Acid Invertase of Melon Fruits: Immunochemical Detection of Acid Invertases

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 theenzymes were examined. The optimum pH values for activity were5.5 and 4.5 for S- and CW-AI, respectively. K_m values of S-and CW-AIs for sucrose were 4.2 and 1.7 mM, and those for raffinosewere 20.0 and 10.5 mM, respectively. The band of a polypeptideof about 70 kDa was detected with antibodies against AI of tomatofruits (CW form) and of mung bean seedlings (S form) by immunoblotanalysis. The 70-kDa polypeptide was isolated from each fractionand further analyzed by limited proteolytic digestion with Staphylococcusaureus V8 protease. The digested polypeptides generated almostidentical profiles. The localization of CW-AI was also studiedand it was found to be an extracellular enzyme. The activitiesof S- and CW-AIs per gram fresh weight increased and reacheda maximum on day 10 after anthesis, and then they decreasedrapidly during maturation of fruits. The relative levels ofS- and CW-AI polypeptides were correlated with the levels ofactivities of S- and CW-AIs. These results show that the decreasein activity of AI during maturation of melon fruits is due toa decline in levels of AI polypeptides in the fruits. (Received April 1, 1992; Accepted September 7, 1992)