Acidophilic β-Galactosidase from Aspergillus niger AHU7120 with Lactose Hydrolytic Activity Under Simulated Gastric Conditions

Abstract

Acidophilic β-galactosidase is a useful enzyme as digestive supplement used to alleviate symptoms of lactose intolerance. Aspergilli are the source of several acidophilic β-galactosidases that retain enzymatic activity under gastric conditions. In this study, we investigated the extracellular acidophilic β-galactosidase activity of six Aspergillus niger strains, AHU7104, AHU7120, AHU7217, AHU7294, AHU7295 and AHU7296; A. niger AHU7120 was selected as an enzyme source. β-Galactosidase from A. niger AHU7120 (AnBGal) was purified from culture supernatant. Its N-terminal sequence was identical to that of An01g12150, which belongs to the glycoside hydrolase family 35, from A. niger CBS 513.88. The DNA sequence of AnBGal was identical to An01g12150. Recombinant AnBGal (rAnBGal) harboring yeast α-factor signal sequence was expressed in Pichia pastoris, and 21.9 mg of purified rAnBGal with 129 U/mg of enzyme activity was isolated from 200 mL of culture supernatant. Native and recombinant AnBGal enzymes showed similar pH optima, pH stability, and kinetics for p-nitrophenyl β-D-galactopyranoside and lactose; rAnBGal showed slightly lower thermal stability than the native enzyme. Lactose in milk was rapidly degraded by rAnBGal at higher pH values (range, 2.0‒3.5), consistent with the pH optimum of AnBGal. We estimated that 3.5 μM AnBGal may degrade ≥ 66% of lactose before gastric half-emptying of ingested milk. These data indicate that AnBGal may help alleviate symptoms of lactose intolerance.