Fruit peels are still regarded as waste and polluted materials for the environment; however, the chemical content in these peels has a broad biological activities [2]. According to Kosasih et al., (2019), phytochemical investigations of sweet orange peel extract (Citrus sinensis L. Osbeck) revealed that orange peel is rich in total polyphenol content. It contains carbohydrates, tannins, saponins, flavonoids, anthocyanins, and β-cyanins, quinones, phenols, coumarins, amino acids, and alkaloids. [4]. This study agrees with our results. However, Abd El-Ghfar and co-workers observed that the total phenolic amount varied greatly from fresh to dried orange peel samples. They also observed that the total phenolic content of orange peel extracted with ethanol was significantly higher (p<`0.05) than in methanol extract [19]. It’s important to highlight that the phytochemical compound content of orange peels varies from one citrus species to another, according to geographic location and the polarity of the solvent used for extraction.
Additionally, polyphenols and flavonoids groups are known to have many health benefits, mostly associated with antioxidant activity [4, 20, 21]. DPPH and FRAP assays are the most common methods used to evaluate the in-vitro antioxidant activity of plant extracts [22, 23]. This study revealed that the methanolic extract of orange peel presented a similar DPPH activity as ascorbic acid. However, the ethanolic extract presented medium DPPH scavenging activity according to aqueous, n-butanol, and ethyl acetate extracts.
In many cases, the effectiveness of antioxidant activity is linked to total phenolic compounds. It was found by Hegazy and co-workers, (2012) that the ethanolic extract of orange peel presented hight levels of phenolic compounds. These results indicate that the reducing power of bioactive compounds is associated with antioxidant activity [24]. Furthermore, the solvent plays a crucial role in the plant constituents’ extraction [24, 25]. It is admitted that methanol and ethanol solvents presented the highest polarity. This property allowed to extract a high yield of phenolic compounds and leads to the highest antioxidant activity (DPPH) if compared to other solvents [24]. The principle of the FRAP method is different from the DPPH assay [23]. According to our results, all orange peel extracts had a reducing power (FRAP), but not at the same level or in the same way as DPPH methods. FRAP analysis indicated a powerful activity of n-butanol and ethyl acetate extracts compared with ascorbic acid. However, the other orange peel extracts presented a weak ferric reducing power according to control. The antioxidant activity of plants can act as primary and secondary antioxidants [26]. Then, it is also dependent on the polarity and type of the extracting solvent, as well as on the test system and the substrate protected by the antioxidant [27].
In this study, polyphenols, flavonoids, tannins and alkaloids of orange peel were assessed by uv-vis HPLC in different solvent of which polyphenols were the major chemical compounds. Preliminary chemical characterization lets to identify ten phytochemical compounds: rutin, (+)-epicatechin, quercetin, (-)-epicatechin, caffeine, caffeic acid, coumarin, ascorbic acid, quinoline, and gallic acid. However, the results obtained in this work cannot be compared with those reported in the literature. Because the retention times depend on many factors, such as extract matrix, solvent composition, and the gradient elution. In plants, the presence of rutin is an indicator of its pharmacological potency. This molecule is endowed with broad biological activities: antioxidant, antimicrobial, anti-inflammatory, anticancer, antidiabetic, etc [10]. In this study, the presence of ascorbic acid, gallic acid, rutin, epicatechin, and quercetin explains and confirms the strong antioxidant activity of orange peel.
The societal health issue of obesity is linked to a number of risk factors, including hyperlipidemia, type 2 diabetes, and hypertension [28]. The discovery of functional foods that can promote weight loss and reduce body fat is an important health objective for people around the world [29, 30]. For that purpose, the protective and corrective effects of orange peel on obese rats were investigated by observing the histopathological changes in the liver, kidney, pancreas, and thyroid. In the present study, the marked liver lipid droplet deposition observed in the obese group was notably ameliorated by orange peel administered as a corrective and protective diet. We deduced that orange peel seems to be more hepatoprotective than corrective. These observations were supported and explained by El-Shazly and colleagues, who reported that supplementation with orange peel isoflavones inhibited adiposity in non-alcoholic fatty liver disease (NAFLD) by up-regulating genes involved in fatty acid β-oxidation, anti-adipogenesis, and the enhancement of leptin and adiponectin mRNA levels involved in the anti-steatohepatitic pathway. However, they added that orange peel also reduced the expression level of mRNA such as steatoses, tumor necrosis factor α, and ghrelin [29].
