Identification of macroprolactin in a patient with asymptomatic hyperprolactinemia as a stable PRL-IgG complex

We characterize the molecular form of PRL in a female patient with asymptomatic, idiopathic hyperprolactinemia. Size exclusion chromatography revealed that PRL was present exclusively in the form of macroprolactin (> 200 kD mol wt). By immunoaffinity purification, Western blot analysis, and binding studies the molecule was identified as a PRL-IgG complex of extremely high stability. Sequence analysis revealed no mutations in the protein coding region of the hPRL gene using hPRL cDNA amplified from the patient's peripheral blood lymphocytes. In spite of full bioactivity in vitro, as determined by Nb2 bioassay, the complex apparently lacks bioactivity in vivo. Its high molecular weight may reduce its access to target organs in the periphery as well as centrally.