Reactivity of the alkaline phosphatases of bovine milk and intestinal mucosa with the substrates phenyl phosphate and o-carboxyphenyl phosphate

Bovine intestinal mucosa alkaline phosphatase reacts equally with the substrates phenyl phosphate and o-carboxyphenyl phosphate. Milk alkaline phosphatase, on the other hand, at a concentration of the substrates of about 8 × 10 −5 moles/l., is 1 3 to 1 4 reactive with the o-carboxyphenyl phosphate as with phenyl phosphate. Analyses of these reactions with a range of substrate concentrations showed that the difference in reactivity is due to a difference in enzyme-substrate dissociation constants, K m. The p K m values for the mucosa phosphatase with both substrates at pH 9.7 is 3.7; with the milk phosphatase and phenyl phosphate the p K m value is 4.6, whereas with o-carboxyphenyl phosphate the value is 2.8. The K m values were also determined for a range of pH values. The difference noted above persisted throughout. K m values obtained with the milk phosphatase in a lipide-containing complex were identical with values obtained with the phosphatase after dissociating with n-butyl alcohol.