Specific binding proteins for human chorionic gonadotrophin in a patient with trophoblastic disease

Human chorionic gonadotrophin (hCG) is present in high concentration in patients with trophoblastic disease. The hCG concentration parallels tumor activity. The first known case of an endogenous binder for hCG in the serum of a patient with trophoblastic disease is reported. The patient's hCG concentration was not detectable when analyzed by a radioimmunoassay using a double antibody method; however, it was positive by a radioimmunoassay using polyethylene glycol precipitation. A high serum blank was also observed in this patient. This led the authors to suspect the presence of an endogenous binder for hCG. Scatchard analysis of the patient's serum revealed a high affinity binder for hCG with Ka = 2 X 10¹¹ M^-1, concentration of binding site = 7 X 10^-12 M, and a lower affinity binder with Ka = 1 X 10¹⁰ M^-1, concentration of binding site = 2 X 10^-11 M. This investigation strongly suggested a specific binder(s) for hCG in the serum of this patient.