The stability of lactase from Streptococcus thermophilus at 55 °C increased 7-fold, 2-fold and 1-5-fold in the presence of lactose, galactose and glucose respectively; maltose had no effect. Total stability over an 8 h period was more than 10-fold better in milk and sweet whey than in lactose solution, owing to the stabilizing influence of the milk proteins and the milk salts. Ovalbumin and reduced glutathione provided some extra stability but were not as effective as the milk components. In the absence of lactose the enzyme was less stable in milk and was not protected at all by sweet whey constituents. None of the milk protein fractions was as effective in the absence of lactose as when it was present. Enhanced thermostability of the enzyme in milk and sweet whey is due to contributions by all major milk components, but binding of lactose to the enzyme is the major factor controlling the extent of stabilization by other components.Hydrolysis of lactose in milk, whey and other dairy products by the enzyme lactase (/ff-galactosidase EC 3.2.
Denaturation of lactase in phosphate buffer was a first‐order process with a half‐life of 1.12 min at 60°C. Denaturation in milk was a biphasic first‐order process with a half‐life of 115 min at 60°C. Electrophoresis, isoelectric focusing and kinetic analysis all indicated the presence of two isozymes. The minor isozyme exhibited 9% of the total activity and was less thermostable in milk than the major isozyme. In the temperature range 53.5–60°C, the rate of denaturation of the major isozyme in milk was more than 100 times less than the rate in buffer. Arrhenius plots of denaturation in milk and buffer were biphasic. Above 56°C, Ea denaturation in milk and buffer was 187 Kcals/mole and 19.8 Kcals/mole, respectively; below 56°C Ea denaturation in milk and buffer was 36.8 Kcals/mole and 158 Kcals/mole, respectively.
Thermal stability of lactase was I70-fold greater in milk than in phosphate buffer at 60°C. Enhanced stability was due primarily to casein, whose cffcct was largely substrate dependent. Casein or lactose alone had little or no effect, but together they stabilized the enzyme more than 400-fold at 60°C. The relative effectiveness of other proteins, as stabilizers, was: bovine serum albumin >caseinate>ovatbumin>soy isolate > u-lactalbumin > lysozyme. Galactose was much less effective than lactose in promoting stability; sucrose and glucose were ineffective. Stability was dependent upon both lactose concentration and casein concentration, and decreased with increasing enzyme concentration. Dithioerythritol increased stability in buffer but had relatively little effect in milk.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.