A review on the phytochemistry and biological activities of Curculigo latifolia Dryand ex. W.Aiton

6.1 Roots

Zabidi et al.⁵⁸ identified the chemical compounds in the root extract of C. latifolia using liquid chromatography–mass spectrometry (LC–MS) analysis, which was found to contain phenolic compounds such as monobenzone, phloridzin, mundulone, scandenin, pomiferin, dimethyl caffeic acid, hordatine A, ubiquinone, hydroquinone, frangulin B, rubratoxin B, and emmotin A. These phytochemicals have a diverse range of bioactive attributes that can play a significant role in exerting pharmacological activities such as anti–diabetic, antimicrobial, anti–inflammatory, antioxidant, and anti–cancer.⁵⁹ For instance, hydroquinone is known to exhibit antioxidant activity by preventing oxidative damage in human cells,⁶⁰ whereas ubiquinone plays a key role in cellular metabolism and protection against lipid peroxidation.⁶¹

Furthermore, Maliwong et al.²³ used high-resolution electrospray mass spectrometry (HR–ESI–MS), which revealed phenolic glycosides, such as molineriosides A–C, curculigoside, curculigoside H, 3–hydroxy–5– methylphenyl 1–O–β–D–glucopyranosyl–(1→6)–β–D–glucopyranoside, nyasicoside, crassifoside B, 1–O–methyl– curculigine, 1–O–methyisolcurculigine, capituloside, and curcapicycloside. Curculigoside is reported as one of the major bioactive compounds in another Curculigo species, Curculigo orchioides, where it was found to promote neuroprotection, anti–arthritic, anti–osteoporosis, and anti–tumor activities.² Curculigoside has also been suggested to exert significant antioxidant activity, which ameliorates learning performance and bone loss in mutated transgenic mice with Alzheimer’s disease.²⁰

A more recent study by Nur et al.⁶² reported the presence of methyl–3–hydroxy–4–methoxybenzoate, sugiol (diterpene), stigmastan–3,6–dione (steroid), aviprin (flavonoid), lucialdehyde B (sesquiterpene), guaiacol (phenol), and smilaxin (saponin). Avipirin is a bioactive compound that exhibits antibacterial, antifungal, and phytotoxic activities.⁶³ Guaiacol has been reported to effectively inhibit human carbonic anhydrase isoenzymes,⁶⁴ and the compound smilaxin has demonstrated immunostimulatory, antiproliferative, and human immunodeficiency virus (HIV) –1–reverse transcriptase inhibitory activities.⁶⁵

6.2 Fruit

Curculin, a sweet protein, is found in the fruit extract of C. latifolia. This protein has sweet–tasting and taste–modifying properties that can enhance the sweetness of acidic or tasteless substances.²⁶ Moreover, its sweet properties make it a promising option for low–calorie sweeteners, which may be beneficial for individuals aiming to reduce their sugar or calorie consumption.³⁴

Moreover, several phytochemicals such as berberine, hordatine A, robustine (alkaloids), frangulin B (anthraquinone), pomiferin (flavonoid), and monobenzone (aromatic hydrocarbons) have been identified.¹⁰ These chemical compounds generally exhibit pharmacological effects. For example, berberine and frangulin B have anti–inflammatory properties,^66,67 pomiferin has high antioxidant activities,⁶⁸ and monobenzone can inhibit the growth of acute myeloid leukemia.⁶⁹

The edible fruit of C. Latifolia could be a promising functional food that can provide health–promoting and taste–enhancing effects owing to the presence of phytochemicals and sweet proteins. However, there are still limited studies on the unique compounds of C. latifolia and curculin; therefore, further investigations into its glycemic index, cholesterol level, and anti-diabetic, anti-inflammatory, and anti–cancer activities will provide more information about it.

