Thymol and Its Derivatives Rich Essential Oils from Asparagus flagellaris and Therapeutic Applications: Review

Abstract

Asparagus flagellaris essential oils of the leaves and roots comprised of twenty-eight (28) and nineteen (19) compounds in total, accounting 97.41 and 97.03% of the oil, respectively, were discovered using GC-MS analysis. According to reports, the EOs are a blend of terpenes, terpene derivatives, non-terpenes, thymol and its derivatives. Additionally, thymol derivatives predominated in the essential oils. When compared to the reference standards Tioconazole and Gentamicin for fungi (28 mm) and bacteria (40–35 mm), respectively, the essential oil exhibited a moderate inhibitory zone (18–10 mm) on the tested organisms. Thu, the essential oils were categorized as bacteriostatic. On the DPPH radical scavenger properties, interaction between the constituents identified in the essential oils demonstrated a distinctive a free radical scavenging activity. The chemical components of A. flagellaris’ essential oils play a key role in both its antioxidant and antibacterial properties.

1. Introduction

Traditional health cures in various societies have a long history dating back to discovery of primitive drugs during their battle against natural disasters, infections, maladies and provender’s. Prehistoric discovered that certain foods had specific characteristics that can help individuals recover from illnesses and live a healthier life [1]. Natural products are not simply mishaps or results of the accommodation of nature but are the characteristic articulation of the increase in the intricacy of an organism [2]. There is ongoing methodology in development of innovative drugs for human diseases from plant origin. Several plants are used to provide treatment [3]. This is especially true in underdeveloped countries where traditional medicine is still used to provide basic health care. A single plant may contain many secondary metabolites with healing activities ranging from anti-glycation, antibacterial to diuretic [4].

Plants are an extraordinary source of drugs and numerous novels organically dynamic compounds particularly used in conventional prescription for management of numerous illnesses. Plants are said to be the most unpredictable substance storage facilities of unfamiliar biodynamic compounds with undiscovered potential for use in present-day medication [5]. Plants with therapeutic and aromatic qualities have been used for millennia and are still helpful in complementary therapies today. Herbs are utilized all around the world, however they are used more frequently in developing countries. Herbs have traditionally been the main route of medication delivery [6]. Numerous fragrant plants that grow in Nigeria have been identified as potential sources of essential oils (EOs) [7]. Due to their well-established usefulness, aromatic plants that yield substantial amounts of EOs can be utilized therapeutically to treat a range of diseases. Strongly fragrant aromatic components that are obtained from aromatic plants make up EOs, a chemical composition. EOs are complex blends of bioactive ingredients with a variety of structural kinds, such as mono-, di-terpenes, sesqui-, sulfur-containing, phenolic and phenylpropanoid compounds [7]. High levels of bacterial drug resistance to existing treatments and the exorbitant costs of recent generations of antibiotics may be overcome thanks to the effective antibacterial capabilities of EOs [8]. The vast array of bioactive elements in EOs interfere with different bacteria’s defense mechanisms by interacting with their cellular enzymes or cell structures, which speeds up the death of microbial cells [9]. EOs and its constituents are used in fragrances, cosmetics, medicines, aromatherapy, dentistry, sanitary products, food preservatives, agriculture and additives, and natural therapies in addition to their remarkable antibacterial characteristics. Due to the billions of dollars in yearly earnings, EOs became a much more enticing topic for both academics and industry [10].

1.1 Terpenoids, thymol and biosynthesis of thymol

The term terpenoid, isoprenoid and terpenes are frequently used interchangeably [11]. Terpenes are hydrocarbons built from isoprene units joined in head-to-tail fashion, while the terpenoids are the oxygen-containing analogues of the terpenes. They are the most diversified group of plant-derived natural products, having a broad range of biological functions. Different types of terpenoids have been isolated from plants and animals [11, 12]. Terpenoids have also been classified based on the functional groups they contain, such as hydrocabons (limonene, Myrecene, α-pinene and β-pinene), aldehydes (geranial, neral), esters (bornile acetate, methyl salicylate), alcohols (menthol, bisabolol), ketones (camphor, thujone), lactones (aesculatine, citroptene), Phenols (limol, carvacrol), ethers (1, 8-cineol) and peroxides (ascaridol).

