CHARACTERISATION AND BIOACTIVITIES OF VOLATILE AND NON-VOLATILE CHEMICAL CONSTITUENTS FROM Calophyllum inophyllum LINN. (GUTTIFERAE) AND Pterocarpus soyauxii TAUB. (FABACEAE)

Abstract

Diabetes and oxidative stress are responsible for an upsurge in the number of neuropathies, retinopathy, nephropathy, cancer, cardiovascular dysfunction and mortality world-wide. Traditionally, Calophyllum inophyllum and Pterocarpus soyauxii are used in the management of diabetes, skin diseases, haemorrhage and common cold in Nigeria. Despite their ethno-medicinal uses, there is sparse information on the chemical constituents responsible for their antidiabetic and antioxidant activities. Hence, this study was designed to isolate and evaluate the antidiabetic and antioxidant activities of volatile and non-volatile chemical constituents of both plants. Calophyllum inophyllum and Pterocarpus soyauxii were collected and authenticated at University of Ibadan and Forest Research Institute, Ibadan. Essential oils (EOs) were extracted from leaf, leaf-stalk, flower, seed, pod, fruit-pulp, stem wood, stem bark, root wood and root bark of C. inophyllum and root parts of P. soyauxii by hydro-distillation using a Clevenger-type apparatus and analysed by gas chromatography-mass spectrometry. The ten parts of C. inophyllum and leaf, stem, root and root bark of P. soyauxii were extracted with methanol and subsequently partitioned with n-hexane and ethyl acetate (EtOAc) to obtain respective fractions. Bioactive fractions were purified using column chromatographic technique and isolated compounds were characterised using Fourier Transform Infrared (FTIR), Nuclear Magnetic Resonance (NMR) and Electron Ionisation Mass Spectrometer (EIMS). In vitro antidiabetic activity was evaluated using α-amylase and α-glucosidase methods, while antioxidant assay was done using 2,2-diphenyl-1-picrylhydrazyl (DPPH) and hydrogen peroxide (H2O2) radical methods. Data were evaluated using regression analysis. A total of 102 and 95 compounds were identified in C. inophyllum and P. soyauxii Essential oils (EOs), respectively. The EOs were dominated by monoterpenes, sesquiterpene and their oxygenated derivatives. The predominant constituents in the EOs were hexadecanal, limonene and cis-cadina-1(6),4-diene. The pod and root bark EtOAc fractions of C. inophyllum and P. soyauxii had higher activities, respectively for both antidiabetic and antioxidant assays compared to hexane fractions. Friedelan-3-one, stigmasterol and 1,3,5-trihydroxy-2-methoxy-xanthone were isolated in the pod EtOAc fraction of C. inophyllum, while 3β-hydroxylup-20(29)-ene, 2,3- dihydroxyfriedelinol, stigmasterol and β-sitosterol, were isolated from P. soyauxii root bark. The most active compound was 1,3,5-trihydroxy-2-methoxy-xanthone and its structural elucidation showed absorption peaks (cm-1) at 3459 (OH), 1653 (C=O) and 1611 (aromatic C=C). The molecular ion at m/z 274.2 [M+] suggesting the molecular formula, C14H10O6. The proton NMR gave signals (ppm) at δ 6.52 (1H, s), δ 6.98 (1H, dd) and δ 7.25 (1H, dd) for three aromatic protons, 12.52 (1H, brs-OH) and 3.45 (3H, sOCH3). The carbon-13 NMR gave eight quaternary carbons, five methine carbons and one methyl carbon signals. Essential oils from both plants have antidiabetic and antioxidant activities with IC50 (mg/mL) ranging from 0.04 to 0.05 and 0.35 to 0.40, respectively. Isolated compounds had good antidiabetic and antioxidant activity with mean IC50 (mg/mL) ranging from 0.03 to 0.04 and 0.15 to 0.44, respectively. Friedelan-3-one and 1,3,5-trihydroxy-2-methoxylxanthone from Calophyllum inophyllum pod as well as 3β-hydroxylup-20(29)-ene and 2,3-dihydroxyvii friedelinol from Pterocarpus soyauxii root bark exhibited antidiabetic and antioxidant activities.