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

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°C, respectively. The optimal pHs were 5, 7, and 9 for the cysteine, metallo, and serine types, respectively. When assayed at 20°C 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 the cysteine and serine types, but less susceptible to the metallo type. Acid treatment of myofibrils also enhanced the digestibility by the 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.