Molecular cloning in the marmoset shows that semenogelin is not the precursor of the TRH-like peptide pGlu-Glu-Pro amide

Two peptides with similar structures to thyrotropin-releasing hormone (TRH), pGlu-Glu-Pro amide and pGlu-Phe-Pro amide, have been identified in human seminal fluid and it has been shown that one of these peptides, pGlu-Glu-Pro amide, has the ability to increase the capacitation of sperm cells, consistent with a role in fertility. In order to select a species in which there is a high degree of expression of the genes that code for 'TRH-like' peptides, we have determined the levels of these peptides in the prostate, pancreas and thyroid of a range of species including rat, rabbit, ox, marmoset, macaque and man. The peptides were extracted from the tissues and purified before determination by RIA with TRH antibody. In addition, trypsin digestion and TRH RIA was used to investigate the presence of N-extended forms. The highest concentrations of TRH-immunoreactive peptides were found in the tissues of the marmoset, Callithrix jacchus. Ion-exchange chromatography demonstrated that marmoset thyroid contained principally authentic TRH, the pancreas contained both TRH and TRH-like peptides while the prostate contained TRH-like peptides alone. Further purification by HPLC showed that the main TRH-immunoreactive peptide in marmoset prostate was pGlu-Glu-Pro amide and a second component was identified as pGlu-Phe-Pro amide. The results indicate that the biosynthesis of these peptides could be studied to advantage in the marmoset. The biosynthetic precursors of the TRH-like peptides have not been identified. To examine whether pGlu-Glu-Pro amide might originate from semenogelin, we determined the sequence of semenogelin in the marmoset. It exhibited a high degree of homology with human semenogelin-I, but in place of the Lys-Gln-Glu-Pro sequence that might give rise to pGlu-Glu-Pro amide, marmoset semenogelin possessed the sequence Ser-Gln-Asp-Gln which cannot serve as a precursor for a TRH-like peptide. Further evidence was obtained by Northern blot analysis of a range of marmoset tissues. The results showed that semenogelin is not present in marmoset prostate. It is concluded that pGlu-Glu-Pro amide originates from a precursor distinct from semenogelin, both in marmoset and in man.

Fertilization promoting peptide: an important regulator of sperm function in vivo?

Fertilization promoting peptide (FPP; pGlu-Glu-ProNH2), a tripeptide structurally related to thyrotrophin releasing hormone, is produced by the prostate gland and released into seminal plasma. Recent studies carried out in vitro have revealed that FPP elicits biologically important responses in both mouse and human spermatozoa. In the presence of physiological concentrations of FPP (50-100 nmol l(-1)), uncapacitated spermatozoa undergo accelerated capacitation and so become potentially fertilizing more quickly, while capacitated spermatozoa are inhibited from undergoing spontaneous acrosomal exocytosis, an event that would make them non-fertilizing. In vivo, these responses would be very important since relatively few spermatozoa reach the site of fertilization; FPP could help to ensure that these were potentially fertilizing cells. A putative receptor (TCP-11) for FPP has been identified in mice. The gene for TCP-11 (which has a human homologue) maps to the t-complex, a region known to contain genes affecting male fertility. Current evidence indicates that FPP and TCP-11 act by modulating the activity of adenylyl cyclase and hence production of cAMP, a signal transduction pathway shown to be important in the acquisition of fertilizing ability. These results suggest that FPP plays an important role in normal fertility and that insufficient FPP could reduce fertility. Prostatic dysfunction can lead to decreased synthesis of FPP and increased synthesis of FPP-related peptides with reduced biological activity, both of which could compromise fertility in vivo. Given that 'male factor' infertility is a common contributor to subfertility in couples, it may prove possible to develop new therapeutic treatments, for at least some males, using FPP. In addition, this ligand-receptor pair could provide a novel target for male contraception.

Static and dynamic roles of extracellular loops in G-protein-coupled receptors: a mechanism for sequential binding of thyrotropin-releasing hormone to its receptor

Small ligands generally bind within the seven transmembrane-spanning helices of G-protein-coupled receptors, but their access to the binding pocket through the closely packed loops has not been elucidated. In this work, a model of the extracellular loops of the thyrotropin-releasing hormone (TRH) receptor (TRHR) was constructed, and molecular dynamics simulations and quasi-harmonic analysis have been performed to study the static and dynamic roles of the extracellular domain. The static analysis based on curvature and electrostatic potential on the surface of TRHR suggests the formation of an initial recognition site between TRH and the surface of its receptor. These results are supported by experimental evidence. A quasi-harmonic analysis of the vibrations of the extracellular loops suggest that the low-frequency motions of the loops will aid the ligand to access its transmembrane binding pocket. We suggest that all small ligands may bind sequentially to the transmembrane pocket by first interacting with the surface binding site and then may be guided into the transmembrane binding pocket by fluctuations in the extracellular loops.

