Evidence for the direct transfer of corticosteroid-binding globulin from plasma to whey in the guinea pig.

Two steroid-binding proteins, corticosteroid-binding globulin (CBG) and progesterone-binding globulin (PBG), are known to be present in the milk whey of lactating guinea pigs. After injection of radioiodinated CBG into the maternal circulation, labeled CBG was found in the milk whey. The labeled whey CBG was identical to its plasma counterpart on the basis of size (sucrose gradients, Sephacryl S-200 gel filtration, and sodium dodecyl sulfate-gel electrophoresis), charge (DEAE-chromatography and polyacrylamide gel electrophoresis), and immunoprecipitability. In contrast, radioiodinated ovalbumin was not transferred to the milk. These results demonstrate that the CBG present in guinea pig whey results from the direct transfer of CBG from plasma to milk.