Analysis of peptide synthesis products by electrospray ionization mass spectrometry.

Electrospray ionization mass spectrometry is an easy, rapid method for the verification of proper peptide synthesis and for the identification of most synthetic by-products. A synthesis-purification scheme has been described that uses mass analysis to (1) confirm the presence of the proper product in the crude peptide mixture, (2) guide the purification process, and (3) confirm the mass and purity of the final product. Even though many of these steps could be performed just as well with other ionization techniques, the liquid-flow characteristics of electrospray source are clearly an advantage when LC-MS is required. In addition, the ease with which fragment ions can be generated to provide structural information, even with the least sophisticated instruments, is a further advantage of ESI-MS. Although much of the operation described here was done manually, many of the steps could be automated with little additional effort (e.g., use of an autosampler). Quadrupole and ion trap instruments are widely available at present and provide the chemist with a variety of instruments from which to choose. Electrospray time-of-flight instruments will be commercially have just become available and should also provide similar results. As electrospray instruments continue to evolve, the instruments display greater performance and enhanced user-friendly interfaces, yet are lower in price and smaller in size. These features should lead to even more widespread use for the characterization of synthetic peptides.

Cyclic RGD peptide analogues as antiplatelet antithrombotics.

Stimulation of platelets activates GPIIbIIIa, the heterodimeric integrin receptor, to bind fibrinogen (Fg), which results in platelet aggregation. GPIIbIIIa/Fg binding inhibitors are potentially suitable for acute use during and after thrombolytic therapy as antithrombotic agents. Incorporation of the tripeptide sequence Arg-Gly-Asp (RGD), a common structural element of many integrin ligands, into cyclic peptides produced a series of peptides of the general structure BrAc-(AA1)-RGD-Cys-OH, which were prepared by solid-phase peptide synthesis. Cyclization was accomplished by reaction of the N-terminal bromoacetyl group with the cysteine sulfhydryl at pH 8 at high dilution, resulting in thioether-bridged cyclic peptides [cyclo-S-Ac-(AA1)-RGD-Cys-OH]. Use of alpha-substituted bromoacetyl groups gave rise to an analogous series of acetyl-substituted thioether-bridged cyclic peptides. Oxidation of the thioethers produced separable diastereomeric sulfoxide-bridged cyclic peptides. After thorough evaluation in a GPIIbIIIa ELISA assay and a platelet aggregation assay, G-4120 (70A; AA1 = D-Tyr; sulfoxide bridge) was selected for further investigation as an antithrombotic agent. G-4120 was equipotent in the platelet aggregation assay to kistrin, a highly potent inhibitor of fibrinogen-mediated platelet aggregation isolated from snake venom (IC50 = 0.15 microM).

Structural determinants of calcium signaling by RGD peptides in rat osteoclasts: integrin-dependent and -independent actions.

Extensive characterization of the vitronectin receptor (VNR), a member of the integrin group of cell adhesion molecules, which is abundantly expressed in osteoclasts, has revealed a role for this receptor in osteoclast adhesion as well as bone resorption. Earlier evidence from our laboratory suggests that VNR is also capable of transducing intracellular signals following receptor ligand interaction, although this function is poorly understood. Thus, addition of peptides containing the minimal tripeptide Arg-Gly-Asp (RGD) integrin recognition sequence elicits transient increases in intracellular free calcium ions, with maximal responses seen with a bone sialoprotein peptide, BSP-IIA. In the present study we have attempted to determine some of the structural requirements for calcium signaling in osteoclasts using derivatives of the peptide PRGDN/T sequence found in bone sialoprotein. While some peptides, such as the parent sequence PRGDN, can induce both signaling and retractile events, it was found that minor structural modifications yielded peptides such as PRADN which elicited a transient increase in intracellular free calcium ions without promoting a reduction in osteoclast spread area (retraction). Conversely, certain other modifications resulted in peptides, such as PrGDN and benzoyl-RGDN, which effect osteoclast retraction, while having minimal Ca2+ signaling capabilities. Osteoclast adhesion, and hence retraction, are known to be RGD-dependent and integrin-dependent events. However, intracellular Ca2+ signaling is RGD-independent and, based on lack of inhibition by an anti-beta 3 integrin antibody F11 and echistatin, very likely integrin-independent. These data suggest that signaling is not always via VNR and as yet unknown receptors on the osteoclast membrane play a role in osteoclast signaling and hence function.

Benzodiazepine peptidomimetic inhibitors of farnesyltransferase.

A structural survey of protein Zn2+ binding geometries was instigated based upon the functional requirement of Ras farnesyltransferase for Zn2+. The Cys-X-X-Cys motif found in Zn(2+)-binding proteins such as aspartate transcarbamylase was used as a template to devise a bidentate-coordination model for Cys-A1-A2-X peptide inhibitors. Accordingly, replacement of the central dipeptide with the hydrophobic scaffold 3-amino-1-carboxymethyl-2,3-dihydro-5- phenyl-1H-1,4-benzodiazepin-2-one (BZA) yielded a peptidomimetic inhibitor, Cys(BZA)Met, of moderate potency (IC50 = 400 nM). N-Methylation of the cysteine amide improved potency almost 100-fold (IC50 = 0.3-1 nM). The increased affinity presumably correlates with a preferred conformation of the inhibitor which maximizes a hydrophobic interaction between the scaffold and the enzyme, and the proper presentation of cysteine and methionine to allow bidentate coordination at Zn2+. These non-peptide inhibitors have been shown to block farnesylation of the Ras protein in intact cells and provide lead compounds for the development of new cancer therapeutic agents.

Growth hormone secretagogues: characterization, efficacy, and minimal bioactive conformation.

Another class of growth hormone (GH) secretagogues has been discovered by altering the backbone structure of a flexible linear GH-releasing peptide (GHRP). In vitro and in vivo characterization confirms these GH secretagogues as the most potent and smallest (M(r) < 500) reported. Anabolic efficacy is demonstrated in rodents with intermittent delivery. A convergent model of the bioactive conformation of GHRPs is developed and is supported by the NMR structure of a highly potent cyclic analog of GHRP-2. The model and functional data provide a logical framework for the further design of low-molecular weight secretagogues and illustrate the utility of an interdisciplinary approach to elucidating potential bound-state conformations of flexible peptide ligands.

Generation of an LFA-1 antagonist by the transfer of the ICAM-1 immunoregulatory epitope to a small molecule.

The protein-protein interaction between leukocyte functional antigen-1 (LFA-1) and intercellular adhesion molecule-1 (ICAM-1) is critical to lymphocyte and immune system function. Here, we report on the transfer of the contiguous, nonlinear epitope of ICAM-1, responsible for its association with LFA-1, to a small-molecule framework. These LFA-1 antagonists bound LFA-1, blocked binding of ICAM-1, and inhibited a mixed lymphocyte reaction (MLR) with potency significantly greater than that of cyclosporine A. Furthermore, in comparison to an antibody to LFA-1, they exhibited significant anti-inflammatory effects in vivo. These results demonstrate the utility of small-molecule mimics of nonlinear protein epitopes and the protein epitopes themselves as leads in the identification of novel pharmaceutical agents.