Origin and Occurrence of Human Chorionic Gonadotropin β-Subunit Core Fragment

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Abstract
Human chorionic gonadotropin (hCG) .beta.-subunit core fragment (.beta. fragment) is present in the urine of pregnant individuals as well as those with trophoblast disease and certain other cancers at concentrations 0.8 (early pregnancy) to 7 (second trimester pregnancy)-fold greater than that of hCG. The core fragment may be directly secreted by trophoblast tissue into the circulation or possibly originates from peripheral degradation of circulating hormone by the kidney. We examined the former hypothesis. We examined 24-h organ cultures of trophoblast tissue from first, second, and third trimester pregnancy. The media from this tissue contained hCG, free .beta.-subunit, and .beta.-fragment. The amount of .beta.-fragment present exceeded that of hCG, as was observed in second and third trimester pregnancy urine. The .beta.-fragment immunoreactive material produced by trophoblast tissue was compared to a standard preparation of urinary .beta.-fragment. The material in medium was identical to the standard .beta.-fragment in its elution pattern from a gel filtration column, from a reverse-phase HPLC column, from an ion-exchange gel, and from an immobilized lectin affinity column, and also by electrophoresis and immunoblotting with fragment-reactive monoclonal antibodies. We conclude that .beta.-fragment can also originate directly from trophoblast tissue, and could be the principal hCG.beta.-immunoreactive molecule secreted.

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