Purification and chemical characterisation of a cell wall-associated b-galactosidase from mature sweet cherry (Prunus avium L.) fruit

Using four different chromatographic steps, b-galactosidase was purified from the ripe fruit of sweet cherry to apparent electrophoretic homogeneity with approximately 131-fold purification. The Prunus avium b-galactosidase showed an apparent molecular mass of about 100 kDa and consisted of four different active polypeptides with pIs of about 7.9, 7.4, 6.9 and 6.4 as estimated by native IEF and bgalactosidase- activity staining. The active polypeptides were individually excised from the gel and subjected to SDS-PAGE. Each of the four native enzymes showing b-galactosidase activity was composed of two polypeptides with an estimated mass of 54 and 33 kDa. Both of these polypeptides were subjected to N-terminal amino acid sequence analysis. The 54 kDa polypeptide of sweet cherry b-galactosidase showed a 43% identity with the 44 kDa subunit of persimmon and apple b-galactosidases and the 48 kDa subunit of carambola galactosidase I. The sweet cherry b-galactosidase exhibited a strict specificity towards p-nitrophenyl b-D-galactopyranoside, a pH optimum of 4.0 and Km and Vmax values of 0.42 mM and 4.12 mmol min1 mg1 of protein respectively with this substrate. The enzyme was also active towards complex glycans. Taken together the results of this study prompted a role for this class of enzymes on sweet cherry fruit ripening and softening.