Preparation of a disulfide-linked dimer of human placental lactogen fragment 1--134 with immunologic and biologic activity.

In order to determine the chemical features of human placental lactogen (hPL) necessary for its biologic activity we prepared the following fragments from the plasmin-cleaved hormone: reduced and alkylated 1--134, reduced and alkylated 141--191, and a 1--134 dimer joined through the single cysteinyl residue at position 53. In a radioimmunoassay using antibodies against native hPL, the two reduced and alkylated fragments produced nonparallel displacement and had less than 1% of the activity of hPL. The ability of reduced and alkylated 1--134 to bind to mammary gland receptors was less than 5% of that of hPL; reduced and alkylated 141--191 showed no detectable activity in the same assay. The 1--134 dimer, in contrast, had 20% of the immunologic activity and 30% of the ability to bind to lactogenic receptors relative to the native hormone. In an in vitro bioassay the lactogenic activity of the 1--134 dimer was equivalent to that of the native hormone. The circular dichroic spectra of hPL, reduced and alkylated 1--134, and 1--134 dimer indicated that the dimer had regained much of the helical content of the native hormone. Antibodies produced to reduced and alkylated 1--134 did not significantly crossreact with either native hPL or 1--134 dimer. From these data we conclude that the information for the lactogenic activity of hPL is contained in the first 134 amino acid residues and that the proper conformation is necessary for its biologic expression.