Immunologic identification of both plasma and human renal inactive renin as prorenin.

Antibodies were raised against a synthetic tetradecapeptide which is a component of the non-renin portion (prosegment) of human renin precursor. Inactive renin from human kidney and plasma strongly adsorbed to a gel coupled to immunoglobulins purified from such an antiserum. These results suggest that renal and circulating inactive human renins contain in their structure the prosegment of prorenin.

Analysis of inactive renin by renin profragment monoclonal antibodies.

Two peptides were synthesized, corresponding to the sequences (-19 to -7) and (-26 to -17) of the prorenin prosegment. Monoclonal antibodies were raised to these sequences and used to characterize human plasma inactive renin. Only anti (-19 to -7) reacted with inactive renin, as measured by direct assay or affinity chromatography. The data were used to evaluate two possible inactive renin stuctures: plasma inactive renin is a truncated prorenin lacking the prosegment N-terminal portion; its spatial conformation masks the N-terminal extremity, preventing interaction of this region with specific antibodies.

A new series of photoactivatable and iodinatable linear vasopressin antagonists.

A series of new linear photoactivatable and iodinatable antagonists of the neuropeptidic hormone vasopressin was designed and synthesized by a combination of PyBOP-mediated Boc/solid-phase peptide synthesis and solution synthesis approaches. These were based on modifications of a previously reported potent and selective antagonist of the vasopressor response (V1a receptor) to [arginine]vasopressin, phenylacetyl-D-Tyr(Me)-Phe-Gln-Asn-Arg-Pro-Arg-Tyr-NH2. (Azidophenyl)alkyl substitutions, of the general structure N3-C6H4(CH2)nCO (n = 0, 1, 2, or 3), were employed in position 1. The seven new analogues are 4-N3-C6H4CO-D-Tyr(Me)-Phe-Gln-Asn-Arg-Pro-Arg-Tyr-NH2 (3), 3-N3-C6H4CO-D-Tyr(Me)-Phe-Gln-Asn-Arg-Pro-Arg-Tyr-NH2 (12), 4-N3-C6H4CH2-CO-D-Tyr(Me)-Phe-Gln-Asn-Arg-Pro-Arg-Tyr-NH2 (13), 3-N3-C6H4CH2CO-D-Tyr(Me)-Phe-Gln-Asn-Arg-Pro-Arg-Tyr-NH2 (14), 4-N3-C6H4(CH2)2CO-D-Tyr(Me)-Phe-Gln-Asn-Arg-Pro-Arg-Tyr-NH2 (15), 3-N3-C6H4(CH2)2CO-D-Tyr(Me)-Phe-Gln-Asn-Arg-Pro-Arg-Tyr-NH2 (16), 4-N3-C6H4-(CH2)3CO-D-Tyr(Me)-Phe-Gln-Asn-Arg-Pro-Arg-Tyr-NH2 (17). All analogues were tested for their affinity of the rat hepatic V1a receptor. Analogues 3 and 12 have a low affinity (Ki approximately 20 nM) and analogues 13-17 show a high affinity (Ki between 0.04 and 0.3 nM). The affinity values appear to be mainly a function of the alkyl chain length and to a lesser extent of the meta or para position of the azido group on the aromatic ring. Analogues 13-17 were iodinated on the Tyr-9 residue, giving compounds 18-22. All these five iodinated derivatives exhibited Ki values of 0.2-1 nM for rat liver membranes. Their affinities for oxytocin and renal V2 vasopressin receptors were much lower. Moreover, all analogues completely antagonized the vasopressin-stimulated inositol phosphates production in WRK1 cells and were devoided of any agonistic potency. Preliminary covalent binding studies showed improved covalent yields as compared to any previously reported results. They are very promising candidates as potential high-affinity, highly selective, photosensitive ligands for the V1a receptor. They could serve as a useful pharmacological tools for studies on the vasopressin binding site.

Design, synthesis and pharmacological characterization of a potent radioiodinated and photoactivatable peptidic oxytocin antagonist.

