Cyclization of disulfide-containing peptides in solid-phase synthesis

Identification of bioneutralization epitopes of human follicle stimulating hormone in the regions 31-52 and 66-75 of its beta-subunit

The crucial role played by follicle stimulating hormone (FSH) in regulating both male and female reproduction and the possibilities of developing contraceptive methods for males by blocking the function of the hormone, makes it important to delineate the hormone-specific bioneutralization epitopes of human follicle stimulating hormone (hFSH) on its beta-subunit. Predictive methods were used to identify the potential surface-oriented regions of hFSH-beta. Peptides corresponding to these regions, i.e. 31-52, 66-75 and 86-95 hFSH-beta, were synthesized, anti-peptide antibodies were elicited in rabbits and the properties of these antisera to bind hFSH and neutralize its biological activity were assessed. Anti-31-52 hFSH-beta antisera bound hFSH specifically, whereas anti-66-75 and anti-86-95 hFSH-beta antisera did not show any detectable binding, proving the region 31-52 hFSH-beta to be a specific antigenic determinant of hFSH. The bioneutralizing abilities of the anti-peptide antibodies were assessed by measuring the hFSH-induced progesterone secretion by rat granulosa cells in vitro. Antibodies to 31-52 and 66-75 hFSH-beta neutralized the bioactivity of hFSH, but anti-86-95 hFSH-beta antibodies did not. Furthermore, the three linear peptides and two disulphide looped peptides of 31-52 hFSH-beta and 86-95 hFSH-beta were also subjected to the in-vitro granulosa cell assay. The linear peptides 31-52 hFSH-beta and 66-75 hFSH-beta and the cyclic 31-52 hFSH-beta disulphide loop peptide significantly inhibited the hFSH-induced progesterone secretion by rat granulosa cells, but the linear 86-95 hFSH-beta peptide and the corresponding cyclic disulphide loop peptide did not. The results clearly show that the regions 31-52 and 66-75 of hFSH-beta harbor bioneutralization epitopes of the hormone. The studies also indicate that cyclization of the linear 31-52 hFSH-beta peptide greatly enhances receptor recognition and that the region 66-75 hFSH-beta may also be involved in hormone-receptor interaction.

Positively charged cyclic hexapeptides, novel blockers for the cardiac sarcolemma Na(+)-Ca2+ exchanger

Positively charged cyclic hexapeptides have been synthesized and tested for their effects on the cardiac sarcolemma Na(+)-Ca2+ exchange activities with a goal to identify a potent blocker. The cyclic hexapeptides, having the different amino acid sequence, contain two arginines (to retain a positive charge), two phenylalanines (to control hydrophobicity), and two cysteines (to form an intramolecular S-S bond). The effect of cyclic hexapeptides were tested on Na(+)-Ca2+ exchange and its partial reaction, the Ca(2+)-Ca2+ exchange, by measuring the 45Ca fluxes in the semi-rapid mixer or monitoring the calcium-sensitive dye Arsenazo III and voltage-sensitive dyes (Oxanol-V or Merocyanine-540). Seven cyclic hexapeptides inhibit Na(+)-Ca2+ exchange with a different potency (IC50 = 2-300 microM). Phe-Arg-Cys-Arg-Cys-Phe-CONH2 (FRCRCFa) inhibits the Na+i-dependent 45Ca uptake (Na(+)-Ca2+ exchange) and Ca2+i-dependent 45Ca uptake (Ca(2+)-Ca2+ exchange) in the isolated cardiac sarcolemma vesicles with IC50 = 10 +/- 2 microM and IC50 = 7 +/- 3 microM, respectively. Interaction of FRCRCFa with a putative inhibitory site does not involve a "slow" binding (a maximal inhibitory effect is already observed after t = 1 s of mixing). The inside positive potential, generated by Na+o-dependent Ca2+ efflux, was monitored by Oxanol-V (A635-A612) or Merocyanine-540 (A570-A500). In both assay systems, FRCRCFa inhibits the Na(+)-Ca2+ exchange with IC50 = 2-3 microM, while a complete inhibition occurs at 20 microM FRCRCFa. The forward (Na+i-dependent Ca2+ influx) and reverse (Na+o-dependent Ca2+ efflux) modes of Na(+)-Ca2+ exchange, monitored by Arsenazo III (A600-A785), are also inhibited by FRCRCFa. The L-Arg4-->D-Arg4 substitution in FRCRCFa does not alter the IC50, meaning that this structural change may increase a proteolytic resistance without a loss of inhibitory potency. At fixed [Na+]i (160 mM) or [Ca2+]i (250 microM) and varying 45Cao (2-200 microM), FRCRCFa decreases Vmax without altering the Km. Therefore, FRCRCFa is a noncompetitive inhibitor in regard to extravesicular Ca2+ either for Na(+)-Ca2+ or Ca(2+)-Ca2+ exchange. It is suggested that FRCRCFa prevents the ion movements through the exchanger rather than the ion binding.

