Acid invertase of melon fruits: immunochemical detection of acid invertases
1992
Iwatsubo, T. | Nakagawa, H. | Ogura, N. | Hirabayashi, T. | Satō, Tōru
Soluble (S) and cell wall-bound (CW) forms of acid invertases (AIs) were partially purified from mesocarp tissue of melon (Cucumis melo L. cv. AMS) fruits and some properties of the enzymes were examined. The optimum pH values for activity were 5.5 and 4.5 for S- and CW-AI, respectively. Km values of S- and CW-AIs for sucrose were 4.2 and 1.7 mM, and those for raffinose were 20.0 and 10.5 mM, respectively. The band of a polypeptide of about 70 kDa was detected with antibodies against AI of tomato fruits (CW form) and of mung bean seedlings (S form) by immunoblot analysis. The 70-kDa polypeptide was isolated from each fraction and further analyzed by limited proteolytic digestion with Staphylococcus aureus V8 protease. The digested polypeptides generated almost identical profiles. The localization of CW-AI was also studied and it was found to be an extracellular enzyme. The activities of S- and CW-AIs per gram fresh weight increased and reached a maximum on day 10 after anthesis, and then they decreased rapidly during maturation of fruits. The relative levels of S- and CW-AI polypeptides were correlated with the levels of activities of S- and CW-AIs. These results show that the decrease in activity of AI during maturation of melon fruits is due to a decline in levels of AI polypeptides in the fruits.
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