Chemical Structure and Biological Activities of Secondary Metabolites from Salicornia europaea L.

Salicornia europaea L. is a halophyte that grows in salt marshes and muddy seashores, which is widely used both as traditional medicine and as an edible vegetable. This salt-tolerant plant is a source of diverse secondary metabolites with several therapeutic properties, including antioxidant, antidiabetic, cytotoxic, anti-inflammatory, and anti-obesity effects. Therefore, this review summarizes the chemical structure and biological activities of secondary metabolites isolated from Salicornia europaea L.


Introduction
Salicornia europaea L., also known as Salicornia herbacea L., is a halophyte belonging to the Chenopodiaceae subfamily with many common names including glasswort, sea beans, sea asparagus, and samphire [1]. As implied by its names, this edible plant tolerates up to 3% salinity [2] and grows in salt marshes and muddy seashores in temperate and subtropical regions worldwide, including the western and southern coasts of Korea [3]. In Asia, S. europaea has been traditionally used as a traditional medicine for constipation, nephropathy, hepatitis, diarrhea, obesity, and diabetes, among other disorders [4,5]. Moreover, in addition to using S. europaea for glass making, its aerial parts are used in salads, pickles, fermented food, and salt substitutes [6,7]. Therefore, due to the many health benefits of S. europaea, several studies proposed the development of this halophyte as a functional food and medicinal plant. Importantly, S. europaea crude extracts reportedly possess several therapeutic properties. For instance, the presence of immunomodulatory compounds in crude extracts was demonstrated via RAW 264.7 macrophage cell line assays [8]. Moreover, the antioxidant activity of S. europaea was measured via the 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging, hydroxyl radical scavenging, and reactive oxygen species (ROS) generation assays [9]. The antihyperglycemic and antihyperlipidemic activities of this plant were also demonstrated in mice fed with a high-fat diet [10] and another study reported that a polysaccharide extract from this plant exhibited anti-inflammatory activity in vitro and in vivo [11].
In addition to the many studies that have characterized the bioactivity of crude S. europaea extracts or individual fractions, many bioactive secondary metabolites have been isolated from this plant. Therefore, this review sought to systematically classify the secondary metabolites isolated from S. europaea according to their chemistry and summarize their biological activities. Most of the biological activities of the secondary metabolites discussed herein were taken from separate studies. Many studies generally classify glasswort-like discussed herein were taken from separate studies. Many studies generally classify glasswort-like species as S. europaea due to the difficulties of taxonomic identification of the genus Salicornia. Therefore, it is worth noting that this manuscript is by no means a comprehensive review of all S. europaea studies, and instead focuses on representative examples.

Caffeoylquinic Acid Derivatives
Caffeoylquinic acid (CQA) derivatives have been reported in many plants including coffee beans, and their various biological activities include antioxidant, antibacterial, anticancer, and antihistaminic effects [49] (Figure 2).
In 2005, Chung et al. isolated and determined the structure of a new natural chlorogenic acid derivative, tungtungmadic acid (3-caffeoyl-4-dihydrocaffeoyl quinic acid, 22) [50]. The plant materials were collected from Busan, in the southern coast of Korea. In this study, tungtungmadic acid displayed a strong antioxidant activity in both DPPH free radical scavenging and iron-induced liver microsomal lipid peroxidation inhibitory assays, with IC 50 values of 5.1 and 9.3 µM, respectively ( Table 2). Studies have also reported that compound 22 can protect plasmid DNA from hydroxyl radical-induced strand breakage. Several other studies on the biological activities of compound 22 have been conducted since its isolation. For instance, compound 22 has also been reported to provide protection against carbon tetrachloride (CC1 4 )-induced hepatic fibrosis and tert-butyl hydroperoxide (t-BHP)induced hepatotoxicity [50,51]. Moreover, this compound possesses anti-inflammatory properties [52], inhibits tumor cell invasion [53], and prevents high-glucose-induced lipid accumulation in human HepG2 cells [54]. Interestingly, the occurrence of compound 22 has not been reported in other sources ( Table 2).

