Pregnancy Serum Contains theβ-Core Fragment of Human Choriogonadotropin*

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Abstract
We have developed a time-resolved immunofluorometric assay (FMA) for quantification of the core fragment of the .beta.-subunit (c.beta.hCG) of hCG. The assay uses two monoclonal antibodies. One antibody is immobilized onto the wall of a microtiter strip well, and the other one is labeled with europium chelate. The assay is sensitive (0.44 pmol/L - 4.6 ng/L), but cross-reaction with free .beta.-subunits of hCG (.beta.hCG) prevents direct determination of c.beta.hCG in serum. To circumvent this limitation we separated hCG and .beta.hCG from c.beta.hCG by gel chromatography and quantified each component in the fractions by specific IFMAs. The high sensitivity of the newly developed IFMA enabled us to demonstrate that serum from pregnant women contains c.beta.hCG and elutes at the same position in gel chromatography as c.beta.hCG purified from urine. The proportion of c.beta.hCG to hCG in pregnancy serum was 0.012-0.045% (mean .+-. SD, 0.028 .+-. 0.01) on a molar basis. Our finding of c.beta.hCG in serum confirms earlier reports suggesting that proteolytic degradation of .beta.hCG in the kidneys may not be the only pathway by which c.beta.hCG is formed.