6.3 Rhizome

In a previous study by Ooi et al.,²⁰ the phenolic composition of the rhizome extract of C. latifolia was determined using high-performance liquid chromatography with diode–array detection (HPLC–DAD). Several active components have been identified, including cinnamic acid, curculigoside, syringic acid, ferulic acid, and protocatechuic acid. The findings also highlighted the efficacy of ethyl acetate solvent in extracting a higher amount of curculigoside and cinnamic acid compared to methanolic rhizome extract.²⁰

In recent studies by Umar et al.⁵⁶ and Umar et al.,³⁷ the chemical composition of the ethanolic rhizome extract of C. latifolia was determined using ultra-high-performance liquid chromatography–Q Exactive hybrid quadrupole–Orbitrap high-resolution accurate mass spectrometry (UHPLC–Q–Orbitrap HRMS). This sensitive and accurate method detected the presence of active components such as lycorine, vanillin, nyasicoside, 4–hydroxy–phenol, and curculigoside B.^39,56 Additionally, the study also identified some isolates that were previously extracted from other Curculigo species such as orchioside B, orcinol glycoside, curculigine C, curculigosaponin G and C, from Curculigo orchiosides,^1,70 capituloside and crassifoside I from Curculigo capitulata,^71,72 and (1S,2R)–O–methylnyasicoside from Curculigo sinensis.⁷³

Mad Nasir et al.⁷⁴ also found Curculigo isolates in methanolic rhizome extracts using ultra–high performance liquid chromatography mass spectrometry (UHPLC–MS). These included curculigine, curculigine M and G, curcupicycloside, sinensigenin A, 1–O–methylisocurculigine, brevicaside B, crassifoside C and D, curculigenin, sinenside B, and orchioside J.⁷⁴

C. latifolia is rich in chemical diversity, some of which can potentially exert various physiological effects. Some of the isolates of Curculigo species that have been reported for their promising biological effects include orcinol glucoside and crassifoside H, which improve depressive behaviors (antidepressant–like effects) in chronic unpredictable mild stress rat models.^75,76 However, many of these compounds have not yet been fully investigated for their biological activity. Therefore, extensive studies on these unique isolates are needed to understand the mechanisms underlying their biological activities, such as antioxidant, anti–diabetic, and antimicrobial activities.

6.4 Leaves

Umar et al.⁵⁶ detected chemical components in the leaves of C. latifolia, where some of the compounds were similar to those found in the rhizome extract. This study identified nyasicoside, vanillin, lycorine, orcinol glycoside, crassifogenin A, behenic acid, pothobanoside C, daucosterol, stigmasterol, curculigine C, curculigosaponin C, E, G, H, and I; crassifoside C, E, and I; curculigoside B and C; and orchioside A and B.^39,56,74 Some of these compounds have been reported to exhibit anti-estrogenic and anti-allergic activities against estrogen–responsive human breast cancer cell lines.⁷⁷ Plant–derived sterols, such as daucosterol and stigmasterol, effectively inhibit cell proliferation and reduce tumor size in patients with prostate, colorectal, and breast cancers.⁷⁸ Additionally, behenic acid showed significant antibacterial activity against Agrobacterium tumefaciens T–37 and Erwinia carotovora EC–1.⁷⁹

Furthermore, in a study by Mad Nasir et al.,⁷⁴ numerous compounds were also detected in the methanolic leaf extract, including orcinoside E, orcinol gentiobioside, sinensigenin B, curculigine M, and (Z)-resveratrol 3,4′-diglucoside. Most of these compounds are derived from glycosides and stilbene groups; however, few studies have investigated their biological effects. Generally, glycosides possess a wide variety of biological attributes, such as anti–inflammatory, antiseptic, anti-rheumatic, and analgesic effects,⁸⁰ whereas stilbenes exhibit cardioprotective, neuroprotective, anti–diabetic, and cancer treatment and prevention activities.⁸¹

Phytochemical screening using liquid chromatography–electrospray ionization mass spectrometry (LC–ESI–MS) by Nur et al.⁶² identified compounds including digiprolactone, 3–tert–butyl–4–methoxyphenol, axedarachin C and 4–O–caffeoylquinic acid–1, and quercetin. Quercetin is regarded as one of the most effective antioxidants that can scavenge free radicals and mitigate diseases associated with oxidative stress such as diabetes, cancer, allergies, inflammation, and gastrointestinal and cardiovascular diseases.⁸²

6.5 Other parts

Umar et al.³⁷ identified the chemical compounds in C. latifolia callus and plantlet leaves, which consist of ecdysterone, a natural anabolic agent that can inhibit the proliferation of breast cancer cells,⁸³ and emodin dianthrone,³⁷ which possesses antidiabetic properties.⁸⁴ Moreover, the petiole, callus, and plantlet leaves showed the presence of breviscapin, a bioactive compound that was previously extracted from Erigeron breviscapus with the ability to enhance cerebral blood flow and microcirculation and resist platelet aggregation.⁸⁵ C. latifolia callus also contained the compound salidroside, which has effective properties in ameliorating memory and emotional behavior in adult mice,^37,86 and theanaphthoquinone, which has been shown to induce cell death in breast cancer cells.^37,87

