2.3.1. Proteases (endopeptidases)
The level of soluble protein in pooled sampleswas determined using
themethod described by Bradford (1976). Trypsin activity was assayed
according to Erlanger et al. (1961), using BAPNA (N-α-Benzoyl-DLarginine
p-nitroanilide) as substrate. The mixtures were incubated at
37 °C and the absorbance of the reaction products was measured at
410 nm. Chymotrypsin activity was measured by the method of
Hummel (1959), asmodified by Applebaum et al. (2001), using BTEE
(N-Benzoyl-L-tyrosine ethyl ester) as substrate. The mixtures were
incubated at 37 °C and the absorbance of the reaction products was
measured at 256 nm. The reaction of trypsinwas stopped by adding 30%
acetic acid. One unit of enzyme activitywas defined as 1 μg nitroanilide
released per minute, using a molar extinction coefficient of 8.8 for
trypsin and of 964 for chymotrypsin.
Leucine aminopeptidase was measured at 37 °C as suggested by
Appel (1974), using leucine P-nitroanilide as substrate. The reaction
of leucine aminopeptidase was stopped by adding 30% acetic acid. The
mixtures were incubated at 37 °C and the absorbance of the reaction
products was measured at 410 nm. One unit of enzyme activity was
defined as 1 μg nitroanilide released per minute, using a molar
extinction coefficient of 8.2.
Acid proteinase (pepsin) activity was evaluated as described by
Sarath et al. (1989), using 2% hemoglobin as substrate. The enzyme
crude extracts and the substrate were incubated at 37 °C and the
absorbance of the reaction products measured at 280 nm. One unit of
enzyme activity was defined as 1 μg tyrosine released per minute,
using the molar extinction coefficient of 0.005.