Caffeoylquinic Acid Derivatives
Caffeoylquinic acid (CQA) derivatives have been reported in many plants including coffee beans, and their various biological activities include antioxidant, antibacterial, an ticancer, and antihistaminic effects [49] (Figure 2). In 2005, Chung et al. isolated and determined the structure of a new natural chloro genic acid derivative, tungtungmadic acid (3-caffeoyl-4-dihydrocaffeoyl quinic acid, 22 [50]. The plant materials were collected from Busan, in the southern coast of Korea. In thi study, tungtungmadic acid displayed a strong antioxidant activity in both DPPH free rad  In 2015, two new (27,28) and four known caffeoylated quinic acids (23, 24, 29, 30) were isolated from S. europaea, and their potential to alleviate high mobility group box 1 (HMGB1)-mediated vascular barrier disruption was evaluated in vitro and in vivo [75]. In this study, farm-raised plant material was obtained from Shinan, which is located on the southern coast of Korea. The new compounds were identified as 3-O-caffeoyl-5-Odihydrocaffeoyl quinic acid (27) and 4,5-di-O-dihydrocaffeoyl quinic acid (28), along with the known compounds 1,3-di-O-caffeoyl quinic acid (29) and 3,5-di-O-dihydrocaffeoyl quinic acid (30). According to this study, compounds 23, 27, 29, and 30 showed vascular protective activities against inflammatory responses induced by HMGB1 in both cellular and animal models. Compound 29 has also been reported to possess antioxidant and cytoprotective effects [76]. This study also showed that the positions of two caffeoyl groups were closely related to their activity. Among the di-O-caffeoylquinic acid compounds, entities with adjacent caffeoyl moieties exhibited better performance than their non-adjacent configured counterparts. In 2016, Cho et al. reported the isolation of three known (27,31,32) and three new caffeoylquinic acid derivatives (33)(34)(35) from methanol extracts of S. europaea collected from Younggwang, in the southwestern coast of Korea [77]. The known compounds included 3-caffeoylquinic acid (31) and 3-caffeoylquinic acid methyl ester (32), and the new compounds were established as 3-caffeoyl-5-dihydrocaffeoylquinic acid methyl ester (33), 3-caffeoyl-4-dihydrocaffeoylquinic acid methyl ester (34), and 3,5-di-dihydrocaffeoylquinic acid methyl ester (35). In this study, all six compounds scavenged DPPH radicals and inhibited CE-OOH formation. The dicaffeoylquinic acid derivatives (27,(33)(34)(35), which have two catechol groups, showed higher activities than the mono-caffeoylquinic acid derivatives (31, 32) and caffeic acid did. Additionally, compound 31 extracted from leaves of Moringa oleifera reportedly possesses moderate influenza A neuraminidase inhibitory activity [78]. This compound also exerts neuroprotective properties via the inhibition of pro-inflammatory responses in activated microglia [79].