Using a segment strategy, we have synthesized four iodinated photoactivatable cyclic peptidic ligands of oxytocin, bearing a beta-mercapto-betabeta-cyclopentamethylene propionic group (Pmp) on their N-terminus. All the syntheses were RP-HPLC monitored, and the compounds were HPLC purified. They were characterized by 1H NMR, MALDI-TOF, or FAB mass spectrometries. The affinities of Pmp-Tyr(Me)-Ile-Thr-Asn-Cys-Gly-Orn-Phe(3I,4N3)-NH2 (20), Pmp-Tyr-Ile-Thr-Asn-Cys-Gly-Orn-Phe(3I,4N3)-NH2 (21), Pmp-Tyr(Me)-Ile-Thr-Asn-Cys-Pro-Orn-Phe(3I,4N3)-NH2 (22), and Pmp-Tyr-Ile-Thr-Asn-Cys-Pro-Orn-Phe(3I,4N3)-NH2 (23) were evaluated as inhibition constants (K(i), in nM) for the human oxytocin receptor expressed in Chinese hamster ovary cells by displacement of a radioiodinated disulfide-cyclized antagonist (Elands et al. Eur. J. Pharmacol. 1987, 147, 197-207). The most potent of them, compound 22, was synthesized by another method in order to allow its radiolabeling by 125I. Its dissociation constant (K(d)) for the human oxytocin receptor, directly measured in saturation studies, was 0.25 +/- 0.04 nM, and its antagonist properties were determined by inactivation of phospholipase C, thus obtaining an inactivation constant (K(inact)) of 0.18 +/- 0.02 nM, evaluated by inositol phosphate accumulation. This compound is a very good tool for the mapping of peptidic antagonist binding sites in the human oxytocin receptor.

Fluorescent pseudo-peptide linear vasopressin antagonists: design, synthesis, and applications.

Fluoresceinyl and rhodamyl groups have been coupled by an amide link to side-chain amino groups at positions 1, 6, and 8 of pseudo-peptide linear vasopressin antagonists (Manning et al. Int. J. Pept. Protein Res. 1992, 40, 261-267) through different positions on the fluorophore, to give tetraethylrhodamyl-DTyr(Me)-Phe-Gln-Asn-Arg-Pro-Arg-Tyr-NH2 (2), 4-HOPh(CH2)2CO-DTyr(Me)-Phe-Gln-Asn-Lys(5-carboxyfl uoresceinyl)-Pro-A rg-NH2 (4), 4-HOPh(CH2)2CO-DTyr(Me)-Phe-Gln-Asn-Lys(5- or 6-carboxytetramethylrhodamyl)-Pro-Arg-NH2 (5, 6), 4-HOPh(CH2)2CO-DTyr(Me)-Phe-Gln-Asn-Arg-Pro-Lys(5- or 6- carboxyfluoresceinyl)-NH2 (8, 9), and 4-HOPh(CH2)2CO-DTyr(Me)-Phe-Gln-Asn-Arg-Pro-Lys(5- or 6- carboxytetramethylrhodamyl)-NH2 (10, 11). The closer to the C-terminus the fluorophore, the higher the affinities of the fluorescent derivatives for the human vasopressin V1a receptor transfected in CHO cells. The compound 10 has a Ki of 70 pM, as determined by competition experiments with [125I]-4-HOPhCH2CO-DTyr(Me)-Phe-Gln-Asn-Arg-Pro-Arg-NH2. It showed a good selectivity for human V1a receptor versus human OT (Ki = 1.2 nM), human vasopressin V1b (Ki approximately 27 nM), and human vasopressin V2 (Ki > 5000 nM) receptor subtypes. All fluorescent analogues were antagonists as shown by the inhibition of vasopressin induced inositol phosphate accumulation. These fluorescent ligands are efficient for labeling cells expressing the human V1a receptor subtype, as shown by flow cytofluorometric experiments or fluorescence microscopy. They are also appropriate tools for structural analysis of the vasopressin receptors by fluorescence.

Visualization of cell surface vasopressin V1a receptors in rat hepatocytes with a fluorescent linear antagonist.

To visualize cell surface V1a vasopressin receptors in rat hepatocytes in the absence of receptor-mediated endocytosis, we used a high-affinity fluorescent linear antagonist, Rhm8-PVA. Epifluorescence microscopy (3CCD camera) and fluorescence spectroscopy were used. Rhm8-PVA alone did not stimulate Ca2+ signals and competitively blocked Ca2+ signals (Kinact of 3.0 nM) evoked by arginine vasopressin (vasopressin). When rat hepatocytes were incubated with 10 nM of Rhm8-PVA for 30 min at 4C, the fluorescent antagonist bound to the surface of cells, presumably the plasma membrane. The V1a receptor specificity of Rhm8-PVA binding was confirmed by its displacement by the nonfluorescent antagonist V4253 and by the natural hormone vasopressin at 4C. Prior vasopressin-mediated endocytosis of V1a receptors at 37C abolished binding of the labeled antagonist, whereas in non-preincubated cells, Rhm8-PVA labeled the cell surface of rat hepatocytes. When cells labeled with Rhm8-PVA at 4C were warmed to 37C to initiate receptor-mediated internalization of the fluorescent complex, Rhm8-PVA remained at the cell surface. Incubation temperature at 4C or 37C had little effect on binding of Rhm8-PVA. We conclude that Rhm8-PVA is unable to evoke receptor-mediated endocytosis and can readily be used to visualize cell surface receptors in living cells.

