Steroid-Binding Proteins in Guinea Pig Milk and Plasma*

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Abstract
The levels of two steroid-binding proteins, progesterone-binding globulin (PBG) (1) and corticosteroid-binding globulin (CBG), present in the plasma of pregnant guinea pigs were determined just before and after parturition. Both PBG (3.37 ± 2.1 μm) and CBG (10.1 ± 1.7 μm) were present in high levels just before parturition and were found to decrease with a half-life of 2 days after delivery. PBG and CBG were also found in the milk whey of lactating guinea pigs but at much lower levels (26.5 ± 12 and 375 ± 18 nm, respectively, on day 1 post partum). The whey content of each protein declined with a halflife of 3 days. The whey steroid-binding proteins were indistinguishable from their plasma counterparts on the basis of ion exchange and gel filtration chromatographies, sucrose gradient centrifugation, susceptibility to heat denaturation, and hormone binding specificity. (Endocrinology106: 1747, 1980)