Triterpenes and the Antimycobacterial Activity of Duroia macrophylla Huber (Rubiaceae)

Duroia macrophylla popularly known as “cabeça-de-urubú,” “apuruí,” or “puruí-grande-da-mata” occurs in the Amazon Forest. Its leaves and branches were collected twice and extracted with dichloromethane and methanol. All extracts were subjected to phytochemical investigation and terpenes and flavonoids were found in all dichloromethane and methanol extracts, respectively. Methanol extracts from both branches (1st collection) and leaves (2nd collection) presented hydrolyzed tannins, yet alkaloids were only detected in the dichloromethane and methanol extracts from branches at the 2nd collection. Phenol compounds were found in both dichloromethane extracts' collections. The action of every extract was assayed against Mycobacterium tuberculosis (RMPr, H37Rv, and INHr strains), showing that the dichloromethane extract from leaves (1st collection) has the major biological activity, with a MIC of 6.25 μg/mL for the INHr strain, 25.0 μg/mL for the RMPr strain, and ≤6.25 μg/mL for the H37Rv strain. The chromatographic fractioning of the dichloromethane extract from leaves (1st collection) yielded the isolation of two triterpenes: oleanolic and ursolic acids, which were identified by NMR analysis and reported for the first time in the Duroia genus.


Introduction
Rubiaceae is the largest family in the Magnoliopsida class, encompassing around 550 genera and 9,000 species being used in several ethnomedicinal practices [1]. The family is characterized by the production of several classes of secondary metabolites with a great pharmacological potential, mainly alkaloids, terpenes, quinovic acid glycosides, flavonoids, and coumarins with antibacterial properties [2]. Rubiaceae plants' secondary metabolites have been investigated scientifically for antimicrobial activities and a large number of plant products have shown to inhibit the growth of pathogenic microorganisms [3][4][5][6]. A literature review article pertaining to Rubiaceae species reveals that 48 out of 611 genera showed a wide range of antibacterial [3] and antimycobacterial activities [4].
The Duroia genus, belonging to Gardenieae tribe and Ixoroideae subfamily, holds about 30 species but few studies have been carried out on this genus. D. hirsuta, which is used as folk healing medicine, is one of these species which has undergone investigation and showed antimycobacterial activity against Mycobacterium phlei [7] and antiviral activity against Herpes simplex viruses (HSV) on in vitro studies [8]. One flavone, one lactone iridoid, one flavonol [9] and one tetracyclic iridoid [10] were isolated from its root petroleum ether and CHCl 3 extract. There is still a large number of species with no chemical or biological study.

NMR Data.
The NMR data was obtained at 295 K on a Bruker AVANCE 400 NMR spectrometer operating at 9.4 Tesla, observing 1 H and 13 C at 400 and 100 MHz, respectively. The spectrometer was equipped with a 5 mm multinuclear direct detection probe, with z-gradient. Onebond (HSQC) and long-range (HMBC) 1 H-13 C NMR correlation experiments were optimized for coupling constants 1 H,C and LR H,C of 140 and 8 Hz, respectively. All NMR chemical shifts were expressed in ppm related to TMS signal at 0.00 ppm as internal reference, and samples were dissolved in CDCl 3 .

Antimycobacterial Activity.
Resazurin microtiter assay (REMA) was used to evaluate the antimycobacterial activity. This method uses resazurin as an oxidoreduction indicator to evaluate the bacterial viability and contamination, in addition to analyzing the antimicrobial activity [15].

Assay
Procedure. Samples were first evaluated in 96well microplates at a 200 g/mL concentration against the three M. tuberculosis strains. The assay started adding 75 L of Middlebrook 7H9 medium enriched with 10% of OADC (oleic acid, albumin, dextrose, and catalase) for M. tuberculosis, 75 L of each extract, and 75 L of inoculum. Then, 200 L of sterile water were added to each peripheral well, so as to avoid medium liquid evaporation when heater-incubated. Finally, the plate was incubated at 37 ∘ C for seven days.

