Flavonoid Composition and Antioxidant Efficacy of Citrus Peels: An Integrated in vitro and in silico Approach toward Potential Neuroprotective Agents

The current study explored the therapeutic significance of different standardized citrus peel extracts as potential neuroprotective agents using integrated in vitro and in silico approaches. Hydroethanolic extract of five industrially important citrus fruit peels were subjected to HPLC-based quantification of estimating major flavonoids of nutraceutical importance. Pharmacological activities like antioxidant and anti-inflammatory activities of the extracts were determined by in vitro assays. Further, The identified bioactive metabolites were subjected to the Prediction of Activity Spectra for Substances program to get a prediction of their biological activity spectrum. Amongst various solvent combinations, 80% ethanol provided maximum (≥ 20% w/w) extract yield. Mandarin peels of Citrus reticulata showed the highest amount of polyphenolics (Citrus reticulata Blanco; 42.24 ± 0.57 mg gallic acid equivalent/g) and flavonoids (Citrus reticulata c.v.;13.08 ± 0.17 mg quercetin equivalent/g) content. The most abundant flavonoid compound present in all the citrus peel was hesperidin, except Citrus reticulata Blanco and Citrus grandis, which showed a considerably high amount of nobiletin and naringin, respectively. Citrus reticulate c.v. peel extract showed potent antioxidant [IC50 = 118.82 ± 1.97 μg/mL in 2,2- diphenyl-1-picrylhydrazyl assay and IC50 = 138.12 ± 2.67 μg/mL in 2,2'-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid assays)], anti-inflammatory (IC50 = 50.61 ± 6.79 μg/mL), and acetylcholinesterase inhibitory ((IC50 = 130.61 ± 2.04μg/mL) activities compared to the other extracts. In silico assessment revealed a high (Pa > 0.7) activity score for free radical scavenging, lipid peroxidase inhibitory, membrane integrity agonistic, anti-inflammatory, antioxidant, and several other important biological activities of the identified flavonoids in the extracts, thus supported neuroprotective potential. Citrus flavonoids naringin, rutin, and tangeretin showed high activity scores for anti-inflammatory activity strengthening the results of in vitro assay. These potentials of citrus peels could be utilized in the development of functional foods and nutraceuticals for neurodegenerative conditions. Furthermore, such a practice will help citrus agro-industrial waste valorisation.