Glycosylated human prolactin: Alterations in glycosylation pattern modify affinity for lactogen receptor and values in prolactin radioimmunoassay

The receptor-binding properties of monomeric nonglycosylated human PRL (hPRL), glycosylated hPRL that does not bind to Concanavalin-A-Sepharose (G1-hPRL) and glycosylated hPRL that binds to Concanavalin-A-Sepharose (G2-hPRL) were tested in the lactating rabbit mammary gland RRA for lactogenic hormones. Variations in the glycosylation pattern of G-hPRL altered its receptor-binding properties, suggesting that the site of glycosylation may be proximal to the receptor-binding region. Relative potencies for the displacement of [l25I]hPRL by hPRL, G1-hPRL, and G2-hPRL were 100%, 40%, and 26%, respectively. Relative potencies for displacement of [125I]G1-hPRL by hPRL, G1-hPRL, and G2-hPRL were 100%, 44%, and 69%, respectively; however, the displacement curve for G2-hPRL was not parallel to the others. When G2-hPRL was radiolabeled, there was no specific binding to lactogenic receptors. The presence of PRL receptor subtypes and/or kinetic cooperativity was suggested by the complexity of the binding isotherms. The...