Flavonoids and Flavanones
Flavonoids and flavonoid glycosides have also been isolated from S. europaea ( Figure 3). In 1982, Arakawa et al. reported the isolation and structural elucidation of 2 -hydroxy-6,7-methylenedioxyisoflavone (36), (−)-(2S)-2 -hydroxy-6,7-methylenedioxyflavanone (37), and 2 ,7-dihydroxy-6-methoxyisoflavone (38) from a methanol extract of S. europaea [80]. The plants for this study were collected from lake Notoro, which is a coastal lagoon by the northern shore of Hokkaido, Japan.   [81]. A cherry blossom derived compound (39) reportedly acted as a potent suppressor of the production of advanced glycation end products (AGEs) and fibroblast apoptosis by AGEs [82]. This quercetin isolated from the leaves of Corchorus olito-   (41), rutin (42), and isorhamnetin 3-O-β-D-glucopyranoside (43) from S. europaea collected from Loire-Atlantique, by the Bay of Biscay, France [81]. A cherry blossom derived compound (39) reportedly acted as a potent suppressor of the production of advanced glycation end products (AGEs) and fibroblast apoptosis by AGEs [82]. This quercetin isolated from the leaves of Corchorus olitorius or the fruit peel of Sicana odorifera acted as an antioxidative agent [83,84]. A broad range of biological effects has been reported for compounds 40-42, including anti-inflammatory, antidiabetic, cardiovascular protection, and anticancer effects [85][86][87]. Compound 43, isolated from S. europaea, has been reported to be a potential agent for the prevention and/or treatment of diabetes [88] and a chemo preventive agent for cancer [89], as well as an anti-oxidant [90] and anti-obesity agent (Table 3) [91].  [92]. However, the activity of compound 43, which contains a methoxy group on the flavonoid B ring, was lower than the activity of compound 40.
In 2011, Kim et al. reported the isolation and antioxidant activities of a novel flavonoid glycoside, isoquercitrin 6"-O-methyloxalate (44), along with the known compounds 41 and 43 [6]. Compounds 41 and 44, which have no substitutions on their B rings, exhibited significant antioxidant activities, whereas the antioxidant activity of compound 43 was comparatively less potent. These results agreed with previous reports that the catechol group of the B ring plays an important role in determining the antioxidant activities of flavonoids [93,94].
Irilin B (51) was recently identified and isolated from S. europaea via antioxidant activity-guided isolation and purification [100]. Compound 51 exhibits a good antioxidant and anti-neuroinflammatory potential. Moreover, compound 51 derived from Chenopodium procerum, an African medicinal plant, reportedly displayed antifungal activity against the plant pathogenic fungus Cladosporium cucumerinum [101]. Interestingly, another study reported the estrogenic activity of this compound from Iris songarica [102].

Chromones
Chromones are benzoannelated γ-pyrone heterocycles that are widely found in nature, particularly in plants ( Figure 4). Several pharmacological properties of chromones, including their anti-allergic, anti-inflammatory, antidiabetic, antitumor, and antimicrobial effects, have been identified thus far [103,104]. astragalin, is found in a wide range of medicinal plants such as wild garlic, tea, Chinese bittersweet, and sundew [99]. Irilin B (51) was recently identified and isolated from S. europaea via antioxidant activity-guided isolation and purification [100]. Compound 51 exhibits a good antioxidant and anti-neuroinflammatory potential. Moreover, compound 51 derived from Chenopodium procerum, an African medicinal plant, reportedly displayed antifungal activity against the plant pathogenic fungus Cladosporium cucumerinum [101]. Interestingly, another study reported the estrogenic activity of this compound from Iris songarica [102].
Most recently, Tuan et al. isolated a new naturally occurring chromone, 7-hydroxy-6,8dimethoxychromone (56) along with compound 53 and 6-methoxychromanone (57) from farm-raised S. europaea [95]. In this study, compounds 53, 56, and 57 inhibited the release of HMGB1, which resulted in improved survival rates of CLP murine models. However, bioactivity study of these chromes are relatively unexplored, as shown in Table 4.
In 2004, Lee et al. isolated and identified compounds 58 and 59 [108]. In this study, plant samples were collected from Mokpo, on the southwestern coast of Korea. Compound 58 has been shown to possess anti-inflammatory, anticancer, hypocholesterolemic, immunomodulatory, antioxidant, neuroprotective, and antidiabetic effects (Table 5) [109]. Moreover, this compound is among the predominant phytosterols in the human diet, along with campesterol and stigmasterol. Furthermore, compound 59 displays anti-osteoarthritic, anti-hypercholesterolemic, cytotoxic, antitumor, hypoglycemic, antioxidant, antimutagenic, and anti-inflammatory properties (  [36]. Compounds 60 and 62 derived from the edible mushroom Cantharellus cibarius have been found to possess potent NF-κB inhibitory activities (Table 5) [112]. However, compound 62 isolated from Agaricus blazei, also known as almond mushroom, has been reported to exert cytotoxic effects [113]. This compound was also isolated from another mushroom genus Trametes and was found to exhibit antimicrobial effects, as well as antibiotic resistance modifying activity [114]. Compound 61 exhibits immunoregulatory [115], anti-inflammatory [116], and anticancer properties [117], and reportedly promotes the proliferation of neural stem cells (Table 5) [118]. O 1 Qualitative bioassay study reported without quantitative data was marked with O.
The pheophorbide compounds 87-89, which are derivatives of chlorophyll a, have been isolated from a methanol extract of S. europaea [32]. Pheophorbide A (87) exhibits a strong antiproliferative activity against A549 and HepG2 cancer cell lines with IC 50 values of 6.15 and 17.56 µM, respectively. (13 2 S)-Hydro-pheophorbide-lactone A (89) has been shown to possess a weak antioxidant activity, with a ferric reducing/antioxidant power (FRAP) value of 79.58 ± 1.69 mM/100 g and a DPPH scavenging rate of 75.33 ± 1.61%.
Compound 87 also possesses antitumor [164][165][166], photodynamic [167][168][169], and anti-inflammatory activities [170]. (13 2 S)-Hydroxy-pheophorbide A (88) exhibits potent photocytotoxicity, but its cytotoxic activity was reportedly lower than that of compound 87 [164,165]. However, compound 88 has a better anti-plasmodial performance than that of compound 87 [171]. The presence of a hydroxyl group at C-13 in compound 88 is the only structural difference between compounds 87 and 88. Therefore, the hydroxyl substitution at the C-13 position might be responsible for the differences in the activities of these compounds.
In addition to the above-described compounds, S. europaea seeds have been reported to possess a wide range of fatty acids including compounds 69-73 [172].

