In this study, the macro (essential)nutrients: sodium (Na), potassium (K), and magnesium (Mg) and micro (nonessential)nutrients: iron (Fe), copper (Cu), manganese (Mn), zinc (Zn), and nickel (Ni) were found in the leave extracts of M. whitei in varying degrees. Consumption of these macrominerals such as Na, K, and Mg and micronutrients such as Fe, Cu, Mn, Zn, and molybdenum (Mo) is very vital for specific physiological functions, while consumption of cadmium (Cd), nickel (Ni), chromium (Cr), lead (Pb), tin (Sn), and mercury (Hg) even in trace amounts results to toxic effects on the tissues of the body [37, 38].
The Amaechi and Egesi (2017) [48] study evaluates the nutritional values of the fruits of M. whitei (Hook. f.) Skeels consumed by some populations among the Izzi clan of Ebonyi state of southeastern part of Nigeria. The results found, among other components, the fruits contained a high amount of moisture (88.20%), total sugar (15.70%), reducing sugars (9.63%), nonreducing sugars (6.13%), the energy value of 40.80 KCal/100 g, very low riboflavin, thiamin (1.53 mg/100 g), and niacin (3.04 mg/100 g). The presence of antioxidants such as vitamins C (14.50 mg/100 g) and E (2.45 μg/g) and mineral elements with potassium and sodium being the most abundant. An insignificant amount of antinutritional phytochemicals which made them conclude that the fruit is not toxic for human consumption. In our study, sodium, potassium, and magnesium were found in the aqueous, ethanolic, and chloroform extracts as the macronutrients in the leaves of M. whitei. The lower levels of sodium and potassium found in this study suggest levels of mineral context may vary in the plant part (roots, stems, leaves, and fruits) compare to the report of Amaechi and Egesi (2017) [48] with the fruits of M. whitei having a higher proportion of sodium and potassium mineral elements. Further findings from this nutritional evaluation of the aqueous, ethanolic, and chloroform extracts of M. whitei leaves revealed the presence of varying amounts of micronutrients such as Fe, Cu, Mn, Zn, Cd, and Ni with lower concentrations of zinc and iron in the three leaf extracts of M. whitei studied than what was found in the wild edible fruit of M. whitei reported by Amaechi and Egesi (2017) [48]. While their study did not analyze copper, manganese, cadmium, and nickel contents of the wild edible fruits, our study did not analyze calcium and phosphorus.
Excess daily intake of sodium may lead to hypertension. However, the low levels of sodium found in the dry leaves of M. whitei (2.4–3.6 mg/100 g) did not occur at an amount that may lead to hypertension nor satisfy the daily sodium requirement of 1.2 to 1.5 g per day [48] assuming 300 g of the plant material, or equivalent of 70 g of the extract of M. whitei, is consumed daily. Consumption of an equivalent amount of the plant material (300 g) daily would also be too low (K: 1.9–4.0 mg/100 g; Mg: 0.2–0.5 mg/100 g; Fe (0.01–0.07 mg/100 g); Cu: 0.01–0.09; Zn: 0.01–0.02 mg/100 g; Mn (0.001–0.01 mg/100 g)) to provide sufficient quantity of the daily requirements of potassium (2.6–3.4 mg/day), magnesium (220–260 mg/day), Fe (17–20.5 mg/day), copper (2–3 mg/day), zinc (7–9.5 mg/day), and Mn (2–5 mg/day) [49, 50]. The work of Ngbolua and Mukeba (2020) [51] on the fresh leaves of M. whitei from northern Angola isolated larger amounts of K (1149.83 mg/100 g) and Cu 3.14 mg/100 g than in the dry leaves of M. whitei which were used in this present study, in addition to Ca (844.87 mg/100 g), P (175.89 mg/100 g), and Se 87.80 mg/100 g which were not analyzed in this study. The low level of these elements in our sample may be due to the loss of some of the nutrients from the leaves during drying and other processing activities or as a result of soil differences, as reported in the work of Janvier et al. [52], on the root bark of M. whitei obtained from five different localities in northern Rwanda which showed different macronutrients and micronutrient contents [52]. This suggests that it would be nutritiously auspicious to consume fresh leaves of M. whitei compared to the dry leaves, except in a situation where the components of these leaves could be concentrated and packaged for daily consumption according to the daily requirements of these micronutrients in the human body.
