Isolation, characterization and significance of papaya β-galactanases to cell wall modification and fruit softening during ripening

beta-Galactosidases (EC 3.2.1.23) from ripe papaya (Carica papaya L. cv. Eksotika) fruits having galactanase activities were fractionated by a combination of cation exchange and gel-filtration chromatography into three isoforms, viz., beta-galactosidase I, II and III. The native proteins of the respective isoforms have apparent molecular masses of 67, 67 and 55 kDa, each showing one predominant polypeptide upon SDS-PAGE of about 31 and 33 kDa for beta-galactosidases I and III, respectively, and of 67 kDa for beta-galactosidase II. The beta-galactosidase I protein, which was undetectable in immature fruits, appeared to be specifically accumulated during ripening. The beta-galactosidase II protein was present in developing fruits, but its lever seemed to decrease with ripening. beta-Galactosidase I seemed to be an important softening enzyme; its activity increased dramatically (4- to 8-fold) to a peak early during ripening and correlated closely with differential softening as related to position in the fruit tissue. The inner mesocarp tissue was softer, and its wall pectins were modified earlier and firmness decreased more rapidly during ripening compared to the outer mesocarp tissue. beta-Galactosidase II also may contribute significantly to softening because of its ability to catalyse increased solubility and depolymerization of pectins as well as through its ability to modify the alkali-soluble hemicellulose fraction of the cell wall. The physiological significance of both beta-galactosidase isoforms may partly be attributed to their functional capacity as beta-(1,4)-galactanases.