Distribution of signaling molecules involved in vasopressin-induced Ca2+ mobilization in rat hepatocyte multiplets.

In freshly isolated rat hepatocyte multiplets, Ca2+ signals in response to vasopressin are highly organized. In this study we used specific probes to visualize, by fluorescence and confocal microscopy, the main signaling molecules involved in vasopressin-mediated Ca2+ responses. V1a receptors were detected with a novel fluorescent antagonist, Rhm8-PVA. The Galphaq/Galpha11, PLCbeta3, PIP2, and InsP3 receptors were detected with specific antibodies. V1a vasopressin receptors and PIP2 were associated with the basolateral membrane and were not detected in the bile canalicular domain. Galphaq/Galpha11, PLCbeta3, and InsP3 receptors were associated with the basolateral membrane and also with other intracellular structures. We used double labeling, Western blotting, and drugs (cytochalasin D, colchicine) known to disorganize the cytoskeleton to demonstrate the partial co-localization of Galphaq/Galpha11 with F-actin.

The discovery of novel vasopressin V1b receptor ligands for pharmacological, functional and structural investigations.

Until recently, pharmacological studies dealing with vasopressin receptor isoforms were severely hampered by the lack of selective agonists or antagonists that recognize the pituitary V(1b) vasopressin receptor. By contrast, many selective vasopressin-related compounds are available for characterization of the vasopressor (V(1a)) or antidiuretic (V(2)) vasopressin receptor subtypes. Recently, SSR149415, a selective nonpeptide molecule, was discovered with nanomolar affinity for mammalian V(1b) receptors and good selectivity for the other vasopressin and oxytocin receptor isoforms. This molecule exhibits potent antagonist properties both in vitro and in vivo. We also designed synthetic peptides derived from [deaminocysteine(1),arginine(8)]vasopressin (dAVP), modified in position 4 by various amino acid residues. Some of these, d[cyclohexylalanine(4)]AVP or d[lysine(4)]AVP, have a high affinity and an excellent selectivity for the human V(1b) receptor subtype. However, they exhibit a mixed V(1b)/V(2) pharmacological profile for the rat vasopressin receptor isoforms. Whatever the species considered, these peptides behave as agonists both in bioassays performed in vitro and in vivo. The d[cyclohexylalanine(4)]AVP was tritiated and represents the first selective radiolabelled ligand available for studying the human V(1b) receptors. The discovery of these new selective V(1b) agonists and V(1b) antagonist allows an accurate pharmacological characterization of all the vasopressin receptor isoforms. As emphasized in this review, attention to the vasopressin and oxytocin receptor species differences is of critical importance in studies with all vasopressin and oxytocin ligands.

[Production and characterization of human antirenin antibodies prepared with synthetic immunogens]. / Production et caractérisation d'anticorps antirénine humaine préparés avec des immunogènes de synthèse.

Specific blockade of renin angiotensin system can be obtained both by enzymatic inhibitors and by passive and active immunization against renin. Recent studies have shown that synthetic peptides mimicking a protein segment can be used as immunogens to elicit antibodies which react with the parent molecule. In order to develop synthetic antirenin antigens we have selected peptidic sequences from human active renin and synthesized the corresponding peptides to produce antibodies able to recognize the entire human molecule and to inhibit its enzymatic activity. Three dimensional models of human renin were used to predict 17 putative epitopes. Then 18 peptides related to 13 potential antigenic determinants were synthesized by solid or liquid phase technic and 11 were shown to be antigenic when tested by their binding to several polyclonal and monoclonal human renin antibodies. The peptides were injected into rabbits and antisera tested by radio-immunoassay (RIA) and enzyme linked immunosorbent assay (ELISA) showed that nine peptides were immunogenic. Mainly, antiserum raised against the peptide mimicking the beta-hairpin 81-50 of active human renin which lies across the catalytic cleft, produced a 25 p. 100 inhibition of plasma renin activity at a 1: 50 dilution. Immunoglobulins, purified from antibodies raised against this epitopes, bound labelled renin and inhibited enzymatic activity of pure human renin on its synthetic tetradecapeptide substrate, in a dose dependent manner.

