Chemical Composition and Antioxidant Activity of Essential Oil of Leucas lanata

Published in Khimiya Prirodnykh Soedinenii, No. 2, March–April, 2023, pp. 325–327.

The genus Leucas R. Br. (Lamiaceae) comprising about 100 species of shrubs, sub-shrubs and herbs, is distributed in tropical to Southern Africa, Arabian Peninsula, Iran to South China, Taiwan, Japan, South East Asia and up to Australia [[1]]. In India, 41 species are reported to occur with maximum species concentration in the Southern Peninsula, of which 23 species are endemic to the country [[2]]. The species of the genus are extensively used in Africa and Asia in the traditional system of medicine for several ailments, suggesting that they have the potential for the discovery of new drugs or leading molecules [[4]]. Several compounds such as labdane-type diterpenes, triterpenes, flavones, lignans, flavonoids, coumarins, steroids, terpenes and fatty acids have been isolated from different Leucas species [[6]–[8]].

Leucas lanata Benth. (Lamiaceae) is a herbaceous perennial plant growing among the grasses on the dry slopes and distributed in South-Central China, East Himalaya, India, Myanmar, Nepal, Pakistan, Philippines, Thailand, Vietnam, and West Himalaya [[9]]. The juice of the whole plant has been used traditionally to treat a variety of diseases like stomach ache, headache and whooping cough [[10]–[12]]. The plant is also used as antidote for reptile poisons [[13]]. The plant extract is also reported to exhibit hepatoprotective, antimicrobial, anti-Parkinson, antioxidant, antiepileptic and wound-healing properties [[9], [14]–[16]]. The GC-MS analysis of leaf extract showed the presence of 6-octadecenoic acid (45.54%), cis-13-octadecenoic acid (42.76%), and l-(+)-ascorbic acid 2,6-dihexadecanoate (2.84%) as predominant constituents [[16]]. A thorough survey of the literature reveals that no work has yet been done on the composition and bioactivity of Leucas lanata essential oil. Hence, the present communication is the first study on the volatile profile and antioxidant activity of the Leucas lanata leaf essential oil.

The fresh leaves of Leucas lanata were collected from Hadagadh Wildlife Sanctuary, Keonjhar District of Odisha, India (21.244812 N, 86.250863 E) in the month of November 2020 and the species was identified by Prof. Pratap Chandra Panda, Taxonomist. The voucher specimen (2022/CBT Dt. 3.11.2021) has been preserved in the Herbarium of Centre for Biotechnology, Siksha 'O' Anusandhan (Deemed to be University), Bhubaneswar. The leaves were shade-dried for 10 days and pulverized. Subsequently, 500 g of the leaves were hydro-distilled for 6 h in a Clevenger apparatus. Following extraction, the essential oil was stored at 4°C until analysis. The essential oil extracted from the leaf of L. lanata was pale yellow in colour with an average oil yield of 0.15% (v/w) on a dry weight basis.

The compound identification was performed on Clarus 580 Gas Chromatograph (Perkin-Elmer, USA) assembled with a SQ8S MS detector. The analysis was carried out on a Elite-5 MS capillary column (30 m × 0.25 mm × 0.25 μm) with helium as the gas carrier (1.0 mL/min). The oven temperature was kept at 60°C, then gradually increased to 220°C at 3°C/min and finally held for 7 min. Injector and ion source temperatures were kept at 250 and 150°C, respectively. The constituents were identified by comparing the mass spectra of detected compound with the in-build NIST Mass Spectral library and by matching the retention indices (RI) with published bibliographic literature [[17]].

The leaf essential oil composition of Leucas lanata is given in Table 1 and the compounds were mentioned according to their elution order in Elite-5 MS column. The GC-MS analysis resulted in the identification of 43 constituents representing 86.08% of the total leaf oil. The analysis showed that the essential oil was rich in sesquiterpene hydrocarbons (48.79%), sesquiterpene alcohols (20.44%), others compounds (13.48%), ether (2.91%), phenylpropanoid (0.28%), and monoterpene alcohol (0.18%). β-Caryophyllene (17.15%), spathulenol (13.43%), (Z,Z)-9,12-octadecadienoic acid (10.60%), calamenene (7.63%), and aromadendrene (6.69%) were the predominant constituents of the essential oil. Other compounds identified with compositions above 1% were β-curcumene (2.17%), caryophyllene oxide (2.12%), β-eudesmol (1.89%), γ-gurjunene (1.87%), allo-aromadendrene (1.78%), t-cadinol (1.49%), eudesm-11-en-4α-ol (1.18%), cyclosativene (1.17%), α-ylangene (1.10%), cyclopentadecanolide (1.03%), β-sesquiphellandrene (1.01%), and bicyclogermacrene (1.00%). A total of 26 components had compositions that were less than 1%. In earlier studies, β-caryophyllene has been reported as the major constituent in the aerial parts of Leucas aspera, Leucas indica, and Leucas stelligera collected from different parts of India with sesquiterpene hydrocarbons as the predominant class of compounds [[18]–[20]].