Conclusions
Salicornia europaea is a popular salt-tolerant plant that has been traditionally used both as a functional food and vegetable seasoning; however, this plant is also known to produce compounds with therapeutic potential. Therefore, this plant is among the most widely recognized halophytes and is farmed in some regions to meet consumer demand.
This review discussed the chemistry and biological activities of S. europaea secondary metabolites reported from 1978 to October 2019. To the best of our knowledge, eighty-nine metabolites have been isolated, including oleanane triterpenoid saponins, caffeoylquinic acid derivatives, flavonoids, chromones, sterols, lignans, and aliphatic compounds. The diverse biological/pharmacological activities of the isolated compounds were also described in this review. Most of the compounds were obtained in small quantities ranges from 0.1 to 10 ppm (Table 9). Only a handful of the isolates were obtained over hundreds of ppm, including 1, 39, and 41 (123,700, and 467 ppm, respectively). However, attention to the direct comparison of the yields is required as these high yields have resulted from dried plant material extraction. Most of the plant samples discussed herein were collected from East Asia, including Korea, Japan, and China, where this plant has been historically used as food and for its therapeutic properties. Nonetheless, research on this plant is not strictly limited to Asia, but includes some regions of Europe as well. Moreover, the study of secondary metabolites from the genus Salicornia encompasses other temperate and subtropical regions worldwide, including America and Africa.
Previous studies on S. europaea have mainly focused on the identification of its secondary metabolites but often fail to provide other details. Specifically, most studies provide only basic descriptions of the collection sites and dates. However, the unique characteristics of this plant, including its seasonal color change, jointed segments, and scale-like stout leaves allow for its easy identification, which facilitates the isolation and identification of the enormous repertoire of secondary metabolites from this plant compared to that of other halophytes.
This review provides important insights that may facilitate the future study of the chemical profiles of this plant. For example, tungtungmadic acid (22) was exclusively isolated from the plant S. europaea and would be a good marker to identify this plant. Moreover, the chemical profile patterns of other secondary metabolites would provide useful references for chemists to identify and study this plant species. Therefore, we expect that future biochemical analyses of S. europaea and other halophytes will lead to the discovery of novel bioactive natural products.