Assessing the biological activities of xanthone derivatives from Swertia macrosperma C.B. Clark

Abstract The anti-microbial and anti-oxidant effects of xanthones extract from Swertia macrosperma C.B. Clark were investigated using extracts from whole plant and partitioned by petroleum ether, ethyl acetate and n-butanol, respectively. Anti-microbial and anti-oxidant activities were detected among different fractions. High-performance liquid chromatography was performed to separate and purify the elutions. The compounds were elucidated by 1H, 13C NMR and LCMS. The ethyl acetate extract showed maximum inhibitory activity in fungal organisms and Gram-positive bacteria, and had the highest anti-oxidant capacity. Eight xanthones were isolated in ethyl acetate fraction from S. macrosperma. Moreover, compounds IV–VI and VIII were isolated from the plant for the first time, and compound VII had the strongest anti-oxidant effect.


Introduction
Xanthone, a natural polyphenolic compound with a simple three-ring skeleton, commonly exists in the plants of Gentianaceae and other plants or fungi (Le Pogam & Boustie 2016). Biosynthesis is also a source of xanthones (Kumar et al. 2015). Xanthones have a wide range of biological and pharmacological properties such as anti-oxidant, anti-inflammatory, anti-microbial, anti-cholinesterase and anti-cytotoxic activities. As the family of Swertia in Gentianaceae, Swertia macrosperma C.B. Clark, called as 'Dazizhangyacai' , is widespread in Yunnan, Guizhou and Tibet of China. The whole plant is used as a medicine, known for its anti-hepatitis virus, anti-pyretic, anti-dotal and anti-diabetic effects (Wang et al. 2012(Wang et al. , 2013Jiang et al. 2015). The main constituents of the Swertia species are xanthones, iridoid glycosides, flavonoides and triterpenoids (Cao et al. 2013). Unlike iridoids and flavonoides, xanthones are not universally present in all plant species investigated in the family of Gentianaceae (Hajimehdipoor et al. 2006). However, xanthones, with a common 9H-xanthen-9-one scaffold, have diverse pharmacological activities (El-Seedi et al. 2010). In the present study, we isolated the xanthones in S. macrosperma C.B. Clark using solvent extraction and column chromatography. Furthermore, we compared the anti-microbial and anti-oxidant activities of different extracts. We also analysed the structure of isolates in ethyl acetate fraction from S. macrosperma C.B. Clark.

Results and discussion
Xanthones have been previously isolated from Swertia species, such as Swertia decussate, Swertia longifolia Boiss, Swertia chirayita and Swertia minor (Patro et al. 2005;Chintalwar & Chattopadhyay 2006;Singh et al. 2012;Uvarani et al. 2012;Jiang et al. 2015;Kshirsagar et al. 2015). We used S. macrosperma C.B. Clark as the object, which mainly located in Tibet, P.R. China. In our study, eight xanthones were isolated from S. macrosperma, and their 13 C and 1 H NMR spectroscopic data are shown in Tables S1 and S2. 13 C and 1 H NMR spectra were shown in Figures S1-S8. All eight xanthones are tetraoxygenated xanthones, belonging to the simple oxygenated xanthones (El-Seedi et al. 2010). Compounds V and VII are free hydroxyxanthones (Tang et al. 2009). The anti-microbial activity of petroleum ether (PE), ethyl acetate (EA), and n-butanol (NB) fraction is shown in Table S3. The results showed that positive anti-bacterial activity of PE and EA fraction against all tested bacterial strains and also the anti-fungal activity against Candida albicans. Moreover, the EA fraction showed the highest inhibitory activity in Staphylococcus aureus, Bacillus subtilis and C. albicans. The NB fraction showed maximum inhibitory activity in Escherichia coli and Salmonella spp (S. spp).
As shown in Table S4, all the fractions showed plasma total anti-oxidant capacity (T-AOC) and anti-lipid peroxidation effect. The EA fraction had the highest anti-lipid peroxidation capacity. As the most powerful anti-oxidant, the EA fraction also played an important role in the spontaneous lipid peroxidation of rabbit liver tissue. We investigated the relationship between T-AOC, inhibition rate (IR) and the concentration of EA fraction. As shown in Table  S5, there was a linear relationship between T-AOC, IR and the concentration of EA fraction (r = 0.907, p = 0.034; r = 0.951, p = 0.013).
Moreover, Compound VII had the strongest anti-oxidant effect in the T-AOC test and the IR was 46.78% at the concentration of 80 μg/mL. Compound VII had the strongest anti-oxidant effect in the eight xanthones (Table S6).

Conclusion
In conclusion, we investigated the anti-microbial activity and anti-oxidant capacity in different fractions, and we found that the EA fraction exhibited the highest anti-microbial activity and anti-oxidant capacity. And we isolated eight xanthones from the EA fraction. Compounds IV-VI and VIII were isolated from the plant for the first time. Compound VII showed the strongest anti-oxidant effect.

Supplementary material
Experimental details relating to this paper are available online, alongside Tables S1-S6 and Figures S1-S8.

Disclosure statement
No potential conflict of interest was reported by the authors.