Haute affinite du serum d'embryon de souris pour les oestrogenes

Abstract

The binding of estrone, 17β-estradiol and 16α, 17β-estriol by mouse embryo and pregnant mouse sera is studied by equilibrium dialysis and gel electrophoresis techniques at different stages of pregnancy. Remarkably high affinities (Pearlman and Crepy indexes) are found for estrone and estradiol in embryo sera, as early as the 12th day of fetal life. This binding activity displays a maximum at 15–18 days of gestation and falls off rapidly after birth. The sera from mothers show little binding activity, except in the last days of pregnancy when the Pearlman-Crepy affinity indexes reach detectable values. Estriol is bound in significant amounts by 15- and 18-day embryo sera. Scatchard analysis of the binding on whole 18-day-embryo sera shows for each estrogen a single class of binding sites. The association constants are approximatively 0.7 · 10 8 M −1 for estradiol, 0.3 · 10 8 M −1 for estrone and 0.02 · 10 8 M −1 for estriol at 25 °C. Analytical electrophoresis in 12% polyacrylamide gels of embryo sera at different ages, previously incubated with the tritiated steroids, demonstrates a single peak of protein-bound radio-activity. The binding protein has the electrophoretic characteristics of mouse α 1-foetoprotein. The estrogen binding properties of mice embryo sera are compared with those described for the rat embryo sera and for purified rat α 1-foetoprotein. The Pearlman-Crepy affinity indexes of the most active sera, the electrophoretic patterns and the association constants for estradiol are similar in the two species. However several significant differences are demonstrated, particularly in binding specificities and rate of disappearance of the binding activity after birth. These results are also discussed in relation to some binding properties of embryo and pregnant serum proteins from man and guinea pig.