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

Monoclonal antibodies to human sex hormone-binding globulin (SHBG): characterization and use in a simple enzyme-linked immunosorbent assay (ELISA) of SHBG in plasma

Four monoclonal antibodies to human sex hormone-binding globulin were raised and characterized. Three of the four antibodies recognised different antigenic determinants on SHBG. Two of the distinct antibodies were useful for Western blotting and recognized a major 48 kDa band in human plasma as well as a 46 kDa minor component. Carbohydrate residues do not form part of the antigenic determinants of these two antibodies, although one of these showed increased signal following removal of N-linked oligosaccharides. Some of the antibodies were selected to form a basis of a same-day, non-competitive, enzyme-linked immunosorbent assay (ELISA) for SHBG in plasma. The assay employs a purified IgG2a SHBG monoclonal antibody adsorbed to the wells of a microtitre plate. After blocking any further adsorption to the plate, standards or diluted patient samples were added for a 5-h incubation at room temperature, after which the plate was washed and antibody-bound SHBG was detected with an anti-SHBG IgG1 monoclonal antibody followed by peroxidase-labeled antimouse-IgG1 and o-phenylenediamine substrate. The assay correlated well with an existing 2-day ELISA for SHBG in plasma using polyclonal antibodies and also correlated with a dihydrosterone (DHT) ligand-binding assay. The monoclonal antibody-based ELISA shows excellent performance characteristics and is unaffected by added testosterone or estradiol.

Genetic variations in human testosterone-estradiol binding globulin

The human testosterone-estradiol-binding globulin (hTeBG) is a plasma heterogeneous glycoprotein with high affinity for a number of circulating steroid hormones. The heterogeneity originates from differential glycosylation of a common protein precursor. Analysis of desialylated hTeBG by isoelectric focusing (IEF) has revealed that microheterogeneity could be partly attributed to variability in sialic acid content or rearrangement of amino acid composition. We have studied this possibility by the analysis of desialylated serum hTeBG by Western blotting of proteins previously separated on IEF-gels. Two distinct well-defined IEF patterns were identified. The most frequent consisted of two major IEF-bands of equal color intensity. The other pattern consisting of four IEF-bands was present in only 5.55% of the total serum samples analyzed. Family studies showed that these phenotypes were autosomally inherited with a simple Mendelian transmission and allele frequencies had an excellent agreement between the observed and expected phenotypes. Androgen affinity constants and serum concentrations of hTeBG variant were similar to those of normal hTeBG. Molecular analyses of each of the exons of hTeBG gene by denaturing gradient gel electrophoresis revealed the presence of a point mutation in exon 8. The studies presented herein confirm and extend previous reports on the existence of structural variants of hTeBG. In addition, the mutation reported in this study is probably the same as that recently identified within numerous ethnic groups throughout the world, thus further supporting the concept of a two allele gene worldwide concoding hTeBG.

Sex hormone-binding globulin/androgen-binding protein: steroid-binding and dimerization domains

Plasma sex hormone-binding globulin (SHBG) and testicular androgen-binding protein (ABP) are homodimeric glycoproteins that share the same primary structure, and differ only with respect to the types of oligosaccharides associated with them. The biological significance of these differences is not understood, but enzymatically deglycosylated SHBG and a non-glycosylated SHBG mutant both bind steroids normally. Various affinity-labelling experiments, and studies of recombinant SHBG mutants have indicated that a region encompassing and including Met-139 in human SHBG represents an important component of its steroid-binding site. Analyses of chimeric proteins comprising various portions of human SHBG and rat ABP have also indicated that residues important for the much higher affinity of human SHBG for steroid ligands are probably located within the N-terminal portion of these molecules. Recent studies of SHBG mutants have confirmed this, and a deletion mutant containing only the first 205 N-terminal residues of human SHBG has been produced which dimerizes and binds steroids appropriately. The introduction of amino-acid substitutions between Lys-134 and Phe-148 of SHBG has also indicated that residues including and immediately N-terminal of Met-139 may influence steroid-binding specificity, while those immediately C-terminal of Met-139 represent at least a part of the dimerization domain. These studies have also demonstrated that dimerization is induced by the presence of steroid ligand in the binding site, and that divalent cations play an important role in this process. Together, these data have led us to conclude that SHBG is a modular protein, which comprises an N-terminal steroid-binding and dimerization domain, and a C-terminal domain containing a highly-conserved consensus sequence for glycosylation that may be required for other biological activities, such as cell-surface recognition.

Estradiol causes the rapid accumulation of cAMP in human prostate

Androgens are widely acknowledged to be central to the pathogenesis of benign prostatic hypertrophy (BPH). However, BPH increases in prevalence as men age, at precisely the stage of life when plasma androgens are decreasing. The decrease in total plasma androgens is amplified by an age-related increase in plasma sex hormone-binding globulin (SHBG) that results in a relatively greater decrease in free androgens than in total androgens. In addition, estrogens have long been suspected to be important in BPH, but a direct effect on the human prostate has never been demonstrated. We present data that are consistent with a role for estradiol, and for a decrease in androgens and an increase in SHBG, in the pathogenesis of BPH. We show that estradiol, but not dihydrotestosterone, acts in concert with SHBG to produce an 8-fold increase in intracellular cAMP in human BPH tissue. This increase is not blocked by an antiestrogen and is not provoked by an estrogen (diethylstilbestrol) that does not bind to SHBG, thus excluding the classic estrogen receptor as being operative in these events. Conversely, dihydrotestosterone, which blocks the binding of estradiol to SHBG, completely negates the effect of estradiol. Finally, we demonstrate that the SHBG-steroid-responsive second-messenger system is primarily localized to the prostatic stromal cells and not to the prostatic epithelial cells. Thus, we have shown a cell-specific, powerful, nontranscriptional effect of estradiol on the human prostate.