[Improvement of intestinal absorption of peptide and protein drugs by chemical modification with fatty acids]

It is well known that the oral bioavailability of peptide and protein drugs is generally poor because they are extensively degraded by proteases in the gastrointestinal tract and impermeable through the intestinal mucosa. Therefore, various approaches have been examined to overcome the delivery problems of these peptides and to improve their absorption via the gastrointestinal tract. Of these approaches, a potentially useful approach to solve these delivery problems may be chemical modification of peptides and proteins to produce prodrugs and analogues. Thus, it is plausible that this approach may protect peptides against degradation by peptidases and other enzymes present at the mucosal barrier and renders the peptides and proteins more lipophilic, resulting in increased bioavailability. From these standpoints, we synthesized lipophilic derivatives of peptides and proteins such as thyrotropin-releasing hormone (TRH), tetragastrin (TG), calcitonin and insulin by chemical modification with fatty acids. The pharmacological activities of these derivatives were relatively high as compared with the native peptides. A significant increase in the intestinal absorption of these derivatives of peptides was observed in comparison with native peptides. Overall, the effects of acylation on the intestinal absorption of these peptides were more predominant in the large intestine than those in the small intestine. In addition, these derivatives were more stable than the parent peptides in homogenates of the various intestinal mucosae. We also examined the intestinal transport characteristics of TG and its acyl derivatives using Caco-2 cell monolayers in order to assess the contribution of enzymatic and transport barriers on their intestinal absorption. The degradation clearance of TG on the apical membrane was decreased by chemical modification with fatty acids. In addition, the permeability clearance of TG was improved by the acylation. On the other hand, the intestinal absorption of thyrotropin releasing hormone (TRH), which is transported by a carrier-mediated process, was also enhanced by chemical modification with lauric acid. In summary, this chemical modification approach may be useful to improve the intestinal absorption of peptide and protein drugs.

A study of a highly specific pyroglutamyl aminopeptidase type-II from the membrane fraction of bovine brain

Pyroglutamyl aminopeptidase type-II is reported to be a highly specific, membrane-bound neuropeptidase, which has the ability to hydrolytically remove the L-pyroglutamyl residue (pGlu) from the N-terminus of thyrotropin releasing hormone (TRH, pGlu-His-Pro-NH2) and closely related tripeptides or tripeptide amides. The primary aim of this study was to purify this enzyme from bovine brain and to compare its characteristics with those previously reported. Following solubilization from the membrane fraction by limited proteolysis with trypsin, the enzyme was purified approximately 3000-fold with a 24% recovery of activity. A native molecular mass of 214,000 Da was determined for the purified enzyme by gel-filtration chromatography. A pH optimum of 6.8-7.6 was observed for the enzyme, with rapid inactivation occurring below pH 4.0 and above pH 9.2. Optimal enzyme activity was observed at 45 degrees C. On the basis of its inhibition, in a time-dependent manner, by metal complexing agents and its subsequent reactivation in the presence of metal ions, the enzyme was identified as a metallopeptidase. Substrate specificity studies revealed that, with the exception of pGlu-Phe-Pro-NH2, pGlu-7-amino-4-methyl-coumarin and pGlu-beta-naphthylamide, the purified enzyme removes N-terminal pGlu from only tri- and tetrapeptides with a histidine residue in the penultimate position. A number of N-terminal pyroglutamyl peptides of varying length were shown to competitively inhibit the enzyme. Of these, luteinizing hormone releasing hormone (LHRH) and LHRH 1-5, although not substrates for the enzyme, were found to be potent inhibitors, with Ki values of 8 and 11 microM, respectively. The study shows that while bovine brain PAPII shares many of the characteristics of PAPII from other mammalian tissues, its substrate specificity is not as narrow as previously reported.

A fertilization promoting peptide (FPP)-related tripeptide competitively inhibits responses to FPP: a cause of male subfertility?