These bioactive compounds are important for managing various diseases and should be included in the human diet because of their ability to provide energy and nutrients and contribute to overall well–being.⁵⁷ Therefore, it is worthwhile to further study the bioactivities of the isolated chemical compounds of C. latifolia to validate the findings for potential use in pharmaceuticals or food supplements.

7.1 Antioxidant activity

Different types of assays have been employed to investigate the antioxidant activity of C. latifolia, and each assay generally involves different chemical reactions and mechanisms.⁸⁸ Some common antioxidant assays that were involved can be seen in Table 5, which include 1,1–diphenyl–2–picrylhydrazyl (DPPH), 2,2’-azino–bis(3–ethylbenzthiazoline–6–sulfonic acid) (ABTS) radical cation scavenging, and ferric reducing antioxidant power (FRAP) assays. In a previous study by Ooi et al.,²⁰ it was reported that C. latifolia rhizome extract exhibited promising scavenging activity for both DPPH and ABTS radicals, with a Trolox–comparable capability. This study suggested that these antioxidant activities were significantly correlated with the bioactive compounds in the extract, such as phenolics and flavonoids.²⁰ The unique features of these compounds, such as their functional groups, configuration, substitution, and amount of hydroxyl groups, essentially facilitate protection against oxidative damage by radical neutralization, iron binding, and reducing power capacities.⁸⁹ The report by Ishak and Zabidi²⁴ is consistent with Ooi et al.,²⁰ where high contents of flavonoids and phenolics in the extracts also corresponded to antioxidant activities. The study demonstrated that the root extract of C. latifolia with high phenolic and flavonoid contents under subcritical water extraction showed DPPH radical scavenging activities of 128.70 mg Trolox equivalents/g sample and ABTS scavenging activity of 66.78 mg Trolox equivalents/g sample.²⁴

Moreover, in the findings of Nur et al.,⁶² different extracts of C. latifolia roots, stems, and leaves reported varying antioxidant capacities, in ABTS study, ethanol and ethyl acetate extracts of C. latifolia exerted the most significant activity by contributing proton radicals to free radical compounds. Meanwhile, β-carotene bleaching assay demonstrated that ethyl acetate extracts of C. latifolia roots, stems, and leaves could effectively prevent β-carotene decomposition upon oxidation of linoleic acid to hydroperoxide compared to the other extracts. In the FRAP assay, the root extracts (ethyl acetate and ethanol) showed the strongest antioxidant activity for reducing Fe³⁺ to Fe²⁺ complexes. Nur et al.⁶² suggested that the hydroxyl groups in the phenolic compounds in the extracts were highly effective in the chelation of iron and subsequently reduced it via a redox reaction.

A recent study by Umar et al.⁵⁶ also suggested that the rhizome extract of C. latifolia showed higher antioxidant activity than the leaf and petiole extracts. In this study, the active components that significantly contributed to the antioxidant activity were evaluated using Fourier transform infrared (FTIR) spectroscopy and UHPLC–Q–Orbitrap HRMS combined with partial least squares (PLS) assay. Based on this analysis, a chemical compound identified as unknown (185) with chemical formula C₄₇H₅₉O₇ was found to be a major contributor to the activity, whereas phenolics (curculigoside B, 2,4– dichloro–5–methoxy–3–methylphenol, orchioside B), cycloartane triterpene (curculigosaponin G), and norlignan compounds (1,1–bis (3,4–dihydroxyphenyl)–1–(2–furan)–methane and (1S,2R)–O–methylnyasicoside) showed only a minor contribution.⁶² Moreover, the presence of phenolics, norlignan, and alkaloid compounds, such as vanillin, crassifoside A, and lycorine, in C. latifolia extracts exhibited antioxidant properties.³⁹ Regarding the functional groups, Umar et al.⁵⁶ determined that compounds with –OH, C=O, C–O, –C–H, C–C, and C–OH groups strongly influenced the antioxidant capacity. This result agrees with the study by Mad Nasir et al.,⁷⁴ which suggested a positive correlation between phenolics and DPPH antioxidant activity of C. latifolia extracts. The report showed that the rhizome extract produced a high DPPH radical scavenging activity having an IC₅₀ value of 18.10 $\pm$ 0.91 $\mathrm{\mu g}/\mathrm{mL}$, which is comparable to that of standard ascorbic acid, which has an IC₅₀ value of 11.49 $\pm$ 0.071 $\mathrm{\mu g}/\mathrm{mL}$. In contrast, the fruit and leaf extracts displayed a lower antioxidant activity against DPPH assay, with IC₅₀ values of 26.99 $\pm$ 1.58 and 547.29 $\pm$ 5.09 $\mathrm{\mu g}/\mathrm{mL}$ respectively.⁷⁴

