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Hypertension is a major public health problem rising across the globe. Inhibition of angiotensin converting enzyme (ACE) is identified as a main therapeutic target in controlling high blood pressure. The current research study investigated the ACE inhibitory property of a flavonoid-rich apple peel extract (FAE), its constituents, selected flavonoids and some quercetin metabolites using a biochemical assay of ACE inhibition and a human umbilical vein endothelial cell (HUVEC) model. FAE, all the tested flavonoids except genistein, and two quercetin metabolites (quercetin-3-O-glucuronic acid and quercetin-3-O-sulfate) significantly (p < 0.05) inhibited ACE. Enzyme kinetic analysis proved that flavonoids are competitive inhibitors over ACE. In the HUVEC model FAE, quercetin-3-O-glucoside and quercetin-3-O-glucuronic acid significantly (p < 0.05) inhibited ACE. Overall, FAE and most of the flavonoids tested showed ACE inhibition in vitro which needs further investigations using animal and human clinical trials.