Fertilization promoting peptide (FPP; pGlu-Glu-ProNH2), a tripeptide structurally related to thyrotrophin releasing hormone (TRH; pGlu-His-ProNH2), is present in the prostate gland and seminal plasma of several mammalian species. FPP has been shown not only to stimulate the capacitation and fertilizing ability of epididymal mouse and ejaculated human spermatozoa, but also to inhibit spontaneous acrosome loss in mouse spermatozoa. These results suggest a possible role in vivo for FPP to maximize the fertilizing potential of the few cells that reach the ampulla. In this study we have investigated the effects of FPP-related peptides on mouse sperm capacitation and the acrosome reaction (using chlortetracycline fluorescence) and in vitro fertilizing ability. Deamidated FPP neither stimulated capacitation when tested at 50-200 nM nor interfered with FPP's stimulation of capacitation. Three neutral peptides (pGlu-Phe-ProNH2, MeO-FPP, pGlu-Gln-ProNH2) were also evaluated. pGlu-Phe-ProNH2, slightly stimulatory when used alone, had no additive effect when used in combination with FPP and the methyl derivative of FPP had no bioactivity itself and did not inhibit responses to FPP. In marked contrast, pGlu-Gln-ProNH2 (Gln-FPP), which had no bioactivity when added to uncapacitated suspensions at 50-100 nM, significantly inhibited FPP's stimulation of capacitation and fertilizing ability in vitro. Furthermore, when Gln-FPP + FPP were added to capacitated suspensions, Gln-FPP prevented FPP's inhibition of spontaneous acrosome loss. Our recent studies have indicated that FPP and adenosine can elicit similar responses but appear to act at different sites. The fact that Gln-FPP inhibited responses to FPP, but not to adenosine, indicates that Gln-FPP is acting at an FPP-specific site. We, therefore, conclude that the specific structure of the FPP molecule is crucial for biological activity. Removal of the terminal amide group abolishes bioactivity and changes to the central amino acid can have significant functional consequences. Since Gln-FPP is a candidate intermediate peptide in the FPP biosynthetic pathway and has been identified in human semen, abnormality in prostate function could lead to release of Gln-FPP along with, or instead of, FPP. Our results suggest that the relative proportions of FPP and related peptides in seminal plasma could have a significant effect on fertility in vivo.

Interactions between conserved residues in transmembrane helices 1, 2, and 7 of the thyrotropin-releasing hormone receptor

The roles of conserved residues in transmembrane helices (TMs) of G protein-coupled receptors have not been well established. A computer-generated model of the thyrotropin-releasing hormone receptor (TRH-R) indicated that conserved Asp-71 (TM-2) could interact with conserved asparagines 316 (TM-7) and 43 (TM-1). To test this model, we constructed mutant TRH-Rs containing polar or alanine substitutions of these residues. The maximal activities of N43A and N316A TRH-Rs were diminished, whereas D71A (Perlman, J. H., Nussenzveig, D. R., Osman, R., and Gershengorn, M. C. (1992) J. Biol. Chem. 267, 24413-24417) and N43A/N316A TRH-Rs were inactive. Computer models of D71A and N43A/N316A TRH-Rs show similar changes from native TRH-R in their TM bundle conformations. The inactivity and the similarity of the computer models of D71A and N43A/N316A TRH-Rs are consistent with the idea that Asp-71 bridges Asn-43 and Asn-316 and suggest that activity is critically dependent on these interactions. The conservation of these residues suggests these specific interactions involving TMs 1, 2, and 7 may be structurally important for all members of the rhodopsin/beta-adrenergic receptor subfamily of G protein-coupled receptors.

Altered ligand dissociation rates in thyrotropin-releasing hormone receptors mutated in glutamine 105 of transmembrane helix III

