**3. Results and Discussion**

#### *3.1. Effect of Phenolic Acids on ACE Inhibition and NO Production*

ACE plays a crucial role in regulating blood pressure [18]. Many synthetic ACE inhibitors are currently being used for the treatment of hypertension. However, these drugs may cause adverse effects. Most natural compounds are safe and do not cause adverse effects. A previous study reported that plant phenolics have the potential to inhibit ACE in vitro [19]. Zhang et al. (2018) demonstrated the ACE inhibition effect of phenolic extracts and fractions derived from lentils, black soybean, and black turtle bean [20]. To confirm the antihypertensive effect of phenolic acids, we measured the ACE inhibitory activity. As shown in Figure 1, among the selected phenolic acids, sinapic acid showed the highest ACE inhibition rate (89%), followed by caffeic acid (78%). In this study, we used the EA.hy 926 endothelial cell line to evaluate the effect of phenolic acids on NO production. Treatment with phenolic acids (50 μM) did not affect the cytotoxicity of endothelial cells (Figure 2A). Reduced NO levels contribute to hypertension and endothelial dysfunction. NO plays an essential role in the vasorelaxation of large arteries [21]. We found that treatment with gallic acid, sinapic acid, and ferulic acid significantly increased NO production by 85.1, 50.5, and 31.9%, respectively, compared with that in the control group cells (Figure 2B). These results indicate that phenolic acids may improve endothelial dysfunction, consequently regulating blood pressure.

**Figure 1.** Inhibitory effect of selected phenolic acids (10 mM) on angiotensin I converting enzyme. Captopril (1.15 μM) was used as positive control. Each value was expressed as the mean ± standard error (*n* = 3). Different letters above the bars indicate significant differences based on the Duncan's test (*p* < 0.05).

**Figure 2.** Effect of selected phenolic acids (50 μM) on cell cytotoxicity (**A**) and NO production (**B**) in EA.hy 926 cells. Quercetin (25 μM) was used as positive control. Each value was expressed as the mean <sup>±</sup> standard error (*<sup>n</sup>* = 3). Statistical significance was analyzed using the Tukey test. ## *<sup>p</sup>* < 0.01 and ### *p* < 0.001 versus nontreated cells.
