Comparison of analytical methods to assay inhibitors of angiotensin I-converting enzyme

The linearity, precision and repeatability of visible spectrophotometric (VSP) and high-performance liquid chromatography (HPLC) methods for analysis of inhibitory activity of angiotensin I-converting enzyme (ACE) were compared by using several inhibitors and Hip-His-Leu (HHL) as substrates. IC50 values (concentration at which ACE activity is inhibited by 50%) of 0.00206±0.00005 μg/mL for captopril, 192±4.53 μg/mL for soybean peptides, and 153±4.29 μg/mL for grass carp peptides determined by the VSP method, and these values were 1.07, 1.07, 1.18 and 1.44-fold, respectively, higher than those from the HPLC method. In addition, the inhibitory constant (Ki value) of captopril was determined to be 7.09 nM and 4.94 nM using VSP and HPLC method, respectively. These results showed that the HPLC method revealed a higher level of sensitivity and precision, suitable for assaying ACE inhibition activity of antihyper-sensitive peptides. In contrast, the VSP method can simultaneously measure several samples with simple operations, suitable for analysis of ACE inhibition activity of food protein enzymatic hydrolysates.