Rapid and simple colorimetric assay for screening angiotensin I-converting enzyme inhibitors.

Angiotensin-converting enzyme (ACE) is one of the main regulators of blood pressure through its action on the renin-angiotensin system. ACE inhibitory peptides from natural materials inhibit ACE activity and have considerable importance as antihypertensive agents. A new chromogenic reaction method for determining hippuric acid (HA) and angiotensin I-converting enzyme (ACE) inhibitor activity was developed. This method is based on the reaction of HA with p-dimethylaminobenzaldehyde in the presence of quinoline, acetate, and acetic anhydride. The red-orange formation product in the reaction has a stable absorbance in the visible region and it was determined at 478 nm. The assay conditions were optimized and by using an ACE concentration of 12 mU/mL in enzymatic reaction, the method was applied to monitor the IC(50) values (the concentration of inhibitor required to inhibit 50% of the ACE activity) for captopril and Saurida elongata (Synodontidae) muscle protein hydrolyzate. With the proposed method, IC(50) values for captopril and Saurida elongata muscle protein hydrolyzate were determined as 0.0123 µM and 0.1648 mg/mL, respectively. Those results correspond to the IC(50) values of 0.0109 µM and 0.1820 mg/mL obtained by high-performance liquid chromatography (HPLC) method. The proposed method is rapid, accurate, reproducible and convenient, and suitable for screening ACE inhibitor peptides from food materials while it does not require HA extraction from the components of the ACE activity assay reaction.