Serum HCG beta-core fragment is masked by associated macromolecules

To determine if beta-core fragment is present in serum from individuals with pregnancy and choriocarcinoma, samples were analyzed by gel filtration and fractions assessed by immunoassays (Mr = 15,000). In agreement with published reports, only minute amounts of beta-core fragment were detected (less than 0.1% of hCG level). A small amount of beta-core fragment activity was identified near the void volume of the column (Mr greater than 60,000). This high molecular weight material was pooled and dissociated with 3M ammonium thiocyanate, and gel filtration repeated. A much more significant beta-core fragment peak was then detected (Mr = 15,000). After dissociation, beta-core fragment was 18% (mol/mol) of the hCG level in early pregnancy serum, 91% in mid-pregnancy serum, 50% in term pregnancy serum, but only 1.3% in choriocarcinoma serum, pre-therapy. We conclude that the major form of beta-core fragment found in serum is associated with other macromolecules, which mask the beta-core fragment epitope to existing hCG beta antibodies. Measurements made on the dissociated beta-core fragment complex are much more in keeping with pregnancy levels found in urine.