Three types of proteinases in Japanese common squid Todarodes pacificus hepatopancreas as studied by using carp myofibrils as substrate

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Three types of proteinases, namely cysteine-, metallo-, and serine-proteinases, were found in squid hepatopancreas by studying the inhibition spectra using carp myofibril as substrate. The cysteine-, metallo-, and serine-types showed the highest activities at 50℃, 35℃, and 40℃, respectively. The optimal pHs were pH 5, pH 7, and pH 9 for the cysteine-, metallo-, and serine-types, respectively. When assayed at 20℃ and pH 7.5, the metallo-type showed the highest activity. The metallo-type was characterized by a high selectivity in the digestion of myosin. Among the three enzymes, the cysteine-type was found to be the most stable against thermal and acid treatments. Heat treated myofibrils were more susceptible to cysteine- and serine-types, but less susceptible to the metallo-type. Acid treatment of myofibrils also enhanced the digestibility by cysteine type. The results indicated that the cysteine-type seemed to be the most suitable enzyme to produce peptides from denatured myofibrils by their random digestion.