Properties ofaProlactin Receptor fromtheRabbit MammaryGland

processes, bothbeing maximal at37°C. 125I-labelled prolactin prepared bytheenzymic iodination procedure with lactoperoxidase binds better toreceptors thandoesthepreparation obtained byusing chloramine-T astheoxidizing agent. Thebinding of125I-labelled prolactin toreceptors is strongly influenced bypHandionic composition butnotbymanylow-molecular-weight compounds tested, e.g. steroids, nucleotides andseveral drugs. Receptor activity issensitive totrypsin andphospholipase Cdigestion, suggesting that protein andphospholipid moieties areessential forthebinding of125I-labelled prolactin. Thebinding of1251_ labelled prolactin toreceptors isasaturable andreversible process. Scatchard and Lineweaver-Burk analyses suggest that1251-labelled prolactin hasahighaffinity forits receptor. Binding of125I-labelled prolactin toreceptors doesnotresult inthedestruction ofthehormone. Considerable prolactin-binding activity isalsoobserved insubcellular fractions isolated fromtheadrenal gland, liver, ovary andkidney ofthepregnant rabbit, afinding that isconsistent withother reported actions ofprolactin inthese organs. Itisgenerally accepted that inmammals oneofthe principal target tissues forprolactin isthemammary gland. Thedirect effects andthemechanism ofaction ofprolactin on mammary growth, differentiation and function havebeenexamined extensively (Cowie & Tindal, 1971). Turkington (1970) reported that prolactin covalently linked toSepharose beadsisbiologically active on mouse mammary epithelial cells andhesuggested that prolactin initiates its effect by an action on thecell membranebecause itispresumedthattheSepharose-prolactin