In this study, phenolic compounds, flavonoids, saponins, tannins, alkaloids, terpenoids, triterpenoids, reducing sugars, and cardiac glycoside were found present, while steroids and anthraquinones were absent in either of the three extracts (aqueous, ethanolic, and chloroform) of the dry leaves of M. whitei. Ngbolua et al. [53], in their mini-review, reported the presence of saponins, flavonoids, tannins, resins, and the absence of cyanogenic glycosides, anthraquinone, alkaloids, and cardiac glycosides in the leaves of M. whitei. This review complimented our study by confirming that the leaves of M. whitei contain saponins, flavonoids, and tannins, in addition to alkaloids, terpenoids, triterpenoids, reducing sugars, and cardiac glycoside not found in the study reported by Ngbolua et al. [53]. The difference might be due to soil differences or type of solvent used in extraction or the difference in ages of the plants used in both studies.
In other parts of the plant besides leaves, Inkoto et al. [54] reported that the roots of Mondia whitei contain phenolic compounds, coumarins, anthraquinones, anthocyanins, tannins, and iridoids using thin-layer chromatography, while Ndukui et al. [55] indicated the presence of saponins, phenols, alkaloids, and tannins in the ethanolic fresh root bark extracts of M. whitei. In the study conducted by Wacho et al. [56], reducing sugars, triterpenes, and steroids were found present in the aqueous and hexane extracts of M. whitei, confirming the presence of reducing sugars and triterpenes isolated from the leaves in the present study. Gbadamosi and Erinoso [57] reported in their study that the roots of M. whitei contained more saponins and tannins than in its leaves, while the leaves contained more flavonoids than the roots. However, in the wild edible fruits of M. whitei, Amaechi and Egesi [48] reported the fruits as containing tannins, phenol, phytate, sterol, carotenoid, oxalate, saponins, flavonoids, alkaloids and went further to quantify the amount of hydrogen cyanide (mg.kg−1) from the fruits of M. whitei on a fresh weight basis. All these reports of different parts of the plant containing different phytochemicals prove that phytochemical compounds vary according to the different parts of the plant, though similar in some instances. This variation in different parts of M. whitei could be due to certain environmental factors, seasonal variations, or differences in varieties of the species [58].
Food has been found to have a profound influence on health. This implies that the consumption of foods of high quality is necessary for body nourishment and protection against processes of inflammation and oxidative stress. Inflammatory and oxidative processes are healed or reduced by phytochemicals present in plant tissues. The use of the different parts of this plant to treat urinary infection, jaundice, headache and diarrhea, and other metabolic and infectious diseases in forms of the antioxidant property and ferric reducing power of MWL extract fractions. The free radical scavenging activity of MWL extract fractions was determined by nitric oxide, 2,2-diphenyl-1-picrylhydrazyl (DPPH), ferric-reducing power, and ABTS assays. The results showed three fractions of MWL extract (aqueous, ethanolic, and chloroform) exhibiting higher percentage inhibition than ascorbic acid against ABTS, respectively, and ascorbic acid displayed higher ferric reducing power than chloroform, ethanolic, and aqueous fractions of MWL extract. The results can be explained in favor of the ability of the different fractions of the MWL extract to effectively and efficiently scavenge for free radicals and mop them up, thereby inhibiting their deleterious effects in breeding inflammatory disorders such as cancer, arthritis, diabetes, hypertension, cardiovascular diseases, atherosclerosis, hyperlipidemia, obesity, aging, Alzheimer’s diseases, and other infectious diseases [59,60,61,62]. The high antioxidant propensity of the extract fractions indicates the positive correlation between the phenolic compounds such as polyphenol, flavonoids, flavonols, tannins, and phenolic terpenes in the leaves of Mondia whitei and their ability to scavenge for and inhibit the free radicals and their inflammatory and oxidative activities such as lipid peroxidation, protein, and DNA damage [56, 63,64,65]. Gbadamosi and Erinoso highlighted the superior antimicrobial and antioxidant activity of the aqueous extracts of both the roots and leaves of