Binding of 3H-cortisol to cytosol and nuclear proteins in liver and thymus of pigs

Abstract

The binding of 3H-cortisol in vitro to protein molecules in the cytosol and nucleus of pig liver and thymus was determined. A comparison of the in vitro binding to previous in vivo binding was made. Also, some comparisons were made between the steroid-protein complexes of the liver, with those in the thymus. The liver and thymus were quickly removed from pigs treated with metyrapone ditartrate and perfused with saline. Tissue minces were prepared and incubated with either 10 −7 M cortisol (125 μCi/μg) or 10 −4 M cortisol (0.125 μCi/μg). After incubation, the tissues were removed, washed, and a nuclear extract and cytosol fraction were obtained. Aliquots of each were fractionated by gel filtration. Radioactivity and protein were determined in each fraction. The binding of cortisol in vitro to protein in the cytosol of the liver and the thymus was similar to results previously reported for in vivo experiments. Therefore, it seems likely that the cytosol macromolecules are able to bind 3H-cortisol in vitro in a way similar to that found in vivo. The elution volumes and specific activities of the cytosol steroid-protein complexes in liver are different from those observed in the thymus cytosol. This indicates that the cytosol proteins which bind cortisol in these two tissues are different, and this may account for the different actions of the steroid in these tissues. The steroid-protein complexes from the nuclei of each tissue had similar elution volumes and similar binding characteristics. Such interaction between the steroid and nuclear macromolecules may be involved in the mechanism of action of cortisol.