The chemical inhibition of acetyl-cholinesterase (AChE) is a potent strategy for addressing signal related neuropathology and natural products are potential sources of compounds with such properties. Essential oil extracts from leaf, seed, stem and rhizome of four medicinal plants [Aframomum melegueta K. Schum, Crassocephalum crepidioides (Benth S. More), Monodora myristica (Gaertn.), and Ocimum gratissimum (Linn)] were tested for acetyl-cholinesterase inhibitory activity (AChEI) using Ellman's colorimentric method and compared to a reference acetyl-cholinesterase inhibitor (galantamine). The seed (IC50 = 6.71 mg/l) and leaf (IC50 = 6.54 mg/l) extracts from O. gratissimum showed values that matched the capacity of the reference inhibitor (IC50 = 6.62 mg/l). The least potent extract was rhizome extracts of A. melegueta (IC50 = 28.97 mg/l) about four times that of the reference inhibitor. Principal component analysis (PCA) showed that the intrinsic properties (bioactive ingredient factor) of each extract (PC1 = 29.50%) was the most important factor defining the difference or similarity in potency to the reference acetyl-cholinesterase inhibitor while ‘dose response’ (PC2 = 11.38%) was the second most important factor. The outstanding AChEI property of O. gratissimum extracts could largely be attributed to the high monoterpene content while the weak potency of rhizome extracts of A. melegueta may be attributed to its predominant concentrations of sesquiterpenes. Since potency could be related to interaction between bioactive components, understanding the interaction between ratios of monoterpene and sesquiterpene in extracts could be important in determining their potency for AChEI.
Interest in plants' essential oils has grown recently due to increased use of natural products in medicine and cosmetics. In spite of its long use, there has been little prior study on the bioactivity and toxicity of the essential oils of Crassocephalum crepidioides (Benth S. More). Thus in this work, the anti-bacterial and brine shrimps' lethality of the essential oils obtained through hydro-distillation were evaluated. The oils were analyzed by gas chromatography and gas chromatography/mass spectrometric techniques. The toxicity of the oils was considered using the brine shrimps' lethality assay at concentrations of 10, 100 and 1000 ppm. The antimicrobial assay was carried out using the agar diffusion method. The bacteria were three strains of Gram-positive bacteria (Staphylococcus aureus, Bacillus subtilis and Salmonella Typhi) and two strains of Gram-negative bacteria (Escherichia coli and Pseudomonas aeruginosa). The main constituents of the leaves essential oil of C. crepidioides were α-caryophyllene (10.29%) and β-cubebene (13.77%), while the stem essential oil were mainly thymol (43.93%) and 4-cyclohexybutyramide (20.94%). The results of the brine shrimps' lethality assay suggest that the essential oil of the stem (LC 50 = 9.10 µg/mL) was just as toxic as the essential oil of the leaves (LC 50 = 9.2 µg/mL). The essential oils were active against all the bacterial strains but low when compared with the standard antibiotic, Gentamicin.
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