A review on metabolites and pharmaceutical potential of food legume crop mung bean (Vigna radiata L. Wilczek)

Nutritional value

Mung bean is primarily grown for its edible seeds that are rich in easily digestible proteins and a good amount of phosphorus and provitamin A (Vir et al., 2018). Various studies conducted using different varieties obtained from different geographical locations have revealed that seeds of mung bean contain moisture (4.10–15.20%), crude proteins (20–32%), crude lipids (0.71–1.85%), crude fibers (3.8–6.15%), ash (0.17–5.87%), and carbohydrates (53.3–67.1%), along with vitamins and minerals (Dahiya et al., 2015). The lipid content of the seed coat has higher amounts of unsaturated fatty acids (72.8% of total lipid content) that include oleic (20.8%), linoleic (16.3%), and linolenic acids (35.7%). The saturated fatty acid content is 27.7% and comprises 14.1% palmitic acid, 4.3% stearic acid, and 9.3% behenic acid (Dahiya et al., 2015). Mung bean oil also contains vitamin E, including tocopherols and tocotrienol. Other vitamins present in mung bean are thiamine, riboflavin, vitamin C, pantothenic acid, and nicotinic acid. Mung bean seeds contain good amounts of minerals such as calcium, potassium, magnesium, and phytin phosphorus. Minerals such as copper, iron, manganese, sodium, and zinc are also present in the bean seeds. Apart from these, some antinutritional factors are present in mung bean seeds, including tannins, phytic acid, hemagglutinin, polyphenols, and trypsin inhibitors (Ganesan and Xu, 2018; Shen et al., 2018) (Table 1). Although the amount of these antinutritional components is very low in seeds, they may hinder the nutritional uptake from the seeds. Phytic acid in particular hinders the uptake of iron and zinc in plant-based diets (Hou et al., 2019). Sprouting of seeds has been reported to reduce the antinutritional content of the seeds and to increase its digestibility and nutritional uptake (Tang et al., 2014). Upon germination, 30.24–58.97% of phytic acid is reported to be hydrolyzed. Apart from germination, other methods to reduce the level of activity of these antinutritional components are soaking and boiling mung bean seeds prior to consumption (Samtiya et al., 2020). Water-soluble components such as raffinose can be removed to a significant extent by discarding the soak water, resulting in the hydrolysis of 93.62% of raffinose (Tang et al., 2014). In combination with cereals such as wheat and rice, mung bean forms a balanced amino acid profile with its high protein levels (19–29%), with high lysine/low methionine amino acid profile of mung bean complementing the low lysine/high methionine and high carbohydrate content of cereals (Vir et al., 2016). This has led to the use of mung bean as baby food because of its rich content of easily digestible proteins, vitamins, and iron (Hou et al., 2019).

Mung bean seeds and sprouts contain different components such as organic acids (citric acid, phosphoric acid), phenolic acids, lipids (γ-tocopherol), and flavonoids that have been recognized to contribute to the pharmaceutical properties (Hou et al., 2019). Different compounds have been isolated from mung bean that belongs to different classes such as flavones, isoflavones, flavonoids, and isoflavonoids (Prokudina et al., 2012). Most of these compounds have been classified as polyphenols because of their polyhydroxy substitutions and were shown to possess antioxidant activity. Vitexin and isovitexin have been reported to be the most abundant flavones present at the concentration of 51.1 and 1.7 mg/g, respectively (Li et al., 2012). Apart from antioxidant activity, these flavonoids are involved in many other plant activities such as early plant development and signaling and protection from stress, insects, and mammalian herbivores (Ganesan and Xu, 2018).

