Structural similarity of molybdate-stabilized steroid receptors in human breast tumors, uteri and leukocytes

Steroid receptor interactions with heat shock protein and immunophilin chaperones

We have provided a historical perspective on a body of steroid receptor research dealing with the structure and physiological significance of the untransformed 9S receptor that has often confused both novice and expert investigators. The frequent controversies and equivocations of earlier studies were due to the fact that the native, hormone-free state of these receptors is a large multiprotein complex that resisted description for many years because of its unstable and dynamic nature. The untransformed 9S state of the steroid and dioxin receptors has provided a unique system for studying the function of the ubiquitous, abundant, and conserved heat shock protein, hsp90. The hormonal control of receptor association with hsp90 provided a method of manipulating the receptor heterocomplex in a manner that was physiologically meaningful. For several steroid receptors, binding to hsp90 was required for the receptor to be in a native hormone-binding state, and for all of the receptors, hormone binding promoted dissociation of the receptor from hsp90 and conversion of the receptor to the DNA-binding state. Although the complexes between tyrosine kinases and hsp90 were discovered earlier, the hormonal regulation or steroid receptor association with hsp90 permitted much more rapid and facile study of hsp90 function. The observations that hsp90 binds to the receptors through their HBDs and that these domains can be fused to structurally different proteins bringing their function under hormonal control provided a powerful linkage between the hormonal regulation of receptor binding to hsp90 and the initial step in steroid hormone action. Because the 9S receptor hsp90 heterocomplexes could be physically stabilized by molybdate, their protein composition could be readily studied, and it became clear that these complexes are multiprotein structures containing a number of unique proteins, such as FKBP51, FKBP52, CyP-40, and p23, that were discovered because of their presence in these structures. Further analysis showed that hsp90 itself exists in a variety of native multiprotein heterocomplexes independent of steroid receptors and other 'substrate' proteins. Cell-free systems can now be used to study the formation of receptor heterocomplexes. As we outlined in the scheme of Fig. 1, the multicomponent receptor-hsp90 heterocomplex assembly system is being reconstituted, and the importance of individual proteins, such as hsp70, p60, and p23, in the assembly process is becoming recognized. It should be noted that our understanding of the mechanism and purpose of steroid receptor heterocomplex assembly is still at an early stage. We can now speculate on the roles of receptor-associated proteins in receptor action, both as individuals and as a group, but their actual functions are still vague or unknown. We can make realistic models about the chaperoning and trafficking of steroid receptors, but we don't yet know how these processes occur, we don't know where chaperoning occurs in the cell (e.g. Is it limited to the cytoplasm? Is it a diffuse process or does chaperoning occur in association with structural elements?), and, with the exception of the requirement for hormone binding, we don't know the extent to which the hsp90-based chaperone system impacts on steroid hormone action. It is not yet clear how far the discovery of this hsp90 heterocomplex assembly system will be extended to the development of a general understanding of protein processing in the cell. Because this assembly system is apparently present in all eukaryotic cells, it probably performs an essential function for many proteins. The bacterial homolog of hsp90 is not an essential protein, but hsp90 is essential in eukaryotes, and recent studies indicate that the development of the cell nucleus from prokaryotic progenitors was accompanied by the duplication of genes for hsp90 and hsp70 (698). (ABSTRACT TRUNCATED)

Characterization of cytosolic glucocorticoid receptor of fetal rat epiphyseal chondrocytes

The cytosolic glucocorticoid receptor of 21st gestational day rat epiphyseal chondrocytes has been evaluated. The receptor, a single class of glucocorticoid binding component approached saturation, utilizing [3H]triamcinolone acetonide ([3H]TA) as the radiolabeled ligand, at approximately 1.8-2.0 x 10(-8) M. The dissociation constant (Kd) reflected high-affinity binding, equaling 4.0 +/- 1.43 x 10(-9) M (n = 7) for [3H]TA. The concentration of receptor estimated from Scatchard analysis was approximately 250 fmol/mg cytosolic protein and when calculated on a sites/cell basis equalled 5800 sites/cell. The relative binding affinities of steroid for receptor were found to be triamcinolone acetonide greater than corticosterone greater than hydrocortisone greater than progesterone greater than medroxyprogesterone acetate much greater than 17 alpha-hydroxyprogesterone much greater than testosterone greater than 17 beta-estradiol. Cytosolic preparations activated in vitro by warming (25 degrees C for 20 min) were shown to exhibit an increased affinity for DNA-cellulose. 46% of the total specifically bound activated ligand-receptor complex was bound to DNA-cellulose. Cytosol maintained at 0-4 degrees C in the presence of 10 mM molybdate or activated in vitro in the presence of molybdate, bound to DNA-cellulose at 8 and 10% respectively. DEAE-Sephadex elution profiles of the nonactivated receptor were indicative of a single binding moiety which eluted from the columns at 0.4 M KCl. Elution profiles of activated receptor were suggestive of an activation induced receptor lability. The 0.4 M KCl peak was diminished, while a concomitant increase in the 0.2 M KCl peak was only modestly discernible. Evaluation of endogenous proteolytic activity in chondrocyte cytosol using [methyl-14C]casein as substrate show a temperature-dependent proteolytic activity with a pH optimum of 5.9-6.65. The proteolytic activity was susceptible to heat inactivation and was inhibitable, by 20 mM EDTA. The sedimentation coefficient of the nonactivated receptor was 9.3s (n = 6) on sucrose density gradients and exhibited steroid specificity and a resistance to activation induced molecular alterations when incubated in the presence of 10 mM molybdate. Receptor activation in vitro, in the absence of molybdate induced an increased receptor susceptibility to proteolytic attack and/or enhanced ligand receptor dissociation as evidenced by a diminution of the 9.3s binding form without a concomitant increase in 5s or 3s receptor fragments.

