Multiple forms of pituitary prolactin, a glycosylated form of porcine prolactin with enhanced biological activity

Abstract

Three forms of prolactin differing in molecular weight (Mr 23,000, 25,000 and 50,000, respectively) and electrophoretic mobility have been isolated and purified from fresh-frozen porcine pituitary glands. The prolactin form with Mr 25,000 is identified as a glycoprotein having an affinity to concanavalin A. The carbohydrate unit containing GlcNAc3, GalNAc1, Man3, Fuc 0.5, Gal 0.4 is linked to asparagine at the 31 position. The amino acid composition and partial primary structure of the glycosylated prolactin are identical to those of the major prolactin form (Mr 23,000). Based on isoelectric focusing and nondenaturing disc-electrophoresis, the glycosylated prolactin appears more acidic than the major form. The glycosylated form of the hormone has 140% of the activity of the non-glycosylated prolactin when measured by the pigeon crop sac assay. The new form accounts for 30-40% of the total monomeric prolactin in the porcine pituitary. The yield was 200 mg purified glycosylated prolactin from 1000 g pituitary gland. The third form of hormone (Mr 50,000) was shown to be a disulphide linked prolactin dimer.