Glutamine 105 in the third transmembrane helix of the thyrotropin-releasing hormone receptor (TRH-R) occupies a position equivalent to a conserved negatively charged residue in receptors for biogenic amines where it acts as counterion interacting with the cationic amine moiety of the ligand. Maximum levels of response to TRH in oocytes expressing wild-type TRH-Rs were indistinguishable from those of oocytes expressing receptors mutated to Glu, Asn, or Asp in position 105. However, the EC50 values for activation of oocyte responses increased more than 500 times in oocytes expressing mutant Glu105 receptors, in which the amido group of Gln105 has been removed by site-directed mutagenesis. Charge effects do not seem to be involved in the huge effect of mutating Gln105 to Glu, since mutation of Gln105 to Asp induces only a 15-fold increase in EC50. Furthermore, no change in EC50 is observed after mutation of Asn110 to Asp. The affinity shift (identified by changes in EC50 values for systems of comparable efficacy) in Glu105 mutant receptors was partially recovered in oocytes expressing Asn105 mutant receptors. These results and those obtained after substitution of Lys, Leu, Tyr, and Ser for Gln105 suggest that the presence and the correct position of the Gln hydrogen bond-donor amido group are important for normal functionality of the receptor. In wild type or Asp105 mutant receptors showing the same maximal responses, decreases in affinity with TRH and methyl-histidyl-TRH correlated with increased dissociation rates of hormone from the receptor. Rapid dilution experiments following subsecond stimulation indicate that the TRH-R is converted rapidly from a form showing fast dissociation kinetics to a form from which the hormone dissociates slowly. Mutation of residue 105 impairs the receptor shift between these two forms. This effect was demonstrated in a direct way by comparing [3H]methyl-histidyl-TRH dissociation rates in COS-7 cells transfected with either wild type or Asp105 mutant TRH-Rs. Thus, residues located in transmembrane helix III positions equivalent to those of the counterions for biogenic amines, regulate hormone-receptor interactions in the TRH receptor (and perhaps other receptors). Furthermore, the nature of the amino acid in these positions may also play a role, directly or indirectly, in conformational changes leading to receptor activation, and hence to signal transduction.

Specific binding sites for rat prepro-TRH-(160-169) on C6 glioma and BN1010 clonal neural cells

A connecting decapeptide corresponding to rat prepro-TRH-(160-169) (Ps4) displays several biological activities that are related or unrelated to TRH. We have previously characterized pituitary binding sites for this connecting peptide and elucidated structural determinants for high peptide binding affinity. In the current study, a series of cell lines was screened for the presence of specific binding sites with a highly potent derivative of Ps4, the monoiodinated radioligand [125I-Tyr0]Ps4. Neuroblastoma x glioma hybrid NG108-15, glioma C6 and neuroblastoma BN1010 cell lines were found to have high-affinity [125I-Tyr0]Ps4 binding sites containing 600, 9700 and 130000 sites/cell, respectively. The specific binding of [125I-Tyr0]Ps4 was rapid, time-dependent, reversible and proportional to the amount of C6 and BN1010 membrane preparation. Furthermore, Scatchard or Hill analysis revealed that [125I-Tyr0]Ps4 was bound by a single population of non-interacting sites with dissociation constants in the subnanomolar range. Competition studies made with Ps4 analogues indicated that [125I-Tyr0]Ps4 binding sites on C6 and BN1010 cells were similar to those previously described on rat pituitary membranes. It is concluded that C6 and BN1010 cells are suited for studies on the intracellular events following binding of the Ps4 and for the molecular characterization of the Ps4 binding sites.

Effects of peptide structure on transport properties of seven thyrotropin releasing hormone (TRH) analogues in a human intestinal cell line (Caco-2)

We conclude that TRH and its analogues permeate mainly via paracellular routes in this particular clone of Caco-2 cells because variation in lipophilicity, polar surface properties, or hydrogen bonding potential-all influencing the transcellular pathway-do not give meaningful relationships. On the other hand, the variations in molecular size of the TRH analogues were too small to detect any influence on the paracellular transport properties. Further studies concerning the solution conformation and the hydrodynamical radii of the molecules probably give more information about the structure-permeability relationship of TRH transport across Caco-2 cell monolayers. The influence of the structural variations of these 7 TRH analogues on the binding affinity to the di- and tripeptide transporter, using another Caco-2 cell clone, are currently under investigation in our laboratory.