In addition, an interesting finding by Farzinebrahimi et al.¹⁵ indicated that the free radical scavenging abilities of tuber and leaf extracts of C. latifolia reached 80% and 60%, respectively. The same study also evaluated a considerable antioxidant activity of 70% and 65% in the callus of tuber and leaf extracts, respectively, suggesting that the plant can be considered to have antioxidant effects.¹⁵ Akkarasiritharattana and Chamutpong¹⁸ found that the aqueous extract of the underground part of C. latifolia, assessed using DPPH and FRAP assays, also showed favorable radical scavenging activity and reducing power ability. However, the ethyl acetate extract of C. latifolia underground part of C. latifolia in the same study resulted in a lower potential. This may be due to the discrepancy in the solvents used, where more polar phenolic and flavonoid compounds possessing antioxidant properties may have been extracted in greater quantities using water than ethyl acetate.¹⁸

With its high antioxidant activity comparable to ascorbic acid, C. latifolia effectively scavenges free radicals, chelates metal iron, and reduces ferric (III) iron to ferrous (II) iron. The antioxidant properties of the plant can protect cells and tissues in the human body from oxidative stress, which is linked to chronic diseases such as diabetes, cancer, and heart disease.⁹⁰ This makes C. latifolia a valuable source of functional foods that can help supplement the daily diet with antioxidants, subsequently preventing the body from harmful free radicals. Nonetheless, investigations of the antioxidant effects of different extracts of various parts of the plant and isolated components are still warranted. Further detailed studies are needed to explore the bioactivities of C. latifolia which will support its medicinal value.

7.2 Antimicrobial activity

Previous studies have reported that different parts of C. latifolia such as the root, stem, and leaf, exert antimicrobial activities against seven different bacterial strains, including Bacillus cereus, Bacillus subtillis, Enterobacter aerogenes, Erwinia sp., Klebsiella sp., Pseudomonas sp. and Staphylococcus aureus.¹⁹ It was demonstrated that by increasing the concentration of all the extracts, with the highest concentration being 100 mg/mL, the elimination and restraint of the microorganisms were gradually more effective.¹⁹ A similar observation was made by Lim and Ibrahim⁹¹ where the root extract was seen to have the ability to inhibit the growth of Candida albicans, with the lowest minimum inhibitory concentration of the extract being 1.56 mg/mL. In another study, the leaves and tubers from intact C. latifolia plant (in vitro) and callus (in vivo) extracts showed considerable antibacterial activity against Pseudomonas aeruginosa and Klebsiella sp.¹⁵ However, the most prominent activity was observed in the tuber extract, which was suggested to contain phytochemicals with antibiotic properties.¹⁵ Furthermore, a study by Mad Nasir et al.⁷⁴ revealed that the methanolic leaf extract had promising antibacterial properties against Staphylococcus aureus (gram-positive bacteria) and Salmonella choleraesuis (gram-negative bacteria), with mean (n=3) inhibition zones of 15.33 mm and 8 mm, respectively. The active phenolic compounds, such as tetrahydromethylmononya– sine, (2R,4S,5S,6R)–2–ethyl–6–(4–methylphenoxy)oxane–3,4,5–triol, and (Z)–resveratrol 3,4′-diglucoside, in the leaf extracts were found to predominantly react with the enzymes and proteins of the microbial cell membrane.¹⁵

Given the antibacterial effects of this plant, additional investigations, such as in–depth research on the chemical constituents of C. latifolia responsible for its antimicrobial activity, are needed. Additionally, since there are limited studies related to the antimicrobial activities of this plant and its different parts, further detailed investigations are required.

