Study of follitropin receptors in testis using a homologous system. Binding of porcine follitropin to plasma membranes from immature porcine testis and correlation with adenylate cyclase stimulation

Ovulation, pregnancy, placentation and husbandry in the African elephant (Loxodonta africana)

The African elephant reproduces so efficiently in the wild that overpopulation is now a serious problem in some game parks in Zimbabwe, Botswana and South Africa. The female reaches puberty between 10 and 12 years of age in the wild and, when in captivity, shows oestrous cycles of 14-15 weeks duration. She readily conceives a singleton in the wild yet her uterus has the capacity for twins. She shows a gestation length of 22 months and, in the wild, shows a population density and feed dependent intercalving interval of 4-8 years. The trophoblast erodes the lumenal epithelium of the endometrium and stimulates upgrowths of blood vessel-containing stromal villi, which develop eventually into the broad, tightly folded lamellae of the zonary, endotheliochorial placenta. Significant quantities of leaked maternal erythrocytes and ferric iron are phagocytosed by specialized trophoblast cells in the haemophagous zones at the lateral edges of the placental band. Although the placenta itself is endocrinologically inert, the foetal gonads, which enlarge greatly during the second half of pregnancy can synthesize 5alpha-dihydryoprogesterone and other 5alpha pregnane derivatives from cholesterol and pregnenolone. These products may synergize with progestagens secreted by the 2-8 large corpora lutea which are always present in the maternal ovaries throughout gestation to maintain the pregnancy state.

Alternative splicing converts the G-protein coupled follitropin receptor gene into a growth factor type I receptor: implications for pleiotropic actions of the hormone

Pituitary follitropin (FSH) has pleiotropic actions on gonads, but it is not certain if all these events are mediated by a single receptor. A single gene for the FSH receptor undergoes extensive alternate splicing generating multiple transcripts, and several of these have been cloned and characterized from the sheep testis. In this study we have investigated the expression in HEK (human embryonic kidney) 293 cells of a cloned cDNA encoding the first eight exons of the FSH receptor along with a carboxyterminal extension that contributed a hypothetical single transmembrane domain. This cDNA, which lacked the conventional seven transmembrane motif of the full-length 695 residue wild-type receptor protein, was also efficiently expressed on the cell surface and exhibited high affinity and specificity for FSH binding. LH did not compete for FSH binding indicating that these structures contained all the motifs necessary for specific hormone recognition. Following hormone binding and affinity crosslinking the deduced M(r) of the expressed receptor was compatible with dimer formation. The expression of these altered FSH receptors on the cell surface was confirmed by immunohistochemistry, which revealed punctate labeling in a pattern comparable to that shown by cells transfected by wild-type receptor cDNA. Addition of FSH stimulated 3H-thymidine incorporation in transfected cells in a biphasic manner. By performing RT-PCR we could show that similar altered receptor transcripts were present in both the ovary and testis. Our results reveal for the first time that the seven transmembrane structure of FSH-receptor is not absolutely necessary for cell surface expression and hormone binding provided other compensating motifs are present in the protein structure for membrane insertion. Some of these features are typical of growth factor receptors. Our investigations also demonstrate that alternate splicing of the FSH receptor gene provides a mechanism for creating receptor diversity and suggest that multiple receptors could be involved in regulation of hormone action.

Topographical differences in human choriogonadotropin antagonist as revealed by monoclonal antibodies

Human choriogonadotropin (hCG), a highly glycosylated hormone loses its biological activity following deglycosylation. The deglycosylated hormone (DG-hCG) binds to gonadal cell receptors without causing complete signal transduction. In order to analyze the molecular features of DG-hCG, we have prepared and studied five monoclonal antibodies (Mab). Two DG-hCG Mabs were highly specific while three others showed some but low cross-reactivity with hCG. Only those that recognized hCG albeit weakly, could inhibit binding of 125I-hCG to receptors or hCG action in MA-10 (mouse Leydig tumor cells). All five Mabs were highly reactive with the free alpha subunit. As these alpha subunit favoring antibodies easily recognized DG-hCG but not the intact hCG configuration, we suggest that loss of sugars in alpha and beta subunits of DG-hCG was responsible for these alterations. Thus, in the molecular topography of DG-hCG, the orientation of the alpha subunit would be different from that in the native hormone and this may be further perturbed following binding to the specific receptor, leading to inefficient coupling to the signal transducing machinery.

