PREGNANCY SERUM CONTAINS THE BETA-CORE FRAGMENT OF HUMAN CHORIO-GONADOTROPIN

We have developed a time-resolved immunofluorometric assay (IFMA) for quantification of the core fragment of the β-subunit (cβhCG) 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 a europium chelate. The assay is sensitive (0.44 pmol/L = 4.6 ng/L), but cross-reaction with free β-subunits of hCG (βhCG) prevents direct determination of cβhCG in serum. To circumvent this limitation we separated hCG and βhCG from cβhCG 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 cβhCG and elutes at the same position in gel chromatography as cβhCG purified from urine. The proportion of cβhCG to hCG in pregnancy serum was 0.012–0.045% (mean ± SD, 0.028 ± 0.01) on a molar basis. Our finding of cβhCG in serum confirms earlier reports suggesting that proteolytic degradation of βhCG in the kidneys may not be the only pathway by which cβhCG is formed. © 1990 by The Endocrine Society.