Teucrium Taxa Used in Kurdistan Region: History
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Contributor:
Faiq Hussain
,
Gianluca Gilardoni
,
,
Zaw Min Thu
,
Giovanni Vidari

Herbal medicine is still widely practiced in the Kurdistan Region, Iraq, especially by people living in villages in mountainous regions. Seven taxa belonging to the genus Teucrium (family Lamiaceae) are commonly employed in the Kurdish traditional medicine, especially to treat jaundice, stomachache and abdominal problems. Teucrium L. is the second-largest genus of the subfamily Ajugoideae in the family Lamiaceae (Labiatae), with a subcosmopolitan distribution and more than 430 taxa with accepted names. Mild climate regions, such as the Mediterranean and the Middle East areas, contain about 90% of the total Teucrium taxa. 

  • Teucrium
  • Kurdish traditional medicine
  • bio-active secondary metabolites

1. Phytochemical Aspects

A survey of the literature reveals that more than 290 compounds have been identified in the essential oils and non-volatile extracts from the Teucrium taxa used in the folk medicine of the Kurdistan Region, Iraq (Table 1). They include mono-, sesqui-, di- and triterpenoids, steroids, flavonoids, phenylethanoid glycosides, etc.
Table 1. Teucrium taxa used in the traditional medicine of the Kurdistan Region, Iraq.
Botanical Name Traditional Uses in the Kurdistan Region, Iraq Growth Places (KRI Districts)
T. chamaedrys L. Eaten as a digestive Baradost, Mosul, Duhok, Gara, Sirsang, Sharanish, Zawita, Khantur, Atrush, Suwara Tuka, Ser Amadiyah, Qara Dagh
T. melissoides Boiss. & Hausskn. ex Boiss. To treat abdominal diseases Qandil, Pushtashan, Rowanduz, Avroman, Zawita
T. oliverianum Ging. ex Benth. Antidiabetic remedy Kirkuk, Hamrin
T. parviflorum Schreb. To treat jaundice, liver disorders, stomachache and to reduce cholesterol level in blood Amadiyah, Sulaimani, Rowandruz
T. polium L. Antirheumatic and to treat abdominal pain Arbil, Safin, Shaqlawa, Kirkuk, Jarmo, Mosul, Sharanish, Suwara Tuka, Zakho, Khantur, Gara, Atrush, Sirsang
T. rigidum Benth. To treat abdominal problems Darband-i Bazian
Teucrium scordium subsp. scordioides (Schreb.) Arcang. (synonym T. scordioides Schreb.) Anti-inflammatory Safin, Baradost, Zawita, Gara
No phytochemical investigation has yet been dedicated to T. rigidum Benth. Essential oils (EOs) are the most analyzed phytochemical parts of the other taxa listed in Table 1, except the oil from T. oliverianum. EOs were isolated by conventional hydrodistillation in a Clevenger apparatus and were analyzed by standard GC-FID and GC-MS techniques. Both polar (Carbowax 20M) and non-polar (OV1 and SE 30) columns were used. EOs can, in general, be divided between those where the main components are sesquiterpene hydrocarbons, such as β-caryophyllene (166) and germacrene D (167), as the oil from T. parviflorum, and the oils where monoterpene hydrocarbons, such as α- (174) and β-pinene (175) predominate, as the oil from T. melissoides. There are significant qualitative and quantitative differences between the EOs of the different taxa and this variability is even intraspecific, which is probably due to the genetic, differing chemotypes, drying conditions, mode of oil distillation, extraction and/or storage, and geographic or climatic factors. The compositions of the EOs isolated from specimens of T. chamaedrys and T. polium collected in different countries, are typical examples of such variability [1]. The high content of menthofuran (188) in the EO from T. scordium subsp. scordioides collected in Serbia [2] indicates that this terpenoid can be considered a chemotaxonomic marker of the oil.
Regarding the structures of non-volatile secondary metabolites, those occurring in T. melissoides and T. parviflorum are still unknown and only a few studies have been dedicated to the contents in extracts from T. oliverianum and T. scordium subspecies scordioides. Instead, the phytochemical aspects of T. chamaedrys and T. polium have been subjected to several investigations. These studies have possibly been promoted by the worldwide occurrence and widespread medicinal uses of the two plants.
