Quantitative determination by HPLC of ent-kaurenoic and grandiflorenic acids in aerial parts of Wedelia paludosa D.C.

ARTIGO

Quantitative determination by HPLC of ent-kaurenoic and grandiflorenic acids in aerial parts of Wedelia paludosa D.C.

R. Batista^I,II; F.C. Braga^II; A.B. OliveiraII,^*

^IDepartamento de Estudos Básicos e Instrumentais, Universidade Estadual do Sudoeste da Bahia, BR 415, km 03, s/n, 45700-000, Itapetinga, BA, Brazil

^IIDepartamento de Produtos Farmacêuticos, Faculdade de Farmácia, Universidade Federal de Minas Gerais, Av. Antônio Carlos 6627, 31270-901, Belo Horizonte, MG, Brazil

ABSTRACT

There are several reports of biological activity for kaurenoic acid (1), which is found in the aerial parts of Wedelia paludosa D.C., a herbaceous plant of the Asteraceae family abundant in Brazil, and that contains grandiflorenic acid (2) also. Both of these diterpenes were shown to cause lysis of trypomastigotes of the protozoa Trypanosoma cruzi, the causative agent of Chagas' disease (American trypanosomiasis). This paper reports the quantitative determination of these diterpene acids in the aerial parts of W. paludosa by an isocratic RP-HPLC method employing 60% acetonitrile in water and UV detection (220 nm). Kaurenoic acid (1) was found to be more abundant (0.85 ± 0.08%) while the amount of grandiflorenic acid (2) determined was almost three times lower (0.32 ± 0.02%). No significant seasonal variation was observed for these compounds in W. paludosa growing in Belo Horizonte (MG, Brazil) what points out to the aerial parts of this species are a good source of these biologically active diterpenes along all the year.

Keywords: Wedelia paludosa, diterpenes, kaurenoic acid, grandiflorenic acid, quantitative determination, RP-HPLC.

INTRODUCTION

W. paludosa D.C. (Asteraceae) is an herbaceous plant species found in many parts of Brazil, especially in the states of Pernambuco, Bahia, Minas Gerais, São Paulo and Santa Catarina. It is often employed for ornamental purposes and is traditionally used to treat a variety of disorders, including cough and painful conditions (Roque et al., 1987). Several biological effects have been described for W. paludosa extracts including antinociceptive (Block et al., 1998a), trypanosomicidal (Chiari et al., 1996), hypoglycemic (Block et al., 1998b; Novaes et al., 2001), antimicrobial (Vieira et al., 2000) and antifungal (Sartori et al., 2003) activity, among others.

Previous phytochemical studies of W. paludosa afforded ent-kaur-16-en-19-oic acid (kaurenoic acid, 1) and ent-kaur-9(11),16-dien-19-oic acid (grandiflorenic acid, 2) as major compounds, while other kaurane diterpenes, triterpenes and sesquiterpene lactones have been isolated as minor constituents (Carvalho et al., 2001; Batista et al., 1999; Ferreira et al., 1994; Roque et al., 1987).

We have previously reported the in vitro anti-Trypanosoma cruzi effect of W. paludosa ethanol extract (Chiari et al., 1996). Bioassay-directed fractionation of this extract afforded 1 and 2. These diterpenes caused total lysis of trypomastigotes of the protozoa T. cruzi, the causative agent of Chagas disease (American trypanosomiasis), at a concentration of 0.68 mg/mL (Batista et al., 1999). Several biological effects have been described for kaurenoic acid and related kaurane diterpenoids (Ghisalberti, 1997). Among more recently reported activity for kaurenoic acid should be mentioned the antimicrobial (Zgoda-Pols et al., 2002), anti-platelet aggregation (Yang et al., 2002), analgesic (Block et al., 1998a), antifungal (Sartori et al., 2003), smooth muscle relaxant (Cunha et al., 2003) and hypoglycemic activity (Bresciani et al., 2004). Significant inhibition of HIV-reverse transcriptase was exhibited by 16a,17-dihydroxy-ent-kauran-19-oic acid from Annona glabra (Chang et al., 1998).

The accumulation of kaurenoic acid in some species make these plants important sources of this diterpene that can be used as starting material in the synthesis of new derivatives for biomedical research. Indeed, it has already been submitted to chemical (Vieira et al., 2002; Bruno et al., 2001) and microbial (Silva et al., 1999; Hanson et al., 1995) transformation, affording several derivatives.

