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.
