Muscle cathepsins of marine fish and invertebrates

Muscle proteases are located mainly in the lysosomes, in the sarcoplasm, and in the extracellular matrix of the connective tissue surrounding each cell. The lysosomal proteases, cathepsins, have optimum activity in the acidic range, although many of them retain high activity also 1 or 2 pH units away from the optimum value. Among the cathepsins there are endopeptidases and exopeptidases. Most cathepsins hydrolyse several proteins of the myofibrils. The major protease of the lysosomes in fish and squid muscles is cathepsin D, an aspartyl endoproteinase. Although it is present in the muscle fibre itself, its generally rather low activity at low temperature limits its significance in tenderization of refrigerated fish of most species. Cathepsin L, a cysteinyl protease, is involved in autolysis and softening in matured chum salmon. Cathepsin B, a cysteinyl carboxypeptidase, is capable to attack also some myofibrillar proteins. Cathepsin A or carboxypeptidase A, and cathepsin C, a dipeptidyl hydrolase and dipeptidyl transferase, contribute to the hydrolysis of muscle proteins in a concerted action with the other cathepsins.