ACE-inhibitory activity was measured by HPLC method (Wang
et al., 2008). The supernatants of soluble fractions (pH 4.3) obtained
from fermentation samples were diluted in distilled
water to the same protein concentration (1 mg/mL) before ACEinhibitory
activity assay. According to the HPLC method, 50 L
of the buffered substrate solution (5 mM HHL in 50 mM HEPES
with 300 mM NaCl, pH 8.3) was mixed with 40 L of the supernatant
sample and pre-incubated at 37 °C for 5 min and then
10 L of ACE (0.1 U/mL) was added and further incubated at
37 °C for 30 min. After incubation, 250 L of 1M HCl was added
to stop the reaction. The amount of hippuric acid (HA) released
in the ACE reaction was measured by RP-HPLC (2690
Waters, Milford, MA, USA) equipped with C18 column
(4.6 × 150 mm, 5 m, Waters) and variable wavelength absorbance
detector set at 228 nm. The elution consisted of 70%
solvent A (0.05% (v/v) trifluoroacetic acid and 0.05% (v/v) triethylamine
in water to maintain pH value at 2.9–3.3) and 25%
(v/v) solvent B (100% (v/v) acetonitrile) and eluted at a flow rate
of 1 mL/min. The inhibitory activity was calculated using the
following equation.
where A is the concentration of HA without adding sample, B
is the concentration of HA with sample added and I is used
to express the percentage of ACE inhibition by sample.
The IC50 value was defined as the concentration of peptide
required to inhibit 50% of the ACE activity under the assayed
conditions, which was estimated by non-linear regression by
fitting data to a four-parameter logistic curve (SPSS20).