[Relationship between prolactin and serum proteins. 1. Protein-bound hormone protected from biodegradation in the...

The authors studied binding of prolactin labeled with 125I in the blood of rats and lactating goats. Not over 20% of the hormone was bound in vitro the first 5 to 10 min of incubation with the blood serum, and a complex with molecular weight of 200 000 appeared which was similar to that formed in vivo. With a longer incubation 125I-prolactin was bound in serum macroglobulin fraction; this was not observed in vivo. Blood protein-bound prolactin retained its immunological activity. Monomeric hormone was absorbed in the kidneys and the mammary glands, with the subsequent rapid degrading. Blood protein-bound prolactin was not absorbed in the kidneys, and was present in the mammary glands in small amounts only.