Disparity between β-Core Levels in Pregnancy Urine and Serum: Implications for the Origin of Urinary β-Core

We used a highly purified preparation of the naturally occurring core fragment of hCGβ (β-core) and a new RIA for β-core to investigate the concentrations and behavior of β-core in serum and urine. We collected serum and 24-h urine specimens from healthy pregnant women during the first trimester of pregnancy. The concentrations of β-core in serum were determined by analysis of fractions eluted from Sephadex G-100. Serum concentrations of β-core immunoreactivity were very low (0.13–1.25 μg/L), while large amounts of β-core were excreted in urine during pregnancy (as much as 4–5 mg/day). Interference with measurement by serum factors did not account for the low levels of β-core immunoreactivity in pregnancy serum. Based on the known urinary clearance rate of β-core in healthy nonpregnant subjects, we calculated that urinary clearance of serum β-core accounts for only about 1% of the β-core in pregnancy urine. We conclude that during pregnancy, the concentrations of β-core in plasma are measurable, but extre...

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