Phenolic contents, in vitro antioxidant activities and biological properties, and HPLC profiles of free and conjugated phenolics extracted from onion, pomegranate, grape, and apple

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ABSTRACTThis study quantified the free and conjugated phenolic content of extracts from selected Liliaceae (white and red onion), Punicaceae (red and pink pomegranate), Vitaceae (zini, red globe, and baladi grape), and Rosaceae (yellow and red apple) cultivated family plants and evaluated their antioxidant activities and beneficial properties toward diabetes and hypertension. Free and bound phenolics were extracted from plants, and total phenolic contents were evaluated. Antioxidant activities were assessed, as well as the inhibitory effects of angiotensin I-converting enzyme (ACE) and α-amylase and α-glucosidase activities. Liquid chromatography–mass spectrometry/mass spectrometry (LC–MS/MS) measured the individual phenolic content in both free and bound extracts. The free phenolic contents in the selected plants were higher than bound phenolic contents, except for in zini and red globe grapes. In onion, the highest antioxidant activity was observed in the bound phenolic extract (49.6–56.9%). In pomegranates, grapes, and apples, the highest antioxidant activities were obtained in free phenolic extracts at 30°C with values of 65.3% to 74.2%, 39.6% to 47.6%, and 49.5% to 55.9%, respectively. ACE was 100% inhibited by the free phenolic extract at 30°C from white onion and by the bound phenolic extract after acid hydrolysis of red onion. The highest α-amylase enzyme inhibitory activity was obtained in free phenolic extracts at 60°C of both red pomegranates and zini grapes. The highest α-glucosidase inhibition activity (99.3–99.6%) was found in free phenolic extracts at 30°C from red and pink pomegranates. Reverse phase HPLC revealed two predominant groups of free and bound phenolics identified in onion, pomegranate, grape, and apple.