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

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Abstract
Native PRL [prolactin] and des-amido forms 1, 2, and 3 were tested for their individual immunochemical and receptor-binding abilities. Apparently 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 < 0.05) decrease in the binding potency of either stored ovine PRL or secreted mouse PRL to a cell membrane receptor preparation. [Radioimmunoassay] indicated that there was a statistically significant (P < 0.05) decrease in PRL immunopotency toward polyclonal antisera only when select residues deamidated. Evidently all the Asn/Gn residues which deamidate to make des-amido forms 1, 2, and 3 constitute part of the receptor-binding domains of both secreted and stored PRL while only a fraction of those same residues form portions of PRL antigenic sites. Thus PRL receptor-binding surface is separated from its antigenic sites, with only partial overlap being indicated. Apparently there are no major structural differences in the receptor-binding domains of secreted and stored PRL. Evidently 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, the process is viewed as a plausible regulatory mechanism for controlling the quantiative action of PRL at a given target organ.