Renin inhibition by synthetic peptides related to mouse and human renin prosegments.

Recently the nucleotide sequences of the genes coding for mouse and human renins have been elucidated and the primary structures of the corresponding renin precursors deduced. The existence of a prosegment suggests that renin is biosynthesized as an inactive zymogen prorenin. Three peptides related to the region 11-19 of the mouse submaxillary prosegment have been synthesized and tested as inhibitors of pure mouse renin acting on a synthetic porcine substrate. Inhibition of renin activity was effective, with IC50s in the 10(-6) M range. Butyloxycarbonyl-leucyl-lysyl-lysyl-methionyl-proline methyl ester (15-19) was the most potent inhibitor with a K value of 2.3 X 10(-6) M at 37 degrees C in 0.5 M citrate/phosphate buffer (pH 6.0). Peptides 16-20 and 9-20 of the human renin prosegment and peptides 11-19 and 15-19 of the mouse prosegment were tested on human plasma renin activity and found to be inhibitory, with IC50s of 3 to 5 X 10(-4) M. These results demonstrate that the renin prosegment is a renin inhibitor and support the hypothesis that prorenin is an inactive zymogen.

Production and characterization of human renin antibodies prepared with synthetic immunogens.

Renin is an aspartyl protease that plays a key role in regulating blood pressure. The aim of this study was to use synthetic peptides to produce antibodies able to recognize the entire human renin molecule and to inhibit its enzymatic activity. Two three-dimensional renin models were used to predict 17 putative epitopes. Then 18 peptides related to 13 potential antigenic determinants were synthesized by solid- or liquid-phase technique, and 11 were shown to be antigenic when tested by their binding to several polyclonal human renin antibodies. The peptides were injected into rabbits, and antisera tested by radio-immunoassay (RIA) and enzyme-linked immunosorbent assay (ELISA) showed that nine peptides were immunogenic. The peptide related to the flap region which lies across the renin catalytic cleft showed potentially useful characteristics. Immunoglobulin G, purified from antibodies raised against this epitope, bound labelled renin and inhibited its enzymatic activity in a dose-dependent manner. This approach constitutes the basis for the development of a synthetic antirenin vaccine able to inhibit the renin-angiotensin system specifically.

Further studies into the Boc/solid-phase synthesis of Ser(P)- and Thr(P)-containing peptides.

The Ser(P)-containing peptide corresponding to phospholamban 11-19, Ac-Ala-Ile-Arg-Arg-Ala-Ser(P)-Thr-Ile-Glu-NH2, was prepared by the use of Boc-Ser(PO3Ph2)-OH in Boc/solid-phase peptide synthesis followed by HF cleavage of the peptide from the polystyrene resin and subsequent platinum-mediated hydrogenolytic cleavage of the phenyl phosphate groups. A study of the HF deprotection step showed that extensive dephosphorylation of the Ser(PO3Ph2)-residue occurred using three commonly used HF conditions and gave rise to large quantities of the Ser-containing peptide. The subsequent study of model peptide systems under standard HF conditions established firstly that the extent of dephosphorylation was dependent on the HF-contact time, and secondly that the Ser(PO3Ph2) residue underwent dephosphorylation at a slightly higher rate than the Thr(PO3Ph2) residue.

[Preparation of antisera against some sequences of somatostatin synthetized on resin. Application to the immunological detection of somatostatin systems: preliminary results (author's transl)]. / Obtention d'antisérums dirigés contre certaines séquences de la somatostatine synthétisées en phase solide, et application à la détection immunocytologique des systèmes à somatostatine: résultats préliminaires.

