Potential antioxidant, anticholinergic, antidiabetic and antiglaucoma activities and molecular docking of spiraeoside as a secondary metabolite of onion (Allium cepa)

Onion contains many dietary and bioactive components including phenolics and flavonoids. Spiraeoside (quercetin-4-O-β-D-glucoside) is one of the most putative flavonoids in onion. Several antioxidant techniques were used in this investigation to assess the antioxidant capabilities of spiraeoside, including 1,1-diphenyl-2-picrylhydrazyl radical (DPPH·) scavenging, N,N-dimethyl-p-phenylenediamine radical (DMPD^•+) scavenging, 2,2'-azinobis-(3-ethylbenzothiazoline-6-sulphonate) radical (ABTS^•+) scavenging activities, cupric ions (Cu²⁺) reducing and potassium ferric cyanide reduction abilities. In contrast, the water-soluble α-tocopherol analogue trolox and the conventional antioxidants butylated hydroxytoluene (BHT), butylated hydroxyanisole (BHA), and α-tocopherol were utilized as the standards for evaluation. Spiraeoside scavenged the DPPH radicals an IC₅₀ of 28.51 μg/mL (r²: 0.9705) meanwhile BHA, BHT, trolox, and α-tocopherol displayed IC₅₀ of 10.10 μg/mL (r²: 0.9015), 25.95 μg/mL (r²: 0.9221), 7.059 μg/mL (r²: 0.9614) and 11.31 μg/mL (r²: 0.9642), accordingly. The results exhibited that spiraeoside had effects similar to BHT, but less potent than α-tocopherol, trolox and BHA. Also, inhibitory effects of spiraeoside were evaluated toward some metabolic enzymes including acetylcholinesterase (AChE), butyrylcholinesterase (BChE), carbonic anhydrase II (CA II) and α-glycosidase, which are related to a number of illnesses, such as Alzheimer's disease (AD), diabetes mellitus and glaucoma disorder. Spiraeoside exhibited IC₅₀ values of 4.44 nM (r²: 0.9610), 7.88 nM (r²: 0.9784), 19.42 nM (r²: 0.9673) and 29.17 mM (r²: 0.9209), respectively against these enzymes. Enzyme inhibition abilities were compared to clinical used inhibitors including acetazolamide (for CA II), tacrine (for AChE and BChE) and acarbose (for α-glycosidase). Spiraeoside demonstrated effective antioxidant, anticholinergic, antidiabetic and antiglaucoma activities. With these properties, it has shown that Spiraeoside has the potential to be a medicine for some metabolic diseases.