Revisiting the Nutritional, Chemical and Biological Potential of Cajanus cajan (L.) Millsp.

The genus Cajanus (Family: Fabaceae) consists of approximately 37 species, and Cajanus cajan (C. cajan) is a significant member of the genus. It is a commercial legume crop widely grown in sub-tropical and semi-arid tropical areas of the world. C. cajan is well known for its folk medicinal uses to treat various disorders, such as toothache, dizziness, diabetes, stomachache, female ailments and chronic infections. These properties have been linked to the presence of several value-added nutritional and bioactive components. Different solvent extracts from C. cajan (leaves, root, stem and seeds) have been evaluated for their phytochemical and biological activities, namely antioxidant, antimicrobial, antidiabetic, neuroprotective, and anti-inflammatory effects. Taken together, and considering the prominent nutraceutical and therapeutic properties of C. cajan, this review article focuses on the important details including ethnomedicinal uses, chemical composition, biological applications and some other medicinal aspects related to C. cajan nutraceutical and pharmacological applications.


Introduction
India contributes significantly to global grain legume production, accounting for approximately 90% of global production and ranking sixth in terms of production and area cultivated [1].Most legume species belong to the Fabaceae or Leguminosae families and are depicted due to their fruits generally known as pods.Recognized for their great significance as dietary supplement for humans and animals, these legumes, such as pea, cowpea, chickpea, soybean, mung bean, beans, fava beans, lentils, peanut and pigeon pea, have been increasingly investigated for nutraceutical purposes [2].
Grain legumes are often considered as nature's treasure offered to mankind and are regarded as "poor man's meat" because of their high quantity of vitamins, minerals, protein (16-50%) and dietary fiber (10-23%) [3].Moreover, grain legumes also play a crucial role in ecological services, due to their biological nitrogen fixation capacity [4].
Cajanus cajan (L.) Millsp. is a leguminous annual woody or perennial plant [5], and the genus Cajanus consists of approximately 37 species out of which C. cajan is an extensively used commercial legume crop [6].It is a native genus from ancient Egypt, Africa, Asia and America, and now it has been widely distributed across the tropical and subtropical regions [7].Globally, C. cajan has been recognized by various names, like Pigeon pea (Australia); red gram, tur, arhar, dal (India); mu dou (China); guando (Brazil) [8], and The cytological analysis of C. cajan showed that it is diploid having 2n = 2x = 22 chromosomes with an average length of 5.73 ± 1.15 µm up to 10.92 ± 2.69 µm and dominantly metacentric in shape, consisting of 14 metacentric and 4 submetacentric chromosomes [14].C. cajan has a genome of size 858 mega-base pairs [15].In the comparative genetic characterization of wild and cultivated C. cajan genotypes, the cultivated species present maximum polymorphic loci [6].

Morphology
From a morphological point of view, C. cajan is a short-lived shrub with erect stems of 1-2 m height [16].Its roots are finely nodulated, lateral and deep rooted of up to 3 m, possessing a root system having a central taproot with several secondary and lateral branches.The branching pattern in C. cajan is determined based on the habitat, spacing and plant genotype.The leaves are lanceolate to elliptical in shape and size, ranging from 6 to 17 cm in length and are around the same breadth.The flowers are usually, yellow to orange in color, present a long peduncle of 1-8 cm long and terminal or axillary racemes (4-12 cm).Calyx: gamosepalous with 5 lobes, Corolla: zygomorphic and bright yellow, Androecium: 10 stamens (4 with short filaments and 6 with long filaments), Gynoecium: ovary (superior, pubescent, 2-9 ovules and monocarpellary), style (long, filiform and glabrous), stigma (incurved & thickened), Seeds: spherical or lens shaped [8].

Traditional Uses
The use of C. cajan for traditional purposes dates since immemorial times, and such information has passed over the generations in order to substantially promote the continuity of knowledge improvement.The diversity and availability in regional flora of plant resources is markedly determined by the use of plant species in folk medicinal practices [17].Various studies have demonstrated that the leaves, seeds, stems and roots of C. cajan have been used in traditional medicine for the treatment of various ailments, including toothache, diabetes, dizziness, baldness and gastrointestinal discomfort in few domains of India, Bangladesh, China and many other nations.In Oman, C. cajan seeds are used for treating many chronic infections, and native people use the juice from leaves to treat various dermatological conditions [18].In ancient times, the floral decoction was used for treating pneumonia, coughs, menstrual disorders, dysentery and bronchitis, while leaf

