Seasonal changes and preliminary characterization of cathepsin D-like activity in sardine (Sardina pilchardus) muscle

The seasonal changes of cathepsin D-like activity in white and dark muscle of sardine were followed and maximum activity was found in April and October, which could be related to the spawning period of this species. Higher catheptic activity was measured in dark muscle and females. The crude extract of this muscle enzyme had an optimum pH and temperature of 3.2 and 55 °C, respectively, and the activity was strongly inhibited by low levels of NaCl (6%). The thermal stability was lower at pH 3.2 than at pH 6.5 and the enzyme was stable in the pH range 3–4.5. The activity of this protease was detected during the ripening process of salted sardine which suggests its participation in this process.