Comparative affinity of embryonal and maternal serums for corticosterone and progesterone in the rat

Abstract

The binding parameters of corticosterone and progesterone have been measured comparatively in the blood sera from rat, embryos and pregnant rats, at the 19th day of pregnancy. The level of corticosterone binding is significantly higher in the sera of foetal rats than in in the sera of pregnant rats. In contrast, the fixation of progesterone appears quantitatively similar in the embryo and in the pregnant rat. Scatchard analysis shows that a single protein species, with high specific affinity for corticosterone is present in the foetal as well as in the pregnant serum. The association constants have similar values, i.e.Kass= 3 to 5 × 108 M‐1 at 25 ° C; these values are characteristic for the corticosterone‐binding globulin of the rat. However, the binding capacity [concentration of binding sites (n) × number of moles of binding protein (M)] is 2 to 3 times higher in the foetal serum. The results are interpreted as indicating that the higher levels of fixation of this steroid in the embryo, compared to the mother, are due to a quantitative rather than qualitative difference between their binding proteins. The systems involved in the binding of progesterone are more complex in the embryo as well as in the pregnant rat. The values of the association constants and the results of competition and heat‐inactivation experiments suggest a probable contribution of the corticosterone binding globulin to these systems. However, the progesterone seems to bind mainly to one or several proteins with low affinity (Kass= 6 × 106 M‐1 at 25 °C) and great binding capacity (n×M= 215 × 10‐8/g of serum proteins). The foetal and pregnant rat sera appear to behave similarly in the process of progesterone binding.