Ion exchange of β-galactosidase: The effect of the immobilization pH on enzyme stability

β-Galactosidase from Aspergillus oryzae has been immobilized at pH 5, 7 and 9 on an aminated support using 5 mM buffer. The immobilization was total in 30 min, maintaining 75–80% of activity. These preparations were inactivated at different pH values and in the presence of 50% ethanol. The stability of the enzyme immobilized at pH 9 was much lower than that of the enzyme immobilized at pH 5 under all studied conditions but the differences decreased as the ionic strength of the inactivation solution increased. The likeliest explanation to these different stabilities depending on the immobilization pH was that the enzyme presented a different orientation on the support. The enzyme immobilized at pH 5 was more stable than the free enzyme at pHs 5 and 9 (by a 2 or a 6 fold factor respectively), while at pH 7 the free enzyme was clearly more stable than the immobilized enzyme.