Purification and characterization of the sex steroid binding protein from macaque serum. Comparison with the human protein

Direct evidence for the localization of the steroid-binding site of the plasma sex steroid-binding protein (SBP or SHBG) at the interface between the subunits

Complete dissociation of dimeric plasma sex steroid-binding protein (SBP or SHBG) was obtained in 6 M urea at 10 degrees C. Removal of urea resulted in the refolding of monomers, followed by reformation of dimeric SBP, which migrates with the same mobility as the native protein. Dimerization does not require Ca+2 or steroid. Renatured monomers yield dimers with dissociation constants for 5 alpha-dihydrotesterone (DHT) and 17 beta-estradiol (E2) indistinguishable from those of native human SBP. This phenomenon was also demonstrated by mixing human and rabbit SBPs that, upon renaturation, form a hybrid dimer composed of one human subunit and one rabbit subunit. The hybrid binds both DHT and E2 in contrast to rSBP, which only binds the androgen. Therefore, we conclude that (1) docking of the two subunits creates an asymmetric steroid-binding site located at the interface between the subunits, and (2) only one face of the dimer defines the specificity for binding E2 by encompassing portion of a structural motif that recognizes the flat ring A of E2. The remaining portion, which recognizes the saturated ring A of DHT, is shared by both faces of the dimer. Because native monomers do not exist alone, the often-asked question of whether the SBP monomer binds steroid can be considered meaningless; steroid-binding activity is expressed only in the dimeric state. Finally, formation of the hybrid indicates that SBP dimerization represents a conserved event during the molecular evolution of SBP, suggesting that the structural elements responsible for dimerization will be homologous in SBPs from other species.

Regulation of sex hormone-binding globulin secretion and gene expression by cycloheximide in vitro

The role of protein synthesis in sex hormone-binding globulin (SHBG) secretion and gene expression was studied in HepG2 cell cultures. Inhibition of protein synthesis by cycloheximide suppressed SHBG levels. Triiodothyronine and estradiol increased SHBG production, and cycloheximide reduced their effects to an extent which correlated with the degree of suppression obtained with the drug alone. Insulin decreased SHBG production, and the effect of the treatment with insulin and cycloheximide together did not differ from that with cycloheximide alone. Cycloheximide did not, alone or with the hormones, decrease SHBG levels more markedly extra- than intracellularly. Therefore, cycloheximide does not impair the secretion of SHBG which is synthesized in the presence of the drug. In contrast to SHBG protein levels, cycloheximide increased SHBG mRNA levels. When the effect of cycloheximide on the rate of SHBG mRNA decay was tested, the drug was found to extend the half-life of SHBG mRNA. Of the hormones, insulin decreased and triiodothyronine modestly increased SHBG mRNA levels, whereas estradiol had no clear effect. Treatment with cycloheximide together with any of the hormones resulted in an increase in SHBG mRNA levels. We conclude that protein synthesis inhibition does not impair the secretion of SHBG produced under such conditions, but stabilizes SHBG mRNA by removing some hepatic protein species involved in the regulation of its degradation.

The plasma sex steroid binding protein (SBP or SHBG). A critical review of recent developments on the structure, molecular biology and function

Significant developments have taken place within the past five years on the characterization, molecular biology and function of the plasma sex steroid-binding protein, SBP (or sex hormone binding globulin, SHBG). During the span of that time, amino acid sequences of two SBPs have been established, amino acid residues in the steroid-binding site have been identified, the structure of the human SBP gene has been deduced and evidence for the possible existence of a SBP membrane receptor has been presented. This review covers the salient aspects of these and other developments including a critical analysis of the various proposed models and interpretations with regards to the structure, evolution, molecular biology and function of SBP.

