ABSTRACTCrude β‐galatosidase was produced by lysis of cells ofStreptococcus thermophilusgrown on deproteinized whey. The enzyme was partially purified by ammonium sulfate precipitation and ion‐exchange chromatography to yield a preparation free of protease activity. Highest activity was observed at pH 7.1, in the presence of potassium and manganese ions. Both monovalent and divalent cations were required for maximum activity but not for stability. The Kmforo‐nitrophenyl‐β‐galactopyranoside and lactose was 0.98 mM and 6.9 mM, respectively. Galactose was a competitive inhibitor with Kiof 60 mM. Gel‐filtration indicated a molecular weight of 530,000. The enzyme seems well suited to hydrolysis of la
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