Hyperstimulated human preovulatory follicular fluid contains placental protein 5 (PP5).

Placental protein 5 (PP5)-like immunoreactivity was found in 22 of 28 fluids aspirated from hyperstimulated preovulatory follicles of women who participated in an in vitro fertilization program. When detectable, the PP5 levels varied from 7 to 67 micrograms/l, which are of the same order of magnitude as those seen in pregnancy sera. In gel filtration, the PP5-like immunoreactivity of follicular fluid eluted in the same volume as purified human placental PP5; and in radioimmunoassay, this substance gave an inhibition curve which was parallel to that of the PP5 standard. A positive correlation was observed between the follicular fluid concentration of estradiol and PP5 (r = 0.60; P less than 0.001), but not between the levels of progesterone, testosterone, or follicular volume and the PP5 concentration. In light of the protease inhibitor nature of PP5, its detection in preovulatory follicular fluid is intriguing in relation to its possible role in the rupture of the follicle.