Growth hormone and prolactin stimulate androgen receptor, insulin-like growth factor-I (IGF-I) and IGF-I receptor levels in the prostate of immature rats

In this study we investigated the involvement of several different pituitary hormones on rat prostate development. 22-day-old Wistar rats, hypophysectomized (hypox) at 19 days of age were supplemented with highly purified human prolactin (hPRL), human luteinizing hormone (hLH), porcine follicle-stimulating hormone (pFSH), and bovine growth hormone (bGH) or with saline. Quantitative analysis of RNAs shows that treatment with either PRL or GH increases significantly steady-state mRNAs levels of the following genes in the prostate: androgen receptor (AR) (respectively 3.5- and 4.8-fold above hypox controls), IGF-I (5- and 2.7-fold), and IGF-I receptor (2.9- and 2.3-fold). LH and FSH, by contrast, have negative effects on these parameters. To test whether the enhancing effect of PRL and GH on AR-mRNA abundance was followed by increased content in the protein itself, binding assays were performed with the androgen agonist [3H]R1881 (131 and 153 fmol/mg protein while hypox controls contained 110 fmol/mg protein). In addition to the well-documented presence of prolactin receptors in prostatic tissues, we have further demonstrated, by means of nuclease S1 protection assays plus dot- and Northern-blot analyses, that a GH receptor mRNA is produced in the immature rat prostate. Moreover, we observed not only strong lactogenic but also purely somatogenic binding to be occurring in the immature prostates. Finally, we have studied IGF-I mRNA content in separated epithelial/stromal cell fractions and have concluded that IGF-I expression is principally located in the prostatic stroma. Taken together, these results suggest that PRL and GH are involved in regulating AR synthesis, at least partially by direct action on the organ. In this context IGF-I appears as a paracrine factor playing a role in epithelium/stroma interactions during prostatic development.

Comparative binding of FSH to chicken and rat testis

The binding characteristics of two highly purified preparations of follicle-stimulating hormone (FSH), one from equine (e) and one from porcine (p), were compared in a chicken testis radioreceptor assay. A two-component binding model was adequate to explain the binding of eFSH or pFSH to the chicken testis homogenate (CTH) FSH binding sites in the Scatchard study. The affinity of eFSH for the FSH binding site of CTH was 10-fold greater than that of pFSH by Scatchard analysis. This is consistent with the observation that in competitive protein binding studies approximately 10-fold greater quantities of pFSH were required to displace [125I] eFSH from the CTH FSH binding site than eFSH. Qualitatively, the order of relative potencies for various gonadotropin preparations was the same in either the CTH or a rat testis receptor radioligand assay. However, quantitatively, higher potency estimates were obtained with the CTH. The ability of LH preparations to displace radioiodinated pFSH or eFSH from CTH was generally very low with the notable exception of equine LH. The relative potency of highly purified eLH was approximately equal to that of the NIH-FSH-S16 standard (0.906 x NIH-FSH-S16) indicating that in the chicken, as in the rat, (Bousfield and Ward, Biochim. Biophys. Acta 885: 327, 1986), eLH has significant FSH activity. It is concluded that the chicken testis is a convenient and suitable source of FSH receptor (binding sites) for the bioassay of FSH, but that quantitative estimates of potency are not directly comparable when different sources of receptor are used. Finally the data show that eFSH has intriguing structural attributes which provide higher affinity to the chicken testis receptor than pFSH.

Purification and characterization of luteinizing hormone from the dromedary (Camelus dromedarius)

Luteinizing hormone (LH) has been purified from 150 dromedary pituitaries and its partial physicochemical, biological and immunological characterization has been achieved. Purification of the hormone was monitored by a porcine LH radioreceptor assay (RRA). In this system, the final camLH preparation exhibited an activity 0.6-fold that of highly purified porcine LH. The acid half-dissociation of camLH at equilibrium was observed at pH 4.2. A homologous camLH RRA was developed using the testicular plasma membrane fraction from prepubertal camels and radioiodinated, highly-purified camLH. Pituitary and chorionic gonadotropins (CG) from several mammalian species were compared to camLH in this system. The equine gonadotropins eLH and eCG were shown to be 6 times less potent in the camel RRA than in the porcine RRA, whereas the LH from other species exhibited similar activities in both systems. This particularity of camel LH receptors offers a new tool for the study of structural features of gonatropin interactions with their receptors.

