Secreted Mouse Prolactin (PRL) and Stored Ovine PRL. II. Role of Amides in Receptor Binding and Immunoreactivity*

Native PRL and des-amido forms 1, 2, and 3 were tested for their individual immunochemical and receptor-binding abilities. The results show that deamidation of either secreted mouse PRL or stored ovine PRL alters their binding in a radioreceptor assay. For each accumulated deamidation there was a statistically significant (P less than 0.05) decrease in the binding potency of either stored ovine PRL or secreted mouse PRL to a cell membrane receptor preparation. RIAs indicated that there was a statistically significant (P less than 0.05) decrease in PRL's immunopotency toward polyclonal antisera only when select residues deamidated. This study suggests that all the asn/gln residues which deamidate in order to make des-amido forms 1, 2, and 3 constitute part of the receptor-binding domains of both secreted and stored PRLs, while only a fraction of those same residues form portions of PRL's antigenic sites. Thus PRL's receptor-binding surface is separated from its antigenic sites, with only partial overlap being indicated. Our data also indicate that there are no major structural differences in the receptor-binding domains of secreted and stored PRLs. We also see that the receptor-binding domain of PRL has been highly conserved throughout evolution. Since the binding affinity of PRL to a membrane-bound receptor can be altered through deamidation, we view the process as a plausible regulatory mechanism for controlling the quantitative action of PRL at a given target organ.