Characterization of a Small Immunoreactive HCG-Like Component Isolated from Urine of a Patient with Choriocarcinoma

Summary

An immunoreactive hCG-like substance with low molecular weight was isolated from the urine of a patient with choriocarcinoma and its characteristic properties were studied. The urinary hCG preparation was fractionated by gel filtration on a Sephadex G75 column and each fraction was assayed for hCG activities by an enzyme linked immunosorbent assay using a monoclonal antibody to hCG (Mab-5D4). Two immunoreactive hCG peaks were obtained. One was coeluted with ordinary hCG and the other was eluted after hCG-α. After refractionation on the same column, the hCG peak (Ag-2) with low molecular weight was radioiodinated with ¹²⁵INA and applied to an immunoaffinity column bound Mab-5D4 for further purification. The purified ¹²⁵I-labelled Ag-2 showed a high binding activity to a conventional rabbit anti-hCG serum and Mab-5D4. This Mab had binding specificity to hCG, hCG-β and LH but not to hCG-α. However, this fraction did not bind to Mab-6E4 which possessed binding activities only to hCG, hCG-α and LH, nor to Mab-2F8 which was specific only to hCG. Autoradiography after SDS-Page of the immune precipitate which was made by ¹²⁵I-labelled Ag-2 and Mab-5D4, revealed that the Ag-2 had a molecular weight of approximately 14,000 daltons.

Lectin (LcH) affinity chromatography of the urinary hCG specimen from the same patient revealed that it contained two kinds of immunoreactive hCG. One did not bind to LcH column but the other did. A small immunoreactive molecule (Ag-2) was detected in the LcH-unbound fraction but not in the LcH-bound fraction.

These results suggest that choriocarcinoma patients excrete in the urine a small LcH-unbound immunoreactive component which contains an antigen epitope common to hCG, hCG-β and LH.

Key words

Small hCG-like molecules - Choriocarcinoma - Monoclonal antibodies - Lectin affinity