A growth hormone/prolactin-binding protein in human milk

In the course of studies of the distribution of GH-binding proteins (GHBP) in biological fluids, we identified a human (h) GH/hPRL-binding component in human milk. To characterize its nature, 16 human milk samples were analyzed by molecular exclusion chromatography after incubation with monomeric [125I]hGH. All samples showed a peak indicative of a hGH-containing complex with a mol wt of 52 kilodaltons (kDa)--considerably smaller than the high affinity GH-binding protein complex in plasma (80-85 kDa). Binding of [125I]hGH was inhibited in a dose-dependent fashion by unlabeled hGH and to a lesser extent by hPRL, but not by oGH. Scatchard analysis yielded a Ka of 2.1 x 10(8) M-1 and a maximum binding capacity of 5.1 micrograms/L for hGH, and a Ka of 0.5 x 10(8) M-1 for hPRL. The derived molecular size of the milk GH/PRL-binding protein (BP) is 30 kDa, assuming 1:1 binding stoichiometry. The milk BP was not immunoprecipitable with any of 4 monoclonal antibodies directed against the hGH receptor or with a polyclonal antiserum directed against the hPRL receptor. The milk BP bound to hGH affinity columns, but unlike the GHBP in human plasma, did not bind to wheat germ lectin columns, suggesting different or no glycosylation. We conclude that human milk contains a high affinity GH/PRLBP that differs from the serum GHBP in its ligand specificity (binding both hGH and hPRL), molecular size, immunological, and glycosylation characteristics. Based on its immunochemical and ligand-binding characteristics, it does not appear to be a truncated GH receptor such as the plasma GHBP. Its lack of immunoreactivity with the one available antiserum also does not support its identity with a truncated PRL receptor. However, it cannot be excluded that the milk BP may represent a proteolytically or otherwise altered truncated form of the PRL receptor (or, less likely, the GH receptor) that maintains some binding activity, but has its immunological epitope(s) disabled. The precise nature and function of this protein remain to be defined.