Electrophoretic analysis of large molecular weight forms of the estrogen receptor in MCF-7 human breast cancer cells

The denatured and nondenatured forms of the estrogen receptor (ER) in MCF-7 human breast cancer cells have been characterized by gel electrophoresis using the covalently attaching ligand [3H]tamoxifen aziridine ([3H]TA). A comparison of the ER binding properties of the recently introduced commercial preparation (Amersham) of [3H]TA with the prototype showed no quantitative or qualitative differences, with both preparations binding efficiently and selectively at either 4 degrees C or 37 degrees C. The nuclear and cytosolic labeled ER, when analyzed by denaturing SDS-PAGE, migrate as 67,000 dalton proteins. [3H]TA labeled ER from cytosol prepared in low salt buffer without protease inhibitors migrates on 5%, 6%, or 7% nondenaturing PAGE as two discrete forms. The molecular weights of these two species, determined by Ferguson plot analysis with multiple standard protein markers, are 260,000 and 67,000 daltons, respectively. The 260 kDa form is dissociated by 0.4 M KCl into the 67 kDa form, suggesting that the larger form is composed of one or more noncovalently attached subunit proteins. When enzyme inhibitors leupeptin and molybdate are included in cytosol prepared in low salt, two high molecular weight [3H]TA labeled peaks are identified, the first 600,000 and the second 430,000 daltons. These two forms migrate together on 4%, 5%, and 6% non-SDS, non-denaturing gels, and are not dissociated when cytosol is made 0.4 M in KCl. These findings indicate that the ER of these human breast cancer cells exists in one or more large discrete forms of mol. wt 430,000-600,000 daltons. These forms are not dissociated by high salt but may be digested by proteases to the subunit 67,000 dalton form. These findings support the proposal that the ER is present as a large molecular weight "holoreceptor", and may be important for our understanding the metabolism and function of the ER in estrogen target tissues and human breast cancer.

Hormone receptor assays: clinical usefulness in the management of carcinoma of the breast

The revision of the subcellular model of hormone action is described, with an incorporation of potential autocrine mechanisms. A general overview of available assay methodologies considers the major disadvantages of earlier methods and describes in detail the current methodologies (sucrose gradient analysis, dextran-coated charcoal assays, ER-EIA, ERICA). A major concern with clinical correlations of response to hormone receptor levels is the quality assurance of the multicentric programs. Results from national and international programs are considered. The clinical correlations are divided into four major categories: (1) the response to hormone deprivation (oophorectomy or adrenalectomy), (2) the development of specific agents which exploit receptor mechanisms (antiestrogens) or inhibit steroid biosynthesis (aminoglutehimide), (3) the rates of recurrence of tumors following mastectomy, and (4) the correlation of hormone receptors with current adjuvant therapies.

Subunit dissociation and activation of wild-type and mutant glucocorticoid receptors

Apparent molecular weights of wild-type and nti ('increased nuclear transfer') mutant glucocorticoid receptors were obtained from Stokes radii and sedimentation coefficients. At low salt concentrations molecular forms of Mr 328,000 and 298,000 of the wild-type and mutant, respectively, were predominant. Increasing ionic strength resulted in receptor dissociation. Dissociated forms of Mr 130,000 and 63,000 of the wild-type and mutant, respectively, were obtained at 300 mM KCl and above. Some metal oxi-anions prevented dissociation. Receptor activation to allow DNA binding produced the dissociated forms which could be separated from non-activated receptors by filtration through DNA-cellulose or by DEAE-cellulose chromatography. Non-activated wild-type and nti receptors eluted from DEAE-cellulose under identical conditions while activated wild-type and nti receptors eluted differently. Partially proteolyzed wild-type receptors behaved identically to nti receptors. We conclude that the large forms of wild-type and nti receptors are heteromeric and contain only one hormone-building polypeptide per complex.

Circadian variation of cytosol glucocorticoid receptors in human polymorphonuclear leukocytes (PMN) and mononuclear cells (MN) in a normal population

Glucocorticoid cytosol and whole cell receptors from human PMN's have been quantified, and compared to those of human MN leukocytes on the same blood sample. The normal cytosol PMN receptor density (N = 15) averaged 1,254 +/- 105 (SE) molecules bound/cell at 0900 h and increased significantly to 1,497 +/- 98 at 2,100 h (P less than 0.02). MN cell cytosol receptor density was 1,198 +/- 145 at 0900 h and increased significantly to 1,551 +/- 117 molecules bound/cell at 2,100 h (P less than 0.01). Corresponding whole cell receptor densities at 0900 h were 2,845 +/- 273 (PMN) and 3,547 +/- 290 (MN) and these did not change significantly at 2,100 h.

CONCLUSIONS

Cytosol receptors in normal human PMN and MN cells increased significantly at 2,100 h from the 0900 h level while serum cortisol levels were dropping. Whole cell receptors in the same PMN and MN cell samples did not change significantly between 0900 and 2,100 h. The normal circadian variation in serum cortisol influences the distribution of the glucocorticoid receptor between the cytosol and the nucleus, but does not influence the amount of receptor available to the whole cell. This is the first time that an endogenous physiological variation of cortisol concentration has been utilized to demonstrate a corresponding change in receptor capacity in vivo.

"Defective" receptors in steroid-resistant conditions may be proteolytic artifacts

The specific question addressed in this report is whether the resistance to steroid treatment of certain tissues or tumors which appear to contain a normal quantity of steroid-binding sites may be due to structural defects in the receptors. This question may be seen as part of the more general question of whether there are intrinsic variations in the structures of receptors for a given class of steroids in different healthy tissues, in healthy vs. malignant tissues or in different types of tumors. Our experimental approach to these questions has involved the stabilization and precise physicochemical characterization of the receptors. To date, we have studied the estrogen and progestin receptors from human breast cancers and benign and malignant gynecologic specimens and the glucocorticoid receptors from several healthy and malignant rodent tissues and from normal human lymphocytes and various types of leukemic cells. Chromatographic and ultracentrifugal analyses in buffers of low ionic strength, containing 20 mM Na2MoO4 as the stabilizer, have revealed each of these receptors to be a large, oligomeric complex, characterized by remarkably similar values of the Stokes radius, sedimentation coefficient, molecular weight and axial ratio. In the absence of adequate stabilization, however, we found that the receptors for three classes of steroids in extracts of some healthy, steroid-responsive tissues, such as rat kidney and human uterine endometrium, are invariably degraded by endogenous proteinases. The extent of such cleavage is increased considerably by freezing the tissues prior to homogenization. Studies designed to distinguish the intact receptors from the products of proteolysis have included the characterization of receptors in cytosols prepared from mixtures of rat liver and kidney. The results strongly support the interpretation that the smaller size of the receptors detected in kidney cytosol reflects their cleavage by the more active proteinases in that tissue. The sizes and shapes of the receptors in cytosols from various tissues were found to be correlated with the activities of specific endopeptidases, assayed fluorometrically with peptidyl derivatives of 7-amino-4-methylcoumarin (AMC). These studies suggested that the receptors are vulnerable to cleavage by "lysine-specific" endopeptidases, detected with t-butyloxycarbonyl-L-valyl-L-leucyl-L-lysyl-AMC. An enzyme of this specificity was partially purified from rat kidney cytosol and tested for its ability to digest the glucocorticoid receptors from rat liver cytosol.(ABSTRACT TRUNCATED AT 400 WORDS)

