TOTAL AMOUNTS OF CIRCULATING HUMAN CHORIONIC-GONADOTROPIN ALPHA-SUBUNIT AND BETA-SUBUNIT CAN BE ASSESSED THROUGHOUT HUMAN-PREGNANCY USING IMMUNORADIOMETRIC ASSAYS CALIBRATED WITH THE UNALTERED AND THERMALLY DISSOCIATED HETERODIMER

The aim of the present study was to determine the variations in the balance between total (free plus combined) circulating alpha and beta subunits of human chorionic gonadotrophin (hCG) throughout human pregnancy. The equivalence between the International Units (IU) of hCG (IRP 75/537) and those assigned to the alpha (IRP 75/569) and beta (IRP 75/551) free subunits was experimentally determined by using intact and thermally dissociated hCG. Heat exposure (2 min at 100 degrees C) of hCG preparations resulted in a complete dissociation of hCG into free, soluble and intact alpha and beta subunits. The hCG and alpha and beta subunit contents of unaltered and heated hCG preparations were assessed by specific immunoradiometric assays. The amount of immunoreactive subunits dissociated by heat from hCG could then be evaluated on a molar basis. Circulating hCG and its free alpha and beta subunits were immunoassayed in 836 blood samples collected from healthy pregnant women at different gestational ages. After conversion of hCG and its subunits into a common IU system, the gestational profiles of the total amounts (h ee plus combined) of alpha- and beta hCG subunits increased together and peaked at 9-10 weeks of gestation. Thereafter, total alpha and beta subunits decreased and subsequently remained stable until term. The decline in total alpha hCG subunit was less marked than that of total beta hCG subunit. The alpha- to beta hCG ratio was equimolar until 10 weeks of gestation when it increased almost fourfold until term (P<0.0001). Finally, the free fraction of the total circulating alpha hCG subunit represented 5-7% in early pregnancy but reached 60-70% in the last trimester (P<0.0001). In contrast, the free fraction of the total beta hCG subunit decreased slightly from 4-5% of total beta subunit in early pregnancy to 2-3% at term (P<0.0001). The present study suggests that thermal dissociation of hCG is a useful method with which to calibrate immunoassays on a molar basis in order to assess circulating levels of the heterodimer and its subunits.