Specific SRIF(1-14) fragments were synthetized on resin using conventional procedures. Rabbits received subcutaneously peptidyl resins in complete Freund adjuvant emulsion. The presence of antibodies was assessed by immunocytochemical and radioimmunological assays. 1. Peptidyl resins lead to antibodies production; their specificity depends on sequence and molecular configuration of the peptide on the resin. Anti-resin antibodies were not detected. 2. In the brain, SRIF(1-4) (in rat) and SRIF(10-13) (in garden-dormouse) can be demonstrated in neurophysins--positive cells of both paraventricular and supraoptic nucleus, but never in hypothalamic or extrahypothalamic SRIF(1-14)--positive neurophysin negative cells. 3. Endocrine cells of pancreatic islets contain SRIF(6-9) (in man) or SRIF(10-13) (in rat, mouse, garden-dormouse); generally, these cells are not detected by SRIF(1-14) anti-serum. Moreover, SRIF(10-13) positive cells are also detected by specific glucagon antibodies. However, it cannot be concluded that SRIF(10-13) antibodies reveal the common Thr-Phe-Thr-Ser fragment in the entire glucagon molecule. It is postulated that antibodies to several SRIF tetrapeptides reveal molecular fragments provided by the functional cleavage of an hypothetical prohormone or by the inactivation of SRIF(1-14) molecule in target cells.

Docking of linear peptide antagonists into the human V(1a) vasopressin receptor. Identification of binding domains by photoaffinity labeling.

A novel photoactivatable linear peptide antagonist selective for the V(1a) vasopressin receptor, [(125)I][Lys(3N(3) Phpa)(8)]HO-LVA, was synthesized, characterized, and used to photolabel the human receptor expressed in Chinese hamster ovary cells. Two specific glycosylated protein species at 85-90 and 46 kDa were covalently labeled, a result identical to that obtained with a previous photosensitive ligand, [(125)I]3N(3)Phpa-LVA (Phalipou, S., Cotte, N. , Carnazzi, E., Seyer, R., Mahe, E., Jard, S., Barberis, C., and Mouillac, B. (1997) J. Biol. Chem. 272, 26536-26544). To identify contact sites between the new photoreactive analogue and the V(1a) receptor, the labeled receptors were digested with Lys-C or Asp-N endoproteinases and chemically cleaved with CNBr. Fragmentation with CNBr, Lyc-C, and Asp-N used alone or in combination, led to the identification of a restricted receptor region spanning the first extracellular loop. The results established that sequence Asp(112)-Pro(120) could be considered as the smallest covalently labeled fragment with [(125)I][Lys(3N(3)Phpa)(8)]HO-LVA. Based on the present experimental result and on previous photoaffinity labeling data obtained with [(125)I]3N(3)Phpa-LVA (covalent attachment to transmembrane domain VII), three-dimensional models of the antagonist-bound receptors were constructed and then verified by site-directed mutagenesis studies. Strikingly, these two linear peptide antagonists, when bound to the V(1a) receptor, could adopt a pseudocyclic conformation similar to that of the cyclic agonists. Despite divergent functional properties, these peptide antagonists could interact with a transmembrane-binding site significantly overlapping that of the natural hormone vasopressin.

Efficient photoaffinity labeling of the rat V1a vasopressin receptor using a linear azidopeptidic antagonist.

We have synthesized and fully characterized by fast-atom-bombardment-mass, NMR and ultraviolet spectroscopies the vasopressin antagonist 3-azidophenylpropionyl-D-Tyr(Me)-Phe-Gln-Asn-Arg-Pro-Arg-Tyr(3I )-NH2. Easily radioiodinatable just before use, it has a high affinity for the natural rat liver V1a receptor [dissociation constant (Kd) = 54 +/- 20 pM; Carnazzi, E., Aumelas, A., Barberis, C., Guillon, G. & Seyer, R. (1994) J. Med. Chem. 37, 1841-1849] and for both the rat vasopressin V1a receptor expressed in Spodoptera frugiperda 9 cells (Sf9 cells, Kd = 688 +/- 35 pM) and in COS-7 cells (Kd = 320 +/- 20 pM). This probe labels specifically the V1a receptors in an ultraviolet-dependent manner, and binds covalently to about 12% of the receptors with high stability over several days, even in dissociation or solubilization conditions. SDS/PAGE studies and autoradiographic analyses of the photolabeled receptors reveal a single band (49.5 kDa) and two bands (63 kDa and 93.6 kDa) for receptor-probe associations obtained in Sf9 and COS-7 cells respectively. These molecular masses are consistent with non-glycosylated and highly glycosylated forms of the receptor, according to each expression system. In rat liver membranes, we have identified apparent molecular masses of about 32, 45 and more than 67 kDa. We finally demonstrated a proteolysis of the receptor that appeared to be Zn2+ and leupeptin sensitive. The high potency of this ligand is promising for the monitoring of the purification of the V1a receptor and for mapping its antagonist-binding site.