Glucocorticoid receptors in human leukemias and related diseases

The evidence to date is compelling that steroid initiated cell lysis involves participation of the glucocorticoid receptor. Not only do the concentrations and specificity of hormones for cell lysis and receptor occupancy correspond, but also steroid resistant cells selected with or without prior mutagenesis often have altered receptors. The glucocorticoid receptor protein from humans and other species is a approximately 95,000 d, thiol group-containing monomer, prone to aggregation when "unactivated." After having bound steroid and been "activated," the monomeric steroid-receptor complex is altered in charge and shape so that its binding to chromatin and DNA is greatly enhanced. Simple measurement of numbers of receptor sites in cells from patients with various blood dyscrasias has given, in some disease, good correlations between high numbers of receptor sites and good therapeutic response. These correlations are strongest for childhood acute lymphoblastic leukemia (ALL) and for non Hodgkins' lymphoma. In other diseases, notably acute myelogenous leukemia, such correlations have not been found. The CEM human ALL line has been used in vitro to study mechanisms of glucocorticoid action and resistance. The requirement for "activated" steroid-receptor complex for cell lysis is shown in these cells by the spontaneous occurrence of steroid resistant, activation-labile receptor mutants. A second category of resistant cells with normal receptors has been defined. Treatment of these "lysis defective" resistant cells with compounds which result in DNA demethylation can render them steroid sensitive. Since DNA demethylation can allow formerly silent genes to become transcribed, it is possible that one or more genes specific for lysis has been "opened" in such cells. Alternatively, DNA demethylation may produce a general biochemical effect on the cell which renders it susceptible to lysis. Mutagenized CEM cells selected for steroid resistance give rise to a third class of mutants, which are deficient in receptor quantity. Each of these classes of steroid resistant cells contains information pertinent to understanding the use of glucocorticoids and the role of glucocorticoid receptors in human leukopathic disease.

Isolation of unliganded steroid receptor proteins by high-performance size-exclusion chromatography. An investigation of steroid-dependent structural alterations

The physicochemical properties of unliganded steroid receptor proteins remain largely unknown primarily due to receptor lability in the absence of specific ligand, especially during prolonged biochemical analyses. We have utilized high-performance size-exclusion chromatography (HPSEC) as a rapid means of investigating the structural properties of cytosolic estrogen receptor proteins in both the presence and absence of [3H]estradiol-17 beta. Cytosols prepared from immature calf uteri were analyzed by HPSEC on an Altex TSK-3000 SW column (600 mm X 7.5 mm I.D.) either before or after incubation with 10 nM [3H]estradiol-17 beta. Postcolumn detection of previously unliganded receptor was accomplished by incubation of fractions (0.38 ml) with 10 nM [3H]estradiol-17 beta for 2-18 h at 0 degrees C. Receptor-bound steroid was separated from free steroid by incubation with small pellets of hydroxylapatite. Nonspecific binding of [3H]estradiol-17 beta in parallel fractions was estimated using an unlabelled competitor (diethylstilbestrol) specific for the estrogen receptor. In low ionic strengths the receptor exists as a single, relatively stable, large form (retention time 34 min). The elution properties of this receptor configuration do not depend on bound steroid ligand. Analysis of receptor at elevated ionic strengths in the presence and absence of steroid ligand suggests that the salt-induced dissociation of receptor components to smaller forms (retention time 47 min) may be partially steroid-dependent. Characterization of receptor in 6 M urea demonstrates the presence of intermediate-sized receptor components (retention time 36-38 min). Analyses of receptor in 6 M urea-0.4 M potassium chloride suggests an inhibition of the more extensive salt-induced dissociation event seen in 0.4 M potassium chloride alone. Furthermore, the intermediate-sized receptor forms seen under these conditions (retention time 41-42 min) are generated in a steroid-dependent manner. The preparation of different molecular forms of biologically active, unliganded estrogen receptor by HPSEC should help further our investigations into the molecular mechanism(s) by which steroid hormones exert their receptor-mediated effects on target cells.

High-performance chromatofocusing of steroid receptor proteins in the presence and absence of steroid. Investigation of steroid-dependent alterations in surface charge heterogeneity

We have previously reported the development of high-performance chromatofocusing (HPCF) systems for rapid evaluation of the surface charge heterogeneity of steroid receptor proteins, each in the presence of its specific steroid ligand. However, the surface charge properties of ligand-free receptor proteins remain largely unknown. We have now employed HPCF to rapidly evaluate the surface charge properties of cytosolic estrogen receptor proteins in both the presence and absence of the ligand ([3H]estradiol-17 beta). All operations were performed at 0-4 degrees C. Cytosols prepared from immature calf uteri were preparatively analyzed by HPCF on a SynChropak AX-500 column (25 cm X 4.6 mm I.D.) either before or after incubation with 5-10 nM [3H]estradiol-17 beta. Elution of receptor was by generation of internal pH gradients (pH 8.1 to 3.2) using Pharmacia Polybuffers 96 and 74. Postcolumn detection of previously unliganded receptor was accomplished by incubation of pH-neutralized (pH 7.4) fractions with 5 nM [3H]estradiol-17 beta in the presence and absence of unlabelled competitor. Specifically bound steroid was determined in each fraction using an hydroxylapatite adsorption assay. Significant surface charge heterogeneity was observed for both unliganded receptor and the steroid-receptor complex. The heterodisperse pattern of receptor surface charge appeared to vary in a steroid-dependent manner. Preformed steroid-receptor complexes eluted primarily between pH 6.5-7 and between pH 5-6, with indications for heterogeneity within both regions. The surface charge distribution of unliganded receptor routinely revealed additional, more acidic eluting (pH 3.8-4.6) receptor forms. Sodium molybdate, a commonly used receptor-stabilizing agent, maintains receptors during HPCF as relatively acidic eluting forms (pH 3.8-5.0). The specific elution profile of molybdate-stabilized receptor also appears steroid-dependent. These data demonstrate that HPCF can be used preparatively to rapidly isolate unliganded receptor forms in a biologically active state.