Hydropathy profiles of predicted thyrotropin-releasing hormone precursors are highly conserved despite low similarity of primary structures

Two types of cDNAs encoding thyrotropin-releasing hormone (TRH) precursors (TRH-A and TRH-B) were amplified from hypothalamic mRNA of sockeye salmon by reverse transcriptase-polymerase chain reaction (RT-PCR). The amplification was achieved using two primers which correspond to TRH progenitor sequence (Lys/Arg-Arg-Gln-His-Pro-Gly-Lys/Arg-Arg). A full length cDNA encoding TRH-A was obtained by 5'- and 3'-RACE methods. It has a length of 1324 base pairs (bp) that contains sequences of 5' and 3' untranslated regions and an open reading frame of 259 codons. The sockeye salmon TRH-A deduced from the nucleotide sequence tandemly contains 8 copies of TRH progenitor sequences. Another cDNA which encodes a part of TRH-B consists of 242 bp, and the sequence homology between TRH-A and -B cDNAs is 90%. The result of Southern blot analysis of sockeye and masu salmon genomic DNAs supported the evidence that there are at least two TRH genes in the salmonid. A RT-PCR analysis of TRH gene expression in various tissues of sockeye salmon showed that strong expression was observed only in the brain. The primary structure of the sockeye salmon TRH-A shares low similarity to those of human, rat and Xenopus TRH precursors (35, 27 and 44%, respectively). However, their hydropathy profiles were almost the same with each other. The profile of sockeye salmon TRH-A showed the presence of two discrete hydrophobic regions, one in the N-terminal region which corresponds to the signal peptide and the other in the C-terminal region. All of the repetitive TRH progenitor sequences are included in three hydrophilic regions easily recognizable. The present results thus suggest that the three-dimensional structures of TRH precursors are highly conserved, although the primary structures of TRH precursors have diverged through the evolutionary pathway of vertebrates.

Enhancement of intestinal transport of thyrotropin-releasing hormone via a carrier-mediated transport system by chemical modification with lauric acid

The transport characteristics of thyrotropin-releasing hormone (TRH) and its chemically modified derivative with lauric acid (Lau-TRH) across the rat small or large intestine were estimated by means of an in vitro everted sac experiment. Both compounds were especially absorbed from the upper small intestine. The penetration of TRH across the upper small intestine was significantly increased by conjugation with lauric acid. Lau-TRH administered to the mucosal side appeared as a native TRH form in the serosal side. On the other hand, a temperature dependency and a directional difference in the transfer rates of these compounds were observed in the everted and non-everted sacs of the upper small intestine. Moreover, the penetration of TRH and Lau-TRH across the upper small intestine was inhibited by 0.25 mM 2,4-dinitrophenol and 10 mM glycylglycine. In addition, Lau-TRH was very stable in the cytosolic fraction of the small intestinal mucosa, while it was gradually converted to the native TRH in the brush-border membrane (BBM) fraction. The binding amounts of TRH to the BBM were remarkably enhanced by the lauric acid conjugation; however, its binding was nonspecific. Therefore, it was suggested that Lau-TRH rapidly bound to the BBM in the small intestine, where Lau-TRH is converted to TRH, and this released TRH is efficiently transported by an oligopeptide transporter which exists in the upper small intestine.

A refined model of the thyrotropin-releasing hormone (TRH) receptor binding pocket. Novel mixed mode Monte Carlo/stochastic dynamics simulations of the complex between TRH and TRH receptor

Previous mutational and computational studies of the thyrotropin-releasing hormone (TRH) receptor identified several residues in its binding pocket [see accompanying paper, Perlman et al. (1996) Biochemistry 35, 7643-7650]. On the basis of the initial model constructed with standard energy minimization techniques, we have conducted 15 mixed mode Monte Carlo/stochastic dynamics (MC-SD) simulations to allow for extended sampling of the conformational states of the ligand and the receptor in the complex. A simulated annealing protocol was adopted in which the complex was cooled from 600 to 310 K in segments of 30 ps of the MC-SD simulations for each change of 100 K. Analysis of the simulation results demonstrated that the mixed mode MC-SD protocol maintained the desired temperature in the constant temperature simulation segments. The elevated temperature and the repeating simulations allowed for adequate sampling of the torsional space of the complex with successful conservation of the general structure and good helicity of the receptor. For the analysis of the interaction between TRH and the binding pocket, TRH was divided into four groups consisting of pyroGlu, His, ProNH2, and the backbone. The pairwise interaction energies of the four separate portions of TRH with the corresponding residues in the receptor provide a physicochemical basis for the understanding of ligand-receptor complexes. The interaction of pyroGlu with Tyr106 shows a bimodal distribution that represents two populations: one with a H-bond and another without it. Asp195 was shown to compete with pyroGlu for the H-bond to Tyr106. Simulations in which Asp195 was interacting with Arg283, thus removing it from the vicinity of Tyr106, resulted in a stable H-bond to pyroGlu. In all simulations His showed a van der Waals attraction to Tyr282 and a weak electrostatic repulsion from Arg 306. The ProNH2 had a strong and frequent H-bonding interaction with Arg306. The backbone carbonyls show a frequent H-bonding interaction with the OH group of Tyr282 and strong, often multiple, interactions with Arg306. Three structures, which maintained these interactions simultaneously, were selected as candidates for ligand-receptor complexes. These show persistent interactions of TRH with Ile 109 and Ile 116 in HX 3 and with Tyr310 and Ser313 in HX 7, which will be tested to refine the structure of the ligand-receptor complex. The superposition of the three structures shows the extent of structural flexibility of the receptor and the ligand in the complex. The backbone of TRH inside the receptor is in an alpha-helical conformation, suggesting that the receptor, through its interaction with the ligand, provides the energy required for the conformational change in the ligand from an extended to the folded form.