Direct effect of plasma sex hormone binding globulin (SHBG) on the metabolic clearance rate of 17 beta-estradiol in the primate

Sex hormone binding globulin (SHBG) has been shown to be a major determinant of testosterone clearance in the primate. It has also been suggested that SHBG would also be a determinant of estradiol clearance (MCR-E2). However, published studies have suggested that the MCR-E2 do not always vary with changes in the level of SHBG. Therefore, the present study was undertaken to address this issue. The baseline MCR-E2 was determined in adult male pigtail macaques (Macaca nemestrina). Following the baseline determination of MCR-E2 the animals were infused with either purified human (h) SHBG or antibody against hSHBG, which also has a high degree of cross-reactivity with primate SHBG. Following the infusions of either hSHBG or anti-SHBG, MCR-E2 was again determined. In addition, luteinizing hormone (LH) was measured using a mouse Leydig cell bioassay. Following the infusion of hSHBG, a marked increase in serum SHBG was noted and the MCR-E2 decreased. Associated with the increase in SHBG, the SHBG bound T levels decreased and LH increased. Following the infusion of antibody, serum SHBG decreased, and the MCR-E2 also decreased. With the decrease in SHBG following the antibody infusion, non-SHBG bound T increased and serum LH fell. This study demonstrates that an increase in the serum SHBG levels is associated with a decrease in MCR-E2, however, an acute decrease in serum SHBG also decreases the MCR-E2. This later result demonstrates that factors in addition to serum SHBG binding may be important in determining the MCR-E2.

2-Iodoestradiol binds with high affinity to human sex hormone binding globulin (SHBG)

Human sex hormone binding globulin (SHBG) binds a set of steroids that differ slightly from each other in structure. Dihydrotestosterone and testosterone are bound with high affinity by SHBG whereas estradiol is bound with a lower affinity. In this work we have studied the binding to human SHBG of the derivatives obtained by substituting iodine in the aromatic A-ring of estradiol. Three A-ring iodinated estradiol derivatives, 2-iodoestradiol, 4-iodoestradiol and 2,4-di-iodoestradiol, were obtained by treating 17 beta-estradiol with NaI and Chloramine T and separating the reaction products by HPLC. Their structures were confirmed by mass spectrometry and 1H-NMR. The corresponding radioactive compounds were obtained with use of Na[125I] in the same synthesizing procedure. Incubation of whole serum, serum albumin and purified SHBG with each of the three [125I]iodoestradiols followed by agarose gel electrophoresis showed only 2-iodoestradiol to have a strong binding to SHBG. This steroid was also bound to albumin, but with a lower affinity. Besides SHBG and albumin, there were no other binders of 2-iodoestradiol in human serum. The affinity constant for the binding of 2-iodoestradiol to purified human SHBG at 37 degrees C and physiological pH was determined by a dextran-coated charcoal method to be 2.4 x 10(9) M-1 (i.e. exceeding that of dihydrotestosterone). It was found that 0.9 mol of 2-iodoestradiol was bound per mol of SHBG dimer (93 kDa) at saturation, and that 2-iodoestradiol competed with dihydrotestosterone for the same binding site of SHBG. It was concluded that 2-iodoestradiol has a remarkably high affinity for human SHBG, and that its gamma-emitting 125I-analog is useful for binding studies of human SHBG.

Characterization of the human sex hormone binding globulin (SHBG) gene and demonstration of two transcripts in both liver and testis

A genomic cosmid clone for human sex hormone binding globulin (SHBG), a liver-secreted plasma glycoprotein that binds sex steroids, was isolated with a previously characterized liver cDNA as probe. Southern blot analysis of genomic DNA indicated that only one SHBG gene is present in the human haploid genome. A 3.8 Kb Xba I-fragment of the clone containing the entire coding region of SHBG was sequenced. The SHBG gene has 8 exons. The 5'-end preceding the translation start site had no TATA box or CAAT box promoter elements. Screening of a human testis cDNA library resulted in the isolation of two distinct cDNA forms. One cDNA was identical with the previously characterized liver SHBG cDNA, thus suggesting that human SHBG and the androgen binding protein (ABP) produced by Sertoli cells are coded for by the same gene. The second cDNA differed from the first by having exon I exchanged with a completely different sequence and exon VII deleted. An exon coding for the 5'-end of this cDNA was found in the cosmid clone 1.5 kb upstream of the first SHBG exon. Primer extension experiments showed the alternatively spliced transcript corresponding to the second cDNA to be present in both liver and testis. From the primary structure of this putative SHBG-gene-related protein, it may be deduced that it is a protein very different from SHBG and probably without steroid binding activity.

