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 sulfate-polyacrylamide 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 enzymeis 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.
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Yamagata et al. (1989) studied this question.