A refined model of the thyrotropin-releasing hormone (TRH) receptor binding pocket. Experimental analysis and energy minimization of the complex between TRH and TRH receptor

Seven transmembrane (TM) spanning, G protein-coupled receptors (GPCRs) appear to bind large glycoprotein hormones predominantly within their extracellular domains, small nonpeptidic ligands within the TM helical bundle, and peptide ligands within the extracellular domains and TM bundle. The tripeptide thyrotropin-releasing hormone (TRH, pyroGlu-His-ProNH2) may bind entirely within the TM bundle of the TRH receptor (TRH-R). We have previously demonstrated direct binding contacts between the pyroGlu of TRH and two residues in TM helix 3 (TM-3) of TRH-R and proposed a model of the binding pocket of TRH-R [Perlman, J. H., Laakkonen, L., Osman, R., & Gershengorn, M. C. (1994) J. Biol. Chem. 269, 23383-23386]. Here, we provide evidence for two additional direct interactions between TRH and TRH-R. One interaction is between the aromatic ring of Tyr 282 of TM-6 and His of TRH. This is based on a large increase in the half-maximally effective concentration (EC50) of TRH for stimulation of inositol phosphate formation by Y282A TRH-R and a loss of selectivity of this mutant receptor for TRH analogs substituted at His. We provide evidence for another interaction between Arg 306 of TM-7 and the terminal carboxamide of TRH. Using four direct interactions as anchors, a refined model of the TRH-R binding pocket was constructed using geometry optimization through energy minimization. A novel method for modeling GPCRs based on Monte Carlo and stochastic dynamics simulations is presented in the accompanying paper [Laakkonen, L. J., Guarnieri, F., Perlman, J. H., Gershengorn, M. C., & Osman, R. (1996) Biochemistry 35, 7651-7663].

Restricted analogues provide evidence of a biologically active conformation of thyrotropin-releasing hormone

Thyrotropin-releasing hormone (TRH) is a tripeptide (< Glu-His-Pro-NH2) that signals through a G protein-coupled receptor. TRH is a highly flexible molecule that can assume many conformations in solution. To attempt to delineate the biologically active conformation of TRH, we synthesized a pair of conformationally restricted cyclohexyl/Ala2-TRH analogues. The diastereomeric analogues use a lactam ring to restrict two of the six free torsional angles of TRH and constrain the X-Pro-NH2 peptide bond to trans. Unrestricted cyclohexyl/Ala2-TRH exhibited a 650-fold lower affinity than TRH for TRH receptor and was 430-fold less potent than TRH in stimulating inositol phosphate second messenger formation. One diastereomer exhibited higher affinity and potency than the unrestricted analogue despite the presence of the methylene bridge and fused ring, whereas the other showed lower affinity and potency. Computer simulations predicted that the positions of the cyclohexyl/Ala2 and Pro-NH2 moieties relative to < glutamate were different in the two analogues and that the conformation of the higher affinity analogue is different from that of trans-TRH in solution but is superimposable on that of trans-TRH found in a model of the TRH/TRH receptor complex. These experimental findings identify a favored relative position of < glutamate and Pro-NH2 in the more active conformation of two diastereomeric analogues of TRH and provide independent support for the model of the TRH/TRH receptor complex.

