In LIPIDTEXT we have developed a concept where protein patterns are studied by electrophoresis (SDS-PAGE) using both intact muscle and model systems.
Using this concept we have tried to find out to what extent the 3 different proteases, cathepsin B, cathepsin L and cathepsin D, all present in rainbow trout muscle, contribute to the softening process. First we have analysed the muscle from fish of different texture by electrophoresis and found proteins whose presence in the muscle relates positively or negatively to the texture. Then we have added purified proteases to muscle extracts from rainbow trout to find out how the different cathepsins influence the texture-related proteins found in the intact muscle. In this way we showed that the cathepsins actually affected many proteins of the muscle. Among those several proteins correlating to texture were affected by either cathepsin B, cathepsin D or both. Although cathepsin L affected many proteins in the muscle only few of the texture-related were among them and this protease therefore it only seems to have a minor influence on the texture of rainbow trout filets.
We could conclude that the balance between the activities of cathepsin B and cathepsin D was important for the textural appearance. Dominance of cathepsin B seemed to preserve texture better than dominance of cathepsin D which led to softening.