According to another previous report, the hepatoprotective effect of Citrus by-product extracts may be due to the presence of phytoconstituents like phenolic compounds, which are related, at least in part, to their antioxidant activity. Antioxidants have been shown to prevent oxidative stress-related liver pathologies directly, by scavenging reactive oxidant substances, and indirectly, as part of the antioxidant defence system [1]. Flavonoids are natural phenolic compounds present in fruit and vegetable species; they play an important role as free radical scavengers or antioxidants in biological systems [31].
The following studies showed that exposing rats to a high-fat diet for three months led to the appearance of pronounced signs of thyroid hypofunction. As is known, one of the main reasons for the development of thyroid hyperplasia is insufficient hormonal secretion, leading to active stimulation of the gland with a subsequent increase in its size. The results of a correlation analysis confirmed a close relationship between the degree of obesity and the severity of histomorphological disorders of the thyroid gland in animals. But in fact, it is difficult to establish the cause or consequence relationship of the excess accumulation of adipose tissue in the body [32]. We must keep in mind that in our experiment, the thyroid disorder observed in the obese group is the consequence of the obesity situation. These results are well supported by the study of El-Sayed, Ibrahim, et al., 2020 [32]. In parallel, the following results demonstrated that thyroid sections of obese rats receiving orange peel as a corrective diet presented a normal follicle structure compared with the control group. However, the thyroid histology of obese rats receiving orange peel as a protective diet presented follicular dystrophy; colloids were small and contained rich resorption vacuoles with thyrocyte hyperplasia. These results confirm that the effect of orange peel is more pronounced as a corrective diet than a protective one on thyroid changes induced by obesity.
Further research showed that rats receiving 1500 mg/kg of citrus sinensis as fresh orange juice for 28 days significantly (p < 0.05) increased TSH and decreased T3 and T4 levels compared with the control group [33]. Furthermore, the same results were observed by Parmar and Kar (2008), who found that 25 mg/kg of citrus sinensis aqueous extract peel administered orally for 15 days was able to significantly reduce T3 and T4 levels in Wistar rats [34] indicating a hypothyroid condition [35]. In fact, the literature reports that many plant extracts are recognised to interfere with thyroid hormone homeostasis in different ways, including the binding of the thyroid stimulating hormone receptor, thyroid iodide transport, and thyroid hormone secretion from the gland. Parmar and Kar (2008) added that the thyroid inhibition property of C. sinensis might also be mediated through one of the above-mentioned mechanisms or through an inhibition of thyroid peroxidase, the main enzyme implicated in thyroid hormone biogenesis. Furthermore, some flavonoids are known to inhibit this enzyme [34, 36]. These funds prove the hypothyroidism effects of orange peel, which are not favourable for obesity management.
Throughout human history, plants were considered as primary source of food and medicine for whole cultures around the world [37]. They contain a wide range of phytochemical compounds, including flavonoids, polyphenols, lignans, and tannins, which makes it difficult to test all these bioactive substances at the same time. Molecular docking provides a solution to explore the activity of a wide range of molecules in a short time. Interactions between the compound and the receptor play a crucial role in the discovery of drugs [18].
It is crucial to be well informed and to conduct in-depth research on the proteins involved in obesity and the traditional herbal medications used to treat this condition. In the present study, ten compounds of orange peel previously identified by HPLC were docked against thyroid receptor beta 7WMH. This latter is one of the most important proteins involved in obesity. The molecular docking results of quercetin, (+)-epicatechin; and (-)-epicatechin with 7WMH receptor protein revealed that each studied ligand had established interactions with three amino acids. In contrast, the co-crystallised ligand used as a control had an interaction with six amino acid residues (Leu 330, Asn 331, Gly 332, Arg 320, Arg 282, and His 435), which allowed this ligand to react as an agonist. In fact, the quality and numbers of amino acids involved in ligand-protein complexation are crucial in the activation or inactivation of the target protein, suggesting that quercetin, (+)-epicatechin, and (-)-epicatechin may be play an antagonist role against 7WMH receptor. In parallel, the effects of flavonoids on thyroid activity have been studied both in vitro and in vivo [38]. In fact, coumarin (10 mg/kg/day, 15 days) [39], quercetin (10 mg/kg/day, 10 days) [40], rutin, naringin, and hesperidin, all identified in the citrus genus and proved to exhibit hypothyroidism activity in animals' models at different levels [38]. Moreover, three of these phytochemical compounds were well identified and docked in the following study. Research is primarily focused on studying a specific group of components separated from the whole or parts of the plant; however, the holistic approach must be considered in the interpretation of phytochemical effects.