Interaction of monoclonal antibodies with growth hormone-binding protein and its complex with growth hormone.

The properties of four independent lines of monoclonal antibodies (MAbs) specific to rat GH-binding protein (GHBP) were examined. Three MAbs, designated GHR-12, GHR-13 and GHR-16, were raised against the entire GHBP molecule. The fourth MAb, designated as GHBP4.3, was raised against the 17 amino acid residues at the C-terminal end of rat GHBP. The interaction of these antibodies with GHBP and their effect on GH binding to GHBP were analysed by conventional competition binding assays and surface plasmon resonance, i.e. with a Biospecific Interaction Analysis (BIAcore) instrument. The binding affinity of these MAbs to GHBP ranged from 29 nmol/l to 30.9 pmol/l. The pair-wise antibody binding to GHBP on BIAcore suggested that GHR-13 and GHR-16 recognized different antigenic determinants while part of the GHR-12 epitope might be shared with the other antibodies. The antibodies inhibited the interaction of GH with GHBP in the competition binding assay. However, in sequential binding on the BIAcore instrument, they were able to bind GHBP after its interaction with GH, indicating that the inhibition observed in the competition binding assay resulted from steric hindrance rather than direct interference with the GH-binding site of GHBP. The present findings, therefore, suggest that these antibodies are useful for investigating GHBP and its interaction with GH.