Estrogen and progestin receptors in human prostatic carcinoma

Cytosol receptors for estrogens (ER) and progestins (PR) were assayed in human prostatic carcinoma (CaP) and benign prostatic hyperplasia (BPH). Specimens were obtained from either the peripheral or the periurethral zone of the prostate. Stringent criteria were used to identify and measure 7-8S specific receptor using sucrose gradient analysis in a vertical tube rotor. Progesterone receptor was found in 14 BPH samples assayed and in 12 of 13 prostate cancers. In contrast, the 7-8S estrogen receptor was found in none of the nine benign samples assayed and in all prostate cancers. BPH samples were taken from either peripheral or periurethral zones and gave similar results. The histology of individual specimens did not correlate with either the ER or PR present, and, in the cancers, there was no correlation between the pathologic stage or the Gleason score and receptor content.

Characterization of an antiestrogen-binding protein in high salt extracts of human breast cancer tissue

An antiestrogen binding protein which binds [3H]tamoxifen (1-[4-(2-dimethylaminoethoxy)-phenyl]1,2-diphenylbut-1(Z)-ene) with high affinity (Kd = 1.1 X 10(-9) M) is present in high salt (0.6 M KCl) extracts of washed breast cancer tissue pellets. Its concentration in high salt extract is higher than its concentration in cytosol. The characteristics of the antiestrogen binding protein from cytosol and salt extract of breast cancer tissue are indistinguishable. It specifically binds triphenylethylene and other nonsteroidal antiestrogens and displays little or no binding affinity for estrogens, progesterone, dihydrotestosterone and cortisol. The antiestrogen binding protein is of unusually large size as judged by gel filtration on agarose 0.5 m and sedimentation analysis on 5-20% sucrose density gradients. Differential centrifugation studies indicate that it is not principally microsomal in origin. This protein is more thermostable than the estrogen receptor from which it can also be distinguished by ion exchange chromatography. The antiestrogen binding protein was eluted from DEAE-Sephacel by 0.05 M KCl indicating that it is less negatively charged than the estrogen receptor which was eluted by 0.1 M KCl. Lipoprotein fractionation of breast cancer cytosol using potassium bromide density gradients did not reveal specific antiestrogen binding activity associated with any recognized class of lipoprotein. Specific [3H]tamoxifen binding sites were pelleted in potassium bromide gradients consistent with the apparent large size of this protein. The physical characteristics of the antiestrogen binding protein in normal human tissue (myometrium) and neoplastic tissue (breast cancer) are remarkably similar, possibly reflecting a highly conserved structure.

Hepatic estrogen receptors and plasma estrogen-binding activity in the Atlantic salmon

Livers of male and female immature Atlantic Salmon (Salmo salar) contain specific high-affinity [3H]estradiol binding sites in cytosol (Kd 2-4 nM, concentration about 0.6 pmol/g liver). Low levels of high-affinity binding are detectable in salt extracts of nuclei of untreated fish, but injections of estradiol result in transient depletion of the cytosol binder and in accumulation of high levels of binding sites in nuclear salt extracts (Kd 5-6 nM; concentration about 6 pmol/g liver). Both the cytosol and nuclear binding sites are temperature sensitive and are optimally assayed by incubation at 2 degrees. Both are specific for estradiol and diethylstilbestrol (DES) and no significant competition by dihydrotestosterone (DHT), progesterone, or hydrocortisone is seen. The triphenylethylene nonsteroidal antiestrogen, 4-hydroxytamoxifen, exhibits an affinity comparable to that of estradiol. The nuclear binding activity sediments with a coefficient of 3.6 S in salt-containing sucrose density gradients, and is stable on storage at -20 degrees for several months. The cytosol binder on the other hand is not stable on sucrose density gradients or on prolonged storage. Salmon plasma contains two [3H]estradiol binding components, one with a relatively high affinity for [3H]estradiol (kd 13 nM) and the other having a much lower affinity but present in high concentrations. The high-affinity plasma binder exhibits distinctive specificity with no affinity for DES or 4-hydroxytamoxifen but some affinity for DHT and progesterone. These properties serve to distinguish the plasma activity from the intrahepatic estrogen binders. The salmon liver estrogen receptor system has many features in common with typical estradiol receptors from other vertebrates. Immature salmon liver appears to be the richest source of hepatic estrogen receptor so far found for any vitellogenic species.

Heteromeric nature of glucocorticoid receptors

The wild-type and a mutant receptor of S49.1 lymphoma cells have been shown by photoaffinity labelling to contain steroid-binding polypeptides of Mr 94 000 and 40 000, respectively. We investigated the molybdate-stabilized forms of these receptors and obtained Mr 325 000 and 285 000, respectively, by gel filtration and sedimentation analysis. Mild chymotrypsin treatment of the large wild-type receptor resulted in a form of about Mr 290 000 which contained a steroid-binding polypeptide of Mr 40 000. The data suggest that the high -Mr forms of glucocorticoid receptors are heteromeric in nature and contain one steroid-binding polypeptide per complex.

Analysis of the steroid receptor of Achlya ambisexualis

We have previously reported the discovery of a specific high-affinity binding protein for the fungal sex steroid pheromone antheridiol in the cytosol of Achlya ambisexualis male cells. In this report, we describe the fractionation of the binding protein from the cytosol by ammonium sulfate precipitation, the optimization of in vitro conditions for radioligand binding assays, and some of the biochemical properties of the binding protein. In the presence of sodium molybdate, the macromolecule has a sedimentation coefficient of 8.3 S in sucrose gradients of low ionic strength, a Stokes radius of 56.6 A (Sephacryl S-300 columns), a molecular weight of approximately 192,000, a frictional ratio of 1.5, and an axial ratio of 8.9. The binding protein can be eluted with 0.24 M KCl as a single peak from DEAE-Sephadex A-25 columns. These results indicate that this steroid-binding protein from a primitive eukaryotic microbe has in vitro biochemical properties that are similar to those of other known steroid receptors in higher organisms.