Minimum Inhibitory Concentration Determination.
The extracts presenting an antimycobacterial activity at the 200 g/mL concentration screening were chosen to evaluate their minimum inhibitory concentration (MIC) value [16]. This value was determined by adding 100 L of medium, 100 L of extract (starting at 200 g/mL concentration on the first well and performing a 1 : 2 microdilution), and 100 L of bacterial inoculum in each well. Also, 200 L of sterile water was added to each peripheral well, in order to avoid medium liquid evaporation when heater-incubated. Then, the plate was incubated at 37 ∘ C for seven days.

Bacterial Viability.
Following the incubation period, 30 L of resazurin (0.02%) was added in each well and incubated for two days at 37 ∘ C. The biological activity was based on the color change, from blue to pink when an oxidoreduction reaction of the reagent occurs due to bacterial growth [15].

Results and Discussion
3.1. Phytochemical Investigation. All Duroia macrophylla extracts were analyzed in order to evaluate the chemical profile [17,18]. Dichloromethane extracts from branches and leaves in both collections showed to be rich in terpenes. Regarding methanolic extracts, only those from branches (1st collection) and leaves (2nd collection) showed the presence of both terpenes and hydrolyzed tannins. Alkaloids were only detected on dichloromethane and methanolic extracts from branches (2nd collection). All methanolic extracts showed the presence of flavonoids. All dichloromethane extracts from branches showed the presence of phenolic compounds. 1 H-NMR spectra analysis showed the presence of aromatic substances in the methanolic extract of branches (1st collection), with several signals between 6.50 and 7.80 ppm.

Substances Isolation and Identification.
Following crude extracts chemical and biological analysis, the dichloromethane extract from leaves (1st collection) was chosen to be fractionated, since it showed to be the most active against the three Mycobacterium tuberculosis strains (RMPr, H37Rv and INHr) ( Table 2). The substance 1 1 H-13 C NMR (HSQC) correlation map showed the hydrogen at 5.31 ppm with the carbon at 122.8, which were identified as the vinilic C-12 carbon of oleanolic acid [7,19] (Table 1). The signal in C 180.0 was assigned to the carboxyl group (C-28).
The 1 H-NMR spectrum from fraction 25-40.6.4.2 showed several signals at the shielded region, between H 0.79 and H 1.72 characteristic of methyl hydrogens, moreover two signals were observed at 3.22 (dd, = 10.8 and 4.9 Hz) and H 5.27 (dd, = 3.6 and 3.5 Hz) characteristic of olefinic hydrogen, which were assigned to H-12 in triterpenes, characterizing the ursanic skeleton of substance 2.
When analyzing the 13 C-NMR spectral data one can find seven methyl carbons (CH 3 ), nine methylene carbons (CH 2 ), seven methine carbons (CH) and seven non-hydrogenated carbons (C), resulting in thirty carbons characteristic of pentacyclic triterpenes. C 179.6 from the carboxylic acid carbon (not hydrogenated), C 137.9 characteristic of unhydrogenated olefinic carbon (sp 2 ) and C 125.9 of hydrogenated olefinic carbon are the major signals characteristic of a ursanic skeleton. These signals represent, carbons C-28, C-13 and C-12 in ursolic acid triterpene, respectively.
On the other hand, the 1 H-13 C NMR (HSQC) correlation map showed correlation of the hydrogen at 5.27 ppm with the carbon at 125.9, which were identified as the vinilic C-12 carbon and the multiplicity of the signals corresponding to It is common to isolate the ursolic acid with oleanolic acid mixture due to molecule similarity, yet a few differences between them enable telling them apart through NMR, due to the difference between the H-18, C-18, C-12, C-13 and C-29 [20] chemical shifts, and mainly on account of H-29 being a doublet for ursolic acid, and a singlet for oleanolic acid.
1 H-NMR spectra and HSQC and HMBC NMR correlation maps overall analysis as well as comparison with literature data [7] enabled the complete structure to be determined as the triterpenes oleanoic and ursolic acids (Table 1) (Figure 2).
The mass spectra analysis of each triterpene isolated showed the molecular ion peak at / = 456 u, and showed the common fragmentation pattern of triterpenes, described in the literature [21]. All these data together confirmed to be the triterpenes, ursolic acid and oleanolic acid. To the best of our knowledge, this is the first report of these two triterpenes in Duroia genus.

