Papaya β-galactosidase/galactanase isoforms in differential cell wall hydrolysis and fruit softening during ripening

The potential significance of the previously reported papaya (Carica papaya L.) β-galactosidase/galactanase (β-d-galactoside galactohydrolase; EC 3.2.1.23) isoforms, β-gal I, II and III, as softening enzymes during ripening was evaluated for hydrolysis of pectins while still structurally attached to unripe fruit cell wall, and hemicelluloses that were already solubilized in 4 M alkali. The enzymes were capable of differentially hydrolyzing the cell wall as evidenced by increased pectin solubility, pectin depolymerization, and degradation of the alkali-soluble hemicelluloses (ASH). This enzyme catalyzed in vitro changes to the cell walls reflecting in part the changes that occur in situ during ripening. β-Galactosidase II was most effective in hydrolyzing pectin, followed by β-gal III and I. The reverse appeared to be true with respect to the hemicelluloses. Hemicellulose, which was already released from any architectural constraints, seemed to be hydrolyzed more extensively than the pectins. The ability of the β-galactanases to markedly hydrolyze pectin and hemicellulose suggests that galactans provide a structural cross-linkage between the cell wall components. Collectively, the results support the case for a functional relevance of the papaya enzymes in softening related changes during ripening.