PLACENTAL PROTEIN 5 (PP5): DEVELOPMENT OF A RADIOIMMUNOASSAY AND MEASUREMENT OF CIRCULATING LEVELS IN NORMAL PREGNANCY

Abstract

A specific and sensitive, double antibody, solid phase radioimmunoassay was developed for measurement of placental protein 5 (PP5) in serum. A sensitivity limit of 0.8 microgram/1 was achieved by pre-incubation with antiserum and use of a small assay volume. Single serum samples from 244 retrospectively proven normal pregnancies were used to construct a normal range of PP5 concentrations. PP5 was detectable at week 7 and rose to reach a maximum mean value of 54 microgram/1 at weeks 36-37. Diurnal variation of PP5 concentration was similar to that encountered from day to day, and slightly greater than variation due to the radioimmunoassay. The decline of serum PP5 concentration post partum was initially rapid (half-life 5-14 minutes), but slowed after 0.5-1 hours; immunoreactive PP5 was still detectable seven days after delivery.