β-Core Fragment Is a Major Form of Immunoreactive Urinary Chorionic Gonadotropin in Human Pregnancy*

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Abstract
A small mol wt fragment of the .beta.-subunit of hCG (.beta.-core fragment) is present in the urine, but not the serum, of pregnant women. We evaluated the relative proportions of this immunoreactive, but biologically inactive, fragment in urine from 15 women at different stages of pregnancy. Freshly voided urine was ultrafiltered and concentrated, and the molecular species of immunoreactive hCG were separated by Sephadex G-100 column chromatography. All urine samples contained the .beta.-core fragment, which eluted after the .alpha.-subunit of hCG. This fragment lacked the carboxy-terminal epitope of hCG, was inactive as an in vitro bioassay, and adsorbed to Concanavalin-A. The .beta.-core fragment was a major form the immunoreactive hCG in urine throughout pregnancy and accounted for over 90% of the immunoreactive hCG in urine from midpregnancy. The excretion pattern of the .beta.-core fragment can account for the low biological to immunological ratio of urinary hCG that occurs at different stages of pregnancy.