Effect of sprouting on dynamic changes in the metabolite content of mung bean

Different studies have shown that upon germination of mung bean, dynamic changes occur in the concentration of metabolites. These changes involve decrease in the concentration of antinutrient components and a significant increase in the free amino acid concentrations along with increased levels of thiamine, riboflavin, niacin, and ascorbic acid, which is favored worldwide and mostly in Asia and North America (Tang et al., 2014). The amount of vitamin C increases up to 1380 mg/kg in sprouts as compared to 50 mg/kg in mung bean dry seeds, thus enhancing iron absorption (Guo et al., 2012). Moreover, the level of reducing sugars and starch reduced significantly with the complete elimination of stachyose and raffinose (Guo et al., 2012). There is a dramatic increase in the concentrations of gallic acid, chlorogenic acid, and coumarin throughout the sprouting process, whereas the catechin level increases at the end of sprouting (Tang et al., 2014). Some phenolic acids such as cinnamic acid, gentisic acid, and p-hydroxybenzoic acid remain present throughout the process of sprouting (Amarowicz et al., 2009). A trans-esterification reaction also occurs that converts crude lipids into fatty acid methyl esters and triglycerides. The (-aminobutyric acid level is enhanced throughout the sprouting process, which is important as it has several health benefits (Moumita et al., 2010). Other pharmaceutically important proteins reported in mung bean are protease inhibitors that have the property to inhibit the proteolytic enzymes and thus regulate proteolytic processes that play a major role in the defense system of plants against different pathogenic organisms (Tang et al., 2014). The level of trypsin inhibitors decreases gradually during germination. The activity of hemagglutinin has been found to decrease by approximately 4% during the germination process (Tang et al., 2014).

Antioxidant effects

Mung bean seeds, sprouts, and even hulls have been shown to possess antioxidant activity. The methanolic extracts of mung bean exhibit high free radical scavenging and reducing power activity because of the presence of a high level of polyphenols (Ganesan and Xu, 2018). Moreover, acetone extracts from sprouts show higher activity than the extracts from seeds because of the occurrence of a higher amount of total phenols and flavonoids (4.5 and 6.8 times more, respectively) than raw seeds (Guo et al., 2012; Paja et al., 2014). The free radical scavenging activity of mung bean seeds’ soup was compared with that of green tea and vitamin C, and it was found that 100 g of mung bean, 36.3 g of dried tea, and 1462 mg of vitamin C showed equivalent level of free radical scavenging activity (Cao et al., 2011). This high antioxidant activity is due to the presence of two major antioxidant compounds, namely vitexin and isovitexin, among which vitexin alone is capable of inhibiting DPPH radicals by approximately 60% at the concentration of 100 :g/ml and was found to be effective in preventing cell death from UV-induced skin damage (Cao et al., 2011). Methanolic extracts of seed coats (obtained from sprouted seeds) were shown to possess high antioxidant potential (80% and 77%) at the concentration of 100 μg/ml when tested by DPPH (2,2-diphenyl-1-picrylhydrazyl) and FRAP (Ferric Reducing Antioxidant Power) methods, respectively (Mehta et al., 2021).

Antimicrobial activity

Biocides or phytochemicals with antimicrobial activity are gaining popularity because of their broad-spectrum activity with no known side effects. Several reports have been published showing the potential of mung bean as an antimicrobial agent (Tang et al., 2014; Ganesan and Xu, 2018; Shen et al., 2018; Mehta et al., 2021) (Table 3). A nonspecific lipid transfer peptide (nsLTP; molecular weight: 9.03 kDa) that shows broad-spectrum antibacterial and antifungal activity has been isolated from mung bean (Shen et al., 2018). It is active against fungi such as Fusarium oxysporum, F. solani, Pythium aphanidermatum, and Sclerotium rolfsii along with the bacterial species Staphylococcus aureus. A cyclophilinlike antifungal protein “mungin” has been isolated from the aqueous extract of seeds that exhibits inhibitory activity against α- and β-glucosidases and shows antifungal activity against fungi such as Rhizoctonia solani, Mycosphaerella arachidicola, Coprinus comatus, Botrytis cinerea, and Fusarium oxysporum (Shen et al., 2018). Chitinase has also been isolated from aqueous extracts of mung bean seeds with a specific activity of 3.81 U/mg and pI of 6.3, and it shows antifungal activity against Fusarium oxysporum, Rhizoctonia solani, Mycosphaerella arachidicola, Pythium aphanidermatum, and Sclerotium rolfsii (Tang et al., 2014). The polyphenol extract from the sprouts exhibited antibacterial properties against Helicobacter pylori, which causes gastro-duodenal diseases in humans (Tang et al., 2014). Recently, methanolic extracts of mung bean seed coat have been shown to exhibit broad-spectrum antibacterial activity against Bacillus cereus, Bacillus subtilis, Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, and S. aureus (Mehta et al., 2021).