Sex steroid-binding protein: identification and comparison of the primary product following cell-free translation of human and monkey (Macaca fascicularis) liver RNA

A very close similarity in molecular, steroid-binding and immunological properties have been demonstrated for the sex steroid-binding proteins of plasma from human (hSBP) and monkey (mSBP): both are glycoproteins composed of two similar subunits able to bind one steroid molecule and to cross-react with the same antibodies. After translation of human and monkey (Macaca fascicularis) liver mRNAs by a wheat-germ embryo extract, in the presence of labelled amino-acids, we have characterized in both cases a single radioactive polypeptide immunologically related to SBP, migrating in SDS-PAGE as a single band and having a molecular weight of about 42,000. This protein could be displaced from the antibody by pure unlabelled SBP in excess. The difference in molecular weight between the in vitro translation product and the native SBP sub-unit is probably due to the absence of glycosylation in the neo-synthesized protein. The radioactivity incorporated into mSBP was 4 times higher than the radioactivity incorporated into hSBP, suggesting that the amount of mRNA for SBP is higher in monkey than in human liver. Our results show that the two sub-units of hSBP and mSBP derive from a common precursor, representing respectively 0.0050% and 0.0013% of the total neosynthesized proteins in monkey and in human liver.

Half-life of plasma sex steroid-binding protein (SBP) in the primate

We have recently shown that the metabolic clearance of testosterone in plasma is directly dependent on sex steroid-binding protein (SBP or SHBG) levels [J. steroid. Biochem. 22, 739 (1986)]. In order to further understand the relationship between these two parameters, we have measured the half-life of SBP in plasma of female rhesus monkeys. SBP was purified to homogeneity from pooled Macaca nemestrina serum, and iodinated with 125I. The labeled protein ([125I]nSBP) was purified by chromatography on DEAE-agarose and fractions identified as immunologically reactive against anti-human SBP were collected. Protein purity of [125I]nSBP was established by SDS gel electrophoresis using an unlabelled Macaca nemestrina SBP as standard. The labeled protein was infused intravenously into two different adult female Macaca mulatta (rhesus) monkeys. Plasma samples were collected at short intervals during the first 24 h after infusion, and then daily for 7-9 days. The clearance profile of labeled SBP in plasma was quantitated by radioactivity measurement and immunoprecipitation. Analysis of the results indicate that the rate of SBP clearance in plasma has two components, the t1/2 (app) of the first component is 7.5 h (r = 0.94), and the t1/2 (app) of the second component is 3.95 days (r = 0.95). Over 90% of the injected 125I-nSBP was removed from plasma within 24 h at a rate corresponding to the t1/2 (app) of the first component. The data indicate that most of the SBP rapidly distributes into extravascular spaces during the first 24 h following infusion, and are consistent with the hypothesis that SBP may be directly involved in sex steroid hormone transport into tissues.

