ORIGIN OF TRANSCORTIN IN THE CHICK-EMBRYO

The synthesis of a transcortin-like molecule in liver of the chick embryo is demonstrated in vitro. Since no contamination by blood CBG occurred and no C-21 steroid binders were detected in the liver cytosol of 5-day-old embryos, liver of embryos at this stage was cultured in a synthetic medium without serum. The steroid binders were detected by equilibrium dialysis after a 4-day culture. Besides non-saturable binding, specific binding with C21-steroids occurred in the liver cytosol (20%) and in the culture medium (80%). Determination of binding constants as well as competition experiments show that corticosterone and progesterone are bound to the culture medium and to the plasma CBG with the same affinity (Ka4 c: 5 × 108 M-1) and specificity. An effect ofestradiol-17β upon the secreted CBG synthesis was observed. Estradiol (10-8 M) either alone or with thyroxine slightly increased CBG secretion in culture medium. © 1979.