Phytochemical composition, antioxidant and antibacterial activities of albizia coriaria leaves from three agroecological zones of Uganda

Abstract

Oxidative stress-induced conditions and bacterial diseases constitute some of the major causes of mortality worldwide. Their treatment is becoming a challenge due to antimicrobial resistance, prohibitive costs, inaccessibility and side effects of the conventional drugs. Thus, traditional medicine is becoming popular in the treatment of these diseases in various parts of the world. The objectives of this study were to;(1) identify the secondary metabolites in extracts of Albizia coriaria leaves (EOACL), (2) determine the total phenolic and total flavonoid contents of EOACL, (3) establish the antioxidant activity of EOACL, (4) evaluate the antibacterial activity of EOACL, and (5) characterize the phytochemicals in the most active EOACL used in traditional treatment of oxidative stress-induced conditions and bacterial diseases in Uganda. The leaves were sampled from Jinja, Kole and Mbarara districts of Uganda, representing the South East, Mid Northern and Southern drylands agroecological zones, respectively. Shade-dried samples were ground into powder and successively extracted with ethyl acetate, ethanol and distilled water. The extracts were chemically profiled using classical phytochemical screening, ultraviolet-visible (UV-Vis) spectroscopy, Fourier transform infrared (FTIR) spectroscopy and gas chromatography-mass spectrometry (GC-MS). The total phenolic content, total flavonoid content, antioxidant and antibacterial activities were determined using; Folin-Ciocalteu method, aluminum chloride assay, 1,1-diphenyl-2-picrylhydrazyl assay and culture-based agar disc diffusion method, respectively. The results obtained varied for the three agroecological zones; Mbarara leaf extracts had many secondary metabolites and exhibited the highest bioactivities, followed by Kole and Jinja extracts. Phytochemical screening results indicated that phenols, alkaloids, saponins, flavonoids, cardiac glycosides and tannins were the major secondary metabolites in EOACL. These results were confirmed by UV-Vis spectra (with absorption maxima of 338 nm, 470 nm, 534 nm, 607 nm and 664 nm) and FTIR spectra which indicated the presence of O−H stretch (3370.27 cm -1 ), C=O (1739.70 cm -1 ), N−H (3261.46 cm -1 ) and aromatic−C=C (1454.48 cm -1 ). Total phenolic and flavonoid contents, and antioxidant activity were found to be highest for ethanolic extracts, with the highest contents (101.72 ± 0.22 mg GAE/ g DW and 13.23 ± 0.03 mg QE/ g DW) and antioxidant potential (IC 50 = 18.65 ± 0.06 mg/mL) being for EOACL from Mbarara district. The high antioxidant potential of EOACL suggests their potential role in the prevention of oxidative stress-induced conditions. Antibacterial screening indicated that ethanolic extracts had the highest antibacterial activities with mean zones of inhibition of 6.00 ± 1.73 to 10.00 ± 1.73 mm, 5.00 ± 1.00 to 12.30 ± 1.53 mm, 17.00 ± 0.00 to 25.00 ± 2.65 mm and 9.00 ± 1.73 to 16.00 ± 1.73 mm for Escherichia coli (E. coli), Staphylococcus aureus, Pseudomonas aeruginosa and Salmonella typhi, respectively. Ethyl acetate EOACL from Kole and Mbarara were active against E. coli with inhibition zones of 3.00 ± 0.00 mm and 4.00 ± 2.00 mm respectively. Ethyl acetate EOACL from Jinja and all the aqueous extracts showed no antibacterial activity. Characterization of fractions of the most active (ethanolic) EOACL using GC-MS led to the identification of nine compounds: lupeol (7), lupenone (8), betulinic acid (9), benzyl alcohol (12), betulin (13), oleanolic acid (14), oleanolic acid acetate (15), undecanol (16) and pterin-6-carboxylic acid (17) of which compounds 13-17 are being reported for the first time in Albizia coriaria. In conclusion, EOACL were established to have compounds with antioxidant and antibacterial activities, giving credence to their use in traditional management of oxidative stress-induced conditions and bacterial diseases. Clinical trials using the active EOACL and the identified compounds should be done.