Purification and thermostability of β-galactosidase (lactase) from an autolytic strain ofStreptococcus salivariussubsp.thermophilus

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Abstract
Summary: β-Galactosidase from an autolytic strain ofStreptococcus salivariussubsp.thermophiluswas purified 109-fold to near homogeneity. The yield of purified enzyme was 41% and the specific activity was 5920-nitrophenylβ-D-galactopyranoside U/mg at 37 °C. Two isozymes were present, but only one subunit was detected, having a mol. wt of 116000. Enzyme stability was 37–83 times greater in milk than in buffer in the range 60–65 °C. At 60 °C the half-life in milk was 146 min. Denaturation in buffer was first-order, but in milk the overall reaction order with respect to enzyme concentration was ˜ 0·5. The activation energy for denaturation was 453 kJ/mol in milk and 372 kJ/mol in buffer. In milk the activation energy for lactose hydrolysis was 35·1 kJ/mol.