Thymol is a naturally occurring monoterpenoid (2-isopropyl-5-methylphenol). The primary active compounds of essential identified from Thymus vulgaris (Lamiaceae) is an isomer of carvacrol. Thymol is a potent antiseptic that have a wide range of applications in home/personal home care products such as mouthwash, hand sanitizer, and acne medicines [13]. Treatment of gastrointestinal and respiratory problems [14, 15, 16, 17]. Ocimum gratissimum, Trachyspermum ammi, Carum copticum, Oliveria decumbens, and Anemopsis californica are among the plants from which thymol has been extracted [18, 19, 20, 21, 22]. Thymol is suggested as a potential natural remedy in these various categories of recognized pharmaceutical uses [23]. Thymol has been used in formulations as an insecticide, fungicide, and medicinal disinfectant, among other non-pharmacological uses [24, 25]. Isopentenyl-pyrophosphate (IPP) and its isomer dimethylallyl-pyrophosphate (DMAPP), both of which are produced by the plastidic methyerythritol pathway and the cytosolic mevalonic pathway which are the five-carbon building blocks from which terpenes are biosynthesized.

The biosynthetic precursor of terpenes in the Mevalonic Acid (MVA) pathway of thymol biosynthesis (Figure 1) is acetyl-Coenzyme A, commonly known as activated acetic acid. It is similar to Claisen condensation and involves the coupling of two acetyl-CoAs to produce acetoacetyl-CoA, a biological equivalent of acetoacetate. In a process that resembles an aldol reaction, acetoacetyl-CoA joins with another equivalent of acetyl-CoA as a carbon nucleophile to produce -hydroxy-methylglutaryl-CoA (HMG-CoA). The next step is an enzymatic reduction of HMG-CoA with nicotinamide adenine dinucleotide in the presence of water, which produces mevalonic acid [26]. Mevalonic acid monophosphate is subsequently phosphorylated by adenosine triphosphate (ATP) to produce mevalonic acid diphosphate. Isopentenylpyrophosphate (IPP), a five-carbon atom intermediate product, is then produced by decarboxylation and drying out. Isomerase, an enzyme with SH group, reacts with the IPP to produce dimethylallylpyrophosphate (DMAPP), another intermediate molecule with five carbon atoms. Geranylpyrophosphate (GPP), a precursor to monoterpenes, is produced when the electrophilic allylic CH₂ group of dimethylallylpyrophosphate (DMAPP) and the nucleophlic methylene group of isopentenylpyrophosphate (IPP) mix. Thymol and its isomer, carvacol, are produced by further reactions and rearrangements inside geranylpyrophosphate (GPP) [26].

Figure 1.

Biosynthesis of thymol via the mevalonate pathway.

