Peptide combinatorial library assay

It is important to further develop the concept of structure-activity relationships to precisely define the structural requirements of glutathione action. Thus, this section introduces the design, synthesis, and screening of a peptide combinatorial library to obtain multiple glutathione analogs. Combinatorial libraries will be composed of mixtures of peptides (consisting of natural or noncoded amino acids) on solid support. After cleavage from the resin, the mixtures of the peptides will be screened directly in different specific assays. 

Houghten RA, Dooley CT. The use of synthetic peptide combinatorial libraries for the determination of peptide ligands in radio-receptor assays opoid peptides. Bioorg. Med. Chem. Lett. 49. 1993 3 405-412. 

Some of the early examples of libraries yielding receptor agonists and antagonists were peptide-based. Houghten, Dooley and co-workers introduced synthetic peptide combinatorial libraries (SPCLs) in which defined mixtures of peptides were synthesized, cleaved from resin and assayed free in solution against either soluble (enzyme) or membrane associated (re-... 

Sompuram SR, Kodela V, Ramanathan H, et al. Synthetic peptides identified from phage-displayed combinatorial libraries as immunodiagnostic assay surrogate quality-control targets. Clin. Chem. 2002 48 410-420. 

In 1991, we first introduced the one-bead one-compound (OBOC ) combinatorial library method.1 Since then, it has been successfully applied to the identification of ligands for a large number of biological targets.2,3 Using well-established on-bead binding or functional assays, the OBOC method is highly efficient and practical. A random library of millions of beads can be rapidly screened in parallel for a specific acceptor molecule (receptor, antibody, enzyme, virus, etc.). The amount of acceptor needed is minute compared to solution phase assay in microtiter plates. The positive beads with active compounds are easily isolated and subjected to structural determination. For peptides that contain natural amino acids and have a free N-terminus, we routinely use an automatic protein sequencer with Edman chemistry, which converts each a-amino acid sequentially to its phenylthiohydantoin (PTH) derivatives, to determine the structure of peptide on the positive beads. 

A variation of the pharmacophore library was seen in an approach used to determine CD4+ T cell epitopes.31 In a 14-mer peptide, the researchers keep residues in positions 1, 4, and 6 constant since they functioned as anchor positions for binding receptor, and proceeded to vary the other residues in the peptide. This allowed the use of a shorter synthetic route compared to one that would be needed if all positions were to be varied. Limiting the size of the library allowed the authors to obtain better assays. By doing a partial release of the peptides, the authors were able to find the final peptide that represented the epitope of the T cell receptor. In another example, the epitope to inhibit stimulation of the thyrotropin receptor also was found via combinatorial libraries.32 Since the synthesis of a totally random hexapeptide library was deemed impractical, the authors opted to hold one position constant while the other five residues were randomized. This method was repeated for each residue in the peptide. The residues that were determined to be the most active were used as a basis for a second-generation library. The only limitation of the library was not the quantity of product synthesized, but to properly pinpoint the peptides in an assay. 

It is in the realm of very large combinatorial libraries that selection rather than screening gains crucial importance. As the focus shifts from randomizing an eight-residue peptide to a 100 amino acid protein (the typical size of a small functional domain, for example a chorismate mutase domain), the number of sequence permutations rises to an astronomical 20100. The ability to assay even a tiny fraction of this sequence space in directed molecular evolution experiments demands selection, even though initial development of an appropriate system may be considerably more involved than the setup of a screening procedure. 

The OBOC combinatorial library method is highly versatile and economical. It also is a form of chemical microarrays. Many investigators successfully have applied the on-bead screening methods in their research. The solution-phase and cell-based assays for OBOC libraries, however, are much less developed and have been applied successfully in only a few laboratories. A need exists to develop robust methods that allow investigators to screen routinely huge OBOC releasable peptide or chemical libraries (e.g., 200,000 compounds) with multiparametric... 

A positional scanning combinatorial library (Fig. 3) can be used in place of iterative synthesis and screening [9]. In this approach, all library mixtures are synthesized and assayed for activity. The most active mixture from each positional library reveals the preferred residue at each position. Because all possible combinations of residues are represented in one of the sublibraries, the approach should be insensitive to synergistic effects associated with the structure. The orthogonal combinatorial library consists of two different libraries made up of the same compounds and synthesized in a way that mixtures from each library will only share one peptide or subunit [10]. In this way, these libraries are nonoverlapping, or orthogonal. 

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