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Li, K.; Wu, F.; Chen, M.; Xiao, Z.; Xu, Y.; Xu, M.; Liu, J.; Xu, D. Identification of Secondary Metabolites in Medicinal Orchids. Encyclopedia. Available online: https://encyclopedia.pub/entry/47025 (accessed on 25 June 2024).
Li K, Wu F, Chen M, Xiao Z, Xu Y, Xu M, et al. Identification of Secondary Metabolites in Medicinal Orchids. Encyclopedia. Available at: https://encyclopedia.pub/entry/47025. Accessed June 25, 2024.
Li, Kunqian, Fengju Wu, Mengzhu Chen, Zhihao Xiao, Ya Xu, Mengwei Xu, Jingyi Liu, Delin Xu. "Identification of Secondary Metabolites in Medicinal Orchids" Encyclopedia, https://encyclopedia.pub/entry/47025 (accessed June 25, 2024).
Li, K., Wu, F., Chen, M., Xiao, Z., Xu, Y., Xu, M., Liu, J., & Xu, D. (2023, July 20). Identification of Secondary Metabolites in Medicinal Orchids. In Encyclopedia. https://encyclopedia.pub/entry/47025
Li, Kunqian, et al. "Identification of Secondary Metabolites in Medicinal Orchids." Encyclopedia. Web. 20 July, 2023.
Identification of Secondary Metabolites in Medicinal Orchids
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The secondary metabolites present in medicinal orchids are diverse and possess a vast array of biological activities. They represent valuable raw materials for modern pharmaceuticals and clinical medicine and have tremendous potential for future development. A systematic collation of secondary metabolites’ composition and a summary of the biological activities of medicinal orchids represent a crucial step in unlocking the potential of these valuable resources in drug development.

medicinal orchid metabolite biosynthesis chemical identification

1. Introduction

Orchids have a long history of medicinal use in China due to their cultivation for over 2000 years. The orchid family is diverse, and so far, over 28,000 species have been identified in 736 genera [1]. In the 2020 edition of the Chinese Pharmacopoeia, several species were recorded as Chinese herbal medicines (CHM). These include Gastrodia elata, Bletilla striata, Dendrobium nobile, D. huoshanense, D. chrysotoxum, D. fimbriatum, D. officinale, Cremastra appendiculata, Dendrobium spp., Pleione bulbocodioides and P. yunnanensis. In China, wild orchids have a long history as medicinal and ornamental plants. Due to their ability to treat diseases and enhance beauty and health, orchids have now become a new economic crop in agriculture. The development of orchids has resulted in several social benefits, particularly in promoting “green and natural” dietary options and daily essentials, including decoction pieces, oral liquids, beauty products, and skincare solutions. Moreover, the active elements identified in orchid plants can be leveraged to formulate new drugs. These genera include Bletilla, Anoectochilus, Liparis and several others. Research findings have shown that the types and quantities of the secondary metabolites present in CHM are known to play a significant role in their pharmacological effects and medicinal quality [2]. To effectively utilize medicinal orchid resources, it is particularly important to excavate their secondary metabolites.
In recent years, secondary metabolites have gained public attention due to their diverse biological activities and pharmacological effects, making them promising candidates for developing new drugs for stubborn diseases. The secondary metabolites found in medicinal orchids are small-molecule substances with various characteristics, including diversity, high specificity and rich biological functions. This makes them a potential source of highly specific active substances to treat stubborn diseases. However, research on medicinal orchids primarily focuses on species identification, classification, conservation and cultivation, as well as compound identification. More attention needs to be paid to the analysis and use of active substances derived from secondary metabolites, which are still in early stages. Despite their potential, medicinal orchids face natural limitations, such as their growth cycle, season, climate and natural accumulation of compounds, resulting in a situation similar to paclitaxel’s in which there is low market supply and high market demand. To address this, it is crucial to elucidate the active secondary metabolites and biosynthesis of medicinal orchid secondary metabolites leading towards resource development and utilization.

2. Identification of Secondary Metabolites

These compounds were first proposed by the German chemist Kossel in 1891, and they are small molecules synthesized by organisms in response to environmental stress and resistance. They serve as plant protectants that aid in disease resistance and defense against natural enemies [3][4]. The biological effects of secondary metabolites are the result of species evolution and adaptation to the environment. The secondary metabolites of medicinal orchids can be classified into three groups: terpenoids, phenols and nitrogen compounds, each of which contain tens of thousands of compounds. Furthermore, the production and distribution of these secondary metabolites are species specific and only occur in particular organs, tissues and developmental stages of the orchids [4]. For instance, D. nobile has the largest number of dendrobine types, whereas D. chrysotoxum has the highest content of moscatilin, with stem tissue being the primary medicinal component. The active substances in traditional Chinese medicine are the material basis for their pharmacological effects. Therefore, the identification and analysis of secondary metabolites are necessary prerequisites for the development and application of medicinal orchids’ active substances.

