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, G1hPRL-, was only slightly retarded by Con-A; the other, G2hPRL-, was adsorbed by Con-A and could be eluted with methyl-D-mannopyranoside, 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 GhPRLs. 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 varyi...