and kept at pH 7.4. All preparations were stabilized under a resting tension of 0.75 g for 1 h. The solution was replaced every 15 min in order to prevent the accumulation of metabolites. The force of contraction was isometrically recorded by a force transducer (Miobath-4, WPI, Sarasota, FL, USA) coupled to an amplifier-recorder (Miobath-4) and to a computer equipped with an analog-to-digital converter board as described earlier. The presence of functional endothelium was assessed by the ability of acetylcholine (10 µM) to induce more than 80% relaxation of vessels pre-contracted with phenylephrine (10 µM). Less than 10% of relaxation to acetylcholine was taken as evidence that the vessel segments were functionally denuded of endothelium. The preparations were exposed to cumulatively concentrations of cryptolepinone (10 −12 -10 −3 M) added to preparations until a maximum response to the drug accumulation was observed as indicated by a plateau response (approximately 3-5 min).
The compounds 13 2 -hydroxyphaeophytin B (5) and 17 3 -ethoxypheophorbide B (6) are being described hering for the fiorst time in the family Malvaceae, and the flavonoid 5,7-dihydroxy-4'methoxyflavone (acacetin, 7) is reported herein for the first time in the genus Sida and the other compounds were isolated for the first time in the species S. rhombifolia. In addition, the vasorelaxant activity of cryptolepinone (8) in rat cranial mesenteric artery with and without functional vascular endothelium is herein reported for the first time.