Identification of the primary translation product of the sex steroid-binding protein from monkey liver mRNA in a cell-free system

The synthesis of monkey (Macaca fascicularis) Sex steroid-Binding Protein (mSBP) in a wheat germ cell-free system in response to liver RNA was demonstrated by use of a specific antiserum raised against purified native human SBP. Antibodies precipitate a single translation product behaving as a 42 kDa protein in sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). Western blots of monkey sera subjected to SDS-PAGE and immunorevelation show that the native mSBP migrates as 2 molecular species (50 and 53 kDa) present in the approximate ratio of 1:10, respectively. The difference in apparent molecular weights of the primary translation product and the reduced mature mSBP may represent glycosylation that occurs post translationally. We describe for the first time the biosynthesis of mSBP at the molecular level and suggest that both components of mSBP derive from a common differentially processed precursor. Its mRNA is poorly represented, since the neosynthesized mSBP represents about 0.005% of the total proteins encoded by liver mRNA.

Molecular characterization of the sex steroid binding protein (SBP) of plasma. Re-examination of rabbit SBP and comparison with the human, macaque and baboon proteins

Physico-chemical characterization of the sex steroid-binding protein, SBP, of rabbit plasma reveals that it is a dimer of mol. wt 85,800 composed of similar subunits of mol. wt 43,000. These data confirm our original proposal for a dimeric structure. The protein contains 9% carbohydrate, comprised of mannose, galactose, N-acetylglucosamine and sialic acid. It is devoid of N-acetylgalactosamine and fucose. The protein binds one molecule of 5 alpha-dihydrotestosterone per dimer with a Kd of 0.89 nM (12 degrees C). Comparison with the human, monkey and baboon SBPs indicates that all these proteins have the same dimeric molecular organization and exhibit microheterogeneity in SDS-PAGE and isoelectricfocusing. Rabbit SBP, however, contains less carbohydrate and has a higher polypeptide molecular weight than all the other SBPs. Spectrophotometric data also indicate that some tryptophan residues are in a different chemical environment than those in other SBPs. The observed microheterogeneity in all four SBP species is due for the most part to variable glycosylation of the subunit and variability at the amino-terminal region of the subunit. Combination of these and other phenomena will generate a significant number of isomeric forms of the SBP subunit which will then interact stoichiometrically to yield active dimeric SBP molecules. These differ slightly from each other depending upon the charge and size of the subunit comprising the dimeric structure, and will result in the observed microheterogeneity of pure SBP preparations. Based on these results along with more recent amino acid sequence data, we conclude that all four SBPs are dimers composed of identical polypeptide chains.

Steroid-binding site of human and rabbit sex steroid binding protein of plasma: fluorescence characterization with equilenin

The interaction of the estrogen d-3-hydroxy-1,3,5(10),6,8-estrapentaen-17-one (equilenin) with the human and rabbit sex steroid binding proteins (hSBP and rSBP, respectively) has been investigated by using fluorescence and absorption spectroscopy. Equilenin competes for the binding of 5 alpha-dihydrotestosterone. The calculated binding constant of equilenin for rSBP is 1.9 X 10(7) M-1 at 4 degrees C, which can be compared with the binding constant of 5.7 X 10(7) M-1 reported for hSBP [Ross, J.B.A., Torres, R., & Petra, P.H. (1982) FEBS Lett. 149, 240]. The results of fluorescence quenching experiments with the collisional quenchers KI and acrylamide indicate that the bound steroid has limited accessibility to the bulk solvent and that there are no anionic surface groups near the steroid-binding site. The fluorescence excitation spectra of SBP-equilenin complexes are similar to the absorption spectra of equilenin in low-dielectric solvents. The fluorescence emission of the SBP-equilenin complexes, however, exhibits wavelength shifts (red shifts) opposite to those of the steroid in low-dielectric solvents or complexed with beta-cyclodextrin (blue shifts) but similar to the red shift produced by addition of the proton acceptor triethylamine to equilenin in cyclohexane. These data indicate that the steroid-binding site of hSBP and rSBP is a nonpolar cavity containing a proton acceptor that participates in a specific interaction, possibly a hydrogen bond, with the 3'-hydroxyl group of the bound steroid.

