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Some Properties of Alkaline Phosphatase of Milk¹

Department of Dairying, University of Alberta, Edmonton, Canada

ABSTRACT

Optimum conditions for measuring the hydrolysis of disodium phenyl phosphate by alkaline phosphatase in raw milk included a 0.02919 M substrate concentration and a hydrolysis period of 20 minutes at 38.5° C. and pH 10.32 ± 0.02. These conditions differ from those specified in the Sanders and Sager method for determination of phosphatase activity in unheated milk.

Some properties of the enzyme, as it occurs in raw milk, were studied. The pH optimum of the catalyzed reaction increased with increasing concentrations of disodium phenyl phosphate. Where the substrate was sodium ß-glycerophosphate, increased substrate concentration did not shift the pH optimum of 9.60 ± 0.02. Michaelis constants were 0.00340 M and 0.00560 M for disodium phenyl phosphate and sodium ß-glycerophosphate, respectively. The energy of activation of the hydrolysis of disodium phenyl phosphate was approximately 9,000 cal / mole. In the absence of substrate, phosphatase was most stable at approximately pH 6.7. A slow decrease in the pH of raw milk resulted in less enzyme inactivation than a rapid decrease. Furthermore, the extent of subsequent restoration of phosphatase activity was greater at the normal pH of milk than at a higher pH. Added magnesium ion did not increase phosphatase activity under the experimental conditions.

¹ The data are part of a thesis presented by the senior author in partial fulfillment of the requirements for the degree of Master of Science at the University of Alberta.

² Present address: University of California, Davis.