Isolation and Characterization of a Possible Native Cucumisin from Developing Melon Fruits and Its Limited Autolysis to Cucumisin

Abstract

A serine proteinase was purified to an electrophoretically homogeneous state from developing fruits of Prince melon (Cucumis melo L. var. Prince).
Its molecular mass was 67 kD by sodium dodecyl suit ate-poh aery lamide gel electrophoresis, which was apparently different from that of cucumisin (50 kD), a serine proteinase previously isolated from Prince melon.
When the purified 67-kD enzyme was incubated at 50°C and pH 7.2, it was split into a 54-kD proteinase and a 14-kD polypeptide by limited autolysis without any loss of the caseinolytic activity. Commercial cucumisin preparation was found to contain the 67- and 54-kD proteinases. The content of each amino acid residue in the 67-kD enzyme was higher than that in the 54-kD proteinase or cucumisin. Thus, it was concluded that the purified 67-kD enzyme is a native form of cucumisin, and that cucumisin is a product derived by the limited autolysis of the 67-kD proteinase.
The 67-kD proteinase was more stable than the 54-kD one at acidic pHs.