Characterization of androgen binding protein in rat epididymal cytosol using a photoaffinity ligand

Photoaffinity labeling of plasma proteins

Photoaffinity labeling is a powerful technique for identifying a target protein. A high degree of labeling specificity can be achieved with this method in comparison to chemical labeling. Human serum albumin (HSA) and α₁-acid glycoprotein (AGP) are two plasma proteins that bind a variety of endogenous and exogenous substances. The ligand binding mechanism of these two proteins is complex. Fatty acids, which are known to be transported in plasma by HSA, cause conformational changes and participate in allosteric ligand binding to HSA. HSA undergoes an N-B transition, a conformational change at alkaline pH, that has been reported to result in increased ligand binding. Attempts have been made to investigate the impact of fatty acids and the N-B transition on ligand binding in HSA using ketoprofen and flunitrazepam as photolabeling agents. Meanwhile, plasma AGP is a mixture of genetic variants of the protein. The photolabeling of AGP with flunitrazepam has been utilized to shed light on the topology of the protein ligand binding site. Furthermore, a review of photoaffinity labeling performed on other major plasma proteins will also be discussed. Using a photoreactive natural ligand as a photolabeling agent to identify target protein in the plasma would reduce non-specific labeling.

Characterization of the oligosaccharides of plasma sex hormone binding globulin from noncirrhotic alcoholic patients

In previous reports we have demonstrated high plasma levels of sex hormone-binding globulin (SHBG) in asymptomatic alcoholic men. In the present work the physicochemical properties of SHBG from plasma of noncirrhotic alcoholic patients have been further compared with SHBG of control subjects. Steroid binding to SHBG was similar for the two groups: alcoholic men, K(d) of 0.62 +/- 0.07 nM and control individuals, K(d) of 0.70 +/- 0.10 nM. The structure of oligosaccharides attached to SHBG from controls and alcoholic men were determined by using serial chromatography. Our data indicated that 7% of SHBG of control individuals was not retarded by the Con-A column, whereas approximately 30% of SHBG of alcoholic men eluted in the void volume of Con A. Approximately 46% of SHBG of alcoholics applied to Con A, possessed biantennary complex oligosaccharides, as indicated by the fact that it could be eluted with methyl-alpha-D-glucopyranoside and by its retention on wheat germ agglutinin; in contrast, when SHBG from control men was analyzed, approximately 51% was eluted with methyl-alpha-D-glucopyranoside. Approximately 9% of the biantennary complex oligosaccharides on SHBG of control men and none of those on SHBG from alcoholic men were fucosylated on the chitobiose core, as determined by chromatography on Lenn culinaris lectin. Galactosylated oligosaccharides were also present on the SHBG fraction as indicated by its interaction with Ricinus communis-I. Approximately 24% of SHBG of alcoholic men and 39% of those on SHBG from control individuals applied to Con-A were retained and could be eluted with methyl-alpha-D-mannopyranoside. Evidence based on the binding on mannoside-eluted SHBG to Con-A, wheat germ agglutinin, and R. communis-I indicated that at least the SHBG in this fraction, from alcoholics or controls, contained two glycosylation sites and that the sites were differentially glycosylated.

Secondary structure and shape of plasma sex steroid-binding protein--comparison with domain G of laminin results in a structural model of plasma sex steroid-binding protein

We have analyzed the secondary structure, shape and dimensions of plasma sex steroid-binding protein (SBP) by CD, size-exclusion chromatography and electron microscopy. CD spectra show extrema at 186 nm and 216 nm characteristic for beta-sheet structures. Analysis with different algorithms indicates 15% alpha-helix, 43% beta-sheet and 10-16% beta-turn structures. An irreversible structural change is observed upon heating above 60 degrees C, which correlates with the loss of steroid-binding activity. As the SBP sequence shows similarity with domains of several multidomain proteins, including laminins, we evaluated the structure of domain G of laminin-1. The CD spectrum shows extrema at 200 nm and 216 nm. Deconvolution results in 13% alpha-helix, 32% beta-sheet and 15% beta-turn structures. Steroid-binding assays indicate that laminin and fragments thereof have no activity. Size-exclusion chromatography reveals that SBP has an extended shape and can be modeled as a cylinder with a length and diameter of 23 nm and 3 nm, respectively. This shape and the dimensions are in agreement with the appearance on electron micrographs. We propose a model for the structure of SBP in which two monomers assemble head to head with the steroid-binding site located in the center of the rod-like particle.

