Chemical Composition and Antioxidant Property of Essential Oil of Callicarpa tomentosa

Published in Khimiya Prirodnykh Soedinenii, No. 4, July–August, 2022, pp. 639–640.

The genus Callicarpa (Lamiaceae) comprises about 140 species of small trees and shrubs distributed in tropical and subtropical locations [[1]]. Several members of the genus Callicarpa are used in the preparation of herbal medicines, insect deterrents, and also fish poisons [[2]]. Phytoconstituents such as monoterpenes, sesquiterpenes, diterpenes, triterpenes, fatty acids, flavonoids, lignans, phenylpropanoids, and phytosterols have been isolated and characterized from the species of Callicarpa [[2]].

Callicarpa tomentosa (L.) L. (Lamiaceae), a medium-sized tree reaching a height of up to 5 m, is found mainly in the Western and Eastern Ghat region of India, Sri Lanka, Indonesia, Myanmar, Nepal, and the Malay Peninsula. In traditional medicine, various parts of this plant are used to treat fever, liver injury, skin infections, and mouth ulcers [[4]]. The juice mixed with common salt is administered as an anthelmintic [[5]], whereas the seeds are used to treat blood loss, hydrocele, malaria, and swellings [[6]], and an aqueous leaf extract is used as an antiseptic to treat wounds, boils, asthma, cough, and ulcers [[7]]. A decoction of the fruit is used to cure bleeding piles [[8]] and the bark of C. tomentosa is being used in the East Indies as a substitute for betel leaf [[9]].

Phytochemicals isolated from the leaf and bark extracts of C. tomentosa include β-sitosterol, maslinic, baurenol, oleanolic, ursolic acids and their methyl ester acetates, lupeol acetate, β-amyrin acetate, and methyl betulinate [[10]]. Little is known about the essential oil constituents of C. tomentosa and their biological activity. To the best of the authors' knowledge, the present study is the first report on the chemical composition and antioxidant activity of the leaf essential oil of Callicarpa tomentosa.

The fresh leaves of Callicarpa tomentosa were collected from Champagarh Reserve Forests (19°52′′19.6′ N, 85°01′′27.6′E) of Khurda Forest Division, Khurda district of Odisha, India in August, 2020. The plant was identified by Prof. Pratap Chandra Panda, and a voucher specimen (No. 1265 Dt. 4.8.2020) was deposited at the Herbarium of Regional Plant Resource Centre, Bhubaneswar, Odisha.

The leaves were shade dried and homogenized into powder. Subsequently, 500 g of the powdered leaves were hydro-distilled for 4 h in a Clevenger apparatus. The essential oil was dried by adding anhydrous Na2SO4 and stored in air-tight glass vials at 4°C until analysis. The leaf essential oil of C. tomentosa was slightly pale yellow in colour with an average oil yield of 0.1% (w/v) on a dry weight basis.

The identification of essential oil constituents was carried out with a Clarus 580 Gas Chromatograph (Perkin-Elmer, USA) equipped with a SQ8S MS detector. The analysis was performed on an Elite-5MS capillary column (30 m × 0.25 mm × 0.25 μm) with helium as the carrier gas at a flow rate of 1.0 mL/min. The column temperature was programmed at 60°C then increased to 220°C at 3°C/min and finally held at 220°C for 7 min. The injector and ion source temperature were kept at 250°C. The essential oil constituents were identified by matching the acquired spectra against the built-in reference of the NIST spectral library and by comparing experimental retention indices (RI) obtained with the published bibliographic literature [[11]].

The chemical composition of C. tomentosa leaf essential oil is presented in Table 1. The constituents are listed according to their order of elution on Elite-5 MS column. Forty-seven constituents representing 99.8% of the total leaf oil, were identified. Eighteen sesquiterpene hydrocarbons (44.4%), 19 oxygenated sesquiterpenes (35.9%), one oxygenated diterpene (8.4%), and two diterpene hydrocarbons (2.0%) were identified in the essential oil. The predominant constituents of the essential oil from C. tomentosa were α-humulene (14.6%), humulene epoxide-II (12.9%), α-eudesmol (7.0%), manool oxide (8.4%), β-caryophyllene (9.5%) and β-chamigrene (5.3%). Other chemical constituents identified with composition more than 2% were α-bisabolol (3.7%), 1-epi-cubenol (2.9%), germacrene-B (2.9%), β-selinene (2.4%), and 9-epi-(E)-caryophyllene (2.1%). The phytoconstituents now obtained are similar to those reported in several other Callicarpa leaf essential oils such as C. bodinieri, C. candicans, C. formosana, C. longifolia, C. petelotii, C. rubella, and C. sinuata, where oxygenated and hydrocarbon sesquiterpenoids have been reported as the major chemical classes [[3]]. α-Humulene and humulene epoxide-II were present in higher concentrations in several Callicarpa species such as C. americana and C. sinuata [[3], [12]]. Humulene epoxide was reported to be the major sesquiterpene in the leaf essential oil of C. nudiflora from China [[13]]. On the other hand, β-caryophyllene was reported as the most abundant component in the essential oil of C. candicans and C. formosana [[3]]. α-Humulene is an important precursor for the biosynthesis of several sesquiterpenoids [[14]]. Humulene epoxide II, was alsoreported C. americana, seems to originate either from biogenesis or oxidation of α-humulene.

Table 1. Chemical Composition of Leaf Essential Oil of Callicarpa tomentosa

aChemical constituents are listed according to their order of elution on Elite-5 MS column; bRetention indices experimentally calculated using straight n-alkane (C8–C40) series on Elite-5 MS column.

The antioxidant potential of C. tomentosa essential oil was evaluated by two different systems, namely DPPH and ABTS assay [[15]]. Butylated hydroxy toluene (BHT) and ascorbic acid were used as the positive control. The antioxidant activity of essential oil was expressed in terms of half maximum inhibitory concentration (IC50) value. A lower IC50 value indicates a higher antioxidant activity. The antioxidant activity of essential oil increased with an increase in the concentration of the essential oil. Among the samples tested for DPPH assay, the antioxidant activity was ranked in the following order: ascorbic acid (6.65 ± 0.30 μg/mL) > C. tomentosa leaf essential oil (17.12 ± 0.36 μg/mL) > BHT (25.67 ± 0.45 μg/mL). Similarly, the ABTS scavenging activity of samples was found to be higher in ascorbic acid (1.61 ± 0.13 μg/mL), followed by C. tomentosa leaf essential oil (8.05 ± 0.23 μg/mL) and BHT (17.67 ± 0.18 μg/mL).

The findings of the study show that C. tomentosa leaf essential oil can be used as a natural antioxidant alternative to synthetic ones. However, further clinical research is required to generate reliable information on the practical effectiveness of C. tomentosa essential oil.

The authors are grateful to Dr. S. C. Si, Dean, Centre for Biotechnology and Dr. M. R. Nayak, President, Siksha 'O' Anusandhan (Deemed to be University) for providing facilities and encouragements.