Solution versus solid-phase cyclization strategies for large sidechain lactam-bridged peptides: a comparative study

A 22-residue peptide with a sidechain lactam bridge involving 18 residues (60-atom cycle) has been synthesized. Three different protection schemes using Fmoc/tBu/cyclohexyl, Fmoc/tBu/allyl or Boc/Bzl/ fluorenylmethyl protecting group combinations have been explored for the solid phase of the linear precursors, which have been subsequently cyclized in solution or in the solid phase. Cyclization yields in solution have been consistently better than on solid phase; however, the solid-phase strategy requires fewer purification steps and therefore global yields are comparable.

Solid-phase synthesis of oxytocin using iodotrichlorosilane as Boc deprotecting agent

Deprotection of the tert-butoxycarbonyl group during solid-phase synthesis of peptides can be conveniently and efficiently carried out using a neutral reagent, silicon tetrachloride/sodium iodide (iodotrichlorosilane). This simple and rapid method has been advantageously employed during the solid-phase synthesis of the pituitary hormone, oxytocin.

Synthesis of a cyclic analogue of the C-loop region of epidermal growth factor, containing 1-aminocyclopropane-1-carboxylic acid

We report the synthesis of a cyclic analogue of epidermal growth factor sequence 33-42 with substitution of 1-aminocyclopropane-1-carboxylic acid for glycine at position 39 (N-acetyl-Cys-Val-Ile-Gly-Tyr-Ser-ACPCA-Asp-Arg-Cys-NH2). The analogue was synthesised by solid-phase methods, using t-Boc chemistry and acid-labile side-chain protecting groups. The use of the 4-methoxybenzyl protecting group for C- and N-terminal cysteine residues resulted in the spontaneous formation of the desired intramolecular disulfide bond after HF deprotection.

Simultaneous multiple synthesis and selective conjugation of cyclized peptides derived from a surface loop of a meningococcal class 1 outer membrane protein

Starting from the alpha-(2,4-dimethoxybenzyl) ester of N-(9-fluorenylmethoxycarbonyl)aspartic acid [Fmoc-Asp-ODmb], side-chain-protected resin-bound Fmoc-peptides containing an N epsilon-1-(4,4-dimethyl-2,6-dioxocyclohexylidene)ethyl lysyl [Lys(Dde)] residue were prepared. The C-terminal dimethoxybenzyl esters of aspartic acid were removed with 1% trifluoroacetic acid and 10% anisole in dichloromethane, followed by Fmoc-cleavage in the usual manner. The resin-bound peptides were then cyclized using 1-benzotriazolyloxy-tris-[N-pyrrolidino]phosphonium hexafluorophosphate (PyBOP) in the presence of N-methylmorpholine. The (dimethyldioxocyclohexylidene)ethyl groups of lysine were removed with 1% hydrazine hydrate in N,N-dimethylacetamide, and the liberated side-chain amino functions were modified by reaction with pentafluorophenyl S-acetylmercaptoacetate (SAMA-OPfp). Finally, the peptides were side-chain deprotected, with exception of the Lys(SAMA) residue, and cleaved from the solid support with trifluoroacetic acid/anisole/water, 95/2.5/2.5. Cyclic peptides comprising 7-14 amino acid residues were obtained employing this procedure. As a model conjugation, cyclo[Thr-Asn-Asn-Asn-Leu-Lys(SAMA)-Thr-Lys-Asp] was coupled with bromoacetamide. The same peptide was also coupled with a bromoacetylpeptide to give a well defined peptide/peptide conjugate. All peptides were conjugated to bromoacetylated tetanus toxoid for immunization purposes.