Nutritional Properties
The nutritional profiling of C. cajan, including of its leaves, seeds, roots and stem, has also been investigated by standard methods to determine the proximate, amino acid and mineral composition (Table 2).The maximum fat (15.00 ± 0.090%), moisture (8.20 ± 0.229%), carbohydrate (40.95 ± 0.244%) and nutritive value (333.73 ± 1.500%) were recorded in seeds, however the highest protein content was found in leaves (31.99 ± 0.070%) (Table 2).Results of the proximate composition of protein isolate, full fat flour and defatted flour derived from C. cajan and its comparisons with wheat flour and yellow-pea flour are shown in Table 3.
The study of amino acids content present in C. cajan reveals that leaves (808.8 ± 10.3 mg/100 g) and roots (871.8 ± 11.2 mg/100 g) contain the highest concentration of glutamine, whereas alanine (1547.8± 3.9 mg/100 g) and aspartic acid (11.56 g/16 gN) were found in maximum amounts in seeds.The lowest concentration of tryptophan was observed in leaves (2.4 ± 0.4 mg/100 g), roots (1.3 ± 0.4 mg/100 g) and seeds (9.5 ± 0.1 mg/100 g).The detail description of the amino acid composition is mentioned in Table 4.To what concerns to mineral composition, the evaluation of C. cajan revealed higher levels of calcium in leaves (33 ± 4.9 mg/100 g), seeds (581 ± 4.3 mg/100 g) and roots (597 ± 2.5 mg/100 g) and lower levels of zinc (2.1 ± 0.9, 0.7 ± 0.2 and 0.7 ± 0.9 mg/100 g, respectively) (Table 5).Due to the nutritional contribution and health benefits of C. cajan it is regarded as an alternative to produce vegetable meat, with high quality standards and appropriate sensory characteristics that allow consumer acceptance and integration of product in daily diet.Moreover, due to its essential nutrient content, this makes an exquisite preference for vegetarian consumers.

Biological Applications
With the growth of world's economy and enhancement in people's living standard, several chronic diseases, like neurological, metabolic, inflammatory, cerebrovascular and cardiovascular disorders have increased rapidly [46].Natural products are widely recognized for their biological or pharmacological potential since ancient times, and recently the interest in their study has re-emerged as upcoming drug candidates.Globally, around 50,000 plants have shown potent therapeutic potentialities [47].According to pharmacological studies, C. cajan leaves have various bioactivities, including antioxidant, antiplasmodial, anticancer, hypoglycemic, insecticidal, neuroprotective and antimicrobial activities [37,48].Moreover, the molecular regulatory mechanism of few biological applications/activity are briefly summarized in Table 8.The most relevant therapeutic applications of C. cajan briefly described below and presented in Figure 2. 50,000 plants have shown potent therapeutic potentialities [47].According to pharmacological studies, C. cajan leaves have various bioactivities, including antioxidant, antiplasmodial, anticancer, hypoglycemic, insecticidal, neuroprotective and antimicrobial activities [37,48].Moreover, the molecular regulatory mechanism of few biological applications/activity are briefly summarized in Table 8.The most relevant therapeutic applications of C. cajan briefly described below and presented in Figure 2.

Antimicrobial Activity
The antimicrobial activity of plants varies pronouncedly depending on chemical constituents presents, hence it is difficult to classify single antimicrobial mechanisms, as they rely on the phytochemical properties of the plant [57].Dinore and Farooqui (2022) [58] investigated the antimicrobial activity of C. cajan leaves methanol extract against Escherichia coli and Candida albicans, and the results indicated a remarkable ability to inhibit the growth of the microorganisms, with minimum inhibitory concentrations (MIC) of 50 µg/mL and minimum fungicidal concentrations (MFC) of 250 µg/mL.Cajanuslactone, one of the most abundant phytoconstituents present in C. cajan leaves is expected to be the responsible for the marked antimicrobial properties [22].The antifungal potential of C. cajan roots were examined by microdilution method to demonstrate the use of plant extract as a novel therapeutic source [59].The ethanolic extract of the roots showed antifungal activity in terms of MIC (Candida albicans 512 µg/mL, Candida krusei 512 µg/mL and Candida tropicalis 512 µg/mL) [59].
In another study, Qi et al. [45] extracted the essential oil from C. cajan leaves by solvent free microwave extraction and reported antimicrobial properties to the extracted oil.The essential oil revealed an effective antimicrobial potential, addressed through determination of MIC and minimum bactericidal concentration (MBC), against Bacillus subtilis (1.06 and 2.12 mg/mL, respectively), and Propionibacterium acnes (0.13 and 0.26 mg/mL, respectively).Additional studies on antimicrobial potential of C. cajan (different parts) are listed in Tables 9 and 10.

