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Receptors for human, simian, ovine, bovine and murine prolactin, human growth hormone and human placental lactogen have been identified in plasma-membrane-containing subcellular particles isolated from rabbit mammary glands. The association and dissociation of (125)I-labelled prolactin are time- and temperature-dependent processes, both being maximal at 37 degrees C. (125)I-labelled prolactin prepared by the enzymic iodination procedure with lactoperoxidase binds better to receptors than does the preparation obtained by using chloramine-t as the oxidizing agent. The binding of (125)I-labelled prolactin to receptors is strongly influenced by pH and ionic composition but not by many low-molecular-weight compounds tested, e.g. steroids, nucleotides and several drugs. Receptor activity is sensitive to trypsin and phospholipase C digestion, suggesting that protein and phospholipid moieties are essential for the binding of (125)I-labelled prolactin. The binding of (125)I-labelled prolactin to receptors is a saturable and reversible process. Scatchard and Lineweaver-Burk analyses suggest that (125)I-labelled prolactin has a high affinity for its receptor. Binding of (125)I-labelled prolactin to receptors does not result in the destruction of the hormone. Considerable prolactin-binding activity is also observed in subcellular fractions isolated from the adrenal gland, liver, ovary and kidney of the pregnant rabbit, a finding that is consistent with other reported actions of prolactin in these organs.

The content of 5-methylcytidylic acid in nuclear DNA and mito chondrial DNA of Xenopus laevis and HeLa cells has been determined. Both nuclear DNA's contain 5-methylcytidylic acid. The 5-methylcytidylic acid content of X. laevis DNA is 1.7 mole percent of total nucleotides, and that of HeLc cell DNA is 0.7 mole percent. In neither mitochondrial DNA could any 5-methylcytidylic acid be detected; the limit of sensitivity was judged at below 0.1 mole percent for X. laevis DNA and below 0.05 mole percent for HeLa cell DNA.