Two forms of glycosylated human prolactin have different pigeon crop sac-stimulating activities

Two forms of glycosylated PRL (G-PRL) which differed in their binding properties to Concanavalin-A (Con-A) were isolated from human pituitary glands. One form, G1-hPRL, was only slightly retarded by Con-A; the other, G2-hPRL, was adsorbed by Con-A and could be eluted with methyl-D-manno-pyranoside, an indication of differing carbohydrate units in the two G-PRLs. Differences in type of glycosylation were also indicated by HPLC peptide mapping of tryptic digests of the two forms. The elution time for the tryptic peptide carrying the asparagine-linked carbohydrate unit varied for the two G-PRLs. The results point to the asparagine at position 31 as being the site of attachment of the carbohydrate. The carbohydrate structure influenced the crop sac-stimulating activity of the G-hPRLs. G1-hPRL had only about one fourth the activity of the reference standard (nonglycosylated ovine PRL, 35 IU/mg). The form that bound to Con-A, G2-hPRL, was equipotent to the reference standard. Because glycosylated forms have varying biological activities and are major components of circulating PRL, the physiological significance of serum concentrations of PRL measured by RIA will have to be reevaluated.