[Demonstration of specific binding sites for carp gonadotropin in ovary membrane preparations of Anguilla (Anguilla anguilla L.)]

Membrane preparations from silver eel ovary bind specifically labeled carp gonadotropin (cGTH 125I); similar preparations from kidney, liver, brain, and muscle exhibit either no or negligible specific binding. cGTH and partially purified eel pituitary extract, but neither hCG nor bTSH, inhibit cGTH 125I specific binding in a dose-dependent way. However, a high membrane concentration can bind hCG 125I and this binding is partly inhibited by excess hCG, thus suggesting the existence of a relatively small number of sites able to recognize this hormone. Studies of either cGTH inhibition of cGTH 125I specific binding or cGTH 125I dose-dependent binding have permitted, according to Scatchard, the binding parameters to be determined. Specific binding seems to have two components: one with low affinity and high capacity, and the other with high affinity (Kd = 1.2 10(-10) M) and low capacity (N = 0.2 fmol/mg ovary).

Study of the superactivity of equine follicle-stimulating hormone in in vitro stimulation of rat Sertoli cells

We have previously shown that equine follicle-stimulating hormone (FSH) stimulates plasminogen activator secretion in Sertoli cells at much lower concentrations than would be expected from its relative binding activity. We have introduced the term 'superactivity' to designate this particular behavior. In the present study, we show that equine FSH triggers a long-lasting (20 h) plasminogen activator secretion, whereas rat, porcine and ovine FSH as well as equine LH and equine choriogonadotropin (CG) provoke a short-term response (2.5 h). Moreover, equine FSH was also shown to be superactive in the stimulation of estradiol secretion and cyclic AMP production. This indicates that the step responsible for the long-term stimulation by equine FSH is not located beyond cAMP accumulation. Equine and porcine FSH were found to be equally stable during incubation with the cells demonstrating that equine FSH superactivity was not due to higher stability. Besides, phosphodiesterase inhibition led to a similar increase in the responses to both hormones. This rules out the possibility that equine FSH superactivity is due to less stimulation of phosphodiesterase activity. All these data strongly suggest that equine FSH exhibits superactivity in rat Sertoli cells by stimulating adenylate cyclase activity for a much longer period of time than do all other gonadotropins. The molecular mechanism of this outstanding behavior remains to be elucidated.

Processing of follitropin and its receptor by cultured pig Sertoli cells. Effect of monensin

Immature pig Sertoli cells, cultured in a chemically defined medium, are able to maintain many of their functional characteristics for at least two weeks. This model was used to investigate the binding, internalization and degradation of 125I-labelled human follitropin (hFSH) and the effects of pig FSH (pFSH) on its own receptors. The binding of 125I-labelled hFSH was dependent on time, temperature and concentration. At 4 degrees C, the apparent steady state was reached in 8-12 h and remained constant for at least 24 h, whereas at 33 degrees C the apparent equilibrium was reached in 4-6 h. Thereafter the total binding declined and by 24 h it was less than 50% of the maximum binding. At 33 degrees C the binding for the hormone to its surface receptor was followed by internalization of the hormone (half-life approximately equal to 1 h) and its degradation (half-life approximately equal to 3 h). The receptor-mediated internalization of hFSH was blocked by phenylarsine oxide. In the presence of the ionophore monensin (20 microM) the rates of binding and internalization were not modified but the degradation rate was much lower (half-life approximately equal to 18 h). Thus, in the presence of monensin, maximum binding increased twofold to threefold, and remained constant for 24 h. This increase was mainly due to an increase of the internalized hormone. When Sertoli cells were exposed to pFSH there was a loss of its own receptor, which was both dose-dependent (ED50 = 250 ng/ml) and time-dependent (t 1/2 = 14 h). Cycloheximide did not modify the FSH-induced down-regulation, whereas monensin enhanced the down-regulation process. These results show that FSH, like other ligands, is internalized and degraded by its target cells and indicate that the hormone-mediated down-regulation is related to the internalization process. However, the discrepancy between the rate of internalization and of hormone-induced down-regulation, suggests that some of the internalized receptors are recycled.