7.3 Anti–diabetic activity

The antidiabetic properties of C. latifolia have been investigated in multiple studies, both through chemical assays and animal model experiments. In a report by Zabidi et al.¹⁰, C. latifolia root extract was found to exhibit a considerably significant inhibition of $\alpha$– glucosidase and dipeptidyl peptidase–4 (DPP (IV)) enzymes, where its percentage inhibition was in close proximity to that of acarbose, a marketed anti–diabetic drug. In addition, it was found that through the synergistic interaction of phytochemicals in the extracts, such as berberine, flavonoid glycoside (phlorizin), isoflavonoid (mundulone and scandenin), and cinnamic acid derivative (dimethyl caffeic acid), insulin secretion and glucose uptake activity were greatly enhanced.¹⁵ The study also suggested that the fruit extract of C. latifolia demonstrated antidiabetic activity, although it was less effective than the root extract. This was possibly because some bioactive compounds in the root extract were absent from the fruit extract, which lowered the efficacy level where fewer synergistic interactions occurred.¹⁵ A similar result was also seen in the study by Ishak and Zabidi,²⁴ where the fruit and root extracts displayed promising antidiabetic effects. However, in this study, the leaf extract of C. latifolia showed a lower potential in activity, which may be due to the lack of some active components.

Furthermore, a study by Umar et al.⁵⁶ found that the rhizome extract exhibited a more significant $\alpha$– glucosidase inhibition than the leaf and petiole extracts. The study suggested that the isolated compound, unknown–85 (C₄₂H₅₁O₆), primarily influenced inhibition, whereas phenolics (orcinol glucoside), cycloartane triterpene (curculigosaponin G, H, and I), and norlignan compounds (1,1–bis (3,4–dihydroxyphenyl)–1–(2–furan)–methane) contributed slightly. Moreover, the fruit of the plant contains a sweet protein known as curculin.³⁴ This protein was suggested to be 500 to 9000 times sweeter than sucrose by weight, and is known for its use as a sugar substitute and taste modifier. Curculin is also recognized as a low–calorie sweetener; thus, it is considered to have a potential in having antidiabetic properties.³⁴ Despite this, there have been no studies to confirm this postulation; therefore, further studies are needed to investigate the glucose uptake, insulin secretion, and $\alpha$–glucosidase and $\alpha$–amylase inhibitory effects of curculin.

Moreover, in an in vivo study, C. latifolia fruit:root extract at a 1:1 ratio was found to decrease glucose and lipid levels, as well as increase insulin and adiponectin levels in streptozotocin (STZ)–induced diabetic rats.⁹² The extract demonstrated its efficiency in inhibiting the disruption of pancreatic $\beta$–cells, which is usually induced by STZ in diabetic rats. This indicates that the fruit and root extract of this plant was capable of scavenging radicals that could cause oxidative stress in cells, suggesting its antioxidant properties.⁹² Another previous study reported the potential for diabetes management linked to an isolated compound from the rhizome extract of C. latifolia known as curculigoside.⁹³ In response to this, it was observed that glucose uptake was improved by the increased availability of glucose transporter type 4 (GLUT4) at the plasma membrane of differentiated 3t3–L1 adipocytes in male Sprague–Dawley rats.⁹³

Based on the reported improvements in glucose uptake, insulin secretion, and the significant $\alpha$–glucosidase and $\alpha$–amylase inhibition effects, it can be deduced that C. latifolia possesses promising anti–diabetic properties. Despite this, the reported studies carried out so far revealed that only aqueous and ethyl acetate extracts of the roots, rhizomes, fruits, and leaves were used. Therefore, various other plant extracts can also be considered for the investigation of their anti–diabetic properties.