Among monoterpenoids, the presence of iridoids, as aglycones and glycosides, in T. chamaedrys, T. oliverianum and T. polium extracts, confirms the observation that they are chemotaxonomic markers of the Lamiaceae family and have been recognized in several genera of the Ajugoideae and Lamioideae subfamilies.
Non-volatile sesquiterpenoids 142–159 were so far isolated from only T. polium [3][4][5]. Most of them belonged to the eudesmane, cadinane and germacrane families.
Diterpenes include representatives of the abeo-abietane and neo-clerodane families. Compounds of the first group (129–141) have been isolated only from T. polium [6]; on the other hand, T. chamaedrys, T. oliverianum and T. polium are rich sources for neo-clerodane diterpenoids. Indeed, these compounds probably represent the most abundant family of specialized metabolites occurring in Teucrium taxa and are considered the chemotaxonomic markers of the genus [7]. The reason of the wide distribution and conservation of neo-clerodanes might be likely due to their potential allelopathic properties [8], a pronounced protective role against herbivore predators and the general antifeedant activity [7][9]. Thus, the last effects may have economic importance against Lepidopterous pests [10]. However, the lactone ring may be absent, as in 47–53 and even the furan moiety may be missing, as in syspirensin B (33). Except for teupolins VII (75) and VIII (76), the carbon C-6 is oxygenated. Other frequently oxygenated carbons are C-2, C-3, C-7, C-12, C-18, C-19 and, rarely, C-10, as in neo-clerodanes 45, 47, 50 and 51. Moreover, the presence of an epoxide, as in 23 and 38, an oxetane, as in 27, 34 and 67, a tetrahydrofuran, as in 47 and 81, a hemiacetal or an acetal, as in 26, 58, 60, 73, 79, a γ-lactone, as in 13, 16, 29, 34, 45, or a δ-lactone ring, as in 28, 83, 86, add to complicate the chemical structures of these diterpenoids.
The NMR data of representative Teucrium sesquiterpenes, neo-clerodane and abeo-abietane diterpenoids are thoroughly discussed in reference [9].
Readers must be aware that some ambiguities exist in the literature about the names and even the stereochemistry of a few neo-clerodane diterpenoids isolated from Teucrium. Typical examples are compound 16, teucvidin (18) and syspirensin A (32). These uncertainties depend on the fact that chemical structures were based mainly on the interpretation of NMR spectra, while only a few ones were firmly confirmed by X-ray analysis [11] or stereoselective total synthesis [12]. 
The group of sterols and triterpenoids comprises compounds widely distributed in plants, such as β-sitosterol, campesterol and oleanolic acid, but also novel triterpene saponins, poliusaposides A-C (163–165), that were isolated from a MeOH extract of T. polium aerial parts.
Flavonoids, in glycosidic and aglycone forms, are relatively abundant in T. chamaedrys and T. polium, while only a couple of compounds, 87 and 92, occurred in T. oliverianum extracts [13][14]. The most characteristic flavonoids are the flavones apigenin (90) and luteolin (95) and a small group of derivatives, including 4′- and 7-O-glycosides and some 6-methoxy flavones. It is worth noting that luteolin 7-O-β-D-(5-O-syringyl)apiofuranosyl-(1→2)-β-D-glucopyranoside (106), isolated from a MeOH extract of T. polium leaves, contains an unprecedented structure [15]. Other classes of flavonoids are rarely represented in the extracts. In fact, the only flavanone isolated so far has been naringenin (99), from T. chamaedrys subsp. chamaedrys, while the only isolated flavonol O-glycoside has been rutin (100), from extracts of T. polium [6][16]. A rare 6,8-di-C-glucoside, vicenin-2, was also isolated from T. polium [17].
Among other phenolic derivatives, only a couple of lignans (125 and 126) have been isolated [18]. This finding is quite interesting because lignans, phenylpropanoid dimers, are produced as a result of plant defense against stress. On the other hand, T. chamaedrys and T. polium are good sources of phenylethanoid glycosides, the verbascoside derivatives 107–117.

