[Critical observations on the use of phenolphthalein monophosphate as a substrate for the determination of prostatic acid phosphatase (author's transl)].

At pH 10, which is in the alkaline measurement range, phenolphthalein is bound by albumin. The resonance system of this acid-base indicator cannot therefore be completely formed, and the colour intensity is correspondingly lower. Albumin also binds phenolphthalein monophosphate at pH 4-8, in the pH range of the enzymic reaction. Depending on the relative concentrations, the substrate concentration is therefore either suboptimal (manifested as a lower catalytic concentration of phosphatase), or the substrate excess is bound by albumin, so that the substrate inhibition of the phosphatase is decreased or abolished. Binding of phenolphthalein and its monophosphate to albumin is partly prevented by the addition of alcohol. These results show that the determination of prostate phosphatase with the substrate phenolphthalein monophosphate is markedly influenced by various factors. The interpretation of the observed colour intensity is therefore difficult, and the calculation of the catalytic concentration is frequently subject to error.