Preparation of 17 alpha-iodoethynylandrosta- and 17 alpha-(2-iodoethenyl)androsta-4,6-dien-17 beta-ol-3-ones as active site-directed photoaffinity ligands for androgen-binding proteins

Unsaturated analogues of androst-4-en-17 beta-ol-3-one, each with a 17 alpha-iodoethynyl or 17 alpha-(2-iodoethenyl) substituent, were prepared, and their relative binding affinities (RBAs) for androgen-binding protein (ABP) were compared with those of 5 alpha-androstan-17 beta-ol-3-one, androst-4-en-17 beta-ol-3-one, androsta-4,6-dien-17 beta-ol-3-one, and androsta-1,4,6-trien-17 beta-ol-3-one. These binding studies indicate that the iodine[125I] analogues of 17 alpha-iodoethynyl and 17 alpha-[(E)-2-iodoethenyl] derivatives of androsta-4,6-dien-17 beta-ol-3-one and androsta-1,4,6-trien-17 beta-ol-3-one will have RBAs at least twice as great as that of 5 alpha-androstan-17 beta-ol-3-one. They can be prepared from 17 alpha-ethynylandrosta-4-en-17 beta-ol-3-one, the final synthetic step using N-[125I]iodosuccinimide, and are potential radioiodinated, active site-directed photoaffinity ligands for ABP and testosterone-binding globulin.

Characterization of Met-139 as the photolabeled amino acid residue in the steroid binding site of sex hormone binding globulin using delta 6 derivatives of either testosterone or estradiol as unsubstituted photoaffinity labeling reagents

Immunopurified human sex hormone binding globulin (SHBG) was photoinactivated and photolabeled by radioinert and radioactive photoaffinity labeling steroids delta 6-testosterone (delta 6-T) and delta 6-estradiol (delta 6-E2). The maximal levels of specific incorporation of these two reagents were 0.50 and 0.33 mol of label/mol of SHBG, respectively. Covalently labeled SHBG fractions were citraconylated, reduced, carboxymethylated, and cleaved by trypsin. Separation of tryptic digests by reverse-phase liquid chromatography gave single radioactive peaks at the same retention times with both steroid reagents. However, the two labeled peptidic fractions could be distinguished by capillary electrophoresis and immunodetection with anti-steroid antibodies, whereas the covalent attachment of radioactivity was confirmed by thin-layer chromatography on silica gel. Edman degradation of the two labeled peptides showed a single sequence His-Pro-Ile-([3H]X)-Arg corresponding to the pentapeptide His-Pro-Ile-Met-Arg 136-140 of SHBG sequence. The coincidence, in both cases, of the absence of an identifiable amino acid residue and of the elution of the most intense peak of radioactivity at the fourth cycle of Edman degradation suggests that the same Met-139 residue was labeled by delta 6-[1,2-3H2]T or by delta 6-[17 alpha-3H]E2. Liquid secondary ion mass spectrometry of the two peptides showed [M+H]+ ions at m/z 939.8 or 923.8, corresponding respectively to the addition of delta 6-T or delta 6-E2 to the pentapeptide. The presence of the steroid molecule in the delta 6-[3H]T-pentapeptide conjugate was confirmed by the difference of 2 mass units with the [M+H]+ peak of the delta 6-[4-14C]T-pentapeptide conjugate.

Photoaffinity labelling of nuclear steroid 5 alpha-reductase of rat ventral prostate

