Etude chimique d’une plante médicinale camerounaise: Acacia polyacantha wild. (Fabaceae). Evaluation des activités antibactérienne et cytotoxique des extraits et de certains composés isolés.

Abstract

The present work describes the chemical study of a medicinal plant of Cameroon: Acacia polyacantha Wild. (Fabaceae). Together with the antibacterial and cytotoxic screenings of its extracts and some isolated compounds. Using chromatographic methods (CC, TLC) and the twenty compounds were isolated from the stem barks and leaves of Acacia polyacantha and their structures were elucidated using modern spectroscopic methods such as 1H, 13C NMR (1D), HMBC, HSQC, COSY (2D), MS, IR and UV. Among these compounds, one of them the saponin named polyacanthoside A or 3-O-[β D-xylopyranosyl-(1→4)-β-D-galactopyranosyl]-oleanolic acid has been described here for the first time. Additionally one saponin: 3-O-[β-D-galactopyranosyl-(1→4)-β-D-galactopyranosyl]-oleanolic acid was isolated for the first time from a plant outside the genus Polyscias. The others eighten known compounds are five flavonoids: epicatechin, apigenin, kaempferol, quercetin-3- O-galactoside, luteolin and one methylgallate ; five pentacyclic triterpens : lupeol, betulinic acid, betulin, β-amyrin, oleanolic acid; four steroids : β-sitosterol, stigmasterol, 3-O-β-D glucopyranosylstigmasterol, spinasterol ; one cyclitol (D-pinitol) : 3-O-methyl-D-chiro-inositol ; and two esters of fatty acid : 2,3-dihydroxypropyl-24’-hydroxytetracosanoate and 2,3-dihydroxypropyltetracosanoate.The crude extracts, fractions of the leaves and some isolated compounds from Acacia polyacantha were evaluated for their antibacteria activities. However, some compounds such as 3-O-[β-D-xylopyranosyl-(1→4)-β-D-galactopyranosyl]-oleanolic acid, β-amyrin, stigmasterol, 3-O-β-D-glucopyranosylstigmasterol, epicatechin, quercetin-3-O-glucoside, 3-O-methyl-D Chiro-inositol, 3-O-[β-D-galactopyranosyl-(1→4)-β-D-galactopyranosyl]-oleanolic acid were evaluated for their antibacteria activities from these studies, broth microdilution method was used for antibacterial assays of samples alone, in assoctiation with an efflux pump inhibitor (EPI), or antibiotics on the tested bacteria. The result shows that the crude extracts of leaves (APL) and stem bark (APB) had minimal inhibitory concentration (MIC) values ≤ 1024 µg/mL on respectively 73.3% and 46.7% of the tested bacteria. MIC values ≤ 512 µg/mL for fractions and ≤ 256 µg/mL for compounds were obtained against 88.9% for both APLb and APLd and against 77.8% tested bacteria for compound 3-O-[β-D-galactopyranosyl-(1→4)-β-D galactopyranosyl]-oleanolic acid. The present study demonstrates the antibacterial potential of Acacia polyacantha and its constituents to combat bacterial infections alone or in combination with EPI or antibiotics. Compounds 3-O-[β-D-xylopyranosyl-(1→4)-β-D-galactopyranosyl]-oleanolic acid and 3-O-[β-D-galactopyranosyl-(1→4)-β-D-galactopyranosyl]-oleanolic acid were tested for their cyto toxicity effects on cells cancer. The recorded IC50 values ranged from 8.90 ± 0.58 μM (towards CCRF-CEM leukemia cells) to 35.21 ± 1.51 μM (towards HepG2 hepa-tocarcinoma cells) for 3-O-[β-D-xylopyranosyl-(1→4)-β-D-galactopyranosyl]-oleanolic acid. For a comparative study of 3-O-[β-D-xylopyranosyl-(1→4)-β-D-galactopyranosyl]-oleanolic acid with compound 3-O-[β-D galactopyranosyl-(1→4)-β-D-galactopyranosyl]-oleanolic acid, a preliminary study was per formed in the sensitive CCRF-CEM leukemia cells. However, 3-O-[β-D-xylopyranosyl-(1→4)-β-D-galactopyranosyl]-oleanolic acid was much more effective with an IC50 value of 8.90 ± 0.58 μM and a compound 3-O-[β-D-galactopyranosyl-(1→4)-β-D-galactopyranosyl]-oleanolic acid showed low cytotoxic effects with a recorded IC50 values 35.16±2.26 μM. Consequently, com pound 3-O-[β-D-galactopyranosyl-(1→4)-β-D-galactopyranosyl]-oleanolic acid was no further tested in other cancer cell lines. This clearly indicates that the presence of galactopyranosyl sugar unit (3-O-[β-D-galactopyranosyl-(1→4)-β-D galactopyranosyl]-oleanolic acid) instead of xy lopyranosyl significantly reduce the cytotoxicity towards CCRF-CEM cells. From this result, 3-O-[β-D-xylopyranosyl-(1→4)-β-D-galactopyranosyl]-oleanolic acid can be used as anticancer drug for both drug sensitive and resistant cancer strains.