Hydrolytic Action of Salmon Cathepsins B and L to Muscle Structural Proteins in Respect of Muscle Softening.

Cathepsins B and L, lysosomal cysteine proteases, are suspected of causing the softening phenomenon of post-mortem muscle of salmon. The proteolytic action of these enzymes on structural proteins of fish muscle was investigated. Cathepsin L was capable of hydrolyzing the major muscle structural proteins, such as connectin, nebulin, myosin, collagen, α-actinin, and troponins T and I. Although cathepsin B hydrolyzed connectin, nebulin, and myosin, the hydrolysis rate was very low and, furthermore, the hydrolytic action was limited to only these proteins. These findings indicate the direct participation of cathepsin L in drastic proteolytic degradation of the fine structure of muscle of the fish.