Red Onion (<i>Allium cepa</i> L.) Methanolic Extract Increases Extracellular Nucleotide Hydrolysis in Rat Serum

Ectonucleotidases are enzymes involved in nucleotides metabolism. The amount of circulating nucleotides may modulate distinct pathophysiological processes in the vasculature, including platelet aggregation and vascular tone. Onion (Allium cepa L.) phenolic compounds modulate enzymatic activity. The present study evaluated the total phenolic content of red onion methanolic extract, its antioxidant capacity, and its ability to interfere in nucleotides hydrolyses in rat serum. Total phenolic content was determined with the Folin-Ciocalteau reagent using gallic acid as a standard, while total flavonoid content was obtained through the aluminum chloride colorimetric method with quercetin as a standard. Antioxidant capacity was evaluated from the ability of the extract to scavenge ABTS·+ and DPPH· radicals. ATP, ADP, AMP, and p-Nph-5\u0027-TMP hydrolyses were colorimetrically determined in response to different onion extract concentrations (0, 125, 250, 500, or 1000 μg/mL). Phenolic content of the extract was 54.35 mg GAE 100 g-1 sample, while flavonoid content was 7.22 mg quercetin g-1 sample. The IC50 value for ABTS·+ was 374.13 ± 7.52 μg/mL, while it was 440.29 ± 15.17 μg/mL for DPPH·. Red onion extract increased ADP and p-Nph-5\u0027-TMP hydrolysis. The results confirmed that red onion contains high content of antioxidant, mainly flavonoids, and high antioxidant capacity. Additionally, biochemical studies suggest that the increased ADP breakdown may be important to regulate vascular processes. As it occurs for other enzymes, the antioxidant capacity of onion extract may neutralize reactive oxygen species (ROS) formation and favor ectonucleotidase activity and the hydrolysis of ADP, a major platelet agonist.