Exchange assay for androgen receptors in the presence of molybdate

Several reports have shown that sodium molybdate stabilizes steroid hormone receptors. We have utilized these observations to develop an exchange assay for the androgen receptor at elevated temperatures. Exchange was found to be complete after 30 min at 30 degrees C. Receptor degradation was negligible during this treatment. Scatchard analysis indicated that the dissociation constant of the androgen receptor was similar both in the absence (Kd = 3.9 nM) and presence (Kd = 2.9 nM) of molybdate. Steroid specificity of the androgen receptor was unaltered by this treatment. The exchange procedure was reproducible, with an interassay variation of 2.45% and intraassay variation less than 10.0%. Using this assay, highest concentrations of androgen binding were measured in androgen target tissues of the rat (Dunning R3327 tumor, prostate and seminal vesicle; 23.37, 20.20 and 19.84 fmol/mg protein respectively). Lower concentrations were observed in other tissues (lung, brain, heart, spleen, liver and kidney; 9.06, 5.63, 3.50, 2.42, 2.33 and 1.36 fmol/mg protein respectively). These results demonstrate that molybdate stabilization of the androgen receptor allows efficient steroid exchange without significant alteration of the receptor's steroid binding properties. Furthermore, this exchange assay can be used to obtain a reasonable measurement of receptor concentrations in different androgen target tissues.

Influence of phosphatase inhibitors and nucleotides on [3H]dexamethasone binding in cytosol of human placenta

The purpose of this investigation was to establish the properties of [3H]dexamethasone binding sites in cytosol of human placenta at term. Cytosol containing 20 mM sodium molybdate (MoO4Na2) was incubated for 120 min at 20 degrees C with 40 nM [3H]dexamethasone. The following properties were observed: (a) a single population of binding sites of high affinity and low capacity was measured by Scatchard analysis; (b) potent glucocorticoids such as dexamethasone and cortisol displaced the tritiated ligand, progesterone showed an intermediate activity, whereas cortisone, testosterone and 17 beta-estradiol were ineffective competitors; (c) ultracentrifugation on 16-41% glycerol gradients containing 20 mM MoO4Na2 yielded sedimentation values of 10.25 +/- 0.35 S (n = 4 placentas); (d) the binding sites could be differentiated from the enzyme 11 beta-hydroxysteroid dehydrogenase, as the activity of the former, but not that of the latter, was greatly dependent on the presence of MoO4Na2 in the incubation medium. Inactivation of binding sites labelled with [3H]dexamethasone by incubation at 20 degrees C was prevented by phosphatase inhibitors such as 20 mM MoO4Na2 (P less than 0.01), 20 mM sodium tungstate (WO4Na2) (P less than 0.01) and to a lower extent by 5 mM ATP and cAMP (P less than 0.05). 50 mM NaF, 5 mM GTP or cGMP had no effect. The protection afforded by MoO4Na2 and WO4Na2 was correlated with a significant inhibition of the activity of acid phosphatase, but not alkaline phosphatase. Neither ATP nor cAMP modified phosphatase activity. It is suggested that binding sites for [3H]dexamethasone in cytosol of human placenta showed properties similar to those described for glucocorticoid receptors in target cells, and that these binding sites are regulated by phosphorylation and dephosphorylation mechanisms.

Detection of a pheromone-binding protein in the aquatic fungus Achlya ambisexualis

Sexual reproduction among the eukaryotic fungi of the genus, Achlya, is controlled by two steroid pheromones. Antheridiol is the pheromone constitutively produced by female cells that induces male sexual differentiation and development. A biologically active tritium-labeled derivative of antheridiol, [1,2-3H]7-deoxy-7-dihydro-antheridiol ( [3H]7dA), has been synthesized. Radioligand-binding studies have revealed the presence of a specific binding protein in the cytosol of male cells that may represent the endogenous receptor for antheridiol. Binding to this macromolecule was characterized by an apparent equilibrium dissociation constant and maximum binding capacity of approx. 7 X 10(-10) M and 1 100-2 000 fmoles/mg protein, respectively. Sedimentation analysis in sucrose gradients revealed that the binding protein distributes in the 8S region under low ionic strength and sodium molybdate-stabilized conditions. Under conditions of high ionic strength, in the presence or absence of 10 mM sodium molybdate, the binding site distributes in the 3.6S region of the gradient. Analysis of radioligand binding in the presence of other steroids and steroid hormones revealed that the binding is specific for antheridiol and its analog.

Immunochemical characterization of wild-type and variant glucocorticoid receptors by monoclonal antibodies

Monoclonal antibodies raised against the rat liver glucocorticoid receptor were used to investigate receptors of wild-type and glucocorticoid-resistant variants of mouse lymphoma cells. Two of the variant types contained receptors of 'nuclear transfer deficient' (nt-) and 'increased nuclear transfer' (nti) phenotypes, respectively, while the third was of the 'receptorless' (r-) phenotype with negligible hormone binding activity. Three monoclonal antibodies of the IgM class and one of the IgG class reacted with both wild-type and nt- receptors but not with the steroid binding form of nti receptors. Some of the antibodies bound the wild-type and nt- receptors more efficiently after activation at 20 degrees C. By use of an immuno-competition assay we were able to detect cross-reacting material in considerable amounts in extracts of nti and r- cell variants. This material was further characterized by gel filtration and immunoblotting. The immunoreactive material of wild-type, nti and r- cells gave a major band of mol. wt. 94 000 upon SDS-gel electrophoresis while the steroid-binding polypeptides of wild-type and nti receptors have mol. wts. of 94 000 and 40 000, respectively. The data show that in S49.1 mouse lymphoma cells the products of two receptor alleles can be distinguished.

Heparin-mediated inactivation and transformation of mammary cytoplasmic glucocorticoid receptor

Glucocorticoid receptor of lactating mouse mammary gland cytosol was exposed to heparin when the receptor was either steroid-free or steroid-bound. Heparin caused a dose-dependent and time-dependent loss of steroid binding activity (inactivation) of the steroid-free receptor; this heparin-induced inactivation was inhibited by molybdate. In contrast, steroid-bound receptor maintained its steroid binding capacity in the presence of heparin but the heparin caused transformation of receptor as detected by increased binding to DNA-cellulose and ATP-Sepharose. Heparin also converted steroid-bound receptor from the 7-8S form to the 4S form. Molybdate inhibited both the heparin-induced transformation and associated conversion to the 4S form.

