Phytochemical Constituents and Pharmacological Activities of Plants from the Genus Adiantum : A Review

Adiantum is a genus of ca. 200 species in the family Adiantaceae, distributed extensively across the world from cool temperate zones to hot tropical regions. A lot of Adiantum species have been used in traditional Chinese medicine to cure human and animal diseases including relief of internal heat or fever, enhancement of urination, removal of urinary calculus, and sundry other curative claims. Chemical studies have shown the presence of various classes of compounds, the main ones being triterpenoids, flavonoids, phenyl propanoids, steroids, alicyclic acids, lipids and long-chain compounds. The extract of this genus as well as pure compounds isolated from it have been demonstrated to possess multiple pharmacological activities such as analgesic, antinociceptive, anti-implantation, and antimicrobial activities. In this review, we have addressed the phytochemistry and pharmacological activities of the Adiantum species in order to collate existing information on this plant as well as highlight its multi-activity properties as a medicinal agent.


INTRODUCTION
Adiantum is a genus of ca. 200 species in the family Adiantaceae, distributed extensively over the world from cool temperate zones to hot tropical regions. As many as 30 species and 5 varieties are found in China [1]. Half of the species have been used in traditional Chinese medicine to cure human and animal diseases including relief of internal heat or fever, enhancement of urination, removal of urinary calculus, elimination of stasis to resolve swelling, relief of cough, cure of diarrhea and stoppage of bleeding, as well as treatment of urinary tract infection, calculus, hepatitis, hemorrhage, fractures, snakebite, burns and scald [2].
According to the literature, Adiantum species are a rich source of triterpenes with various structural skeletons. Besides, flavonoids, phenyl propanoids and sterols have been isolated from the genus Adiantum [3][4][5][6][7][8][9][10][11][12][13][14]. These compounds have been reported to show various bioactivities, such as analgesic, antinociceptive, anti-implantation, and antimicrobial activities. In this review, we summarize the current knowledge of the phytochemistry of the plants as well as the compounds that have been isolated from the genus Adiantum. The biological activities of this genus have also been addressed.

PHYTOCHEMISTRY
Since the 1960s, 124 compounds, including terpenoids, flavonoids, phenyl propanoids, steroids, alicyclic acids, lipids and long-chain compounds have been reportedly isolated from the genus. Triterpenoids and flavonoids are the dominant constituents within the genus Adiantum.

Lipids
The betaine lipid diacylglyceryl-O-4'-(N,N,N,trimethyl)homoserine was isolated from A. capillus-veneris [58]. Positional analysis of the fatty acids by lipase treatment showed that palmitic acid is esterified at position 1, and linoleic, linolenic, and arachidonic acids at position 2 of the glycerol moiety of the lipid. Although the trimethylhomoserine lipid has been found in some algal species, this is the first report that it exists in a vascular plant [58].

PHARMACOLOGICAL ACTIVITIES
The extract of the Adiantum species as well as pure compounds isolated from them, have been demonstrated to possess multiple pharmacological activities including analgesic, antinociceptive, anti-implantation, and antimicrobial activities.

Analgesic activity
Hexane fraction from A. cuneatum as well as filicene (58) and filicenal (61), given intraperitoneally, exhibited potent analgesic activity when evaluated in two models of pain in mice -writhing test and formalin-induced pain. 58 presented an ID 50 value of 19.5 µmol/kg body weight (writhing test), being about 7-fold more active than some reference drugs, such as acetyl salicylic acid and acetaminophen, with the dose of 73.0-247.0, and 140-250 µmol/kg respectively. It also inhibited both phases (neurogenic and inflammatory) of the formalin test at 10 mg/kg (24 µmol/kg). The results confirm and justify the popular use of this plant for the treatment of sorrowful conditions [35]. Ethanol extract of A. venustum demonstrated good analgesic activity with 100 mg/kg when compared with 50 mg/kg [63].

Anti-inflammatory activity
The chronic anti-inflammatory activity of the ethanol extract of A. venustum has been evaluated by carrageen-induced paw edema method. The results, at the two dose levels tested in rats, indicate significant antiinflammatory activity. Maximum inhibition of inflammation was 71.15 % recorded with 100 mg/kg of plant extract. A further decrease in dose level (50 mg/kg) produced an even greater decrease in anti-inflammatory activity [63].

Anti-implantation activity
Petroleum ether extracts of A. capillus and isoadiantone (22) were reported to be active as inhibitors of postcoital implantation in rats [38].

Antimicrobial activity
The The methanol extract of A. trapiziforme inhibited the growth of Bacillus megaterium and Staphylococcus aureus B-43-5. Older plants showed more pronounced activity than young ones and fertile fronds had greater activity than vegetative ones [67].

Antiviral activity
Using vesicular stomatitis virus in monkey cell cultures as test organism, the extracts of A. capillus-veneris was found to exhibit antiviral activity [69].

Agglutinating activity
Lectin from the leaves of A. flabellulatum had a characteristic of glycoproteins, exihibiting agglutinating activity on rabbit erythrocytes, as well as human erythrocytes of A, B, or O groups, but had no activity on turtle erythrocytes. It agglutinated cells of unicellular alga (Chlorella pyrenoidosa), natural or heat-treated cells of yeast (Saccharomyces cerbvisiae) and heat-treated cells of Bacillus subtilis [70]. Its' highest agglutining activity on chicken's red blood cell reached 2 10 . In addition, it was specific for not only some marine algae, as well as bacterial, yeast and tumor cells but also for two species of plant harmful germ and bacteria, Helminthosporium turcicum and Pseudomonas solanacoarum [71].

Insect-molting hormone activity
Leaf material from 64 New Zealand ferns was examined for insect molting hormone activity by using the housefly larvae for bioassay. Activity was found in most species including the genera Adiantum [72].

Other activities
Total flavonoids from A. capiuaris-veneris showed high scavenging activity on hydroxyl radicals [73]. El-Tantawy et al determined the antidiabetic and diuretic effects of the alcohol and aqueous extracts of A. capillus-veneris as well as the isolated mucilage [55]. Melos et al evaluated the allelopathic potentials of the crude ethanol extract of A. tetraphyllum and its fractions against Lactuca sativa (lettuce) and Allium cepa (onion) seeds [3]. The average time of germination of lettuce and onion seeds, when subjected to the crude ethanol extract as well as hexane and AcOEt fractions of A. tetraphyllum, respectively, at a concentration of 1000 mg/L, was significantly (p < 0,05) longer than that for the control.

CONCLUSION
Chemical studies on Adiantum species have revealed that the typical constituents of this genus are terpenoids and flavonoids. Among them, some exhibit strong bioactivities, especially analgesic, antinociceptive, antiimplantation, and antimicrobial activities.
Further phytochemical and biological studies should be carried out on this genus in order to elucidate their active principles and mechanisms of action of the active constituents.