Diethylstilbestrol and Estradiol Binding to Serum Albumin and Pregnancy Plasma of Rat and Human*

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Abstract
The equilibrium binding of diethylstilbestrol (DES) and 17β-estradiol (E2) to plasma proteins has been characterized. DES exhibits a 10- to 20-fold greater binding affinity index for bovine serum albumin and rat plasma than E2. As expected, E2 gave high values for binding to plasma from pregnant mice or rats, reflecting the presence of α-fetoprotein. DES bound to these samples as it did to bovine serum albumin and rat plasma. These results suggested that DES interacts weakly with α-fetoprotein. This was verified by Scatchard plots of DES and E2 binding to rat and human pregnancy plasma. High affinity, low capacity binding was demonstrated with E2 but not with DES. The significantly lower binding of DES suggests that increased delivery of DES to the fetus may be at least partially responsible for the transplacental toxicity and carcinogenicity of DES.