Molecular organization of the sex steroid-binding protein (SBP) of human plasma

Several years ago this laboratory presented evidence that SBP is a dimer composed of two subunits having similar molecular weights. The question of whether or not these subunits are identical and therefore products of a single gene remained unanswered. We now report that the two polypeptide chains are identical and that SBP is a homodimer. The experimental approach was to reduce and [14C]alkylate cystine residues in human SBP, digest the product with trypsin or cyanogen bromide and determine the number of unique amino acid sequences around each [14C]carboxymethylcysteine residue. Only four unique sequences were found when all the radioactive peptides were analyzed. Since there are eight half-cystine residues per dimer, the results support a homodimeric structure.

Direct effect of sex steroid-binding protein (SBP) of plasma on the metabolic clearance rate of testosterone in the rhesus macaque

We report direct evidence for the effect of the sex steroid-binding protein (SBP) on the metabolic clearance rate of testosterone (MCRT). Pure rhesus SBP or human SBP was infused intravenously into three different cycling female rhesus monkeys. MCRT was measured before and after SBP had reached 150-300% of basal levels. A decrease in MCRT was observed in all cases. The effect of SBP on MCRT was tested further in four additional cycling females by infusing immunoaffinity-purified monospecific human SBP antibodies known to cross-react with rhesus SBP. SBP dropped to 54, 40, 4 and 2% of basal levels with a concomitant increase of 118, 190, 320 and 640% of basal MCRT. In one of these animals, pure rabbit SBP was administered after the anti-human SBP infusion resulting in a decrease in MCRT. The magnitude of the SBP effect on MCRT is related to the distribution of testosterone (T) bound to SBP and albumin in the plasma. Calculations show that as long as the percent of T bound to SBP is equal or higher than the percent of T bound to albumin, the influence on MCRT is small. However, if SBP is reduced to the extent that T is bound mostly to albumin, the redistribution of T is associated with a dramatic increase in MCRT. We conclude that under normal conditions each animal has an optimum concentration of plasma SBP which binds a maximum amount of T. If SBP increases above this level, little effect on MCRT will result. However, a drop below the optimum level, as is the case in certain physiological or clinical conditions, will produce a large increase in the clearance of T.

Size isomers of testosterone-estradiol-binding globulin exist in the plasma of individual men and women

We isolated testosterone-estradiol-binding globulin TeBG rapidly and in high yield from pooled pregnancy plasma. It showed two bands on sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS PAGE). Both bands stained with three different monoclonal antibodies to TeBG, thus demonstrating their immunological similarity. Freshly drawn, individual sera, from men, women, and pregnant patients were submitted to microaffinity chromatography, a procedure which partially purifies TeBG in approximately 4 hr. The partially purified plasma was submitted to SDS PAGE, followed by immunoblotting. The blotted TeBG exhibited the same two bands seen in the isolated, purified protein. The size heterogeneity observed in TeBG purified to: proteolysis occurring during isolation; a peculiarity of pregnancy plasma; or heterogeneity attendant upon the use of pooled plasma for isolation.

Purification of testosterone-oestradiol-binding globulins from mammalian sera by anion-exchange high performance liquid chromatography

Testosterone-oestradiol-binding globulin (TeBG) has been isolated from serum or plasma of several species using procedures that yielded highly purified protein, but which required multiple and tedious chromatographic steps. In this report we describe a procedure for the isolation of TeBG which involves two chromatographic steps: androgen affinity chromatography followed by anion-exchange high performance liquid chromatography (anion-exchange HPLC). The purity of the final product was confirmed by silver staining following fractionation on sodium dodecyl sulphate-containing polyacrylamide gels. The size heterogeneity and specific binding activity of TeBGs purified from human, rabbit, or bull serum (or plasma) by this technique was indistinguishable from preparations obtained by conventional chromatography. The present technique shortened the entire purification procedure to about 5 working days and yielded milligram quantities of highly purified protein. Bases on our experience with serum or plasma from the human, rabbit, and bull, this approach should be suitable for isolation of TeBG from a wide range of species.