Rapid uptake and binding of estradiol-17β-6-(O-carboxymethyl)oxime:: 125I-labeled BSA by female rat liver

To investigate potential membrane-mediated responses to estrogen, a membrane-impermeant, radioiodinated, steroid-BSA conjugate-estradiol-17 beta-6-(O-carboxymethyl)oxime: I-125-labeled BSA (17 beta-E-6-I-125-BSA)-or related steroid conjugates, or I-125-BSA was injected into female Sprague-Dawley rats, and tissues were collected at varying times postinjection. The liver, adrenal, and spleen displayed the most prominent uptake of 17 beta-E-6-I-125-BSA, reaching a maximum of 43 times blood levels in sonicated liver samples at 5 min postinjection, but no uptake of I-125-BSA. Isolation of liver membranes by differential centrifugation showed that over 50% of recovered radioactivity was in association with microsomes and plasmalemma (P3 fraction) at 30 sec postinjection. By 60 min postinjection, over 75% of recovered radioactivity was in association with mitochondrial and lysosomal membranes (P2 fraction), and less than 10% remained in the P3 fraction, In vitro competition assays demonstrated two binding sites in liver P3 fractions, The spleen and liver also showed saturable binding in vivo. These data suggest the presence of at least one membrane-binding protein for estrogen in liver, adrenal, and spleen. Initial studies of affinity-purified liver P3 fractions using ligand blots indicated the presence of two binding proteins, These potential membrane estrogen-binding proteins may be involved in a very rapid shuttling of estrogen from the plasmalemma to mitochondria and lysosomes.