Anti-inflammatory and immunomodulatory activity

Inflammation may occur because of several causes such as infection, foreign body stimulus, and tissue damage and has been associated with nitric oxide and proin ammatory cytokines produced by the activation of the immune system (Ali et al., 2014; Soufli et al., 2016). Mung bean has been found to be beneficial for treating various inflammatory responses, and this is evident from its use in folk remedies and cuisine, particularly in Asian countries (Lee et al., 2011). Clinical evidence of the antiinflammatory activity was obtained by analyzing the effect of the ethanolic extract of mung bean on lipopolysaccharide-stimulated macrophages. The study showed a noncytotoxic decrease in macrophage activity through the suppression of pro-inflammatory genes such as Apoe and Bcl2a1a (Yeap et al., 2012). This activity was found to be associated with the presence of polyphenols, vitexin, isovitexin, and gallic acid in the extract. In another study, treatment of cells with the polyphenol fraction (3.7 mg/ml) of ethanolic extract of mung bean seeds was shown to drastically downregulate pro-inflammatory cytokines such as IL-6, IL-1β, IL-12β, TNF-α, and inducible NO synthase (Lee et al., 2011). The anti-inflammatory response was also shown by aqueous extracts obtained from germinated and fermented mung bean seeds when dose-dependently tested under in vitro (measured by inhibition of nitric oxide level in cultured cell supernatant) and in vivo (inhibition of arachidonic acid-induced ear edema in mice) conditions (Ali et al., 2014). The in vitro study revealed a decrease in NO level up to 21.6 and 40.3% for germinated and fermented seed extracts, respectively, while in the in vivo study, both the extracts when used at the concentration of 1000 mg/kg body weight led to a significant reduction in edema (Ali et al., 2014). Likewise, anti-inflammatory effect of mung bean has also been shown by other researchers either by blocking The cell proliferation and stabilizing p27^KIP1 in

Th cells after antigenic stimulation (Lee et al., 2013) or by suppressing LPS-induced IL-1b mRNA expression that leads to a reduced inflammatory response (Zhang et al., 2013). In another study, mung bean was shown to stabilize the liposomal membrane, which prevented the extracellular release of the lysosomal components and thus inhibited an inflammatory response (Venkateshwarlu et al., 2016). Other studies have shown a 30% decrease in triacylglycerol and total cholesterol levels in the muscle, which was correlated with the suppression of lipogenic genes such as ACC, C/EBP alpha, PGC-1 alpha, and PPAR gamma, subsequently leading to decreased inflammatory response (Inhae et al., 2015). Thus, these results suggest the potential use of mung bean in the reduction of symptoms of inflammatory diseases such as diabetes, allergies, rheumatoid arthritis, autoimmune disease, asthma, cancer, and other inflammatory disorders (Bellik et al., 2012). The aqueous extracts of mung bean also exhibited immunomodulatory activity as evaluated by the BrdU (bromodeoxyuridine) immunoassay, and it was found that 20 μg/ml of phytic acid, genistein, and syringic acid significantly suppressed interleukin-10 secretion and promoted interferon-γ secretion, thereby inducing Th-1 (helper T lymphocytes)-predominant response (Cherng et al., 2007). Thus, it can be concluded that the non-nutritional components of the mung bean play an important role in human immune modulation. The antiallergic activity of mung bean sprout extracts was also determined by assaying mast cell degranulation and histamine release, and passive cutaneous anaphylaxis (PCA) reaction. The study concluded that flavonoids (kaempferol-3-O-rutinoside, isovitexin, and isoquercitrin) of mung bean play a major role in antiallergic activity (Li et al., 2016).