Kaurenoic acid (1) is not commercially available, and it is of interest to find easily accessible plant sources for its production. With this purpose, its content has been previously assayed in species of Xylopia (Annonaceae) (Melo et al., 2001), Rabdosia (Lamiaceae) (Kubo et al., 1982) and in Annona glabra (Annonaceae) (Oliveira et al., 2002), by HPLC methods. Furthermore, kaurenoic acid (1) has been determined by GC methods in Asteraceae species, including Mikania glomerata (Vilegas et al., 1997) and Wedelia paludosa (Bresciani et al., 2000; Bresciani et al., 2004). Therefore, the main aim of the present work was to develop and validate an HPLC method to quantify both kaurenoic (1) and grandiflorenic (2) acids in aerial parts of W. paludosa as well as to evaluate their seasonal variation.

MATERIAL AND METHODS

NMR data

¹H and ¹³C NMR spectra were recorded on a Bruker Advance DPX-200 spectrometer (¹H 200 MHz and ¹³C 50 MHz) using TMS as internal standard for both nuclei. Samples were dissolved in CDCl₃. Chemical shifts (d) are given in ppm and J couplings in Hertz (Hz).

Reference substances

Kaurenoic (1) and grandiflorenic (2) acids were previously isolated from the aerial parts of W. paludosa (Batista et al., 1999). The identity and the purity of these diterpenes were checked by HPLC, ¹H and ¹³C NMR analysis.

Plant material

Samples of aerial parts of W. paludosa were collected in Belo Horizonte, Brazil, in January (summer, sample 1), May (autumn, sample 2), August (winter, sample 3) and December (spring, sample 4) 1998, and a voucher specimen is deposited at the UFMG herbarium (BHCB 19033). The plant material was dried at 40 °C for 72h, powdered and stored at -15ºC until use.

Extraction of the kaurane diterpenes

An aliquot of the plant material was accurately weighed (400 mg) and extracted only with hexane (30 mL) under sonication for 40 min, at room temperature. After removing the hexane in a rotary evaporator (40 °C), the residue was dissolved in CH₃CN (2 mL). The solution was filtered on Adsorbex RP-18 cartridges (Merck, Germany), previously conditioned with CH₃CN (2 mL), and then centrifuged at 10,000 rpm during 5 min, prior to injection onto the HPLC apparatus.

Chromatographic system

Analyses were carried out on a Merck-Hitachi apparatus (Germany) composed of a pump L-6200A, automatic injector AS-2000A, UV-VIS detector L-4250 and integrator D-2500. An ODS column (150 ´ 4.0 mm I.D., 5 mm) was employed (Merck, Germany) at a temperature of 35°C, flow rate of 1.0 ml/min and wavelength of 220 nm. Isocratic elution by 60% CH₃CN in water was employed. Solvents used were of HPLC grade (Merck, Darmstadt, Germany) and were degassed by sonication before use. Analyses were performed in triplicate and each sample was injected onto the HPLC apparatus in duplicate.

Calibration graphs

Five point calibration curves were obtained from CH₃CN standard solutions of kaurenoic (1.0 mg/mL) and grandiflorenic (0.5 mg/mL) acids, injected in the range 5-40 mL. The solutions were analyzed and the corresponding peak areas were compared against the mass of the injected kauranes. Each point of the graph was the mean of five measurements. Linear regression equations were obtained by computer analysis, employing Sigma Plot software (Jandell Co., USA). Identification of the diterpenoid peaks in the extracts was made by comparison with the retention time of standard solutions injected in the same conditions.

Recovery studies

Appropriate amounts of kaurenoic and grandiflorenic acids were added to plant material, corresponding to 50, 75 and 100% contents of those acids in the previously assayed material (Table 1). The follow-up extractions and HPLC analysis were carried out as described above. The recovery was determined as follows (Chen et al., 2003):

Recovery (%) = (A – B)/C x 100%

where, A is the amount detected, B is the amount of sample without added standards, and C is the amount of the standards added. Recovery studies were performed in 5 replicates for each concentration.

RESULTS AND DISCUSSION

Kaurenoic (1) and grandiflorenic (2) acids were characterized by ¹H and ¹³C NMR spectra. Characteristic signals were observed for the hydrogens of an exocyclic double bond (1, d 4.73 and 4.79, 1H each, s; 2, d 4.79 and 4.91, 1H each, s) and of the methyl groups attached to C-4 (1, d 1.25, s, 3H; 2, d 1.23, s, 3H) and C-10 (1, d 0.95, s, 3H; 2, d 1.02, s, 3H) of the kaurane skeleton. Furthermore, the ¹H NMR spectrum of 2 presented a triplet at d 5.24 (1H, J 3.3 Hz) which was assigned to H-11. ¹H and ¹³C NMR data for 1 and 2 (Tables 2 and 3) are in agreement with those previously reported (Reynolds, 1984; Silva et al., 1999).