Antimycobacterial Activity.
All extracts showed activity against M. tuberculosis at least for one strain, except for the methanol extract of branches (1st collection) ( Table 2). The dichloromethane extract of leaves (1st collection) showed the highest activity, with a MIC of 6.25 g/mL for INHr strain, 25.0 g/mL for RMPr strain and ≤6.25 g/mL for H37Rv strain. Triterpenes oleanoic and ursolic acids were isolated from this extract. The methanolic extract of leaves (2nd collection) that showed a MIC of 12.5 g/mL for INHr strain was the second most active one ( Table 2).
The wide variety of natural products chemical structures plays a major role on the development of new antimycobacterial drugs generations, as shown in the extensive literature  revision made by Copp [22], from 1990 to 2002, which uncovered 352 substances isolated from natural products (terrestrial and marine) presenting an antimycobacterial activity and a MIC ≤ 64 g/mL. The highest activity of the dichloromethane extract from leaves (1st collection) in this work could be attributed to the presence of terpenes. Several studies, such as those performed by Newton et al. [23], Cantrell et al. [24], Copp [22], Seidel and Taylor [25], Aguiar et al. [26], and Higuchi et al. [27], showed terpenes to be responsible for the antimycobacterial activity.
Extracts and compounds from other Rubiaceae species, such as Duroia hirsuta and Psychotria vellosiana, showed activity against Mycobacterium phlei [6] and M. tuberculosis and M. kansasii, respectively [28]. According to some authors, the antimycobacterial activity can also be related to the presence of alkaloids, normally found in Rubiaceae species [29,30].  (Table 3). Fraction 25-40.6 was active against the three strains, with a MIC of 200 g/mL and their fractionation yielded the substances 1 and 2 corresponding to the triterpenes oleanoic and ursolic acids, respectively.
Studies conducted by Higuchi et al. [27] reported the oleanolic and ursolic acids' mixture MIC to be 62.5 g/mL. Other studies showed the growth inhibitory activity against Mycobacterium tuberculosis with a MIC of 16 g/mL for oleanolic acid and 50 g/mL for ursolic acid [24].
Ge et al. [31] also obtained a better MIC value for oleanolic acid (MIC of 28.7 g/mL) than that for ursolic acid (MIC of 41.9 g/mL). These latter examples showed each isolated compound to exhibit a better MIC than the mixture's.
The high lipophilicity of terpenes is probably the main factor that allows their penetration through the mycobacterial cell wall [27].
Other studies showed these substances inhibited 99% the growth of M. tuberculosis H37Rv [32]. The literature data reported that oleanolic acid has a synergistic effect when combined with isoniazid, rifampicin, or ethambutol (first line antitubercular drugs) [32].
According to Pauli et al. [33], a crude extract MIC may or not be a reliable antimycobacterial activity indicator since such extracts could hold active compound antagonist substances decreasing the MIC. Otherwise, a crude extract could hold compound agonists with increasing effects on MIC, the so called synergism effect. According to the author an extract with high activity (lower MIC) could present large amounts of compounds with moderated antimycobacterial activity. In other scenario, crude extracts with moderated MIC could hold small amounts of chemically active compounds. When the extract loses its activity during the fractionation, it could be due to a synergism effect between the substances on it. Therefore, the combined action of two or more substances can result on a biological effect higher than any single one's. Hence, more thorough studies are necessary to find which substances should be mixed in order to attain the desirable antimycobacterial activity.