Antidiabetic effects

Mung bean seeds or sprouts were found to have a significant effect on patients with diabetes. The oral feeding of seeds and ethanolic extracts of sprouts to type 2 diabetic mice lowered their blood glucose level and affected the level of plasma C-peptide, triglycerides, total cholesterol, blood urea nitrogen, and glucagon. Moreover, there was a marked improvement in insulin immunoreactive levels and increased glucose tolerance (Ganesan and Xu, 2018). The ethanolic extracts of sprouts were more effective in controlling high sugar levels because of their high content of starch-hydrolyzing enzymes (α-amylase and α-glycosidase) inhibitors. This inhibition potential of extracts has correlation with high phenolic content that reduced the intestinal absorption of carbohydrates and eventually lowered the blood glucose level (Randhir and Shetty, 2007; Yao et al., 2013; Luo et al., 2016). Randhir and Shetty (2007) reported that α-amylase and Helicobacter pylori growth inhibition were related to diabetic management and peptic ulcer management, respectively. Diabetic conditions were also associated with enhancement in the production of reactive oxygen species (ROS) that caused malfunction of β-cells and enhanced resistance to insulin, which further promoted type 2 diabetes (Ganesan and Xu, 2018). In this context, a study revealed that the phenolic compounds and flavonoids of mung bean lowered the formation of ROS and showed scavenging activity against free radicals, thus modulating hyperglycemia (Yeap et al., 2012). Furthermore, mung bean starch containing 32% amylose was considered to lower glycemia and thus help in glucose metabolism in persons with diabetes (Ren et al., 2015).

Lipid metabolism modulator activity

Various experimental studies (in vitro or animal experiments) showed that feeding mung bean in any form (extract/powdered/fermented or other approach) improved lipid metabolism (Tang et al., 2014; Yeap et al., 2015; Asrullah et al., 2016; Lopes et al., 2018; Liu et al., 2019). According to these studies, mung bean constituents (proteins, phenolics, and flavonoids) dose-dependently decreased blood total cholesterol, triglyceride, and low-density lipoprotein (LDL) levels and increased high-density lipid levels. A study involving rats and mice showed a significant decline in cholesterol level; this decline was thought to be due to the presence of cholesterol-like compounds such as phytosterol in mung bean, which prevented the synthesis and absorption of cholesterol (Asrullah et al., 2016). In a study by Yao et al. (2014), mung bean exhibited total cholesterol reducing property as shown by increased fecal excretion of sterol and bile acids and decline in intestinal uptake and synthesis of cholesterol. In another in vitro and animal study, ethanolic extracts and flavonoids (vitexin and isovitexin) of mung bean significantly lowered fat accumulation by decreasing the expression of genes involved in lipogenesis such as enzyme acetyl-CoA carboxylase (which involved in fatty acid synthesis), peroxisome proliferator-activated receptor-ligand-activated transcription factors (enhancing fatty acid storage), and the CCAAT/enhancer-binding protein which act as a fat metabolismregulating transcriptional factor. (Inhae et al., 2015). Mung bean starch and fiber fermentation by gut microflora has been reported to yield high content of shortchain fatty acids that reduced the levels of total cholesterol, LDL, and triglycerides in blood plasma (London et al., 2014; Reverri et al., 2017). Antioxidants of mung bean have also been shown to exert a hypolipidemic effect on hypercholesterolemic mice (Yeap et al., 2015; Liu et al., 2019).

Antihypertensive effect

Among the synthetic drugs used to regulate blood pressure, angiotensin-converting enzyme (ACE) inhibitors play an important role (Sonklin et al., 2020). In this context, mung bean was also found to affect the renin angiotensin system through its ACE inhibitor peptides. This resulted in lowering of the ACE enzyme activity and reduced the conversion of plasma angiotensin-I into angiotensin-II, wherein the latter one is involved in increasing blood pressure by acting directly on blood vessels, sympathetic nerves, and adrenal glands (Li et al., 2005; Xie et al., 2019; Sonklin et al., 2020). The resultant inhibition led to the control of hypertension. In some studies, by using sprout powder, juice, and concentrated extracts, it was found that the systolic blood pressure reduced significantly in short-term (3–6 h) and long-term (30 days) studies in rats; thus, mung bean showed potential in the prevention and management of hypertension (Hsu et al., 2011; Sonklin et al., 2020).

