Evidence for the Presence of Human Chorionic Gonadotropin (hCG) and Freeβ-Subunit of hCG in the Human Pituitary

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Abstract
Previous studies have indicated that the pituitary gland may produce free a-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 β-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 reactivi-ties with a panel of polyclonal and monoclonal antibodies specific for holo-hCG, β-subunit of hCG, α-subunit, or hCG/LH. In addition to the expected LH and α-subunit, we detected materials which eluted from the column in positions very similar to those of cochromatographed 125I-hCG tracer and hCG-β (NIH CR123-β), and which showed immunoreactivity in specific immunoradiometric assays for holo-hCG and hCG-β, respectively. Holo-hCG and hCG-β 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-β-subunit activity could be enriched (∼500 times) by affinity chromatography on an hCG antibody-coupled Sepharose column. When subjected to isoelectric focusing in granulated gel holo-hCG and hCG-β-subunit of pituitary origin were focused in the pI-range of pregnancy hCG and pregnancy hCG-β-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 (∼25%) was much higher than that found in pregnancy hCG. On the basis of immunoreactivites, the content of holo-hCG in our pituitary extract was estimated to be 60 μg/g, and that of free β-subunit 45 μg/g; for comparison, LH was approximately 20 mg/g, and free α-subunit 1.6 mg/g. In addition, we could demonstrate the presence of both holo-hCG- and free hCG-β-subunit-like immunoreactivity in NaCl-extracts from single pituitaries of two postmenopausal women. In these studies a second hCG-β-immunoreactive material eluting far behind the hCG-β-position was found. Chromatography of purified LH-β-subunit, which crossreacts 1.56% in the hCG-β IRMA, yielded an elution pattern clearly distinguishable from that of the hCG-β-immunoreactive substances. Spiking of the pituitary extract with hCG-β and a β-fragment of hCG similar to the so-called β-core fragment resulted in a rise of the first and second hCG-β-activity peak, respectively.Taken together, the immunological and the various chromatographic studies provide strong evidence for the existence of both free hCG-β-subunit and holo-hCG in the human pituitary gland. The properties of hCG- and hCG-β-like materials found in the human pituitary compared favorably with those of the respective purified standard preparations of pregnancy sources and of urinary material from a postmenopausal woman. Apparently, the human pituitary is able to produce both an α-subunit, which has been well recognized, and an hCG-β-subunit as described in this report, and to assemble the two subunits to produce holo-hCG.