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 (K_i 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.