Purification of β-Core Fragment from Pregnancy Urine and Demonstration That Its Carbohydrate Moieties Differ from Those of Native Human Chorionic Gonadotropin-β

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Abstract
Pregnancy urine contains large quantities of hCG, free .beta.-subunit, free .alpha.-subunit, and a population of fragments of .beta.-subunit known as .beta.-core. This .beta.-core population, which can account for as much as 70% of the total .beta.-immunoreactivity in pregnancy urine, is of interest as both a normal metabolite of pregnancy and a potential marker for malignancy. We have purified the .beta.-core fragment from pregnancy urine (P-core) and have characterized it with respect to size and carbohydrate composition. P-Core was purified by chromatography on Sephadex G-100, Concanavalin A (Con A)-Sepharose, DEAE-Sephacel, and Sephadex G-75 (superfine). The purified P-core has an apparent mol wt of 17,500 and 17,000, as determined by gel filtration in Sephadex G-75 (superfine) and sodium dodecyl sulfate-polyacrylamide gel electrophoresis under nonreducing conditions, respectively. The sialic acid content of P-core was assayed chemically and was less than 0.07 .mu.mol sialic acid/.mu.mole P-core. For comparison to P-core, we have prepared a trypsin fragment of .beta.-subunit that retains the .beta.-core immunological determinant recognized by SB6 antiserum and lacks the carboxy-terminal immunological determinant. We have designated this .beta.-core molecule as T-core (tryptic fragment of .beta.-subunit) to distinguish it from the .beta.-core molecule that we have purified from pregnancy urine (i.e. P-core). Most of the P-core and T-core molecules bind to Con A (84% and 86%, respectively). The Con A-bound material was used for subsequent characterizations. Neither P-core or T-core binds to DEAE using conditions under which intact hCG.beta. binds to DEAE. A variety of agarose-bound lectins were used to further investigate the carbohydrate nature of the Con A-bound P-core and T-core molecules. The lectin binding data indicate that the antennae on P-core do not contain appreciable sialic acid or galactose, in contrast to the antennae on T-core, which contain both. We conclude that P-core, the naturally occurring .beta.-core fragment in pregnancy, has been processed to a form in which nearly all of the sialic acid and galactose residues are removed, but the Con A-binding site (consisting of the core sugars) and most of the core fucose are retained.