Structure, function, and regulation of androgen-binding protein/sex hormone-binding globulin

Despite over 20 years of research, the functions of ABP and SHBG remain elusive. The major reason for this lack of knowledge has been the unavailability of natural mutants with clinical defects for study. There is strong evidence that these binding proteins do act to modulate the gene regulatory actions of nuclear sex steroid receptors by controlling the availability of androgens and estrogens. In plasma, SHBG controls the metabolic clearance rate of sex steroids. In addition there is strong evidence that they have a much broader function. The identification of plasma membrane receptors in target tissues and the finding of homologous domains in several developmental proteins support other functions. Moreover, other experiments suggest the proteins may actually be hormones or growth factors. These findings are not compatible with a model that has the proteins only regulating free steroid hormone levels. Obviously, much more experimentation will be necessary to reveal the functions of ABP and SHBG. The recent discoveries have offered several clues to their functions and open new routes for study. These experiments, coupled with newly developed techniques, such as gene knockout by homologous recombination, make one optimistic that the functions of these unique proteins will be deciphered in the near future.

Synthesis and characterization by 1H and 13C nuclear magnetic resonance spectroscopy of 17 alpha-hexanoic derivatives of 5 alpha-dihydrotestosterone and testosterone

The synthesis and characterization of 17 alpha-(6'-hexanoic acid) derivatives of 5 alpha-dihydrotestosterone and testosterone, useful as ligands for affinity chromatography purification or as precursors for affinity-labeling of androgen-binding proteins, is described. Alkynylation of 3-ethylenedioxy-, 3 beta-hydroxy-, and 3 beta,5-dihydroxy-5 alpha-androstan-17-one precursors with the potassium derivative of 5-hexyn-1-ol led to the corresponding 17 alpha-(6'-hydroxyhex-1'-ynyl) derivatives, which were hydrogenated over 10% Pt-C catalyst to give 17 alpha-(6'-hydroxyhexyl) derivatives. Chromic acid oxidation of the primary hydroxy group of the 3-ethylenedioxy-17-hexyl intermediate into carboxylic acid followed by acid cleavage of the 3-ketal group gave 17 alpha-(5'-carboxypentyl)-5 alpha-dihydrotestosterone, which was also obtained directly by chromic acid oxidation of the 3 beta-hydroxy intermediate. Chromic acid oxidation of the primary hydroxy group of the 3 beta,5 alpha-dihydroxy precursor resulted in a 5 alpha-hydroxy-3-oxo intermediate, which was dehydrated to give 17 alpha-(5'-carboxypentyl)testosterone. The 17 alpha configuration of these derivatives and of synthetic precursors was established by comparing their molecular rotations and their 1H and 13C nuclear magnetic resonance (NMR) spectra including solvent effects, with data reported for 17 alpha- or 17 beta-substituted steroid analogs as well as with 1H and 13C NMR reference data recorded in this work for 17 alpha-ethynyltestosterone, 17 alpha-ethynyl-19-nortestosterone, 17 alpha-ethyl-19-nortestosterone, 17 alpha-methyltestosterone, and 17 alpha-methyl-5 alpha-dihydrotestosterone.

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.

Genetic polymorphism of the human sex hormone-binding globulin: evidence of an isoelectric focusing variant with normal androgen-binding affinities

Human sex hormone-binding globulin (hSHBG) is a plasma glycoprotein composed of two identical subunits. The protein, which has high affinity for testosterone and estradiol has been purified to homogeneity. In this study we have investigated, on neuraminidase-treated serum samples, the presence of genetic variations of hSHBG by polyacrylamide gel isoelectric focusing (IEF). Based on IEF analyses of 110 serum samples from adult Mexican individuals we have identified two distinct IEF-patterns. The most frequent phenotype (95.45%) was characterized by two IEF-bands with pIs of 6.50 and 6.63, respectively. In five serum samples, a different 4-band pattern with pIs of 6.50, 6.63, 6.70 and 6.76 was identified. Family studies showed that this pattern was genetically determined. The frequency of this variant was 4.55%, and the observed phenotypes were consistent with the expression of an autosomal genetic system. The estimated gene frequencies for both alleles were shown to be in genetic equilibrium. Affinity constants, binding kinetics and serum concentrations of hSHBG from individuals having a 4-band pattern were similar to those obtained in individuals with a 2-band pattern, thus suggesting that the mechanism responsible for the generation of polymorphic variants of hSHBG reported herein did not involve the steroid binding site of the molecule. These findings may be of broad interest, as other serum binding proteins express genetic variants, which may permit their further structural and functional subclassification.