Abstract
In the previous paper1), it was reported that the optimal pH of casein digestion by the extract of cuttle-fish liver lie in the range of 5.0-6.0 and at 2.5. Two kinds of proteinases with different pH optima at 5.0-6.0 and 2.5 respectively seem to be existed in the liver at least. Accordingly, this work was undertaken to see several characteristic properties of these proteinases.
Both the proteinase with the optimal pH at 5.0-6.0 and the proteinase with the optimal pH at 2.5 were found to be activated by such reducing agents as NaCN, cysteine, ascorbic acid, and glutathione (Table 1).
On the other hand, these actions were inhibited by metallic ions, such as Mn…, Mg…, Co…, Cu…, and Zn…, and by such thiol reagents as monoiodoacetate, and p-chloromercuribenzoate (PCMB). (Tables 2 and 3)
However, it was observed that there are some differences in the effects of activators or inhibitors on the activities of these proteinases; in the former, its action is more powerfully activated by the reducing agents than that in the latter and is more strongly inhibited by the metal ions and the thiol reagents.
As shown in Fig. 2, it was found that the inhibitions by PCMB are readily reversed by the addition of cysteine in the both.
From the results, it may be concluded that cuttle-fish liver contains two kinds of cathepsinlike enzymes of which natures resemble each other, and each proteinase possesses sulfhydryl radical as one of the active centers.