In order to get more information on the molecular structure of the rat prostatic 5 alpha-reductase (3-oxo-5 alpha-steroid: NADP+ 4-ene-oxidoreductase, EC 1.3:1.22) a systematic photoaffinity labelling study has been performed. To irreversibly freeze the status quo of interaction, either testosterone, the physiological ligand, or diazo-MAPD (21-diazo-4-methyl-4-aza-5 alpha-pregnane-3,20-dione), a specific 5 alpha-reductase inhibitor, was irradiated with isolated nuclei or with purified nuclear membranes or with solubilized nuclear membrane proteins and checked for optimal labelling conditions. The principal substances covalently labelled were phospholipids and at a minor ratio proteins. Analysis by SDS-PAGE and autoradiofluorography revealed two labelled polypeptides with molecular weights of 20 kDa and 26 kDa. The following evidence indicates that these polypeptides might be derived from the enzyme 5 alpha-reductase: both proteins are labelled only when specific ligands for 5 alpha-reductase are used; binding can be reduced by the addition of an excess of unlabelled ligand; enzyme activity is irreversibly suppressed when irradiated in the presence of these ligands; only subcellular fractions containing 5 alpha-reductase reveal the labelled proteins; in all 5 alpha-reductase containing preparations with increasing specific activity, independent of the polypeptide pattern, the same proteins are labelled.

The extracellular sex steroid-binding proteins of testis and liver. Structure-function studies

Though the existence of extracellular sex steroid-binding proteins has been known for a number of years, we are still only on the threshold of understanding their biological role. Through efforts such as those described above, we are beginning to examine the structure of these macromolecules and correlating them with present known functions. As our understanding of the function of these proteins evolves, we will be further able to ascribe structural domains.

Protein secretions of Sertoli cells

As was stated in the introduction, many of the functions of the Sertoli cells are apparently carried out by the protein secretions of these cells. The use of Sertoli cell cultures and appropriate biochemical and immunological techniques has allowed the characterization of some of these secretion products. It is likely that many of the functions of the Sertoli cells are necessary because of the presence of the blood-testis barrier. Many growth and nutritive factors which are necessary for cell viability are available to most cells via the serum. The germinal cells within the adluminal compartment do not have access to serum factors and one of the functions of the Sertoli cells is to synthesize serum-like components and secrete them into the adluminal compartment. The historical description of Sertoli cells as "nurse cells" thus appears to have been accurate. The nurse-cell function is most clearly demonstrated by the proposed mechanism by which germinal cells obtain ferric ions. The Sertoli cells have developed a system to move serum-derived iron through their own cytoplasm and to secrete it bound to newly synthesized testicular transferrin molecules which can deliver it to specific receptors on the germinal cell surface (Huggenvik et al., 1984). Functionally, all of the secreted proteins from Sertoli cells which have been characterized or proposed fall into one of five basic classes. First, Sertoli cells secrete a number of transport proteins including transferrin, ceruloplasmin, and ABP. The proposed function of these proteins is the transport of Fe3+, Cu2+, and androgens to the germinal cells or to the epididymis (ABP). Second, Sertoli cells synthesize and secrete a number of proteins which have a hormone-like or growth factor-like activity. AMH is a clear and well-documented example of this type of product while the evidence for inhibin, somatomedin C, EGF-like growth factor, and seminiferous growth factor will require further corroboration. Third, Sertoli cells secrete proteins which have enzymatic activities. Plasminogen activator is the best characterized example of this class of products and the alpha-lactalbumin-like activity is of potential interest. The fourth class of Sertoli cell secretion products includes those proteins which contribute to the basement membrane, namely, type IV collagen and laminin. Finally, there is a very important group of Sertoli cell secretion products for which there is, as yet, no evidence for a defined function. This group includes SGP-1 and SGP-2 which are the major sertoli cell products in rats and which have been well-characterized biochemically.(ABSTRACT TRUNCATED AT 400 WORDS)

Purification and partial characterization of a corticosteroid-binding globulin from hamster serum