2. Bioactivity and Pharmacological Properties

Antioxidant and free radical-scavenging properties have been determined for most extracts and isolated phytochemicals described, including EOs [1][19], iridoid glycosides [20], abeo-abietanes [6], phenyl ethanoid glycosides [20][21] and flavonoids [4][15][20]. However, such antioxidant action has usually been evaluated through various standard in vitro assays, in cell-free systems, which included cupric reducing antioxidant capacity (CUPRAC) assay [4], DPPH scavenging (DPPH), reducing power (RP), xanthine oxidase inhibitory effect (XOI) and antioxidant activity in a linoleic acid system (ALP) [20]. Therefore, these evaluations have limited pharmacological relation and limit the validation of the established biological action. Moreover, the antioxidant activity of an extract may be due to a synergistic effect of the various components through different antioxidant mechanisms,
Antibacterial and other bioactivities of EOs have been described in detail in a previous review [1]. An interesting potential application of the antimicrobial power of EOs, for example that from T. polium, concerns the implementation as preservative additives in the food industry in order to fight microbial contaminations and development [1][22] and lipid oxidation [23].
A study of the relationship between structure and antioxidant effects has been performed on verbascoside (114) and derivatives. The activity varied, depending on the glycosylation and methylation patterns. It was observed that increasing sugar units with accompanying free-phenolic-hydroxyl pairs increased antioxidant activity, while hydroxyl methylation decreases this effect [4]. Thus, poliumosides 110 and 116 showed the highest antioxidant capacity. It was suggested that phenolic hydroxyl pairs form hydrogen bond between adjacent groups that can stabilize phenoxy radical intermediates. Instead, hydroxyl methylation destabilizes the intermediate by disrupting hydrogen bonding [4]. Noteworthy, compounds 114 and 116 with free ortho-dihydroxyl groups showed higher activity than the positive controls, trolox and α-tocopherol [4]. A similar trend was observed with the radical scavenging activity of a group of flavonoids isolated from T. polium [15][20]. Luteolin (95) and luteolin-based compounds with a free ortho-dihydroxy group in ring B elicited a massive reduction of the radical species, whereas luteolin 4′-O-glucoside (93) and apigenin (90) showed very low or no radical scavenging and reducing properties. This finding underlined that the structural feature responsible for the observed activity is the presence of a C-2–C-3 double bond, a carbonyl group at C-4 and, more importantly, a free ortho-dihydroxy (catechol-type) substitution in the flavone B-ring. The authors suggested that the formation of flavonoid phenoxy radicals may be stabilized by the mesomeric equilibrium to ortho-semiquinone structures [20]. In this regard, the higher scavenging and reducing activities of compounds 110 and 116, with respect to flavonoid glycosides, could be attributed to the presence of a second phenolic ring in the caffeoyl residue with an ortho-dihydroxy group [20]. Flavonoid aglycones showed more potent antiradical action than their corresponding O-glycosides in the A or B ring and a disaccharide moiety bound to the C-7 of the A-ring weakens the antiradical effect, as observed for luteolin 7-O-rutinoside (101) and luteolin 7-O-neohesperidoside (102), compared to luteolin 7-O-glucoside (103) and luteolin (95). In contrast to these findings, T. polium flavonoids 101–103 showed a lower xanthine oxidase inhibitory (XOI) activity than compound 93. It was suggested that the presence of a free C-4′ hydroxyl group in compounds 101–103 makes them more easily ionizable and, therefore, less able than flavonoid 93 to interact with the hydrophobic channel, which is the main access to XO active site [20].
Among other bioactive metabolites, the iridoid harpagide (1) exerted a wide number of biological activities such as cytotoxic, anti-inflammatory, anti-osteoporotic and neuroprotective effects [24]. Similarly, 8-O-acetylharpagide (2) showed vasoconstrictor [25], antitumoral, antiviral, antibacterial and anti-inflammatory activities. Notably, an inconsistent connection between anti-tumor and antioxidant/radical scavenging activity was observed for the iridoids 2 and teucardoside (3) and poliumoside (116), which resulted in varied effects on several cancer cell lines. This finding is consistent with some in vivo studies [4]. Moreover, iridoids 2 and 3 exhibited an extraordinary ability to inhibit lipid peroxidation [20]. It was suggested that the presence of a free hydroxyl group on C5 of the iridoid 2 could be responsible for the higher antioxidant ability than compound 3 [20].