Nuclear magnetic resonance studies of thyrotropin-releasing hormone (TRH) and analogues incorporating D-histidine and 4-hydroxy-L-proline

NMR studies have been used to examine conformational effects in thyrotropin-releasing hormone (TRH), the epimer incorporating D-His, and their analogues where trans- and cis-4-hydroxy-L-proline replace L-proline (Pro). In all six compounds the observed overall conformation of the major conformer around the Pro-His amide bond, and the observed increase of the cis/trans ratio between the conformers when L-His is replaced by D-His, can be accommodated by assuming that a ten-membered ring is formed by hydrogen bonding between the N-H of the Pro carboxamide function and the N pi-atom of the His imidazole nucleus.

The TRH-like peptides in rabbit testis are different from the TRH-like peptide in the prostate

Human seminal fluid contains a number of tripeptide amides with similar structures to thyrotropin releasing hormone (TRH), two of which have been identified as pGlu-Glu-Pro amide and pGlu-Phe-Pro amide. To determine whether these peptides originate in the same tissues and have the same molecular origin, TRH-immunoreactive peptides were extracted from the prostate and testis of the rabbit, purified by ion exchange chromatography and HPLC, and identified by co-chromatography with 3H-labelled marker peptides. In addition, trypsin digestion was used to release TRH-like tripeptides from N-extended forms of these peptides. The sole TRH-like peptide in the prostate was shown to be pGlu-Glu-Pro amide; it was not accompanied by a detectable amount of pGlu-Phe-Pro amide. The prostate also appeared to contain a very small amount of N-extended forms of these peptides. In contrast to the prostate, the testis contained high concentrations of N-extended forms of pGlu-Phe-Pro amide but essentially no tripeptide. The testis also contained N-extended forms of two other neutral TRH-like peptides which were less hydrophobic than pGlu-Phe-Pro amide. Neither the prostate nor the testis contained a significant amount of TRH. The results show that in the rabbit the TRH-like peptides pGlu-Glu-Pro amide and pGlu-Phe-Pro amide occur in different tissues and appear to be formed from different precursors.

A thyrotropin-releasing hormone analogue: pGlu-Phe-D-Pro-psi [CN4]-NMe at 293 and 107 K

Data have been measured at two temperatures, 293 K and 107 K, for a crystal of a thyrotropin-releasing hormone analogue, pGlu-Phe-D-Pro-psi [CN4]-NMe, C20H25N7O3, and the structures solved and refined. The tripeptide contains a tetrazole ring which mimics a cis-peptide bond at the C terminus. An intermolecular hydrogen bond exists between two molecules related by the twofold screw axis, resulting in infinite chains of hydrogen-bonded peptide molecules. Because of the folding and packing of the molecules, there are no intermolecular contacts of less than 4 A to the N atom of the phenylalanine residue.

Hydrolytic cleavage of pyroglutamyl-peptide bond. I. The susceptibility of pyroglutamyl-peptide bond to dilute hydrochloric acid

The susceptibility of the pyroglutamyl-peptide bond in some biologically active peptides, dog neuromedin U-8 fragment (pGlu-Phe-Leu-Phe-Arg-Pro-Arg-OH), human big gastrin fragment (pGlu-Leu-Gly-Pro-OH) and thyrotropin releasing hormone (TRH) fragments (pGlu-His-Pro-OH, pGlu-His-OH), to 1 N HCl under mild conditions and/or at 60 degrees C was studied. It was found that the N-terminal portion of pGlu-peptides is extremely labile to acid hydrolysis, giving not only the ring-opened product of the pyrrolidone moiety of the pGlu residue, but also the cleavage product of the pGlu-peptide linkage. The ring-opening reaction predominated over the cleavage reaction in hydrolysis of the four peptides in 1 N HCl at 60 degrees C. The ring-opening reaction and the cleavage reaction of pGlu-peptide linkage proceeded faster than the cleavage of internal peptide bonds. The rate of hydrolysis was affected by the reaction temperature, and the ring-operating reaction was greatly diminished at 4 degrees C in comparison with the cleavage reaction. Thus, the phenomenon that the pGlu-peptide bond is susceptible to dilute HCl as compared to the other peptide bond appears to be a general one.

