Some forms of big big prolactin behave as a complex of monomeric prolactin with an immunoglobulin G in patients with macroprolactinemia or prolactinoma.

The purpose of this study was to characterize the structure of big big PRL (bb-PRL) in patients with macroprolactinemia or prolactinomas. Serum samples from these patients were fractionated by Sephadex G-100 chromatography, and PRL was measured in the eluate by an immunoradiometric assay. The fractions containing bb-PRL were subjected to affinity chromatography with an antihuman immunoglobulin G (IgG) agarose column. PRL and IgG were assayed in the fractions obtained after affinity chromatography by immunoradiometric assay and radial immunodiffusion, respectively. bb-PRL was also immunoprecipitated with an antihuman PRL antibody, followed by polyacrylamide gel electrophoresis and Western blotting. We found that an average of 60% (range, 27-87%) of bb-PRL from patients with macroprolactinemia was retained by the agarose, indicating that this form of PRL contains an IgG. In one of the patients with prolactinoma, the big big form constituted 76% of the total PRL immunoreactivity. Most (75%) of the bb-PRL from this patient behaved as an IgG after affinity chromatography. In the two other patients with prolactinoma, we found that 22% and 25% of the bb-PRL, which represented only 2% and 3% of the total PRL in the serum, reacted as an IgG. In both groups of patients, the 23-kilodalton form of PRL was detected after the immunoprecipitation of bb-PRL. These results show that bb-PRL is in part a complex of 23-kilodalton PRL with an IgG and not an antibody mimicking the actions of PRL, as has been demonstrated for some large forms of growth-hormone.