The radiometric assay of alkaline phosphatase activity with 125-I-labelled phenolphthalein monophosphate.

The coupling of 125-I to phenolphthalein monophosphate is described. Hydrolytic cleavage of the phosphate group by alkaline phosphatases (EC 3.1.-3.1) releases labelled phenolphthalein which is extracted into ethyl acetate at pH 10. This forms the basis of a sensitive assay for the enzyme with the advantages of sensitivity and rapidity from the use of a gamma-emitter. Results with Escherichia coli and calf intestine alkaline phosphatases are presented.