The quantitative RP-HPLC determination of both kaurenoic (1) and grandiflorenic (2) acids in W. paludosa aerial parts is reported here for the first time. The results for repeatability showed relative standard deviations of 1.34% for 1 and 1.84% for 2 (n = 5). The results from recovery studies of 1 and 2 ranged from 83.0 to 101.0%. The relative standard deviations (R.S.D.) of recovery of 1 and 2 ranged between 1.9% and 4.6% (Table 1). Good response linearity was obtained for both diterpenes with r² values of 0.9998 (1) and 0.9999 (2) (peak area vs. mass). The linear equations obtained for 1 and 2 were y = 32208.5x + 2432.8 and y =108421.3x + 25392.8 respectively. The quantitative limit of the method was established as 1.25 mg for 1 (RSD = 2.07%) and 0.63 mg for 2 (RSD = 2.67%).

A typical chromatogram obtained in the present study is shown in Figure 1. Attempts to improve resolution of kaurenoic acid, including modifications of the strength and selectivity of eluents, were unsuccessful. According to Snyder et al. (1997), resolution R_s > 2.0 between the peaks of interest is desired for good quantitative determination. While it is feasible to determine two peaks with R_s < 1.5, there will be some overlap of peak area. Based on the valley between the peak of 1 and that one of the earlier eluted compound, a resolution of R_s = 0.98 was estimated for these two adjacent bands. In such cases, the error is always less than 1% for the larger band (1) and can be ignored (Snyder et al., 1997). Hence, the chromatographic conditions employed are adequate for quantitative determination of kaurenes 1 and 2 in aerial parts of W. paludosa, expressed as a percentage basis of dry plant mass (Table 4).

Kaurenoic acid (1) was shown to be an abundant diterpene in the aerial parts of W. paludosa both in dry (May and August) and rainy (December and January) periods of a year with contents ranging from 0.74 ± 0.02% (January, sample 1) to 0.95 ± 0.04% (December, sample 4), with an average concentration of 0.85 ± 0.08% for the dry plant. Previously reported contents of kaurenoic acid (1) are shown in Table 5, along with the results obtained in the present work. The evaluation of the concentration of 1 in roots, leaves, stems and flowers of W. paludosa in different seasons by HRG/FID has shown a remarkable seasonal variation for plants growing in Florianópolis (southern Brazil). Higher contents were observed in the roots and stems in the autumn (0.67% and 0.50%, respectively) while in the winter the concentrations were much lower, especially in the roots (0.0005%) (Bresciani et al., 2004). However, for plants growing in Belo Horizonte (south east), the seasonal variation was slight (Table 4) which could be explained by the climatic differences between these regions, particularly during the winter time, with rather lower temperatures in Florianópolis than in Belo Horizonte.

The aerial parts of W. paludosa present the highest amount of kaurenoic acid among the species indicated in Table 5, excepting X. frutescens seeds. Despite the higher content of 1 in X. frutescens seeds, W. paludosa should be considered a better source of kaurenoic acid, since it is an easily acessible perennial herb, cultivated with ornamental purposes in different regions of Brazil, whereas X. frutescens is a tree found only in the Amazon region. Furthermore, overexploitation of X. frutescens seeds as source of pharmacologically active compounds may endanger the species.

The amounts of grandiflorenic acid (2) were similar in the four samples of W. paludosa assayed and its average concentration was 0.32 ± 0.02%, almost three times less than kaurenoic acid (1). In the best of our knowledge, this is the first report on the quantitative determination of grandiflorenic acid (2) and the first for both the diterpenoid acids 1 and 2 in W. paludosa, by HPLC.

In conclusion, our results show that Wedelia paludosa is a valuable source of kaurenoic (1) and grandiflorenic (2) acids.

ACKNOWLEDGEMENTS

We thank UESB for supporting R. Batista during his Doctorate at The Curso de Pós-Graduação em Ciências Farmacêuticas, Faculdade de Farmácia, UFMG, Belo Horizonte, Brazil, and CNPq for Research Fellowships to ABO and FCB.

Received 01/16/05

Accepted 04/29/05

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Publication Dates

• Publication in this collection
  26 May 2008
• Date of issue
  June 2005

History

• Accepted
  29 Apr 2005
• Received
  16 Jan 2005