Table 1. Chemical Composition of Leaf Essential Oil of Leucas lanata

Compound a	RI b	%	Compound a	RIb	%
α-Terpineol	1189	0.18	Germacrene-B	1552	0.78
Eugenol	1337	0.28	Spathulenol	1572	13.43
Cyclosativene	1364	1.17	Caryophyllene oxide	1598	2.12
α-Ylangene	1377	1.10	Humulene epioxide II	1616	0.79
β-Cubebene	1395	0.33	γ-Eudesmol	1631	0.66
Longifolene	1404	0.21	t-Cadinol	1633	1.49
β-caryophyllene	1409	17.15	β-Eudesmol	1642	1.89
β-Copaene	1422	0.45	α-Cadinol	1655	0.71
β-Gurjunene	1424	0.32	Eudesm-11-en-4α-ol	1661	1.18
α-Guaiene	1429	0.49	β-Bisabolol	1668	0.56
Aromadendrene	1442	6.69	Occidenol	1692	0.52
α-Humulene	1445	0.39	n-Tetradecanoic acid	1722	0.27
allo-Aromadendrene	1464	1.78	Amorpha-4,7(11)-diene	1801	0.52
Bicyclogermacrene	1469	1.00	Cyclopentadecanolide	1833	1.03
γ-Muurolene	1472	0.27	Methyl hexadecanoate	1917	0.24
trans-Cadina-1(6),4-diene	1476	0.66	Hexadecanoic acid	1970	0.66
γ-Gurjunene	1479	1.87	(Z,Z)-9,12-Octadecadienoic acid	2123	10.60
Germacrene-D	1490	0.57	n-Tricosane	2324	0.68
γ-Himachalene	1497	0.87	Total identified		86.08
β-Selinene	1500	0.36	Sesquiterpene hydrocarbons		48.79
β-Curcumene	1507	2.17	Sesquiterpene alcohols		20.44
Calamenene	1510	7.63	Ether		2.91
7-epi-α-Selinene	1520	0.78	Phenylpropanoid		0.28
β-Sesquiphellandrene	1530	1.01	Monoterpene alcohol		0.18
α-Calacorene	1545	0.22	Others		13.48

a Compounds listed in order of elution from Elite-5 MS column; b retention indices calculated from homologous series of (C8–C20) n-alkane on Elite-5 MS column.

Similarly, a high content of β-caryophyllene followed by α-humulene and germacrene D were identified in the essential oil of the aerial parts of Leucas zeylanica collected from Vietnam [[21]]. While iso-menthone was found to be the major constituent of the essential oil of Leucas glabrata in Tanzania [[22]]; camphor was the dominant compound of Leucas virgata essential oil [[23]]. The presence of caryophyllene oxide, humulene epoxide II, allo-aromadendrene, germacrene D and sabinene have been identified in other Leucas species [[18], [21]–[23]].

The antioxidant potential of essential oil was evaluated using DPPH and ABTS assay, according to the method described by Jena et al. [[24]]. Ascorbic acid and BHT were taken as positive controls. The lower the IC50 value, the better the antioxidant activity of the sample. Among the samples tested for the DPPH assay, the leaf essential oil exhibited moderate antioxidant ability (93.49 ± 2.16 μg/mL) as compared to ascorbic acid (5.61 ± 0.15 μg/mL) and BHT (21.94 ± 0.35 μg/mL). Similarly, the leaf essential oil of L. lanata exhibited moderate ABTS free radical scavenging ability (87.21 ± 1.73 μg/mL) compared to ascorbic acid (2.72 ± 0.05 μg/mL) and BHT (18.11 ± 0.13 μg/mL).

The compounds like β-caryophyllene, allo-aromadendrene and caryophyllene oxide present in the essential oil of Leucas lanata might be responsible for the antioxidant activity [[25]]. The sesquiterpene hydrocarbon and β-caryophyllene might be suitable for some therapeutic applications as it is reported to inhibit CCl4-mediated liver fibrosis and reduce the activation of hepatic cells [[27]].

Acknowledgment

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 encouragement. In addition, Bibhuti Bhusan Champati and Sudipta Jena contributed equally to this work.

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By Bibhuti Bhusan Champati; Sudipta Jena; Asit Ray; Swagat Mohanty; Ambika Sahoo; Prabhat Kumar Das; Subrat Kumar Kar; Tirthabrata Sahoo; Sanghamitra Nayak and Pratap Chandra Panda

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