In vitro proteolysis of myofibrillar and sarcoplasmic proteins of white muscle of sea bass (Dicentrarchus labrax L.): effects of cathepsins B, D and L

The purpose of this study was to obtain additional information regarding proteolysis mechanisms and disorganization of fish myofibrils resulting in a loss of flesh quality. The ability of cathepsins to degrade in vitro myofibrillar and sarcoplasmic proteins from fish muscle was investigated in order to explain their role in post mortem softening. This led to the identification of substrates of the enzymes. Cathepsins degraded myosin heavy chain and α-actinin. Tropomyosin and actin were only susceptible to the action of cathepsin L. Troponin T (assumed 32 kDa component) was resistant only to the action of cathepsin D. Desmin was degraded by cathepsins B and L. Slight changes of some other myofibrillar or cytosolic proteins were also observed (creatine kinase and other unidentified proteins). When compared with protein modifications observed in stored post mortem muscle, these results suggest that cathepsin D (if location is in the cytosol and if pH conditions for activity are met in post mortem muscle) could be involved in a post mortem myofibrillar degradation mechanism.