Characterization of a receptor for human chorionic gonadotrophin in luteinized rat ovaries

Abstract

Radioiodine-labeled human chorionic gonadotrophin (HCG) binds in a highly specific manner to sedimentable components from luteinized ovaries of rats in which pseudopregnancy has been induced by treatment with pregnant mare's serum gonadotropin and HCG, but not to similar fractions from parotid or adrenal glands. Of the other iodinated compounds tested (asialo HCG, oxidized HCG, the [alpha] and [beta] subunits of HCG and bovine serum albumin) only asialo HCG was bound to a significant extent to subcellular fractions from ovarian homogenates. Binding [131I]HCG to the crude 800 x g pellet from ovarian homogenates was inhibited by non-labeled HCG. Ovine follicle stimulating hormone, human growth hormone, ovine prolactin, human thyroid stimulating hormone and the [alpha] and [beta] subunits of HCG were virtually ineffective as competitors for HCG binding sites. The possibility that at least a portion of the binding to sedimentable components may be due to plasma membrane contamination of the fractions is not excluded.The amount of hormone bound to receptor increased with increasing duration of incubation, temperature of incubation, and hormone concentration. Kinetic studies indicated that the concentration of binding sites is approximately 2 [middle dot] 10-14 M and that the Kd for the radioiodinated hormone was on the order of 10-10 M. This simple, specific, high affinity binding system may provide a tool for investigating initial steps in the action of HCG.