Our objective was to characterize and purify the corticosteroid-binding proteins in hamster pregnancy serum. When [3H]cortisol-labeled pregnancy and proestrous serum were subjected to native polyacrylamide gel electrophoresis, a single peak of specific steroid-binding activity was detected in each, with identical electrophoretic mobility. The steroid-binding affinity (Ka = 1.07.10(8) M-1 for cortisol) is typical of corticosteroid-binding globulin from other species, but the steroid-binding specificity (cortisol greater than testosterone greater than progesterone) is not. An ultraviolet photoaffinity-labeling protocol was developed using 17 beta-hydroxy-4,6-[1,2-3H]androstadiene-3-one ([3H]androstadienolone), permitting analysis of ultraviolet photoaffinity-labeled proestrous and pregnancy serum by two-dimensional polyacrylamide gel electrophoresis and fluorography. Both sera contained the same labeled protein species. Corticosteroid-binding globulin was purified from pregnancy serum by DEAE-cellulose chromatography followed by steroid affinity chromatography on androstadienolone-17 beta-hemisuccinate-ethylenediamine-AffiGel 10. The purified protein (Mr = 62,250; pI = 3.95; n = 1; Stokes radius = 3.5; S = 4-5) was determined to be a glycoprotein. When analyzed by gel filtration and two-dimensional polyacrylamide gel electrophoresis, purified corticosteroid-binding globulin behaved the same as in unfractionated serum, and when ultraviolet photoaffinity-labeled with [3H]androstadienolone, purified corticosteroid-binding globulin produced the same fluorogram spot pattern seen in unfractionated serum. A specific corticosteroid-binding globulin antiserum was raised in rabbits, and this antiserum reacted with a single spot in Western blots of unfractionated serum. Thus, hamster pregnancy serum was determined to have one corticosteroid-binding protein. This protein is identical to the corticosteroid-binding globulin found in proestrous serum, with a higher titer in pregnancy serum. No other steroid-binding component is observed in hamster serum.

The apparent molecular weight of androgen-binding protein (ABP) in the blood of immature rats differs from that of ABP in the epididymis

When androgen-binding protein (ABP) in unfractionated immature (20-day old) male rat serum was covalently labeled with the site-specific photoaffinity ligand [3H]17 beta-hydroxy-4,6-androstadien-3-one and analyzed on 5.6% polyacrylamide tube gels containing SDS (SDS-PAGE), a protein of Mr 33,700 +/- 1200 was shown to be specifically labeled. Rat epididymal ABP from unfractionated cytosol analyzed under identical conditions exhibited two androgen-specific peaks of radioactivity, Mr 49,900 +/- 600 and Mr 44,100 +/- 800, which correspond to the previously described subunits of ABP. The apparent molecular weight differences between serum and epididymal ABP were further assessed on preparations of serum ABP that had been partially purified by chromatography on Affi-Gel blue (to remove albumin) and on Sephadex G-150 (to remove other proteins). When these preparations of ABP were photolabeled and analyzed by SDS-PAGE as above, two subunits of Mr 61,700 +/- 1300 and Mr 47,100 +/- 700 were resolved. Serum and epididymal ABP were further purified by androgen affinity chromatography. When these preparations were subjected to SDS-PAGE on slab gels containing 10% polyacrylamide and identified by fluorography of photolabeled ABP or by immunochemical localization following electrophoretic transfer to nitrocellulose, differences in the apparent molecular weight of ABP from the two sources persisted. Immunochemical localization studies on ABPs that had been desialylated with neuraminidase indicated that there was an increased mobility of the subunits, as one would anticipate from removal of carbohydrate. Differences in apparent molecular weight of ABPs from the two sources are likely due to differences in glycosylation.

Further characterization of a steroid receptor-active protease from the mature rabbit epididymis

The nucleomyofibrillar fraction of mature rabbit epididymides contains a salt-extractable and leupeptin-sensitive protease that alters the sedimentation coefficient of cytosolic steroid receptors. We refer to this modification as receptor conversion. The substrate used in these studies was cytosolic estrogen receptor obtained from frozen rabbit uteri. The unactivated form of the receptor exists as an oligomer under hypotonic (0.01 M KCl) conditions (S20,w congruent to 9.6, Stokes radius (Rs) congruent to 7.4 nm, Mr congruent to 320,000) and dissociates under hypertonic (0.4 M KCl) conditions to yield the steroid-binding monomer (S20,w congruent to 4.7, Rs congruent to 5.1 nm, Mr congruent to 104,000). According to analysis under hypotonic conditions, the epididymal protease disrupts the oligomeric architecture of the receptor and reduces the size of the steroid-binding monomer (S20,w congruent to 3.2, Rs congruent to 3.0 nm, Mr congruent to 42,000). The epididymal protease had no detectable effect on the structure of the proteins used as standards for the ultracentrifugal or gel filtration analyses. Although inhibited by leupeptin, the epididymal enzyme is not a typical thiol protease since it was unaffected by thiol-blocking agents (iodoacetamide and N-ethylmaleimide), and was partially inhibited by thiol-reducing agents (monothioglycerol and dithiothreitol). Calcium and magnesium ions alone, or in combination with ATP, had no effect on the activity of the protease. However, both cations selectively suppressed recovery of the oligomeric receptor form. These results, in conjunction with those from previous studies, serve to distinguish the epididymal protease from receptor-active proteases described in extracts of other animal tissues. Molybdate, at a concentration of 50 mM, blocked receptor conversion. The ability of the receptor to be stabilized by molybdate was lost following conversion. Finally, the epididymal protease appears to remove a portion of the estrogen receptor that is necessary for nucleotide-binding.