Direct demonstration of intrinsic follicle-stimulating hormone receptor-binding activity in acid-treated equine luteinizing hormone

After dissociating equine gonadotropins as a function of time at pH 3, we examined them by radioligand assay and sodium dodecyl sulfate polyacrylamide gel electrophoresis under nondissociating conditions (low, 0.1% SDS). Equine follicle-stimulating hormone (FSH) rapidly lost its receptor-binding activity, and low SDS-polyacrylamide gels demonstrated dissociation into subunits. Maximum dissociation occurred after 20-30 min of pH 3 incubation. Equine luteinizing hormone (LH), however, retained most biologic activity and was largely intact after 72 h of pH 3 incubation. Dose-response curves of acid-treated equine LH and FSH and intact equine LH and FSH were compared in five types of radioligand receptor assays. LH and FSH receptor-binding activities of equine LH were unaffected by pH 3. Equine LH showed 19- and 32-times more activity in the rat testis FSH assay than it did in chicken or horse FSH assays, respectively, directly demonstrating the intrinsic FSH receptor-binding activity of equine LH and the relative lack of specificity for these hormone preparations of the rat FSH receptor. Acid-treated 95% of its biologic activity in FSH assays. In LH assays, the slight (0.2%) activity of equine FSH was relatively unaffected by acid treatment, suggesting that contamination by equine LH accounts for this activity.

Low molecular weight FSH binding inhibitor in bovine testis

Purification of low Mr (less than 5000) inhibitors of FSH binding to receptors has been hampered by their low concentration in tissue extracts and physiologic fluids. The calf testis represents an accessible and economic source of large quantities of tissue, and was therefore studied as a source of FSH binding inhibitors. Supernatants (27,000 X g) of calf testis homogenates inhibited binding of [125I]-hFSH as well as [125I]-hCG to membrane-bound receptor from the same source. FSH binding inhibitor was concentrated from large volumes of testis supernatant by precipitation of inert material with metaphosphoric acid, concentration/desalting of the resulting supernatant by ultrafiltration (Amicon UM-05 membranes) and lyophilization. Separation of FSH binding inhibitor and LH(hCG) binding inhibitor activities could be achieved by molecular sieving on Sephadex G-25. A partially purified fraction with inhibitors of FSH binding activity (ED50 = 44 micrograms protein) and free of LH(hCG) binding inhibitor activity (no activity at 800 micrograms protein) emerged with a Ve/Vo of 1.9, reflecting an apparent Mr of about 1500. Inhibitors of LH(hCG) binding activity emerged with the column outer volume. Rechromatography of the FSH binding inhibitor fraction on G-25 indicated two closely associated peaks of activity. These could be further resolved by gel filtration through BioGel P2, to give a salt-free fraction with an FSH binding inhibitor activity (ED50) of 24 micrograms protein. The inhibitor was heat-labile, losing 80% of its activity after 2 hours at 100 C. The testicular low molecular weight FSH binding inhibitor is similar to bovine follicular fluid and serum FSH binding inhibitor by several parameters.(ABSTRACT TRUNCATED AT 250 WORDS)

The ontogeny of hypothalamic pituitary function in the pig. I. Pituitary LH and FSH in the fetus and neonate

Pituitaries were collected from fetal and postnatal pigs from day 55 p.c. until 6 weeks after birth at closely spaced intervals. LH and FSH in individual pituitaries were quantified by both homologous RIA and homologous RRA. No significant difference was found between results obtained by RIA and RRA. Both LH and FSH are first detected by RIA and RRA in the porcine fetal pituitary at 75 days p.c. Thereafter both LH and FSH pituitary content rises until term. LH pituitary concentration in both male and female fetuses and FSH pituitary concentration in males exhibit a peak just before birth. FSH pituitary concentration in females rises until birth and thereafter remains elevated. A statistically significant sex difference was found postnatally with regard to FSH content and concentration but not for LH.

Characterization of the testicular binding site for iodinated rat FSH in the turtle, Chrysemys picta

1. To correlate the morphological observations with the known gonadotropic activity of FSH in the turtle testis, studies of the binding of iodinated FSH were conducted. 2. These demonstrated the presence of gonadotropin-binding sites of high affinity (apparent Kd = 10(-10) M) for [125I]rFSH in turtle testicular membrane preparations. 3. Although these sites did not bind iodinated human LH or avian LH, these hormones, as well as PMSG and FSH, were effective competitive inhibitors of the binding of the radioligand. 4. Binding of the radioligand to the testis was influenced by duration of incubation and temperature. 5. Binding activity was lost after incubation with proteolytic enzymes (trypsin, pronase) but not with DNAase, lipase, collagenase and neuraminidase. 6. The binding exhibited target organ specificity (no binding observed in brain, epididymis, lung, muscle and pancreas). 7. In addition, the number of binding sites varied according to the stage spermatogenesis, being highest when the tubules contained spermatocytes and spermatids, intermediate when the tubules consisted to Sertoli cells and spermatogonia and lowest st spermiation.

