Phenolphthalein monophosphate as a substrate for serum alkaline phosphatase. An appraisal

The procedure for serum alkaline phosphatase determination employing phenol-phthalein monophosphate as substrate was evaluated under routine clinical laboratory conditions. A straight-line response to increasing serum concentration was observed, and the reaction was shown to be Mg+ + dependent. The method proved to be extremely simple, and the results obtained correlated well with those given by a standard routine procedure involving determination of the liberated inorganic phos-phate. 1N41ETHODS for time tletem-mimmahon of serummm alkaline phosphatase ac-tivity depend upon hydrolysis of an orgamiic pimosphate, after which the inorganic phosphorus or the organic moiety produced is measured. Most of the methods in which time organic phosphorus is measured em-ploy $-glycerophosphate as substrate (1, 2), while a number of differ-ent substrates have been recommended for use iii methods which meas-ure the organic moiety. King amid Armstrong (3) immtroduced phenyl phosphate and measured time liberated phenol with the Fohin-Ciocalteu reagent. Later time need for proteimi precipitation was elimimmated by the application of the 4-amirio-antipyrine metimod for the determination of phenol (4,5). Further simplificatiomm resulted from time introduction of p-mtro-phenyl phosphate simice the liberated p-mmitrophemiol may conveniently be determined colorimetrically after the additiomi of alkali (6-8). This procedure, however, does not give a linear relationship between colo