Quantitative isolation of bovine prolactin was accomplished by immunoaffinity chromatography using clonal antibody as the stationary ligand. The phosphorylated and non-phosphorylated (native) prolactins contained in the immunopurified preparations were separated by chromatofocusing. Isolates from individual pituitaries revealed that phosphorylated prolactin represented between 20 and 80% of the total prolactin. The stoichiometry of phosphate in phosphorylated prolactin was 1.4:1 when determined by amino acid analysis after preparation of the S-ethylcysteine derivative. One major phosphorylation site, serine-90, and two minor sites, serine-26 and -34, were determined by mapping and sequencing studies. Serine-90 was conserved in prolactins, growth hormones and placental lactogens. Serine-26 and -34 were conserved in prolactins, but were not found in growth hormones or placental lactogens. Absorption spectroscopy of the aromatic amino acid residues indicated that phosphorylation of prolactin was associated with a unique structural conformation.

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