Synthesis and enzymic hydrolysis of cyclic peptides containing an anthranilic acid residue

Anthranilic acid-containing cyclic tetrapeptides: at the crossroads of conformational rigidity and synthetic accessibility

Each amino acid in a peptide contributes three atom units to main-chains, hence natural cyclic peptides can be 9, 12, 15, …. i.e. 3n membered-rings, where n is the number of amino acids. Cyclic peptides that are 9 or 12-membered ring compounds tend to be hard to prepare because of strain, while their one amino acid homologs (15-membered cyclic pentapeptides) are not conformationally homogeneous unless constrained by strategically placed proline or d-amino acid residues. We hypothesized that replacing one genetically encoded amino acid in a cyclic tetrapeptide with a rigid β-amino acid would render peptidomimetic designs that rest at a useful crossroads between synthetic accessibility and conformational rigidity. Thus this research explored non-proline containing 13-membered ring peptides 1 featuring one anthranilic acid (Anth) residue. Twelve cyclic peptides of this type were prepared, and in doing so the viability of both solution- and solid-phase methods was demonstrated. The library produced contained a complete set of four diastereoisomers of the sequence 1aaf (i.e. cyclo-AlaAlaPheAnth). Without exception, these four diastereoisomers each adopted one predominant conformation in solution; basically these conformations feature amide N-H vectors puckering above and below the equatorial plane, and approximately oriented their N-H[combining low line] atoms towards the polar axis. Moreover, the shapes of these conformers varied in a logical and predictable way (NOE, temperature coefficient, D/H exchange, circular dichroism). Comparisons were made of the side-chain orientations presented by compounds 1aaa in solution with ideal secondary structures and protein-protein interaction interfaces. Various 1aaa stereoisomers in solution present side-chains in similar orientations to regular and inverse γ-turns, and to the most common β-turns (types I and II). Consistent with this, compounds 1aaa have a tendency to mimic various turns and bends at protein-protein interfaces. Finally, proteolytic- and hydrolytic stabilities of the compounds at different pHs indicate they are robust relative to related linear peptides, and rates of permeability through an artificial membrane indicate their structures are conducive to cell permeability.

A novel cyclic octapeptide as an analogue of cyclic peptide in triostin A

This paper reported an ongoing study of cyclic peptides as carriers of potential anti-tumor agents. In an effort to carry out anti-cancer drug design, we synthesized another novel cyclic peptide as the analogue of the cyclic peptide in Triostin A. The linear peptide chains were synthesized by coupling protected amino acid residues according to Pfp/DCC methods (Pfp: Pentafluorophenol, DCC: N,N'-Dicyclohexyl-carbodiimide) in solution. After deblocking the Boc- group of the linear octapeptide chain, the cyclic product was achieved by employing diphenylphosphoryl azide (DPPA) as cyclic agent at low temperature in DMF. Further study on cyclic octapeptide-drug conjugates is in progress.

A cyclopeptidic suicide substrate preferentially inactivates urokinase-type plasminogen activator

c[Arg-aB-(CH2+SCH3 phi)-Gly4] was designed and studied as a mechanism-based inactivator (suicide substrate) for plasminogen activators (u-PA and t-PA) and plasmin. This compound inhibited u-PA and fulfills criteria expected for the involvement of an enzyme-activated inhibitor: first-order and irreversible process, saturation kinetics, protection by substrate. The limiting first-order rate constant kinact and the apparent enzyme-inhibitor dissociation constant KI were 0.021 s-1 and 9 microM, respectively at pH 7.5 and 25 degrees C. The activation of plasminogen by u-PA is compromised after this enzyme has been treated by the reagent. Plasmin and t-PA were inactivated 40- and 2330-fold less efficiently than u-PA, respectively.

Synthesis of a cyclic hexapeptide with sequence corresponding to murine tumor necrosis factor-(127-132) as a novel potential antitumor agent

A cyclic hexapeptide cyclo(Lys-Gly-Asp-Gln-Leu-Ser-) 10 was synthesized stepwise in solution by acylation of peptide ester trifluoroacetates directly with preactivated Boc-amino acids using the DCC/HOBt method; the final cyclization reaction was performed using the pentafluorophenyl ester method in solution (1-4). This peptide is a cyclic derivative of murine tumor necrosis factor-(127-132) and is designed as a potential antitumor agent. The cyclic peptide 10 displayed weak cytotoxic activity on three of the four human tumor cell lines tested.

Reaction of thrombin and proteinases of the fibrinolytic system with a mechanism-based inhibitor, 3,4-dihydro-3-benzyl-6-chloromethylcoumarin

Thrombin, plasmin and tissue plasminogen activator (one- and two-chain forms) were examined with respect to their reaction with the suicide substrate, 3,4-dihydro-3-benzyl-6-chloromethylcoumarin, at 4 degrees C. The enzymes were irreversibly inhibited and the apparent second-order rate constants ki/Ki were 31,000, 316, 187 and 250 M-1.s-1, respectively. The extent of fibrin clot lysis induced by urokinase and two-chain tissue plasminogen activator was considerably decreased after treatment of these enzymes with the dihydrocoumarin derivative (molar excess of inhibitor over enzyme ranging from 6 to 21 for urokinase and 50 to 1500 for tissue plasminogen activator). This inhibitor has been tested as anticoagulant in human plasma and was effective at prolonging the prothrombin time from 12 to 40 s.

Synthesis of cyclic peptide homologs of glutathione as potential antitumor agents

Two cyclic tripeptide homologs, cyclo(Glu[Cys-beta-Ala-]-OH) 8a, and cyclo(Glu[Cys-Gaba-]-OH) 8b, were synthesized by the pentafluorophenyl ester method in solution. These cyclic peptides are cyclo homologs of glutathione and are designed as potential antitumor agents. The 1H- and 13C-n.m.r. spectral parameters of cyclo(Glu[Cys(Bzl)-beta-Ala-]-OH) 7a were measured in DMSO-d6 and a possible conformation has been proposed. The cyclic peptide 8a showed low cytotoxic activities against three human tumor cell lines: KB, HeLa, and Colo 205.

Susceptibility of plasminogen activators to suicide inactivation

Halomethylated derivatives of dihydrocoumarins are efficient enzyme-activated inhibitors ("suicide" substrates) of plasminogen activators. Kinetic analysis indicate that the one-chain and two-chain forms of the human plasminogen activator are inhibited by 3,4-dihydro-3-benzyl-6-chloromethylcoumarin through a mechanism-based inactivation characterized by the following kinetic parameters (4 degrees C, pH 6.8) : k2 equal to 0.02 s-1 and 0.03 s-1 (for one- and two-chain tissue plasminogen activators, respectively) and Ki equal to 0.16 mM for both forms. Human urokinase and human tissue-type plasminogen activator can be discriminated on the basis of their inhibition by this suicide substrate. The design of a new series of suicide substrates of serine proteases (functionalized cyclopeptides possessing a potential alkylating function closely related to that found in halomethylated derivatives of dihydrocoumarins) is described.