ISOLATION, CHARACTERIZATION AND SIGNIFICANCE OF PAPAYA BETA-GALACTANASES TO CELL-WALL MODIFICATION AND FRUIT SOFTENING DURING RIPENING

beta-Galactosidases (EC 3.2.1.23) from ripe papaya (Carica papaya L. c v. Eksotika) fruits having galactanase activities were fractionated by a combination of cation exchange and gel-filtration chromatography in to 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-P AGE of about 31 and 33 kDa for beta-galactosidases I and III, respecti vely, and of 67 kDa for beta-galactosidase II. The beta-galactosidase I protein, which was undetectable in immature fruits, appeared to be s pecifically accumulated during ripening. The beta-galactosidase II pro tein was present in developing fruits, but its lever seemed to decreas e with ripening. beta-Galactosidase I seemed to be an important soften ing enzyme; its activity increased dramatically (4- to 8-fold) to a pe ak early during ripening and correlated closely with differential soft ening as related to position in the fruit tissue. The inner mesocarp t issue was softer, and its wall pectins were modified earlier and firmn ess decreased more rapidly during ripening compared to the outer mesoc arp 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 physio logical significance of both beta-galactosidase isoforms may partly be attributed to their functional capacity as beta-(1,4)-galactanases.