Affinity labelling of steroid hormone receptors

Affinity labelling techniques have proved indispensable for the study of reversible biological recognition systems, since they conserve ligand-receptor interaction by covalent linkage. Using photo- and electrophilic labelling, it has become possible to unequivocally identify steroid hormone receptors and their proteolytic degradation products and it is simple to establish receptor peptide maps even in crude receptor preparations. The isolation of receptor proteins has been greatly simplified, as their integrity can be analyzed at any step of a purification protocol by SDS-PAGE analysis after crosslinking. Moreover, affinity-labelled receptors can be purified under denaturing conditions, e.g., in high-resolving preparative SDS-PAGE, and the material obtained can be efficiently used to generate anti-receptor antibodies. Peptide mapping after crosslinking of related receptors has been used to assess the degree of structural homology between different forms of steroid hormone receptors and receptors of different species. Peptide sequence analysis of purified crosslinked receptor protein and anti-receptor antibodies have provided the basis for cloning corresponding genes. Techniques have been established to demonstrate--via crosslinking--that the cloned DNA sequences correspond to the receptor gene binding the correct ligand. The analytical and preparative crosslinking methods developed for steroid receptors are potentially important for the study of any system in which signal transduction proceeds via the reversible interaction between biological macromolecules.

Demonstration of heavy and light protomers of human testosterone-estradiol-binding globulin

Human testosterone-estradiol-binding globulin (hTeBG) was purified from pregnancy serum by sequential ammonium sulfate precipitation, affinity chromatography, and hydroxylapatite chromatography. An overall purification of 2800-fold was achieved with a 27% total yield. Apparent homogeneity of the final product was shown by polyacrylamide gel electrophoresis with or without sodium dodecyl sulfate (SDS). The equilibrium dissociation constant (Kd) at 4 degrees C for 5 alpha-dihydrotestosterone (DHT) was estimated to be 1.94 +/- 0.95 X 10(-9) M. Analysis of the purified protein revealed microheterogeneity with regard to size on polyacrylamide gel in the presence of SDS and to charge on isoelectric focusing gels. The apparent molecular weight of native hTeBG determined by gradient gel electrophoresis was 115,000. SDS-polyacrylamide gel electrophoresis indicated that hTeBG is comprised of two molecular weight components of 53,000 and 46,000, which are designated as heavy (hTeBGH) and light (hTeBGL) protomers, respectively. Photolysis of purified hTeBG with [1,2-3H]17 beta-hydroxy-4,6-androstadien-3-one [( 3H]delta 6-testosterone) resulted in specific labeling of its binding sites. Analysis of photolabeled products by SDS-polyacrylamide gel electrophoresis revealed two radioactive products with electrophoretic mobilities identical to those of the hTeBGH and hTeBGL. The ratio of hTeBGH to hTeBGL was about 10:1. The H and the L protomers were separated and examined by peptide mapping using protease V8 and chymotrypsin. Comparison of the fragmentation patterns produced by these proteases revealed that hTeBGH and hTeBGL components were nearly identical. Removal of sialic acid or carbohydrate residues from hTeBG did not affect the presence of two molecular components. Isoelectric focusing of native hTeBG demonstrated three isoelectric variants with pIs at 4.75, 4.85 and 4.90. After treatment with neuraminidase and other glycosidases, only two isoelectric species were observed with more alkaline pIs. Although purified hTeBG appeared heterogeneous with regard to size and charge, it was remarkably homogeneous in its ability to absorb to Concanavalin A-Sepharose. We conclude that hTeBg, like the androgen binding proteins of the rabbit and rat, is a dimer whose monomer exhibits two protomeric forms.