Throughout Northern and Southern Nigeria, A. flagellaris (Kunth) Bak., an aromatic herb, is found [27]. Asparagaceae is its family. It has arching spiny branchlets and grows to a height of roughly 1 m as seen in Figure 1 [28]. It is ascendant, generally erect plant. Africa as a whole uses it for a wide range of medical purposes. In both the guinea worm remedy and the ointment for hair development, the branchlets (cladodes) comprise the major component [27]. Gonorrhea, syphilis and other sexually transmitted diseases (STDs), as well as diarrhea and urinary infections, are all treated using the stem and leaves [27]. The roots are employed in Unani medicine as laxatives, tonics, aphrodisiacs, galactagogues, and treatments for renal and liver problems [29]. This herb was used to treat cholera, rheumatism and diuresis by the ancient Greeks, Romans, Indians and Chinese [30]. Indians utilized it to enhance fertility, relieve menstruation cramps, and boost nursing mothers’ milk production. It works to boost kidney cellular activity, which in turn speeds up urine production. Chinese pharmacists store sparagus roots for their loved ones because they think it will make them more compassionate and loving [30]. A. flagellaris’ stem bark and leaves were subjected to a phytochemical screening, which revealed a sizable quantity of flavonoid and a modest amount of carbohydrate, as well as trace amounts of ketones, pentose, and cardiac glycoside [28]. Mshelia et al. [30] reported the inhibition of six microorganisms viz. Corynebacteria, Escherichia coli, Klebsiella, Shigella dysenteriae, Neisseria gonorrheae and Candida albicans at different molarity of A. flagellaris ethanol extract, while the aqueous extract was susceptible to five organisms namely Streptococcus pyogenes, Corynebacteria, Neisseria gonorrhoeae Proteus spp. and Treponema pallidum.

2. Chemical constituents of A. flagellaris

The yield (w/w %) of the essential oils extracted from A. flagellaris leaves and roots using the hydrodistillation method was only 0.80 and 0.44%, respectively, based on the dry weight of the plant samples [31, 32]. The volatile oils had a distinctive aroma of thyme. The GC-MS technique was used to examine the chemicals found in the essential oils of A. flagellaris leaves and roots. In Tables 1 and 2, the detected ingredients and the total ion concentration (TIC) in % were summarized. Twenty-eight (28) and nineteen (19) compounds in total, accounting for 97.41 and 97.03% of the oil, respectively, were discovered using GC-MS analysis of leaves and root EOs. According to reports, the EOs are a blend of terpenes, terpene derivatives, non-terpenes and thymol and its derivatives.

S/N	Compounds identified	TIC (%)
1	1-(2,5-dimethoxyphenyl)-Ethanone	0.35
2	2’-Hydroxy-1′-acetonaphthone	1.77
3	2-isopropenyl-(+)-2-carene	0.24
4	8,9-dehydro-4-hydroxy thymol methylether	1.49
5	Cis-β-Farnesene	0.26
6	Durenol	3.29
7	Euparin	0.64
8	Isophosphinoline	0.76
9	n-octylesterundecanoic acid	0.98
10	Thymol methyl ether	25.8
11	Thymolhydroquinone dimethylether	12.72
12	Thymolisobutyrate	4.33
13	Thymyl tiglate	34.73
14	Thymyl-2-methylbutyrate	5.47
15	α-Bisabolol	1.36
16	α-Humulene	0.37
17	α-Santalene	0.8
18	β-Bisabolene	0.88
19	δ-Cadinene	0.79
	Total	97.03

Table 1.

Constituents of A. flagellaris roots EO.

S/N	Compounds identified	TIC (%)
1	Thymol methyl ether	9.42
2	Bornyl acetate	0.44
3	Thymol	1.71
4	Thymol hydroquinone dimethyl ether	10.52
5	Germacrene D	3.83
6	β-Copaene	0.87
7	β-Bisabolene	0.92
8	δ-Cadinene	2.69
9	cis-Farnesol	1.51
10	ϒ-Cadine-1,4-dinene	0.37
11	α-Muurolene	0.83
12	Valencene	1.39
13	Thymyl-2-methylbutyrate	17.34
14	α-Patchoulene	2.13
15	Thymyl tiglate	18.49
16	6-(3-Isopropenyl-2-methyl-1-cyclopropen-1-yl)-6-methyl-2-heptanol	3.4
17	7-epi-cis-sesquisabinene hydrate	5.46
18	ϒ-Muurolene	2.01
19	Germacra-4(15),5,10(14)-trien-1α-ol	0.31
20	8,9-Dihydro-cycloisolongifolene	0.39
21	2-Ethylbenzimidazole	1.64
22	3-Chloro-4-fluorophenol	2.91
23	5-Propyl-1,3-benzodioxole	4.59
24	Longiverbenone	2
25	2-Allyl-p-cresol	0.37
26	(E)-Cinnamyl alcohol	1.22
27	Phytol	0.75
28	8-Cedre-13-ol	0.31
	Total	97.82

Table 2.