7.4 Other bioactivities

Ooi et al.⁹⁴ investigated the lipid profile of C. latifolia rhizome extract and reported improvements in total cholesterol levels and increased high–density lipoprotein (HDL) levels in obese diabetic rats. Additionally, the lipoprotein cholesterol ratio of HDL and low–density lipoprotein (LDL) (HDL:LDL) was ameliorated by treatment with 200 mg/kg body weight rhizome extract. This indicated that the extract could potentially help reduce the risk of cardiovascular disease, especially in obese patients.⁹⁴ Similarly, Ishak et al.⁹² found that treatment with 50, 100, and 200 mg/kg body weight of fruit improved the lipid profile of obese diabetic rats. The findings demonstrated a significant decrease in total cholesterol, triglycerides, and LDL and an increase in HDL. These findings suggest that C. latifolia is a promising plant for controlling cholesterol levels in the body, which is important for reducing the risk of heart disease and stroke, which are more prevalent in individuals with obesity.

C. latifolia root and leaf extracts have also been demonstrated to improve sperm quality in mice (Mus musculus). According to a study by Jaafar et al.,⁹⁵ mice fed with C. latifolia root extract showed a greater increase in sperm motility when compared to the control group, whereas the mice fed with C. latifolia leaf extract showed a higher sperm count and viability.

In addition, a cytotoxicity test was carried out on the root extract of this plant, and Lim and Ibrahim⁹¹ suggested that the extract demonstrated no toxicity against brine shrimp eggs, with a lethal concentration (LC₅₀) of 2.25 mg/mL. This finding indicated that the extract had a minimal negative impact on living cells and was less likely to cause a reduction in cell viability or cell death.⁹⁶ Moreover, a cell viability assessment, which measures the proportion of healthy functional cells, was conducted by Zabidi et al.¹⁰ via the MTT (3-[4,5–dimethylthiazol–2–yl] –2,5 diphenyl tetrazolium bromide) assay. The study showed the ability of 3T3–L1 cells to survive at IC₂₀ (153.21 ± 9.65 μg/ml) and IC₅₀ (561.42 ± 6.22 μg/ml) of C. latifolia root extract and IC₂₀ (295.67 ± 7.43μg/ml) and IC₅₀ (495.67 ± 11.31 μg/ml) of fruits extract. Meanwhile, MTT assay by Ooi et al.⁹³ found the IC₂₀ and IC₅₀ values for ethyl acetate fraction of rhizome extract were 111.73 ± 9.57 and 509.59 ± 49.75 mg/mL, respectively. These findings suggest that C. latifolia does not have toxic effects on cells, suggesting that it is safe for use or consumption.

Previous reports have suggested that C. latifolia compounds have anti–aging and ultraviolet protection properties. An in silico anti–elastase study was conducted by Nur et al.⁶² through molecular docking to predict the interactions between ligands (C. latifolia compounds) and the elastase target protein (1B0F). From the study, sugiol, aviprin, 4–O–caffeoylquinic acid–1, quercetin and 5,7,3,5–tetrahydroxyflavanone compounds of C. latifolia demonstrated the best interaction against the target protein, where the binding free energies of the compounds were <–6 kcal/mol. This indicated that the compounds of the plant have promising potential to exert anti–elastase activity, which could promote anti–aging effects. In another study by Nur et al.,⁹⁷ C. latifolia compounds, pomiferin, scandenin, mundulon, and rubratoxin were found to have a negative binding affinity energy against matrix metalloproteinases, MMP–1 (collagenase) and MMP–9 (gelatinase), while 1,1,6–trimethyl–1,2–dihydronaphthalene, orcinol glucoside, and sugiol compounds effectively inhibited the target proteins hyaluronidase. The ability to inhibit target proteins is mainly attributed to the functional groups in the chemical structures of the active compounds, which can readily interact with the amino acid residues of the target proteins.⁹⁷

Furthermore, Nur et al.⁹⁸ revealed that hexane, ethyl acetate, and ethanol extracts of Curculigo latifolia leaves, roots, and stems have promising bioactivity in ultraviolet protection against UVA and UVB rays. The study found that the hexane leaf extract demonstrated the highest intensity in absorbing UVA and UVB radiation, with absorption values of 1.192 and 1.804, respectively. Additionally, it was also indicated that leaf hexane, root ethyl acetate, stem ethyl acetate and leaf ethyl acetate extracts at concentration 250 ppm showed the most prominent ultra–protective effect with sun protection factor (SPF) values of 23.65, 16.5, 22.5 and 23.03 respectively. Based on the reported findings, it can be suggested that C. latifolia plants have properties that could be used to make sunscreen products and protect the skin against erythema, pigmentation, and harmful UV rays.⁹⁸

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Not applicable – review paper.