Characterization and molecular cloning of a heterodimeric β-galactosidase from the probiotic strain Lactobacillus acidophilus R22

beta-Galactosidase from the probiotic strain Lactobacillus acidophilus R22 was purified to apparent homogeneity by ammonium sulphate fractionation, hydrophobic interaction, and affinity chromatography. The enzyme is a heterodimer consisting of two subunits of 35 and 72 kDa, as determined by gel electrophoresis. The optimum temperature of beta-galactosidase activity was 55 degrees C (10-min assay) and the range of pH 6.5-8, respectively, for both o-nitrophenyl-beta-D-galactopyranoside (oNPG) and lactose hydrolysis. The K-m and V-max values for lactose and oNPG were 4.04 +/- 0.26 mM, 28.8 +/- 0.2 mu mol D-glucose released per min per mg protein, and 0.73 +/- 0.07 mM, 361 +/- 12 mu mol o-nitrophenol released per min per mg protein, respectively. The enzyme was inhibited by high concentrations of oNPG with K-i,K-s=31.7 +/- 3.5 mM. The enzyme showed no specific requirements for metal ions, with the exception of Mg2+, which enhanced both activity and stability. The genes encoding this heterodimeric enzyme, lacL and lacM, were cloned, and compared with other beta-galactosidases from lactobacilli. beta-Galactosidase from L. acidophilus was used for the synthesis of prebiotic galacto-oligosaccharides (GOS) from lactose, with the maximum GOS yield of 38.5% of total sugars at about 75% lactose conversion.