Structure and subunit dissociation of the mouse glucocorticoid receptor: rapid analysis using vertical tube rotor sucrose gradients

The structure and subunit dissociation of the glucocorticoid receptor from the mouse AtT-20 pituitary tumor cell line was analyzed on sucrose gradients using a Beckman VTi 80 vertical tube rotor. This technique afforded a very rapid analysis (65 min) of the variously sedimenting forms compared to swinging-bucket rotor sucrose gradients, which take 16 h to run. Thus, it was possible to detect and study the molybdatestabilized, oligomeric, untransformed receptor (9.1 S) in the presence of 0.3 M KCl. Under similar conditions using the swinging-bucket rotor, only the monomeric, transformed species (3.8 S) was observed. That is, artifactual subunit dissociation was minimized using the vertical tube rotor, allowing the study of the receptor structure in a more native state. Further studies demonstrated that Sephadex LH-20 chromatography causes receptor transformation. Thus, dextran-charcoal adsorption is preferred for the removal of unbound hormone under certain circumstances. Finally, using vertical tube rotor sucrose gradients, it was determined that the transformation of the mouse AtT-20 glucocorticoid receptor involves a conversion of the oligomeric, 9.1 S, untransformed species to a 5.2 S, transformed moiety. This suggests that the 5.2 S, intermediate transformed species may be the physiologically relevant form of this gene regulatory protein.

Characterization of 7-8S progestin binding protein in human prostate using vertical tube rotor

The specific binding of the synthetic progestin, 17 alpha-methyl[3H]promegestone (R5020), to the cytosol of human benign hyperplastic prostate has been studied in sucrose density gradients using a vertical tube rotor. The cytosol of human prostate was shown to contain substantial amounts of a 7-8S macromolecule with a high affinity (Kd = 0.5-1 nM) for R5020 which is saturated at low concentrations (10 nM). The conventional technique of sucrose density gradient analysis in a swinging bucket rotor was not suitable for reproducible optimal analysis of a 7-8S high affinity complex. The use of the salt, Na2MoO4, had a stabilizing effect on the complex. Comparison of saturation analysis assays using dextran charcoal assay and vertical tube rotor assay showed that the charcoal assay can give an over-estimation of the 7-8S saturable binding. Progestational steroids competed with R5020 for binding to 7-8S, whereas androgenic steroids, with the exception of 19-nor-testosterone, did not compete. Incubation of cytosol at elevated temperatures in the presence of DNA-cellulose resulted in the binding of the hormone-protein complex to DNA-cellulose. High ionic strength buffer was required to extract the complex which sedimented at 4.5S in sucrose gradients prepared in 0.4 M KCl. Based on the data presented, progestin binding in human prostate is clearly similar in physical chemical properties to progesterone receptors in "classical" target tissues. However, rapid sucrose gradient analysis with a vertical tube rotor is preferred over conventional techniques to evaluate progestin receptor binding in human prostate.

Glucocorticoid receptors in the gill tissue of fish

The presence of glucocorticoid-binding macromolecular receptors was demonstrated in the Na2MO4 (10 mM)-stabilized gill cytosol of the American eel, Anguilla rostrata and in that of the trout, Salmo gairdneri. In all experiments, tritiated triamcinolone acetonide [( 3H]TA) was used as ligand. In the eel, the steroid was bound with a KD of 2.84 +/- 0.4 nM and an Nmax of 188 +/- 34 fmol/mg protein. The binding parameters for the trout cytosol were KD = 1.43 +/- 0.13 nM; Nmax = 271 +/- 113 fmol/mg protein. Competition studies with [3H]TA-labeled eel gill cytosol and radioinert steroids gave the following binding hierarchy: TA greater than dexamethasone greater than cortisol greater than 11-deoxycortisol greater than 21-deoxycortisol. Aldosterone, estrogens, or androgens did not complete. The eel gill receptor was deactivated by prior treatment with trypsin or mersalyl. RNase was without effect, but DNase degraded the receptor except when used in the presence of trypsin inhibitor. The eel gill TA-receptor complex sedimented on a linear (10-30%) sucrose gradient with a single peak at 7.0 S or 3.5 S, in hypotonic or hypertonic (0.4 M KCl) gradients, respectively. The eel ligand-receptor complex did not bind, following heat activation, to DNA-cellulose or phospho-cellulose, though it bound to DEAE-cellulose. In this respect, it behaved similarly to the eel intestinal mucosal TA-receptor complex, described previously. The initiation of dissociation of the eel receptor-[3H]TA complex with excess TA yielded pseudo-first-order dissociation kinetics (k-1 at 0 degree C: 2.39 X 10(-5) S-1), while the association kinetics of the receptor with the ligand was of second order (k + 1: 2.51 X 10(4) M-1 S-1). Sepharose column chromatography indicated a molecular weight of 334,690 Da. Calculation of the Stokes radius gave a value of 84.5 A and the frictional ratio, calculated from the molecular weight, was 1.84. From these data it was concluded that the gills of these two euryhaline teleosts contain tetrapod-type glucocorticoid receptors. These studies are the first to demonstrate these steroid recognition molecules in fish gill. The presence of receptors in the fish gill tissue are in agreement with the physiological action of corticosteroids in allowing adaptation of the animals to habitats of different salinity.

Characterization and immunocytochemical demonstration of glucocorticoid receptor using antisera specific to transformed receptor

Cytosolic and nuclear forms of the glucocorticoid receptor were characterized using immunochemical techniques. Antibodies were raised in rabbits to an Mr 58,000 fragment of the transformed (DNA-binding) glucocorticoid receptor purified from rat liver cytosol by DNA-cellulose chromatography and polyacrylamide gel electrophoresis. Antibodies reacted with the transformed receptor form in a radioimmunoassay for glucocorticoid receptor. Western blot analysis of antibody reactivity revealed a single Mr 185,000 receptor form in rat liver cytosol but a smaller Mr 85,000 form in nucleosol, indicating the Mr 85,000 form is the transformed receptor. Furthermore, western blot analysis indicates that the Mr 185,000 receptor undergoes proteolysis during receptor purification and in vitro transformation processes by generating immunochemically similar proteins of smaller molecular weights. An identical Mr 185,000 glucocorticoid receptor was detected in cytosols of four rat tissues; liver, brain, adrenal medulla, and thymus. The glucocorticoid receptor was localized to the cytoplasm and nucleus of rat adrenal medulla cells by immunohistochemistry, demonstrating the existence in vivo of the transformed receptor and translocation of the receptor from cytoplasm to nucleus.