Constituents of A. flagellaris leaves EO.

Thymol and its derivatives made up the majority of the leaves essential oil (EO), which included 57.48% of it, along with its derivatives (thymol-2-methyl butyrate, 17.34%, 5-thymyl tiglate, 18.49%, thymol hydroquinone dimethyl ether, 10.52%). Additionally, the leaves essential oil (EO) has 12.99% oxygenated sesquiterpenes of therapeutic significance, including 7-epi-cis-sesquisabinene hydrate, 6-(3-isopropenyl-2-methyl-1-cyclopropen-1-yl)-6-methyl-2-heptanol and others. Germacra-4(15), 5,10(14)-trien-1-ol, cis-Farnesol, 8-Cedre-13-ol and Longiverbenone. 15.04% hydrocarbon sesquiterpenes (β-Copaene, Germacrene D, ϒ-Muurolene,δ-Cadiene, β-Bisabolene, α-uurolene, ϒ-Cadine-1,4-diene, Valence, 8,9-dihydro-cycloisolongifolene and α-Patchoulene), 6.62% oxygenated monoterpenes (5-Propyl-1,3-benzodioxol, Bornyl acetate, (E)-Cinnamyl alcohol and 2-Allyl-cresol), 0.75% oxygenated diterpene (phytol) and 4.55% non-terpenes/non-terpenoids.

Nevertheless, the majority of the EO’s roots (84.54%) were thymol derivatives (thymol hydroquinone dimethylether, 12.72%, thymol methyl ether, 25.80%, thymyl-2-methylbutyrate, thymol isobutyrate, 8, 9-dehydro-4-hydroxy thymol dimethylether, 5.4% and thymyl tiglate, 34.73%). 1.36% oxygenated sesquiterpenes (Bisabolol), 3.10% oxygenated monoterpenes (1-(2, 5-dimethoxyphenyl and Durenol)-Ethanone), 3.10% hydrogen sesquiterpenes (Santalene, Cis-Farnesene, Bisabolene and Humulene), and 10.99% non-terpenes/non-terpenoids. However, thymol derivatives were shown to be the main component of the EOs of A. montana’s subsurface organs [33, 34].

3. Therapeutic application of A. flagellaris essential oils

Antioxidants of renown, thymol and its derivatives also have intriguing inhibitory potential against some bacteria [31, 32]. BHA and Ascorbic acid (93.09–90.40%) were competing with the essential oils of A. flagellaris leaves and roots, which had a substantial scavenging power from 91.04–88.06% and 90.04–50.73%, respectively (92.60–90.68%). Also noted was the concentration dependence of the antioxidant properties. The chemical compounds of the essential oil, thymol and its derivatives, which make up the majority of the mixture, may be accountable for the antioxidant property seen. However, the analysis of variance revealed no distinction in antioxidant between the essential oil of the leaves and the benchmarks that was significant (P < 0.05).

The essential oils from the leaves and roots of A. flagellaris were examined for its antibacterial properties against 10 pathogenic organisms at various concentrations (1000–62.5 μg/mL) in comparison to reference standards (gentamicin and tioconazole). The antibacterial ability of the essential oils were evaluated by measuring the inhibitory zones surrounding the well. The antibacterial activity revealed that none of the examined organisms were inhibited by the essential oils at 1000 μg/mL. R. spp. had the lowest inhibition at 250 μg/mL. (10 mm). The bactericidal capabilities of A. flagellaris leaves and roots essential oil were substantially different (P < 0.05) from reference standards (gentamicin and tioconazole). While only a small amount or lower concentration of the oil is needed to attack a active site in the organism, a high concentration will cause accumulation and blockage of the sensitive site and, as a result, have no effect [16]. This may explain why the oils are insensitive to the tested microorganisms at a higher concentration (1000 μg/mL).

