PURIFICATION AND PROPERTIES OF AN EXO-(1-]3)-BETA-D-GALACTANASE FROM ASPERGILLUS-NIGER

An exo-(1 --> 3)-beta-D-galactanase was purified by six chromatographic steps from a culture supernatant of Aspergillus niger. Its apparent molecular mass was 66 kDa, as estimated by SDS-PAGE analysis. The purified enzyme had no detectable activity on various p-nitrophenyl glycosides and on native plant polysaccharides but exhibited a high activity on a (1 --> 3)-beta-D-linked galactan backbone obtained after partial acid hydrolysis and two Smith degradations of gum arabic. The optimum conditions were pH 4.5 and 40-50 degrees C. The enzyme had a Michaelis constant (K-m) of 1.9 mg/mL for the beta-(1 --> 3)-D-galactan with a maximum reaction velocity (V-max) of 1380 nkat/mg. The study of the reaction products obtained after enzyme treatment of two galactans derived from gum arabic through one or two Smith degradations showed that it was an exo-(1 --> 3)-beta-D-galactanase able to by-pass the branching points of galactan backbones and thus to release the side-chains of type II arabinogalactans in an undegraded form.