The effect of molybdate on the intracellular distribution of estrogen receptor in mammary tumors

The addition of sodium molybdate (20 mM) to human breast tumor homogenates results in an increased yield of estrogen receptor sites in the cytosol, but reduces the number of sites detectable in salt extracts of purified nuclei. Only about 25% of the increase in cytosol receptor can be accounted for by the loss of nuclear sites. Addition of molybdate to isolated nuclear salt extracts has no inhibitory effect on the estrogen receptor assay, but addition of the oxyanion to the cytosol after separation of the initial nuclear pellet still gives a significant increase in estradiol binding capacity. Furthermore, addition of molybdate to the suspension from the first nuclear pellet still results in loss of binding sites in the nuclear salt extract. This rules out the possibility that the negative effect of molybdate on nuclear receptor recovery is by its inhibition of cytosol receptor activation and translocation during tissue preparation. The number of nuclei isolated from estrogen receptor-containing tumors does not differ significantly in the presence or absence of molybdate. Estrogen receptor-negative tumors however yield fewer nuclei, particularly when they are processed in the presence of molybdate. We conclude that when homogenization of tumors is carried out in the presence of molybdate, a significant fraction of estrogen receptor which would normally be present in purified nuclei is lost, perhaps by leakage through the nuclear membrane.

Hydrodynamic and biochemical correlates of the activation of the glucocorticoid-receptor complex

Possible changes in the size and shape of the glucocorticoid-receptor complex (GRC) following activation remain poorly documented, due to the lability and possible activation of the receptor during the determination of these hydrodynamic parameters. In the present study molybdate was used to stabilize the GRC, thus preventing these uncontrolled transformations. Cytosol prepared from mouse whole brains was incubated for 18 h at 0-2 degrees C with [3H]triamcinolone acetonide (+/- molybdate). Activation was then initiated by incubation at 22 degrees C for variable times and quenched at 0 degree C by adding molybdate. The Stokes radius and sedimentation coefficient of the GRC declined from 77 A and 9.2 S before activation to 58 A and 3.8 S after activation. These measurements remained consistent after recycling GRC between sedimentation and gel filtration procedures and correspond to a 3-fold reduction in the relative molecular mass. The loss and formation of the 297 and 92 kDa species, respectively, after different durations of activation correlated nearly perfectly with increased binding of GRC to DNA-cellulose (DNA-C). The observed size change also correlated well with decreased adsorption to DEAE-cellulose filters (DE-81) and increased adsorption to glass fiber filters (GF/C). The increased adsorption to GF/C may reflect an increase in hydrophobicity which, with extended durations of activation, leads to increased aggregation and reduced binding to DNA-C, but not to a change in adsorption to DE-81. We propose that during activation the 297 kDa form of the GRC splits to form a 92 kDa species that displays an increased affinity for DNA.

Characterization of uterine estrogen receptors by size-exclusion and ion-exchange high-performance liquid chromatography

This report presents the first application of ion-exchange high-performance liquid chromatography in the study of ER from the rabbit uterus. In the presence of sodium molybdate (20 mM), native ER was eluted as a sharp peak at 0.29 M NaCl by a linear salt gradient, but without molybdate, it resolved into 4 major peaks. Molybdate-stabilized ER from the DEAE column, similar to ER from crude cytosol, sedimented at the 6-8S region in low salt and 4S region in high salt linear sucrose gradients, and was excluded from size-exclusion HPLC. In contrast, dissociated ER subunits from DEAE eluates ranged from 3.5 to 4.5S, and showed differences in molecular weights in a size-exclusion column. These results show that the native ER is a large molecule which dissociates into smaller subunits in the absence of molybdate; each of the steroid-bound moieties differs in molecular weight and surface charge from the native molecule.

Rat uterine progesterone receptor: stabilization of hormone-binding components for biochemical analyses

A method was developed for quantitative recovery of the labile rat uterine progesterone receptor hormone-binding components. Initial conditions were established by the sucrose gradient procedure. Upon centrifugation through low-salt 5-20% sucrose gradients prepared in 10% glycerol, the well-known 6-8 S progesterone receptor components were observed either when cytosol was prelabeled with [3H]17,21-dimethyl-19-nor-4,9-pregnadiene-3,20-dione [( 3H]R5020) or when prelabeled with [3H]progesterone followed by postlabeling the fractions collected after centrifugation with either [3H]progesterone or [3H]R5020. Recovery of progesterone receptor binding was improved by prelabeling with [3H]R5020, by adding 1.5 mM ethylene glycol bis(beta-aminoethylether)N,N'-tetraacetic acid (EGTA) to all buffers, and at high tissue concentrations. Under these conditions quantitative conversion of the receptor to specific [3H]R5020-binding 4S components was achieved with 150 or 400 mM KCl. Similar conditions proved unsuitable for receptor analysis by gel filtration (Bio-Rad agarose A0.5M or A1.5M), apparently due to [3H]R5020 dissociation from the receptor in the large volume of elution buffer. However, excellent receptor recovery (97.2 +/- 6.7%) was achieved by including 10 nM unlabeled progesterone in all preparation and elution buffers. Receptors were then detected by the addition of 5 nM [3H]R5020 to the column fractions, exchange incubation for 3-6 h at 4 degrees C, and subsequent separation of bound and free steroid by the hydroxyapatite assay. This method resulted in a consistent elution pattern suggestive of receptor heterogeneity. Identity of the peak(s) as progesterone receptor components(s) was confirmed by the lack of competition by 2 microM cortisol when added either to cytosol or during the postlabeling-exchange process. Neither the qualitative nor quantitative results of the column profiles were changed substantially in the presence of 20 mM molybdate. Although the receptor structure has yet to be established, both statistical analysis of the column profiles by computer curve-fitting procedures and rechromatography of peak fractions suggested that the rat uterine progesterone receptor may be composed of multiple components. This ligand-stabilization/postlabeling-exchange procedure provides a method for further studies of progesterone receptor biochemistry in mammalian systems. Additionally, similar procedures may stabilize other labile ligand-binding proteins for biochemical analyses and/or purification.

Glucocorticoid receptors of wild-type and variant mouse lymphoma cells

Glucocorticoid receptors of wild-type mouse lymphoma cells and two glucocorticoid resistant variants of nt- and nti phenotypes, respectively, were investigated. Photoaffinity labelling of receptor complexes with a radiolabelled glucocorticoid of high affinity enabled us to analyse crude receptor preparations by SDS gel electrophoresis. Wild-type and nt- receptors yielded radiolabelled polypeptide bands of 98,000 mol. wt while nti receptors had a mol. wt of 42,000. Monoclonal antibodies raised against purified rat liver glucocorticoid receptors reacted with wild-type and nt- receptors but not with nti receptors. Partial proteolysis of wild-type receptors with alpha-chymotrypsin resulted in a fragment of 39,000 mol. wt which contained the steroid binding site but had increased affinity for DNA indistinguishable from nti receptors. Mild proteolysis with trypsin yielded smaller fragments which contained the steroid binding site but did not bind to DNA. A model of the wild-type receptor is discussed.

