[Determination of serum activity of placental alkaline phosphatase]

The authors have developed an enzymatic method for the measurement of serum placental alkaline phosphatase (PLAP) based on the hydrolysis of paranitrophenyl phosphate into para-nitrophenol. The specificity for the isoenzyme involved is achieved by means of its characteristics thermostability. After one hour incubation at 60 degrees C, PLAP still maintains its full activity, while other isoenzymes are completely inactivated. The sensitivity of the method was improved (less than 1 U/l) by optimizing parameters such as the volume of specimen, the nature of the buffer (2-methyl-amino-ethanol), the reaction time and the pH. Analysis of the method performances has revealed a lower detection limit of 0.12 U/l, a linearity from 0 to 50 U/l, a precision lower than 10% for most of the analytical range and a complete enzyme recovery. The reference range was estimated from 0 U/l to 0.33 U/l in a group of 125 non-smokers without any cancerous disease.