Antitumor effects

Although the precise mechanisms that regulate the prevention of cancer are fully recognized, several studies have proved that mung bean exerts antitumor effects through different mechanisms of action. One of these mechanisms is the action of nucleases R-TBN1 and R-HBN1 present in mung bean that were effective against the human myeloblastic leukemia cell line ML-2 (Matousek et al., 2009). These plant-derived nucleases were reported to be 10 times more potent than the bovine seminal ribonuclease and less toxic with high efficiency and stability for use as a biochemical agent for antitumor or cytostatic effect (Matousek et al., 2009). Mung bean also showed dose-dependent antiproliferative effect as determined by the MTT assay against various cancer cell lines such as tongue squamous cell carcinoma cell line (CAL27), prostate cancer cell line (DU145), breast cancer cell line (MCF-7), blood cancer cell line (HL-60), and ovary cancer cell line (SK-OV-3) (Xu and Chang, 2012). Another mechanism of action of mung bean was through trypsin inhibitors that affected the proliferation and metastasis of human colon cancer cells (SW480). The effect was confirmed through the wound healing assay in which the effect of a purified active peptide fragment of trypsin inhibitor was tested on a cancer cell line, and it was found that the concentration of 10 μmol/l effectively exhibited wound healing property with 50% reduction as compared to control (Zhao et al., 2012). In another study, acidified methanol extracts of sprouted mung bean were shown to induce apoptosis and cell cycle arrest/slowdown. This study concluded that the Cdk-inhibitor proteins (p21, p27, and p53), TNF-α, and interferon-β (IFN-β) may be the key regulatory factors responsible for this regulation (Ha dh et al., 2012). In another study, fermented mung bean extract (aqueous) was shown to halt the progression of breast cancer by decreasing the mitotic division of tumor cells through the promotion of IL-2 and IFN-γ production and cytotoxicity (Yeap et al., 2013). The components of mung bean, mainly starch, dietary fibers, phenolic compounds, and microconstituents such as phytic acid, vitexin, isovitexin, protease inhibitors, and saponins, have been shown to possess antioxidant and anticarcinogenic properties in various in vitro and animal studies (Hangen and Bennink, 2003; Patterson et al., 2009; Makhafola et al., 2016; Alam et al., 2017; Barahuie et al., 2017; Park et., 2017). Recently, by performing SRB (Sulforhodamine B) assay, methanolic extracts of seed coat (obtained from sprouted seeds) were shown to exhibit anticancer activity against the human lung cancer cell line HOP-62; the extract at 80 μg/ml concentration showed 65% cell growth inhibition activity (Mehta et al., 2021).

Hepatoprotective activity

The liver acts as a vital organ in the process of metabolism, storage, and excretion of various metabolites, and its dysfunction is associated with various ailments such as direct hepatic damage, inflammation, and fibrosis (Alshammari et al., 2018; Lopes et al., 2018). Consumption of mung bean in different forms such as sprouted, cooked, or fermented beans has been shown to exhibit significant hepatoprotective activity in a dose-dependent manner by decreasing enzyme activities (asparagine aminotransferase and alanine aminotransferase) and preventing excess deposition of fat (Lopes et al., 2018). The occurrence of nonalcoholic fatty liver disease is promoted by oxidative stress and ROS; in this context, mung bean was shown to overcome oxidative stress, lower the formation of ROS, and increase scavenging activity against free radicals (Yeap et al., 2012; Golabi et al., 2017; Alshammari et al., 2018). In alcohol-induced liver injury, aqueous extracts of mung bean were shown to significantly decline the serum level of enzymes (alanine aminotransferase and aspartate aminotransferase), total cholesterol, triglyceride, nitric oxide, and malondialdehyde and increase FRAP and superoxide dismutase activities (Ali et al., 2013). In another study involving an alcohol-induced liver injury (mouse) model, flavonoids were shown to exhibit significant hepatoprotective activity because of their antioxidant potential and through suppression of hepatic lipid accumulation (Liu et al., 2015).

Other biological properties

In addition to the above mentioned activities, mung bean also possesses other biological properties that might be significant for human health. These include antinociceptive effect (pain-relieving), where mung bean extracts inhibited the production of nitric oxide, a potent pain mediator (Ali et al., 2014); detoxification activity, where mung bean polyphenol extracts prevented cardiovascular diseases by acting against Al-induced biotoxicity through the ROS-JNK and NF-κB-mediated caspase pathways (Cheng et al., 2017); and alcohol dehydrogenase and tyrosinase inhibition activity, where the ethyl acetate extracts of mung bean showed activity against melanogenesis and thus could be used in cosmetics (Jeong et al., 2016).