Estrogen receptor in human benign prostatic hyperplasia

Estrogens have been proposed as a major etiological factor in the pathogenesis of benign prostatic hyperplasia in man. The presence of estrogen receptor in benign prostatic hyperplasia would support this concept. Using the receptor stabilizer, sodium molybdate, and a hydroxylapatite assay we assayed human benign prostatic hyperplasia for the presence of cytosolic estrogen receptor. For comparison, we assayed estrogen receptor in cytosols of prostatic cancer and normal tissue, and we also measured androgen receptor and progesterone receptor concentrations in the 3 tissue types. Estrogen receptor was present in 8 of 15 benign prostatic hyperplasia specimens at a mean concentration of 9.2 fmol./mg. protein for the estrogen-receptor-positive samples. Sucrose gradient analysis of the estrogen receptor of benign prostatic hyperplasia revealed that it sedimented in the region of 8S, and steroid specificity studies confirmed that the binding to estrogen receptor was estrogen-specific. Estrogen receptor was also found in normal (3 of 3) and malignant (4 of 6) tissues, and all tissues were positive for androgen receptor. The presence of estrogen receptor in human benign prostatic hyperplasia supports the proposal that circulating estrogens may have a role in the pathogenesis of this disorder.

The effects of ionizing irradiation on the sedimentation coefficient of cytoplasmic steroid receptors in rat mammary tumors

The effects of ionizing irradiation on the sedimentation coefficients of both estrogen receptor (ER) and progesterone receptor (PgR) have been examined in comparison to the effects of proteolysis. DMBA-induced rat mammary tumors were subjected to a treatment of 20 Gy and the ER and PgR concentrations were determined at different time intervals after irradiation. On a 5-20% sucrose gradient the ER sedimented as 9-11 and 4-5 S molecular forms, while PgR sedimented as a small 8-9 S peak and a major 4-5 S peak. Radiotherapy particularly reduced the 4-5 S sedimentation peaks of both receptors but did not initiate any new sedimentation forms. Although the 4-5 S ER receptor concentrations remained low, both progesterone receptor forms appeared to recover by 60 days after treatment. As these effects could be due to the release of proteolytic enzymes following irradiation of tumors, the receptors from untreated tumors were exposed to different concentrations of trypsin. The effects of trypsin were identical for ER and for PgR, and proved to be dependent on the trypsin concentration. Only concentrations of trypsin up to 30 micrograms/ml resulted in a reduction of 9-11 S ER or 8-9 S PgR forms which was accompanied by a simultaneous increase in the 4-5 S peaks, resulting in no change in total binding sites. Still higher trypsinization (300-3000 micrograms/ml) also reduced the 4-5 S ER and PgR fractions. In the presence or the absence of sodium molybdate, a stabilizer of the faster sedimenting forms of the receptor, no alterations were observed in the position of, or the total number of binding sites of, the sucrose gradient fractions from control or irradiated tumors. The irradiation effects appear to be due either to damage of the cytosolic ER receptor, thereby preventing its participation in the induction of de novo synthesis of ER and PgR, or to the non-specific damage of transcription and/or translation systems.

Physicochemical characterization and transformation of the cytosolic glucocorticoid receptor from rabbit liver

Characterization of the glucocorticoid receptor from rabbit liver cytosol was studied in vitro. Binding of [3H]-dexamethasone showed a high affinity (KD = 2.4 . 10(-9)M) and a concentration of binding sites of 0.3 . 10(-12) mol/mg proteins. Association and dissociation rate constants for [3H]-dexamethasone were respectively 1.6 . 10(5)M-1 . min-1 and 2.5 . 10(-4) min-1. Competition experiments showed that dexamethasone and triamcinolone acetonide were the most effective competitors while progesterone and aldosterone competed very poorly. The stabilities of bound and unbound forms were investigated with and without molybdate and other oxyanions. Calibrated gel filtration gave a stokes radius of 5.6 nm and an apparent mol. wt of 280,000. The untransformed complex sedimented at 9 s in 5-20% sucrose density gradients and the transformed species sedimented near 4 s. The behaviour of the three forms of receptor, untransformed, transformed and molybdate-stabilized was studied on ionic exchangers. Transformation of the [3H]-dexamethasone--receptor complex was shown to occur both with high ionic strength and elevated temperature. The transformation step was almost completely inhibited by tungstate and molybdate while vanadate revealed a very slight inhibitory effect.

Subunit dissociation as a possible mechanism of glucocorticoid receptor activation

For the elucidation of the mechanism of steroid hormone receptor activation, the hydrodynamic properties of the unactivated and activated forms of the nonproteolyzed glucocorticoid receptor from the mouse AtT-20 pituitary tumor cell line were determined. The unactivated, molybdate-stabilized receptor has the following properties: sedimentation coefficient = 9 S; Rs = 8.3 nm; Mr = 317 000; f/f0 = 1.70; axial ratio (prolate ellipsoid) = 14. The activated monomeric receptor has a sedimentation coefficient of 3.2 S, a Stokes radius of 6 nm, a molecular weight of 81 000, a frictional ratio of 1.93, and an axial ratio (prolate ellipsoid) of 18. A receptor species of intermediate size was detected when the analysis was performed in buffer containing both 0.3 M KCl and 20mM Na2MoO4. Its characteristics are as follows: sedimentation coefficient = 5 S; Rs = 8.3 nm; Mr = 176 000; f/f0 = 2.06; axial ratio (prolate ellipsoid) = 22. A preliminary study seemed to indicate that this is an activated form of the receptor. On the basis of the molecular weights, it is likely that the unactivated receptor is a tetramer of identical hormone-binding subunits (Mr = 81 000) while the intermediate form is a homodimer. Alternatively, non-hormone-binding components (receptor-binding factors) may be involved in forming the multimeric, nonactivated receptor complex. In either case, the dissociation of a multimeric, nonactivated receptor into subunits appears to be a possible mechanism of receptor activation. Finally, the addition of high concentrations of 1-thioglycerol promoted activation. Thus, sulfhydryl groups may be involved in receptor subunit interaction.