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Metabolism and Enzymology

The Role of -Galactosidases in the Modification of Cell Wall Components during Muskmelon Fruit Ripening

Department of Agricultural Chemistry, Obihiro University of Agriculture and Veterinary Medicine, Obihiro, Hokkaido 080, Japan

The changes in activities of soluble -galactosidase and two forms of wall-bound -galactosidases extracted with NaCl and EDTA were investigated throughout the development of muskmelon (Cucumis melo L. cv Prince) fruits. DEAE-cellulose ion-exchange chromatography of soluble -galactosidase revealed the presence of two isoforms. Soluble isoform I was detected in all stages throughout the fruit development, whereas soluble isoform II appeared around 34 d after anthesis when fruit ripening initiated. Both NaCl- and EDTA-released -galactosidase activities also increased as ripening proceeded. The soluble and wall-bound forms behaved differently upon ion-exchange chromatography. Enzymological properties such as optimum pH, optimum temperature, K_m values for p-nitrophenyl -D-galactopyranoside, and inhibition by metal ions were nearly similar in all forms. Molecular sizes of pectic polymers and hemicelluloses extracted from fruit mesocarp cell walls were shifted from larger to smaller polymers during ripening, as determined by gel filtration profiles. NaCl-released -galactosidase from cell walls of ripe fruits had the ability to degrade in vitro the pectin extracted from preripe fruit cell walls to smaller sizes of pectin similar to those that were observed in ripe cell walls in situ. Both soluble isoform I and II were able to degrade in vitro the 5% KOH-extractable hemicellulose from preripe fruit cell walls to sizes of molecules similar to those that were observed in ripe cell walls in situ. Soluble isoform I and the NaCl-released form from ripe fruits were able to modify in vitro 24% KOH-extractable hemicellulose from preripe cell walls to sizes of molecules similar to those that were observed in ripe fruits in situ.

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