CHEMICAL CONSTITUENTS, ANTICARIES, ANTIOXIDANT AND CYTOTOXIC ACTIVITIES OF THE ROOT OF Mezoneuron benthamianum BAILL.

Abstract

Chewing sticks are the common means of maintaining oral hygiene in Africa. The roots of Mezoneuron benthamianumare commonly used as chewing stick in the southwestof Nigeria. However, their usage in preventing dental caries, a disease caused by bacteria especially Streptococcus mutans, has not been investigated.This study was therefore designed to identify the chemical constituents, the anticaries and other biological activities of M. benthamianum. Roots of M. benthamianum were purchased in Ibadan and authenticated at the University of Ibadan herbarium (No.: UIH-22401).The pulverized root sample (2.7 kg) was extracted with methanol to obtain a crude extract which was successively partitioned with n-hexane, dichloromethane and ethyl acetate. The fractions were subjected to column and thin layer chromatography to obtain pure compounds. Thechemical structures of the isolated compounds were elucidated by the use of Infrared (IR), Nuclear Magnetic Resonance (NMR) and mass spectroscopy. The extracts and isolated compounds were evaluated for antibacterial activities against Streptococcus mutans, Pseudomonas aeruginosa, Escherichia coli and Staphyloccus aureus at concentrations ranging from 0.0125 to 20.0 mg/L.Their antioxidant and cytotoxic activities were determined using 2,2-diphenyl-1-picrylhydrazyl methodand brine shrimpbioassay. Data were analysed using descriptive statistics. The successive partitioning of the crude methanol extract (200.0 g) of M. benthamianum yielded 5.0 g (n-hexane),14.0 g (dichloromethane) and 70 g (ethyl acetate) fractions. Column chromatographic purification of the n-hexane and ethyl acetate fractions gave six compounds which were elucidated to be taepeeninA, nortaepeeninA, stigmasterol, trans-resveratrol, piceatannol and gallic acid. The most abundant isolated compound, trans-resveratrol, showed the IR absorptions at 3421 (O-H) and 1633 cm-1 (aromatic C=C). The mass spectrum gave the (MH)+ion at m/z229.0, suggesting the molecular formula C14H12O3. The 1HNMR spectrum displayed four sets of doublet signals indicating two ortho-coupled aromatic protons (δ 7.35, d,J = 8.6 Hz, H-2’, H-6’ and 6.78, d, J = 8.6 Hz, H-3’, H-5’) and two trans-olefinic protons(δ 6.97, d,J = 16.3 Hz, H-8 and 6.81, d,J = 16.4 Hz, H-7). The 13C NMR spectrum showed the presence of seven aromatic methine carbons (δ 102.62, C-4; 105.80, C-2, C-6; 116.48, C-3’, C-5’ and 128.83, C-2’, C-6’), two olefinic methine carbons (δ 126.95, C-7 and 129.40, C-8) and five aromatic quaternary carbons (δ 130.39, C-1’;141.32, C-1; 158.27, C-4’ and 159.57, C-3, C-5),suggestive of fourteen carbon atoms. The trans-resveratrol and piceatannol exhibited strong antimicrobial activity against S.mutans, P. aeruginosa, E. coli and S. aureus with trans-resveratrol having the same minimum inhibitory concentration (MIC) of 0.025 mg/mL against all the organisms and piceatannol with MIC ranging from 0.025 – 0.30 mg/mL. They also showed potent antioxidant activity with IC50 of 35.81 and 30.35 µM,when compared with ascorbic acid (IC50 = 38.20 µM) and cytotoxic activity against brine shrimp with LC50 of 7.15 and 99.99 µg/ml. Taepeenin A, nortaepeenin A and piceatannol were isolated from Mezoneuron benthamianum. The trans-resveratrol and piceatannol were identified as the major compoundswhich may be responsible for itsanticaries activity.