Cellular processing of follicle-stimulating hormone by Sertoli cells in serum-free culture

Specific binding of radioiodinated human follicle-stimulating hormone [( 125I]hFSH) to receptors on the cell surface of immature rat Sertoli cells in serum-free culture (4-7 days) was concentration-, time- and temperature-dependent with an apparent steady state of binding stained after 2 h at 4, 18 or 32 degrees C. Analysis of [125I]hFSH binding data indicated an apparent Ka of 3 X 10(9) M-1 with approximately 670 fmoles binding sites/mg protein. The radioiodinated hFSH (60-70 microCi/micrograms) bound to receptor and stimulated estradiol production, thus confirming its biological activity and Sertoli cell viability. Approximately 50% of the specifically bound radioactivity was lost from the cell surface after 0.25 h at 32 degrees C, 1 h at 18 degrees C or 4 h at 4 degrees C. Most (greater than 90%) of the receptor-bound hormone was removed from the cell surface and degraded to [125I]mono- and diiodotyrosine within 4 h at 32 degrees C. FSH degradation was inhibited at low temperatures (4 degrees C) and by the lysosomotropic agents chloroquine, NH4Cl or concanavalin A. Bacitracin and methylamine, inhibitors of internalization, were effective in blocking degradation but not loss of membrane-bound [125I]hFSH. Thus it appears that, as with small peptide and simple protein hormones, complex glycoprotein hormone-receptor complexes, such as FSH Sertoli cell receptors, are endocytosed by normal target cells and subsequently degraded, most probably within the lysosomes.

Solubilization and characterization of an angiotensin II binding protein from liver

Binding sites with high affinity for angiotensin II were solubilized from hepatic membranes by treatment with digitonin. Binding of radioiodinated angiotensin II was assayed by gel filtration and independently by a technique exploiting the failure of activated charcoal to adsorb the bound ligand. The binding protein was partially purified using ammonium sulfate fractionation followed by gel filtration, and in the presence of protease inhibitors, the isolated binding protein preparation did not catalyze degradation of the angiotensin II. Binding to the membranes as well as to the solubilized preparation was specific and saturable. The membranes exhibited a single set of high-affinity binding sites with a Kd of 0.5 nM. The solubilized preparation, also showed the presence of a single class of high-affinity binding sites (Kd = 10.5 nM). Displacement studies using angiotensin I as well as various fragments, agonists and antagonists of angiotensin II disclosed a structure-activity profile similar to that found with intact membranes. Dissociation of angiotensin II from the soluble macromolecular complex was slow but was enhanced at non-physiological pH values or in the presence of 4.5 M urea, or 1% sodium dodecyl sulfate. Covalent binding of the radioiodinated angiotensin II to a single, specific macromolecular component was achieved by treatment with disuccinimidyl suberate. The apparent molecular weight of this reduced, denatured radioactive protein was estimated at about 68 000 by polyacrylamide gel electrophoresis.

[Determination of substances inhibiting and stimulating the binding of FSH to its receptors]

We describe a method to estimate binding of human 125I-FSH to a preparation of bovine testicular receptors. Various experimental conditions are tested and the validity of the method is demonstrated. Using this method, the presence of biological substances modifying the FSH binding is measured in various fractions of ram retetestis fluid submitted to several steps of purification by chromatography. FSH receptor binding inhibitor (FSHRBI) activity is obtained in a low molecular weight fraction and FSH receptor binding stimulator activity in a larger one. These cybernin activities are isolated in fractions different from the ones observed with inhibin and GnRH like activities.

