Radioimmunoassay for a Progestagen-Associated Protein of the Human Endometrium*

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Abstract
We have previously demonstrated the presence of a progestagen-associated endometrial protein (PEP) in women. In this communication, we report the development of a RIA for PEP. We describe the use of this method for determining changes in PEP concentration that occur in the endometrium, amniotic fluid, and serum during the normal menstrual cycle and pregnancy. This assay used a partially purified PEP as the reference standard, purified radioiodinated PEP as the tracer, and a goat antibody to PEP as the specific antibody. The assay appears to be highly specific, since the tracer did not bind to antibodies to several known pregnancy-associated plasma, uterine, or placental proteins, including human transferrin, α1-anti-trypsin, ceruloplasmin, human PRL, placental protein SP1, pregnancy zone protein, α-fetoprotein, hCG, pregnancy-associated plasma proteins, and rabbit uteroglobin. Also, hCG, human PRL, and rabbit uteroglobin did not compete for the binding of the tracer to anti-PEP. The detection threshold of the RIA varied from 4–8 ng in the absence of serum and from 8–20 ng reference antigen/assay tube in the presence of adult human serum (0.5 ml/tube). The RIA was highly reproducible, since the interassay coefficients of variation for the slopes of the dose-response curves generated in the absence or presence of human male serum did not exceed 5%. The mean concentration of PEP in the midsecretory phase endometria (n = 11) was 48-fold higher than that in the proliferative phase endometria (n = 10), and the concentration in the uterine decidua (n = 11) of early pregnancy (6–10 weeks) was about 24-fold higher than that in the midsecretory phase endometria. The mean level of PEP in amniotic fluid samples obtained during 14–18 weeks of pregnancy (n = 10) was 5- to 15-fold higher than that obtained during 20–30 weeks (n = 5) or 34–42 weeks (n = 10). PEP was not detected in the sera (n = 21) of cycling women. However, it was detected in the pregnancy sera, the mean concentration in the sera obtained during weeks 6–10 (n = 28) being 5- to 7-fold higher than that during weeks 15–28 (n = 18) or weeks 29–40 (n = 7). Thus, this preliminary study strongly suggests that the synthesis and secretion of PEP increase rather dramatically during the first trimester of pregnancy and decline rapidly thereafter.