Immunochemical Studies of Guinea Pig Progesterone- Binding Plasma Protein*

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Abstract
An antiserum specific for guinea pig progesterone-binding plasma protein (PBP) was prepared. Using a very sensitive immunoenzymatic assay, PBP could be detected not only in pregnant guinea pig plasma (970 .mu.g/ml of plasma at day 40-60 of pregnancy), but also in the plasma of fetuses (2.77 .mu.g/ml), umbilical arteries (l.79 .mu.g/ml), umbilical vein (2.90 .mu.g/ml) and in amniotic fluid (0.47 .mu.g/ml). The protein was also found in low concentration in the plasma of nonpregnant females (2.10 .mu.g/ml) and males (1.56 .mu.g/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 globin and human sex steroid-binding plasma protein. There was no cross reaction between guinea pig PBP and PBP of other pregnant hystricomorphs (viscacha [Lagostomus maximus], degu [Octodon degus], and coypu [Myocastor coypus]). No evidence was found of an interaction between guinea pig uterine progesterone receptor and the anti-PBP antiserum.