based on their optimal pH (acid or alkaline) and
the tissue of production (gastric, pancreatic and
mucosal) (Halver and Hardy, 2002). Proteases
reported in this study were serine proteases
(alkaline protease) because the kcat–pH of serine
proteinase–catalyzed reactions showed a bell–
shaped curve reflecting a maximum activity close
to pH 8 (Wong, 1995). The result was similar to
that reported by Amaral et al. (2005) who showed
that the optimal pH for the soluble enzyme
(trypsin) extracted from the intestines of Nile
tilapia was 8.0. In stomach-less fish like tilapia,
digestion of protein is undertaken by alkaline or
serine proteases, which are most active in an
alkaline environment. However, the highest
activity levels of protease in tilapia at all sizes were
not that different and all were found in the lower
intestine