CHEMICAL CONSTITUENTS AND BIOLOGICAL ACTIVITIES OF ESSENTIAL OILS FROM SELECTED NIGERIAN MEDICINAL PLANTS

Abstract

Oxidative stress and microbial infections are major threats to humans. Toxicity, side effects and resistance to the existing drugs necessitate the need for alternatives. Essential Oils (EOs) of plants, which are complex mixtures of terpenes and terpenoids, have demonstrated multiple pharmacological activities. However, there is dearth of information on the EOs of most plants used as antioxidant and in the treatment of microbial infections. Therefore, this study was designed to determine the chemical constituents and biological activities of EOs from selected medicinal plants in Nigeria. Tecoma stans (TS) (FHI 112524), Plumeria acuminata (PA) (FHI 112495), Plumeria rubra (PR) (FHI 112567), Delonix regia (DRe) (FHI 112523), Gliricidia sepium (GS) (FHI 112496), Duranta repens (DR) (FHI 112525), Zanthoxylum zanthoxyloides (ZZ) (FHI 112982), Ceiba pentandra (CP) (FHI 112962) and Annona muricata (AM) (FHI 112526) were collected within University of Ibadan and authenticated at Forest Research Institute of Nigeria. Different plant parts (leaves, stem, stembark, fruits, twig, seeds, flowers, heartwood, root) were air-dried, pulverised and subjected to hydrodistillation. The EOs were analysed for their chemical constituents using gas chromatographymass spectrometry. Antioxidant properties and cytotoxicity of the oils were respectively determined using 2,2-diphenyl-1-picryhydrazyl radical-scavenging and brine shrimp lethality assays, with reference to the standards. Antimicrobial activities were also determined using agar-diffusion method against selected fungi (Candida albicans, Aspergillus niger, Fusarium solani) and bacteria (Staphylococcus aureus, Escherichia coli, Salmonella typhi, Leclericia adecarboxylata, Morganella morganii, Citrobacter freundii, Klebsiella pneumoniae) using Ketoconazole and Gentamycin as reference drugs. Data were analysed using descriptive statistics. The yields of the EOs ranged from 0.06 to 0.62 % (w/w). Dominant terpenes in the EOs were linalool (11.4%, stem), α-terpineol (11.7%, seed) in TS; linalool (16.1%, leaf), β-eudesmol (43.0%, root) in PA; spathulenol (34.7%), carryophyllene (17.8%) in PR stem; phytol (12.7%) in DRe root; humulene epoxide II (17.5%) in GS stem; geranyl acetone (4.3%) in DR leaf. Others were α-pinene (38.5%, stembark), caryophyllene oxide (27.0%, leaf) in ZZ; β-elemene (18.5%, stem) α-eudesmol (21.1%, heartwood), β-caryophyllene (28.7%, stembark) in CP; E-caryophyllene (28.2%) in AM leaf. Prominent non-terpenes were pentadecanal (51.2%) in PR leaf; (9Z)-docosenamide (51.6%) in DRe twig; palmitic acid (55.7%, root), styrene (52.5%, fruit) in DR. Thus, the chemical constituents were established for Tecoma stans, Delonix regia and Ceiba pentandra essential oils. The EOs of TS (seed, stem), DR (fruit, stem) and AM (leaf) displayed higher antioxidant activities (IC50 6.44 - 43.22 µg/mL) than control (IC50: BHA- 45.11 µg/mL, α-tocopherol-81.58µg/mL). Cytotoxicities of all the EOs were high (LC50 5.8992-89.8190 ppm) compared with standard values. The EOs exhibited antifungal activities (7.8±0.4 - 22.1±0.1 mm) compared with Ketoconazole (10.3±0.4 – 21.0±1.4 mm). However, only EOs of PA exhibited antibacterial activities (7.9±0.0 - 12.1±0.1 mm) comparable with Gentamycin (8.0±0.0 - 12.5±0.1 mm). The essential oils of selected plants had antioxidant properties, were cytotoxic and possessed antimicrobial activities. Essential oils of Tecoma stans (stem, seed), Duranta repens (fruit, stem) and Annona muricata (leaf) could be alternative sources of antioxidants, while Plumeria acuminata (leaf, root, flower) oils could act as a natural antimicrobial agent.