Inhibition of FSH action on granulosa cells by low molecular weight components of follicular fluid

Immature female rats were injected with a single dose (10 IU) of pregnant mare serum gonadotropin to induce growth and maturation of ovarian follicles. Using such ovaries as a model, we tested the effects of low molecular weight subfractions of charcoal-absorbed bovine follicular fluid (FF-c) on (a) radioiodinated human FSH (125I-hFSH) binding to ovarian homogenates, (b) ovine FSH-stimulated adenylate cyclase activity in granulosa cell homogenates and (c) cAMP production by intact granulosa cells. The follicular fluid was fractionated by ultrafiltration through membranes of differing pore-sized into molecular weight components of 1000-5000 (passing Amicon H1P-5 hollow fibers but retained by Amicon UM-2 membrane) and 500-1000 (passing Amicon UM-2 membrane but retained by Amicon Um-05 membrane). These low molecular weight fractions inhibited 125I-hFSH binding to ovarian receptors, FSH-stimulated cAMP production by rat granulosa cells and FSH-stimulated, as well as fluoride-ion-stimulated adenylate cyclase activity in granulosa cell homogenates. Inhibition of FSH-stimulated adenylate cyclase activity by the FF subfractions was non-competitive as determined by double reciprocal plot analysis. Our results suggest that modulation of FSH effects on granulosa cells may be mediated by low molecular weight constituents of follicular fluid.

Familial inappropriate TSH secretion: evidence suggesting a dissociated pituitary resistance to T3 and T4

The study of pituitary thyroid relationships of a 40-year-old woman, suffering from recurrent goiter, is presented. The characteristics of this case were the following: i) constant hyperthyrotropinemia (range 8.2 to 29 micro u/ml) despite high thyroid hormone levels (mean total T4:13.4 microgram/dl; mean total T3:255 ng/dl) and clinical euthyroidism; ii) this TSH hypersecretion is not regulated by an excess of T4, while a suppressive effect is easily observed with T3. Together with the clinical effects observed with administration of T3 and T4, this suggests that thyroid hormone resistance predominates at the pituitary level, for T4 while not T3; iii) evidence is presented for the familial distribution of the trait and its mode of transmission. The clinical aspect and the pattern of resistance exhibited by the propositus and her progeny appear distinct from those of previous observations of thyroid hormone resistance.

Physico-chemical properties of pregnant mare serum gonadotropin

Pregnant mare serum gonadotropin exhibits a dissociation at acid pH as shown by the drop of s20,w values from 3.52 S at pH 8.1 to 2.52 S at pH 2.0. The dissociation is accompanied by an absorbance change with a maximum at 287 nm and a parallel loss of both follicle-stimulating hormone (FSH) and luteinizing hormone (LH) activities as followed by radioreceptor assays. The apparent pKa of the acid transition is 3.45 with an extremely slow and temperature-dependent rate at pH 2.0 (1.8 . 10(-4) s-1 at 37 degrees C). By gel filtration the molecular weight of the active hormone is estimated to be 45 000 (rather than the previously reported 53 000-64 000). The active conformation of the hormone includes beta sheet structure (34%) as for other gonadotropin hormones with a minor but significative amount of alpha-helix. Four tyrosine residues were titrated, two of pKa = 10.3 and two of pKa = 11 out of a total of seven tyrosines. The parallel changes in FSH and LH activities during the preparation and the acid transition suggest that the two biological activities are intrinsic properties of the same molecular entity.

The effect of ionic environment in specific FSH binding to plasma membrane receptor

Studies were undertaken to define the effects of ionic strength and ion selectivity on specific binding of 125I-hFSH to both its particulate and Triton X-100 solubilized calf testis plasma membrane receptor. Plasma membrane vesicles migrated with a negative charge on free flow electrophoresis. There was no evidence of formation of inside-out vesicles. Specific FSH binding to the plasma membrane vesicles with both monovalent and divalent cations, used to reduce charge repulsion between the negatively charged FSH molecule and membrane vesicles, revealed similar binding maxima at an ionic strength range of 0.03 to 0.05. However, Mg2+ consistently resulted in 25-30% greater specific binding than with any other cation. That increased FSH binding suggested unmasking of specific FSH plasma membrane binding sites. However, specific FSH binding to solubilized plasma membrane vesicles was unaffected by ionic strength or ion selectivity. Enzymatic removal of sialic acid from plasma membrane vesicles significantly enhanced specific FSH binding. In previous studies, we had observed that sialic acid was not exposed within the FSH binding site. Consequently, enhanced FSH binding associated with desialylation of plasma membrane vesicles probably reflected modification of parareceptor sites which affected the conformation of the specific FSH receptor.

