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

Abstract

Three forms of prolactin differing in molecular weight (M_r 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 M_r 25 000 is identified as a glycoprotein having an affinity to concanavalin A. The carbohydrate unit containing GlcN Ac₃, GalNAc₁, Man₃, Fuc₀₅, Gal₀₄ 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 (M_r 23 000). Based on isoelectric focusing and non-denaturing 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 1000g pituitary gland. The third form of hormone (M_r 50 000) was shown to be a disulphide linkéed prolactin dimer.