2.1. Alkaloids

Alkaloids are the most commonly found nitrogen-containing compounds in the secondary metabolites of medicinal orchids and an essential source of bioactive compounds. More than 50 species of orchids have yielded the discovery of over 140 alkaloids [5], which can be classified into various groups based on their structural characteristics, such as sesquiterpenes, indolizines, amides and indoles. In 2019, 52 alkaloid components were detected in 19 orchid species [6]. However, the alkaloid content in medicinal orchids is typically low, with only five out of thirty-five Dendrobium plants having an alkaloid content of over 0.1% [7]. This paper summarizes 27 newly identified alkaloids, whose chemistry structure are shown in Figure 1. It is worth noting that three new alkaloids were identified from D. nobile, including a new pair of amide tautomers [8]. Four new indolizine alkaloids were identified from D. crepidatum [9][10]. Furthermore, a pair of enantiomers and three new indolizine alkaloids have been discovered from D. crepidatum [11]. These twenty-seven alkaloids are listed in Table 1 and include eight sesquiterpene alkaloids (1–8), two amides (9–10), eleven indolizines (11–21), four indoles (22–25), a spiral-shaped alkaloid (26) and a Lycodine-type Lycopodium alkaloid (27).
Figure 1. The chemical structure of alkaloids found in orchids.

2.2. Phenanthrenes

Phenanthrenes are a class of substances with three benzene rings as the parent ring and are commonly found in medicinal orchids. Based on their structural properties, they can be classified into simple phenanthrene, dihydrophenanthrene (DHP), phenanthraquinone (ketone), phenanthrene furan and phenanthrene dimer. Table 1 summarizes the 39 phenanthrenes that were identified from medicinal orchids between 2018 and 2023 (for their chemical structures, refer to Figure 2). Notably, a new DHP trimer was identified from B. striata [12], and silk grass was found to contain six new DHP [13]. Furthermore, monolithic orchids were discovered to have four optically rotating phenanthraquinones [14], and two new phenanthraquinones were found in Dendrobium flowers [15]. Additionally, three secondary metabolites were identified for the first time from plants in the genus Bletilla [12]. These thirty-nine phenanthrenes are listed in Table 1 and include one DHP dimer (28), twenty DHP (29–48), seven simple phenanthrenes (49–55) and nine phenanthraquinones (56–64), as well as furans (65) and biphenanthrene (66).
Figure 2. The chemical structure of phenanthrene found in orchids.

2.3. Bibenzylates

Bibenzyl is an essential precursor for synthesizing phenanthrene substances, with 1,2-diphenylethane as the parent ring. Among medicinal orchids, the bibenzyls of Dendrobium have been studied in depth. For instance, eight bibenzyl compounds were isolated from D. officinale leaves [16], and its stems contained 15 bibenzyl compounds, including dendrocandin X and 3,4′-dihydroxy-4,5-dimethoxybibenzyl, which were isolated from D. officinale for the first time [17]. Additionally, dendrocandin Y, a new bibenzyl derivative, was discovered from the stem extract [18]. Using UPLC-Q-TOF-MS technology, five bibenzyloids, including densiflorol A, aloifol I and isomoniliformin A, were identified in D. pendulum for the first time [19]. The discovery of new benzyl compounds in medicinal orchids serves as a research resource for developing their medicinal value. Table 1 (67–101) summarizes the information of 35 bibenzyls (for their chemical structures, refer to Figure 3). In Table 1, composition (67) was identified as new from Bletilla, whereas compositions (75, 83, 89) were determined to be new from Dendrobium. Three new substances (90, 98, 101) were found in D. plicatile [20], D. hercoglossum [21] and D. hancockii [22], respectively, within the genus.
Figure 3. The chemical structure of bibenzyl substances found in orchids.

2.4. Other Secondary Metabolites

Medicinal orchids encompass a wide range of species, each characterized by a diverse array of secondary metabolites. These metabolites can be further categorized into various material types. For instance, flavonoids can be further sub-classified into flavanones, flavonols and other categories. These flavonoids are widely distributed in medicinal orchid plants and are a primary source of plant pigments. In fact, 66 flavonoids have been detected in 24 orchid plants [23], while 34 different species of Dendrobium have been found to contain flavonoids [24], including D. devonianum [25], D. officinale [26][27][28], D. fimbriatum [29] and D. huoshanense [30]. In the family of Orchidaceae, many types of secondary metabolites still exist, and 54 species are summarized in Table 1 (102–155) (for their chemical structures, refer to Figure 4).
Table 1. A total of 155 components were first identified from medicinal orchids in 2018–2023.
Figure 4. The chemical structure of other substances found in orchids.

References

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