Isolation of cytoplasmic glucocorticoid-binding protein(s) from rat liver by means of affinity chromatography, and its partial characterization

Studies on high affinity binding of mineralo- and glucocorticoids in rabbit aorta cytosol

High affinity, specific binding-sites to mineralocorticoids and glucocorticoids, with characteristics of steroid receptors, have been found in rabbit aorta cytosol. Binding parameters (dissociation constants and number of binding sites per mg of cytosol protein) were determined from Scatchard plots, after statistical treatment of the data with the aid of a computer program, for the following tritiated steroids: 11-desoxycorticosterone (DOC), aldosterone (Aldo), progesterone (Prog), corticosterone (BK), cortisol (FK) and dexamethasone (Dex). The specificity of binding was then examined by means of steroid competition studies. The results of these experiments indicate that three different types of high-affinity binding sites to adrenal steroids are present in aorta cytosol: Type A, with the highest affinity for DOC; Type B, with the highest affinity for FK; Type C, with the highest affinity for Dex. In accordance with the relative competitive potencies of various steroids for these binding sites, Type A is designated as the "arterial mineralocorticoid binder", clearly differing in its binding characteristics from the cytoplasmic mineralocorticoid binders in known target tissues to these steroids (e.g. the renal receptor), while Type C is designated as the "arterial glucocorticoid binder", closely resembling the classical glucocorticoid receptor in known target tissues to glucocorticoids. Type B exhibited some of the binding characteristics of transcortin and may represent a modified, intracellular transcortin. While Types B and C are present also in the cytosol of inferior vena cava. Type A was only in the aorta. The role of these binders is not known at present. Arguments are presented in favor of a hypothesis that the Type A (mineralocorticoid) binder represents an arterial wall; and that, under certain conditions, this action leads to an increased contractility of arterial and arteriolar smooth muscles, increased peripheral resistance and hypertension.

Electron spin resonance study of human transcortin: Thiol groups and binding site topography

A series of cortisol analogs bearing a nitroxide free radical on C-17 side chains with a variation of distance between the steroid D-ring and the spin label from 7.4 to 17.6 A has been synthesized. These analogs were found to retain a good affinity for the specific corticosteroid binding site of purified human transcortin. The spin-labeled cortisol analogs were used to probe the human transcortin binding site structure by electron spin resonance (ESR) spectroscopy. A total depth of approx. 25 A was estimated for the binding site crevice. Use of sulfhydryl reagents (N-ethylmaleimide, p-chloromercuribenzoate) showed that a maximum of two sulfhydryl groups were titratable after reduction and denaturation of the protein. One of these thiol groups appeared to be involved in the cortisol binding site and could not be detected in the presence of bound steroid. ESR study of its environment, using spin-labeled N-ethylmaleimide reagents of various side-chain lengths, led to the conclusion that this thiol was at a depth of approx. 15 A or more in the binding site cavity. The second sulfhydryl group may be present in an oxidized form in the purified native transcortin, since it became titratable only after reductive treatment of the protein. ESR study showed that this thiol may be located in a crevice at approx. 15 A from the protein surface. These findings are compatible with a structural organization of the transcortin cortisol binding site, taking into account tentative models previously proposed by others.

The kinetics of the precipitation of chemically modified alpha S1-casein by calcium

The aggregation of alpha S1-casein by Ca is viewed as resulting from a reduction in the net negative charge by the binding of positively charged Ca2+. This reduction in charge leads to the equivalent of isoelectric precipitation. A quantitative relationship between the rate of aggregation of the casein and the monomer net charge is predicted in this isoelectric precipitation model. Precipitation results are presented for alpha S1-casein labelled with dansyl chloride. Providing the changes in charge brought about by the modification are taken into account, the precipitation behaviour of these caseins can be reconciled with that of native alpha S1-casein. This emphasises the role of electrostatic repulsion in this process and lend further support to the isoelectric precipitation model.

Inhibition of endogenous RNA polymerase activity from thymus chromatin by transcortin-hydrocortisone complex

A transortin-hydrocortisone complex has been isolated from human serum by affinity chromatography on oxidized corticosterone coupled to AH-Sepharose 4B. The influence of this complex and of hydrocortisone alone on endogenous RNA polymerase activity from thymus chromatin have been tested. Results show that hydrocortisone alone has no effect on RNA polymerase activity from thymus chromatin. Under the same experimental conditions, The transcortin-hydrocortisone complex induces an important decrease in the incorporation of UMP into RNA. The dose response of thymic RNA polymerase to transcortin-hydrocortisone complex and the effects of alpha-amanitin on this response are also reported.