Also, it is assumed that the protein bound at higher concentrations generating resistance to the tested microorganisms, and this was diminished as the concentration of the oil reduced. Furthermore, it has been stated that an antimicrobial’s microbiologic activity is restricted to its non-protein-bound portion [31]. The interactions between the compounds present in the essential oils of the plant’s leaves and roots could explain the study’s findings regarding the degree of antibacterial activity. Therefore, these chemical elements may affect a number of bacterial cell target locations [35]. The essential oil’s function might be regarded to as a “bacteriostatic antibiotic” since it prevents bacteria from multiplying by inhibiting bacterial protein synthesis, DNA replication, or other processes involved in cellular metabolism. This class of drugs is known as bacteriostatic antibiotics, and it includes tetracyclines, sulfonamides, spectinomycin, and others [35]. This result is in line with earlier findings on leaf extracts [6] and Marchese et al. [20] claims that all thyme essential oils are bacteriostatic.

However, those from the leaves and roots of A. flagellaris with noteworthy chemical compositions, such as thymol methyl ether, bornyl acetate, thymol β-bisabolene, were also found to be the most common ingredients in the root essential oil of Chaerophyllum villosum [36, 37]. Significant antioxidant and antibacterial activities were significantly reduced by these components. Germacrene D, farnesol, bornyl acetate and β-bisabolene have all been suggested as the possible causes of the antibacterial and antioxidant effects of Eupatorium adenoporum essential oil [38]. All yeasts and filamentous fungi (F. oxysporum f. sp. albedinis and M. ramanianus) were responsive to Thymol, Thymol Methyl Ether, α-Muurolene, Germacrene D, and α-Muurolene in Thymus fontanesii [39]. Thymol and thymol methyl ether were two of the key components of Thymus vulgaris essential oil that have antioxidant and antibacterial effects [40].

The essential oil’s comparatively large content of thymol derivatives may be responsible for the majority of the reported therapeutic advantages of A. flagellaris in folk medicine. The literature has in-depth descriptions of strong thymol derivative antiseptic, anti-inflammatory, antibacterial, antifungal, flavoring and antispasmodic properties [38]. Other minor essential oil constituents are of phytochemical relevance because of their combined impact in suppressing the biological activities observed in this study.

4. Conclusion

The essential oils of the leaves and roots of A. flagellaris and their chemical makeup, antioxidant and antibacterial capabilities. The oil samples from the leaves and roots, respectively, contained 28 and 19, mostly oxygenated monoterpenes, oxygenated sesquiterpenes, hydrocarbon sesquiterpenes, oxygenated diterpenes and thymol and its derivatives. The essential oil shown significant antioxidant activity when compared to common medicines. The essential oil at a concentration between 500 and 125 μg/mL exhibited moderate activity against all the tested microorganisms, according to the inhibition zones (10–18 mm) obtained from the antimicrobial assay, in comparison to the standard drugs (gentamicin; 10 μg/mL and tioconazole; 0.7 mg/mL), which gave 26-40 mm inhibition zones. The chemical elements and biological activity of the A. flagellaris leaves and roots essential oil support the ethnomedicinal usage of this plant for treating syphilis, gonorrhea and other sexually transmitted disorders.

Acknowledgments

The authors appreciate the Federal University of Petroleum Resources in Effurun, Delta State, Nigeria, and University of Ibadan, Nigeria, respectively, for providing the laboratory space and equipment necessary for the extraction and analysis of the essential oils.

Conflict of interest

Oluwakayode O. Odeja, Michael Gabriel Ibok and Ejike O. Okpala declare that they have no conflict of interest.

Acronyms and abbreviations

EOs	essential oils
TIC	total ion concentration
STDs	sexually transmitted diseases
BHA	butyl hydroxy anisole
MVA	mevalonic acid