Solution synthesis of antigenic and inhibiting peptides of the human renin prosegment-1. [Tyr47, Nle53] preprorenin-(47-60) peptide methyl ester.

The [Tyr47, Nle53] preprorenin (47-60) peptide methyl ester, a tetradecapeptide related to the human renin prosegment, has been synthesized using a three-segment coupling strategy. Selective deprotection of the segments before coupling allowed an easy removal of the final tetradecapeptide side chain-protecting groups by acidolysis and an easy purification. Antibodies raised against this peptide bound the plasmatic inactive renin.

Solution synthesis of antigenic and inhibiting peptides of the human renin prosegment-2. [Tyr40] preprorenin-(40-50) peptide methyl ester.

The [Tyr40] preprorenin (40-50) peptide methyl ester, an undecapeptide related to the human renin prosegment, has been synthesized using a stepwise strategy with hydrogenolisable protections on the side chains. The final deprotection was very difficult as observed by 1H NMR and reversed phase HPLC. 2D 1H NMR spectroscopy of the purified peptide allowed the assignment of all protons.

Iodination of vasopressin analogues with agonistic and antagonistic properties: effects on biological properties and affinity for vascular and renal vasopressin receptors.

Twelve L- and D-tyrosine-containing vasopressin analogues were prepared in their mono- and diiodinated forms. These include six arginine vasopressin (AVP) vascular (V1) type antagonists/antidiuretic (V2) agonists, four V1/V2 antagonists, and two V1/V2 agonists, one of which is AVP itself. Ten peptides were iodinated on the tyrosyl residue in position 2; two were iodinated on a tyrosyl amide residue replacing the glycyl amide residue at position 9. All peptides were tested both for their biological activities in vivo (rat vasopressor and antidiuretic tests) and for their ability to bind to vasopressin receptors of the V1 (vascular) and V2 (renal) types from rat liver and rat kidney membranes, respectively. It is shown that monoiodination of the tyrosyl residue in the vasopressin analogues that were tested either preserves or reduces to a highly variable extent the in vivo and in vitro biological activities of these analogues. In most cases diiodonitation resulted in a marked decrease in biological activity. The effects of iodination on the affinity of vasopressin analogues for hepatic V1 receptors and renal V2 receptors were more related to the affinity of the noniodinated peptide for these receptors than to the biological properties (antagonist versus agonist) of the tested analogues, the nature (L versus D) of the iodinated tyrosyl residue, or the position (2 versus 9) at which this residue was introduced. The loss of affinity due to iodination was usually more pronounced for peptides exhibiting high affinity for vasopressin receptors. However, we show that among the monoiodinated peptides some (especially monoiodinated [2-D-Tyrosine]-AVP) retained enough affinity for vasopressin binding sites to suggest that their radioiodinated conterparts would be promising labeled ligands for use in studies in vasopressin receptors.

Characterization of the angiotensin II receptor.

We present a brief overview of the present knowledge on the structural and molecular properties of angiotensin II receptors and the various attempts to determine their primary structures, with special reference to our strategy for receptor purification. The strategy involves covalent labeling of the receptor with synthetic biotinylated photoactivatable probes, followed by indirect affinity chromatography on immobilized streptavidin. The various applications of these probes to the study of structural and molecular properties and to the cell biology of angiotensin II receptors are discussed.

Repetitive BOP coupling (REBOP) in solid phase peptide synthesis. Luliberin synthesis as model.

The coupling reagent (benzotriazol-1-yloxy)tris-(dimethylamino)phosphonium (BOP) hexafluorophosphate was tested in the synthesis of luliberin (LH-RH) with inexpensive classically protected Boc-amino acids, in slight excess, and benzhydryl amino resin, without any other additive. The good solubility of this reagent and its by-products is of particular interest for automated peptide synthesis. [D-His2]LH-RH was also synthesized and compared with LH-RH by proton nuclear magnetic resonance spectroscopy. As shown by the biological tests and the high performance liquid chromatography study, unprotected pyroGlu and Boc-His can be used without any significant racemization but Boc-His(Boc) was found to be preferable since it gave no detectable racemization and no by-products. The difficult isolation of the minority D-derivative from the crude preparation of LH-RH was resolved by a recycling procedure in reversed phase HPLC.