Angiotensin-Converting Enzyme Inhibitory Activity of Selected Phenolic Acids, Flavonoids, Their <i>O</i>-Glucosides, and Low-Molecular-Weight Phenolic Metabolites in Relation to Their Oxidation Potentials

<b>Background/Objectives</b>: In this study, the angiotensin-converting enzyme (ACE) inhibitory activity of selected phenolic acids, flavonoids, their <i>O</i>-glucosides, and low-molecular-weight phenolic metabolites was addressed to show their importance against blood hypertension. <b>Methods</b>: A fluorescence assay was used for the determination of the ACE inhibitory activity, whereas the first anodic peak oxidation potential (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mrow><mi mathvariant="normal">E</mi></mrow><mrow><mi mathvariant="normal">p</mi><mi mathvariant="normal">a</mi></mrow></msub></mrow></semantics></math></inline-formula>) was provided by the differential pulse voltammetry (DPV) method. The relationship between the ACE inhibitory activity and <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mrow><mi mathvariant="normal">E</mi></mrow><mrow><mi mathvariant="normal">p</mi><mi mathvariant="normal">a</mi></mrow></msub></mrow></semantics></math></inline-formula> was evaluated. <b>Results</b>: Phenolic acids showed a very low ACE inhibitory activity, and their rank was chlorogenic acid > p-coumaric acid > sinapic acid > gentisic acid > ferulic acid > syringic acid > vanillic acid > protocatechuic acid > caffeic acid. The low-molecular-weight phenolic metabolites of flavonoids showed a moderate ACE inhibitory activity. In contrast, flavonoid aglicones had the highest ACE inhibitory activity, and the order was luteolin > quercetin > kaempferol > cyanidin > delphinidin > pelargonin > naringenin. A lower inhibition activity was noted for quercetin-3-<i>O</i>-glucoside, luteolin-4′-<i>O</i>-glucosides, cyanidin-3-<i>O</i>-glucoside, and pelargonidin-3-<i>O</i>-glucosides, whereas a higher ACE inhibition activity was observed for 7-<i>O</i>-glucosides of luteolin, apigenin, and kaempferol. A lack of correlation was found between the IC₅₀ of phenolic acids, low-molecular-weight phenolic metabolites, and their <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mrow><mi mathvariant="normal">E</mi></mrow><mrow><mi mathvariant="normal">p</mi><mi mathvariant="normal">a</mi></mrow></msub></mrow></semantics></math></inline-formula> values. In contrast, weak positive correlations were found between the IC₅₀ of aglicons, 3-<i>O</i>-glucosides, 7-<i>O</i>-glucosides, and their <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mrow><mi mathvariant="normal">E</mi></mrow><mrow><mi mathvariant="normal">p</mi><mi mathvariant="normal">a</mi></mrow></msub></mrow></semantics></math></inline-formula> values provided by the DPV (r = 0.61, r = 0.66 and r = 0.88, respectively). <b>Conclusions</b>: This study expands our knowledge of the ACE inhibitory activity of phenolic compounds.