Size heterogeneity of rat pituitary prolactin

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Abstract
The occurrence of multiple forms of rat prolactin with different MW (size heterogeneity) was studied with anterior pituitary extracts, purified rat prolactin and 125I-labeled rat prolactin. In each case, 3 main forms of the hormone were detected by gel filtration on Sephadex G-100: a major one (80-90%) corresponding to monomeric prolactin (MW 22,000-25,000), a peak (8-20%) that could be a dimer (MW 45,000-50,000) and a small quantity (1-5%) of a component of much greater MW. On freezing and thawing of 125I-labeled rat prolactin, there was little interconversion of monomer and dimer peaks, but both were converted substantially to very high MW material. All 3 peaks of 125I-labeled rat prolactin could be precipitated by anti-(rat prolactin) serum and all 3 gave similar patterns of radioactive peptides after digestion with chymotrypsin followed by high-voltage paper electrophoresis. On sodium dodecyl sulfate/polyacrylamide-gel electrophoresis, the monomer peak of 125I-labeled prolactin migrated as a single component of MW 22,000, the very high MW peak largely dissociated to a component running in the same position as the monomer, and the dimer peak migrated partly as a component of MW 45,000 and partly as a component migrating with monomeric prolactin. No treatment was found that could dissociate the dimer peak completely to monomeric prolactin.