Immunochemical studies of guinea pig progesterone-binding plasma protein

An antiserum specific for guinea pig progesterone-binding plasma protein (PBP) has been prepared. Using a very sensitive immunoenzymatic assay, PBP could be detected not only in pregnant guinea pig plasma (970 microgram/ml of plasma at day 40--60 of pregnancy), but also in the plasma of fetuses (2.77 micrograms/ml), umbilical arteries (1.79 micrograms/ml), umbilical vein (2.90 micrograms/ml), and in amniotic fluid (0.47 microgram/ml). The protein was also found in low concentration in the plasma of nonpregnant females (2.10 micrograms/ml) and males (1.56 micrograms/ml). The antiserum was used to search for immunological similarities between various steroid-binding proteins. No cross reaction was found with cavian or human corticosteroid-binding globulin and human sex steroid-binding plasma protein. There was no cross reaction between guinea pig PBP and PBP of other pregnant hystricomorphs (viscacha, degu, and coypu). Moreover, no evidence was found of an interaction between guinea pig uterine progesterone receptor and the anti-PBP antiserum.