Antioxidant Activity
Different studies also have been performed to assess the antioxidant potential of different parts of C. cajan.Aggarwal et al. (2015) reported the antioxidant potential of C. cajan ethanol seed extract using ferric reducing antioxidant power (FRAP) assay.The results obtained revealed a concentration-dependent antioxidant activity (concentration 25 to 450 µg, 4.4 to 43.0 µM) [64].

Anti-Diabetic Activity
The antidiabetic potential of C. cajan methanol root extract was addressed by Nahar et al. (2014) in alloxan-induced diabetic Swiss albino mice.The experimental mice were treated with C. cajan extract up to 5 days (200 and 400 mg/kg bw, orally).Glucose tolerance test and hyperglycemic effect studies (involving diabetes induction in mixed sex Swiss albino mice by injection of aqueous alloxan monohydrate, 55 mg/kg, intravenously) were carried out on tested animals, along with determination of the antioxidant activity.The results showed a rapid decline in fasting serum glucose level (p < 0.001) and blood glucose level (p < 0.001) in 5 days.On the basis of these results, the plant extract evidenced potent hypoglycemic and antioxidant properties compared to other species (e.g., Tamarindus indica seeds) [65].

Tyrosinase Inhibitory Activity
C. cajan root, stems and seeds were also addressed for its ability to inhibit tyrosinase activity, and for that water, dichloromethane and methanol extracts were prepared.The IC 50 values of the extracts varied from 3.55-12.43mg/mL, whereas the maximum inhibitory capacity was reported for methanol root extract (IC 50 = 3.55 mg/mL) [38].

Neuroprotective Activity
A variety of naturally-occurring bioactive compounds are currently being explored for their therapeutic potential in neurodegenerative diseases, but only a few are known to have benefits [68].The use of plant extracts and their bioactive constituents are one of the promising approaches for the treatment of neurological diseases [69].C. cajan was also exploited for their neuroprotective abilities.The presence of stilbenoids is able to induce apoptotic neuronal death by Aβ 25-35 injection in mice and cause elevation in choline acetyltransferase (ChAT) and superoxide dismutase (SOD) activity in the cortex and hippocampus [70].In a study with injured larvae of zebrafish, cajanin stilbene acid (CSA) and its derivative were found to decline the migration and production of primitive macrophages and neutrophiles [71], being thus proposed that C. cajan may be a promissory source of biomolecules with neuroprotective abilities.

Conclusions and Future Prospects
Pigeon pea (C.cajan) is one of the most commonly and widely used, tropical and subtropical legume due to its nutrient packed edible seeds, might being effectively used for food and medicinal purposes.However, it is an underutilized/neglected legume species.As yet, several flavonoids, isoflavonoids, tannins, phenolics and proteins have been isolated from various plant parts, and their therapeutic properties have also been confirmed, but many pure and bioactive components were still not taken into consideration.Several studies have identified that the phytochemicals present display excellent bioactive effects for a plethora of human conditions.
A number of extensive research has been done only on extracts rather than isolated fractions and oils, that indicates necessity of further study in this direction.Moreover, majority of studies are limited to in vitro screening, with only a few focusing on in vivo testing.As a result, advanced research is required to explore new phytopharmaceuticals based on C. cajan.Clinical trials should be conducted to assess the toxicity profile of C. cajan in humans in respect of antioxidant activity, antimicrobial activity, anthelminthic activity, anti-inflammatory activity, antidiabetic activity and immunomodulatory aspects.The current review article aims to concentrate attention of researchers as well as pharmaceutical industries on untouched and unexplored aspects related to C. cajan and may serve as a crucial link towards the establishment of C. cajan as a therapeutic drug.Although, as it is a leguminous plant and plays a major role in biological nitrogen fixation, further more relevant knowledge regarding the characteristics of soil, indigenous microbes and plant species-specific responses is required to establish the inoculant for maximum ecological restoration benefits and to support future adoption of this practice.

Table 2 .
Proximate composition of Cajanus cajan from different countries.

Table 3 .
Comparison of Cajanus cajan flour proximate composition with generally used flours.

Table 4 .
Amino acid composition of different parts of Cajanus cajan.

Table 6 .
Bioactive components present in Cajanus cajan from different regions.

Table 8 .
Molecular regulatory mechanism of few biological activities of Cajanus cajan.

Table 8 .
Molecular regulatory mechanism of few biological activities of Cajanus cajan.
ZI: Zone of inhibition; MIC: Minimum inhibitory concentration; "-": No zone of inhibition and MIC.