Application of a spray drying technique in the production of TRH-containing injectable sustained-release microparticles of biodegradable polymers

Copoly (dl-lactic/glycolic acid) microparticles for sustained release of a water-soluble drug (Thyrotropin releasing hormone: TRH) were prepared by a spray drying method. A higher entrapment ratio was achieved with the spray drying method with the in-water drying method. In order to avoid agglomeration of the microparticles, a double-nozzle spray drying method was designed using mannitol as an anti-adherent. The surface of the spray-dried microparticles was coated with mannitol, and the extent of agglomeration was decreased. Acetonitrile was the most suitable solvent for microencapsulation using the double-nozzle spray drying method because the initial burst of TRH from the microparticles during the first day was the smallest. When PLGA with a weight-average molecular weight of 14,000 was used, constant release of TRH continued for one month with a small initial burst. In conclusion, the production of biodegradable microparticles by the double-nozzle spray drying method appears to be an attractive alternative to conventional microencapsulation methods.

Identification of two classes of prolactin-releasing factors in intermediate lobe tumors from transgenic mice

Targeted tumorigenesis, using the POMC gene promoter ligated to the simian virus 40 large T antigen, generated transgenic mice with massive tumors of the intermediate lobe (IL) of the pituitary. Inoculation of nude mice with the IL tumor cells resulted in very large secondary tumors. As the IL from several species produces a potent PRL-releasing factor (PRF), it was of interest to determine whether IL tumors from these mice also contain PRF. The objectives were to 1) measure serum PRL levels in mice with IL tumors, 2) determine whether these tumors contain PRF and examine its chromatographic properties, and 3) analyze whether this PRF is related to POMC, its derivatives, or other PRL secretagogues. Serum PRL levels were 5- to 6-fold higher in transgenic than in control mice. Primary and secondary IL tumors were acid extracted and successively fractionated using Sephadex G-100 gel filtration and reverse phase and gel permeation HPLC. PRF activity was determined using short term incubation of tissue extracts or column fractions with GH3 cells. Crude tumor extracts exhibited a strong and dose-dependent PRF activity. Upon chromatography, the PRF activity from either primary or secondary tumors resolved into two classes of compounds: a big PRF with an estimated mol wt of 70-80 kilodaltons and two small, very hydrophobic peptides. The elution profiles of the three PRFs differed from those of beta-endorphin, alpha MSH, beta MSH, ACTH, TRH, oxytocin, angiotensin II, vasoactive intestinal polypeptide, or corticotropin-like intermediate peptide. In summary, we have identified an animal model with IL tumors that has hyperprolactinemia and overproduces PRF. Two classes of PRFs, big and small, were resolved which differ from POMC derivatives and known regulators of PRL release. These data suggest that PRF is produced by melanotrophs, but is not a product of the POMC gene. The IL tumors should provide an excellent source for the purification and structural elucidation of PRFs.

Production and characterization of monoclonal and polyclonal antibodies against thyrotropin-releasing hormone

In this article, two mouse monoclonal antibodies (83-7B5-A1 and 83-6B6-A10) and three rabbit polyclonal antibodies (1118, 8572, and 8577) directed against thyrotropin-releasing hormone (TRH; pGlu-His-Pro-NH2) are described. The anti-TRH antibodies were raised by immunization with a TRH-bovine serum albumin conjugate obtained by coupling of the CO2H group of pGlu-His-Pro-OH to NH2 groups in the protein. The monoclonal antibodies were produced by hybridoma clones obtained by the fusion of SP2/0 myeloma cells with spleen cells of an immunized BALB/c mouse. Both monoclonal antibodies were of the IgG1 (kappa) subclass. Characterization of the anti-TRH antibodies showed that in general they are specific for the pGlu-His moiety. The cross-reactivities for the TRH-like peptides [Glu1]TRH, [Glu2]TRH, and [Phe2]TRH are low, while alterations at the Pro-NH2 moiety of TRH are recognized to varying extents. The specificities of these antibodies are markedly different from those previously obtained using TRH coupled through the histidine residue to protein as the immunogen.

The electrophoretic mobility of tripeptides as a function of pH and ionic strength: comparison with iontophoretic flux data

Capillary electrophoresis (CE) is an extremely efficient separations tool which can also be used to determine fundamental molecular parameters, e.g., the electrophoretic mobility of a molecule. We have studied the changes in the CE estimated electrophoretic mobility of thyrotropin releasing hormone (TRH) as a function of pH and ionic strength. Further, we have used CE to estimate the mobilities of two synthetic analogs of TRH to examine the behavior of positive (basic) and negative (acidic) peptides under the conditions of this work. These data were then compared with literature values of iontophoretic flux of these molecules under similar formulation conditions. Our results suggest that CE could potentially assist formulation optimization for the iontophoretic delivery of peptides.