Evidence for the presence of human chorionic gonadotropin (hCG) and free beta-subunit of hCG in the human pituitary.

Abstract

Previous studies have indicated that the pituitary gland may produce free alpha-subunit and small quantities of hCG in addition to other glycoprotein hormones. Since synthesis of holo-hCG requires the presence of both subunits, we have investigated the occurrence in human pituitary of free beta-subunit of hCG, in addition to intact holo-hCG. We processed a pituitary extract by fractionated ammonium sulfate precipitation followed by sequential chromatography on Sephadex G-100 and Ultrogel AcA 44. The fractions obtained were assessed for their reactivities with a panel of polyclonal and monoclonal antibodies specific for holo-hCG, beta-subunit of hCG, alpha-subunit, or hCG/LH. In addition to the expected LH and alpha-subunit, we detected materials which eluted from the column in positions very similar to those of cochromatographed 125I-hCG tracer and hCG-beta (NIH CR123-beta), and which showed immunoreactivity in specific immunoradiometric assays for holo-hCG and hCG-beta, respectively. Holo-hCG and hCG-beta material derived from the urine of a postmenopausal woman showed behaviors on the column similar to the pituitary forms. Both the pituitary holo-hCG- and free hCG-beta-subunit activity could be enriched (approximately 500 times) by affinity chromatography on an hCG antibody-coupled Sepharose column. When subjected to isoelectric focusing in granulated gel holo-hCG and hCG-beta-subunit of pituitary origin were focused in the pI-range of pregnancy hCG and pregnancy hCG-beta-subunit, respectively. Like pregnancy hCG, most (75%) of the pituitary hCG was bound to a column of Con A-Sepharose; however, the Con A-nonbinding hCG fraction (approximately 25%) was much higher than that found in pregnancy hCG. On the basis of immunoreactivities, the content of holo-hCG in our pituitary extract was estimated to be 60 micrograms/g, and that of free beta-subunit 45 micrograms/g; for comparison, LH was approximately 20 mg/g, and free alpha-subunit 1.6 mg/g. In addition, we could demonstrate the presence of both holo-hCG- and free hCG-beta-subunit-like immunoreactivity in NaCl-extracts from single pituitaries of two postmenopausal women. In these studies a second hCG-beta-immunoreactive material eluting far behind the hCG-beta-position was found. Chromatography of purified LH-beta-subunit, which crossreacts 1.56% in the hCG-beta IRMA, yielded an elution pattern clearly distinguishable from that of the hCG-beta-immunoreactive substances.(ABSTRACT TRUNCATED AT 400 WORDS)