Human chorionic gonadotrophin early pregnancy levels are more closely related to changes in β-subunit trophoblast production than to variations in α-subunit production

Summary Human chorionic gonadotrophin (hCG) is composed of two non-covalently bound α and β subunits synthesized from separate mRNAs. The hCG heterodimer and uncombined subunits are secreted during early gestation by trophoblast into the maternal bloodstream and into the exocoelomic cavity on the opposite side of the trophoblastic layer. The aim was to compare the relative amount of hCG and its free subunits in these compartments. Levels of hCG were similar in coelomic fluid and in maternal serum collected from the same women. By contrast, levels of free subunits were higher in coelomic fluid than in maternal serum: 186-fold for free αhCG subunit and 34-fold for free βhCG subunit. These enormous gradients are likely to be related to differences in the clearance rates of hCG and its subunits between maternal and exocoelomic compartments. Considering coelomic fluid as a metabolic cul-de-sac into which hCG and its subunits accumulate and are slowly metabolized, their levels in this fluid may be reasonably considered as a direct reflection of their trophoblastic production. This hypothesis suggests that the amount of free α subunit is in formidable excess when compared to intact hCG and free βhCG subunit and that only a small fraction ( 90% of βhCG.