Human endometrium and menstrual fluid contain placental protein 5 (PP5).

Placental protein 5 (PP5), a serine protease inhibitor, was found in the endometrium and menstrual fluid of non-pregnant women. PP5 was present in all 16 endometrial samples taken at various phases of the menstrual cycle. In the secretory phase, the endometrial PP5 content was higher (median 17.4 micrograms/g protein; n = 8) than in the proliferative phase (median 3.8 micrograms/g protein; n = 8). In gel filtration, endometrial tissue homogenates yielded one immunoreactive peak corresponding to a mol. wt of 28,000 daltons, whereas placental PP5 eluted at 32,000 daltons. In sodium dodecyl sulphate polyacrylamide gel electrophoresis, endometrial PP5 comigrated with purified placental PP5 corresponding to a mol. wt of 30,000 daltons. All menstrual fluid samples (n = 14) contained immunoreactive PP5 at concentrations from 77 to 1150 micrograms/l. The mol. wt of menstrual fluid PP5-immunoreactivity was 13,000 daltons. The dose-response curves of purified PP5 standard and endometrial and menstrual fluid PP5 were parallel in the PP5 radioimmunoassay. The higher concentration of PP5 in secretory endometrium indicates association of PP5 with endocrine events of the menstrual cycle.