Studies on the nature of interaction between follicle-stimulating hormone and the testicular receptors

Two previous reports from this laboratory showed that the binding of 125I-labeled human choriogonadotropin and 125I-labeled human luteinizing hormone to rat testicular receptors is partially irreversible and the binding parameters obtained from Scatchard analysis of the data can be drastically altered simply by changing the reaction volume of the binding assays (Chen, C.J.H., Lindeman, J.G., Trowbridge, C.G. and Bhalla, V.K. (1979) Biochim, Biophys. Acta 584, 407--435; Bhalla, V.K., Trowbridge, C.G., Chen, C.J.H., Lindeman, J.G. and Rojas, F.J. (1979) Biochim, Biophys. Acta 584, 436--453). It is reported herein that the binding reaction between follicle-stimulating hormone and testicular receptors displays very similar characteristics. The results support the previous conclusion that receptor concentrations fluctuate in the membranes and that the extent of their loss from tissue membranes in vitro is dependent upon time and temperature of incubation, the volume of buffer present, and the quantity of hormone used.

Gonadal receptors. II. Effects of time and reaction volume upon the binding of human chorionic gonadotropin and human luteinizing hormone to particulate receptors

The effect of reaction volume upon the binding of gonadotropins by particulate receptors was studied. Two experimental approaches were used: one involved increasing the reaction volume of the binding assay (i.e. diluting the hormone and receptor concentrations and will be referred to as buffer coincubation studies) and the other involved incubating the testicular homogenate in various buffer volumes prior to the binding assay (buffer preincubation stidies). The results showed that the number of hormone binding sites inferred from Scatchard analysis was inversely related to the reaction volume in the coincubation as well as in the preincubation studies. Time-dependent dissociation of receptors from the intact testis was demonstrated by perifusion studies and the loss of receptors from intact testis correlated with the appearance of soluble factors (Bhalla, V.K., Haskell, J., Grier, H. and Mahesh, V.B. (1976) J. Biol. Chem. 251, 4947--4957) in the eluate obtained. The results obtained along with those presented in the preceding manuscript (Chen, C.J.H., Lindeman, J.G., Trowbridge, C.G. and Bhalla, V.K. (1979) Biochim. Biophys. Acta 284, 407--435) question the validity of the rapid equilibrium model which assumes reversible hormone occupancy of a fixed number of receptor sites. An alternate binding model is proposed herein and its implications are discussed.

1H nuclear-magnetic-resonance studies of porcine lutropin and its alpha and beta subunits

The titration curves of the histidine residues of porcine lutropin and its isolated alpha and beta subunits have been determined by following the pH-dependence of the imidazole C-2 proton resonances. The isolated alpha subunit contains a buried histidine, whose C-2 proton does not exchange with solvent, and which has the unusually low pK of 3.3. In the native hormone all the histidine residues have relatively normal pK values (between 5.7 and 6.2). The four histidine C-2 proton resonances have been assigned to specific residues in the amino-acid sequence, by means of deuterium and tritium exchange experiments on the alpha subunit and its des(92-96) derivative. The histidine with a pK of 3.3 is identified as His-alpha87. The effects of pH on tyrosine and methyl proton resonances show that the titration of His-87 in the isolated alpha subunit is accompanied by a significant conformational change which involves loosening of the protein structure but which is not a normal unfolding transition. The role of conformational changes in the generation of biological activity by subunit association in the glycoprotein hormones is discussed.

Porcine follitropin. Isolation and characterization of the native hormone and its alpha and beta subunits

The properties of porcine follitropin and its subunits which have not yet been characterized are presented. The porcine follitropin obtained has a biological potency of 81 times the National Institutes of Health Porcine Follitropin P-1 preparation. Its contamination by lutropin and thyrotropin amounted to 1 and 0.5 percent by weight respectively, as measured by radioimmunoassay. The alpha and beta subunits of porcine follitropin were obtained by incubation in an acidic urea solution followed by anion exchange chromatography. The amino acid composition of porcine follitropin alpha subunit was found to be identical to that of alpha chain of porcine lutropin and thyrotropin. These porcine alpha chains differ, nevertheless, markedly in their carbohydrate composition particularly with respect to their mannose and galactose contents. The amino-terminal residue of the follitropin alpha subunit is threonyl. The carboxy-terminal end of the alpha chain is of variable length. Cysteyl residue was detected at the aminoterminal end of the follitropin beta chain with glutamic acid at its carboxy-terminal end. Cross-contamination of the alpha and beta subunit preparations was measured by specific radioimmunoassay and amounted to 0.5 and 0.1 percent by weight respectively.