Binding studies with intact rat prolactin and a 16K fragment of the hormone

A cleaved form of PRL has been found in the pituitary of rodents and humans and in human blood. A 16K fragment derived from cleaved rat (r) PRL has mitogenic and lactogenic activities in PRL bioassays. In this report we analyzed whether 16K rPRL acts through PRL and/or specific 16K receptors. The 16K PRL displaced 23K PRL label with a potency that varied among different tissues. With liver membranes the fragment had only 5-6% the potency of intact PRL, but in whole brain its potency was similar to that of the 23K form. By contrast, 16K PRL was 30 times more effective at displacing [125I]23K rPRL from the kidney membranes than was intact PRL. [125I]16K PRL bound specifically to membranes of kidney, liver, brain, and skeletal muscle, and it was not displaced by intact or cleaved 23K rPRL, intact ovine PRL, or rat or bovine GH. The specific binding of 16K PRL to the kidney changed with incubation time, temperature, and pH and was significantly higher than that of 23K PRL to brain and kidney membranes. However, the specific binding of the 16K was lower in liver and skeletal muscle. The binding sites for 16K PRL in the kidney had a higher affinity than did those for 23K PRL (Kd = 6.9 x 10(-9) vs. 0.8 x 10(-8), respectively), but the opposite was found in the other tissues. The specific 16K PRL-binding sites in different tissues may represent receptors that mediate actions of the fragment that are unshared by intact PRL. Furthermore, 16K PRL binds to 23K receptors with an activity that varies among the target tissues. These results support the notion of distinct PRL isoreceptors in different tissues.