The saponin glycosides poliusaposides A-C (163–165) were evaluated for anticancer effects by a National Cancer Institute 60 human tumor cell line screen (http://dtp.nci.nih.gov/branches/btb/ivclsp.html, latest accessed on 24 April 2022) [26]. Poliusaposide C (165) completely inhibited the growth of a breast (MDA-MB-468) and colon cancer line (HCC-2998) and partially inhibited the growth of a colon (COLO 205), renal (A498) and melanoma cancer (SK-MEL-498) cell line [26]. The other two saponins were considerably less active. By analogy with previously reported bidesmosidic saponins, it was suggested that the increased activity of 165, compared to 163 and 164, was linked to the presence of multiple apiose units, the apiose branching in the oligosaccharide moiety attached to C-28, the difference in glycan chain polarity and the increased aglycone hydroxylation, due to the reduction of the triterpenoid carboxylic acid group to a primary alcohol [26].
Verbascoside (114) exhibited anti-inflammatory, immunosuppressive, anti-infective and protein kinase C inhibitory properties [27]; moreover, compound 114 and forsythoside B (111) exerted strong neuroprotective and antiseptic effects [28][29].
The hypoglycemic effect of T. polium has been accounted for by its constituents that increase insulin release [30]. In this context, the effect of the major flavonoids occurring in T. polium extract, rutin (100) and apigenin (90), on insulin secretion at various glucose concentrations was investigated [31]. The two flavonoids demonstrated protective effects on β-cell destruction in a model of streptozotocin-induced diabetes, due to the antioxidant activity [31]. Among other flavonoids isolated from T. polium, cirsiliol (87) showed good relaxant, sedative and hypnotic effects [28]. Moreover, 3′,4′,5-trihydroxy-6,7-dimethoxyflavone and 5,6,7,3′,4′-pentahydroxyflavone, carrying a 3′,4′-dihydroxy B-ring pattern, showed an interesting inhibitory activity against the biofilm-forming Staphylococcus aureus strain AH133. It was suggested that due to the antibacterial activity, these flavonoids can potentially be used for coating medical devices such as catheters [18]. Antibacterial activity of the crude extract of T. polium, as well as of isolated flavonoid salvigenin (94) and sesquiterpenoids 151 and 154, was observed with Staphylococcus aureus anti-biofilm activity in the low μM range [5]. It should be noted that biofilm formation is a physical strategy that bacteria employ to effectively block the penetration and toxicity of antibiotics. Thus, blocking or retarding the formation of biofilms improves the efficacy of antibiotics.
Shawky in a network pharmacology-based analysis showed that T. polium had potential anti-cancer effects against A375 human melanoma cells, TRAIL-resistant Huh7 cells and gastric cancer cells, likely due to luteolin (95) occurrence in the plant. In fact, luteolin inhibited the proliferation and induced the apoptosis of A375 human melanoma cells by reducing the expression of MMP-2 and MMP-9 proteinases through the PI3K/AKT pathway [32].
Moreover, it was found that the aqueous extract of T. polium aerial parts, given intraperitoneally, reduced significantly the serum levels of cholesterol and triglycerides in hyperlipidemic rats. It was suggested that the presence of flavonoid and terpenoid constituents may play a role in the observed hypolipidemic effects [33].
Although many crude extracts or partially purified fractions from Teucrium have showed various beneficial biological and pharmacological effects, the use of herbal remedies prepared from Teucrium plants should be considered with caution unless the safety has been demonstrated by rigorous scientific evidence. A paradigmatic example is T. chamaedrys, commonly known as ‘germander’, which has long been used as dietary supplement for facilitating weight loss or as a hypoglycemic aid. However, acute and chronic hepatitis and even fatal cirrhosis were observed in patients who had consumed the plant as a tea for 3–8 weeks [34][35]. Subsequently, similar hepatotoxicity was observed with other members of the Teucrium genus, including the widely used T. polium [36][37]. These toxic effects have mainly been associated with the presence of abundant neo-clerodane diterpenoids, especially teucrin A (19) [38]. In fact, Lekehal and collaborators, while studying the in vivo hepatotoxicity of teucrin A (19) and teuchamaedryn A (20), as well as the furano diterpenoid fraction of T. chamaedrys, demonstrated that the furan ring of neo-clerodane diterpenoids is bioactivated by CYP3A (cytochrome P450 enzymes) into electrophilic metabolites that covalently bind to hepatocellular proteins, deplete GSH and cytoskeleton associated protein thiols and lead to formation of plasma membrane blebs and apoptosis in rat hepatocytes [36][39]. 1,4-Enedials or possible epoxide precursors are the likely reactive toxic metabolites [37]. In contrast to these findings, 18 neo-clerodane diterpenes containing a 3-substituted furan ring, isolated from T. polium, showed low toxicity at the highest test concentration (200 μM) against HepG2 human cells, which provided a useful model to study the function of the CYP3A4 enzyme [40]. The authors of the study suggested that a high concentration of neo-clerodanes in the crude extract or synergistic effect of the neo-clerodanes with one another or with other phytochemicals present in the plant might produce hepatotoxic effects. Other lines of evidence implicate immune-mediated pathways in initiating liver injury. In other cases, autoantibodies were present [36][41].

This entry is adapted from the peer-reviewed paper 10.3390/molecules27103116

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