M. whitei, explaining the possible reason to be due to higher solubility of the active phytochemical constituents in the water [57]. This fact accentuates the expected effective role of the extracts of different plant parts in managing various metabolic and infectious ailments. This study assessed the antioxidant effects of phytochemical components of M. whitei and found that the aqueous, ethanolic, and chloroform fractions of the plant possess a high level of free radical scavenging activity against DPPH, NO, ABTS, and ferric-reducing power, which inflicts damage on macromolecules such as lipids, proteins, and DNA. It could be explained that the phenolic compounds in the leaves of M. whitei are responsible for this protection against macromolecular damage occasioned by the overriding influences of free radicals released by oxidative stress and metabolic processes in biological systems. This foregoing explanation is consistent with the fact that the aqueous leaf fraction contained the highest amount of total phenol and ethanolic fraction with the highest amount of flavonoids and flavonols, which have been reported by Ghosh et al. [66] to be invariably responsible for their antioxidant activity because of the hydroxyl group in the phenolic and flavonoid and flavonol constituents in the plant. The cytotoxic impact of DOX and 5-FU, well-known anticancer medicines, was confirmed by our investigation. The in vitro assay indicates MWL to have significant cytotoxic activity on HeLa and HT-29 cell lines due to the presence of mineral nutrients and phytochemical and bioactive components. The cell viability reduction of MWL agrees with the cytotoxic effect report of other medicinal researchers [45, 67,68,69] and opposite our previous study on the cytotoxic effect of B. coriacea Engl. (BC) on AsPC-1 cells [44]. However, it is important to identify the precise mechanism of action that triggers the cytotoxicity activity of MWL.
In the current study, in silico analysis was considered to gain insight into the molecular mechanism of anti-toxicity via antioxidant potentials of M. whitei leave extracts. Plant extracts have been explored following in silico studies [70, 71] to define the exact molecules underlying their therapeutic effects. We used a standard inhibitor to predict exact binding pockets. The choice of target protein was based on existing reports on overexpression of CathB in cancers [72] linked to oxidative damages. Cancers of the breast, cervix, bladder, stomach, colon, ovary, lung, prostate, and thyroid have all been associated with the overexpression of cathepsin B [73]. Ten of the eighteen bioactives identified in MWL showed potent inhibition of CathB revealing their molecular target with the binding energy of −4.4 to −8.3 Kcal/Mol. Interactions within the pockets revealed similar kinds of bonds. Although the molecular interactions revealed different amino acids in the pocket, we suggest that Trp221 and Gln23 could be the most critical target. For instance, all the potential inhibitors shared similar interactions compared to the standard inhibitors within the pocket, albeit the Trp221 and Gln23. Perhaps these amino acids play a critical role in the synthesis of nucleic acids translated to proteins like Cathepsin B in cancer cells to foster metastasis [74]. Again, malignant ovarian tumor cystic fluid had considerably greater cathepsin B and C levels [75]. Numerous investigations have shown that increased cathepsin B levels are associated with accelerated angiogenesis, invasion, and metastasis.
Furthermore, increased cathepsin B expression in the tumor epithelial cells of colorectal carcinomas was linked to a considerably reduced patient survival time [76]. Although stromal cells and normal epithelial cells nearby the tumors can express cathepsin B, the expression is often highest along the tumor’s advancing edge [77]. Interestingly, existing literature has demonstrated that amino acid restrictions play active roles in cancer interventions [78,79,80]. In the current study, we suggest that the inhibition of the amino acids in the binding pockets by the MWL bioactive can lead to reduced tumor vascularity, increased tumor cell mortality, decreased cell proliferation, and poor tumor invasion [81, 82]. Similarly, this has been reported in cathepsin B-deficient animals [83]. Therefore, we hypothesize that amino acid inhibition, especially in cancer cells, could be a critical molecular target of Mondia whitei leaves used to manage various diseases.