Combinatorial peptide libraries: robotic synthesis and analysis by nuclear magnetic resonance, mass spectrometry, tandem mass spectrometry, and high-performance capillary electrophoresis techniques

Total chemical synthesis, refolding, and crystallographic structure of fully active immunophilin calstabin 2 (FKBP12.6)

Synthetic biology (or chemical biology) is a growing field to which the chemical synthesis of proteins, particularly enzymes, makes a fundamental contribution. However, the chemical synthesis of catalytically active proteins (enzymes) remains poorly documented because it is difficult to obtain enough material for biochemical experiments. We chose calstabin, a 107-amino-acid proline isomerase, as a model. We synthesized the enzyme using the native chemical ligation approach and obtained several tens of milligrams. The polypeptide was refolded properly, and we characterized its biophysical properties, measured its catalytic activity, and then crystallized it in order to obtain its tridimensional structure after X-ray diffraction. The refolded enzyme was compared to the recombinant, wild-type enzyme. In addition, as a first step of validating the whole process, we incorporated exotic amino acids into the N-terminus. Surprisingly, none of the changes altered the catalytic activities of the corresponding mutants. Using this body of techniques, avenues are now open to further obtain enzymes modified with exotic amino acids in a way that is only barely accessible by molecular biology, obtaining detailed information on the structure-function relationship of enzymes reachable by complete chemical synthesis.

Progress in liquid chromatography-mass spectrometry instrumentation and its impact on high-throughput screening

In the past 10 years, liquid chromatography-mass spectrometry (LC-MS) has rapidly matured to become a very powerful and useful analytical tool that is widely applied in many areas of chemistry, pharmaceutical sciences and biochemistry. In this paper, recent instrumental developments in LC-MS-related interfacing, ionization and mass analysis are reviewed from the perspective of the application of LC-MS in high-throughput screening of combinatorial libraries and the related high-throughput quantitative bioanalysis in early drug-discovery studies, such as early adsorption, distribution, metabolism and excretion studies.

Melanin-concentrating hormone and its receptors: state of the art

Melanin-concentrating hormone (MCH) is a cyclic neuropeptide of nineteen amino acids in mammals. Its involvement in the feeding behaviour has been well established during the last few years. A first receptor subtype, now termed MCHIR, was discovered in 1999, following the desorphanisation of the SLCI orphan receptor, using either reverse pharmacology or systematic screening of agonist candidates. A second MCH receptor, MCH2R, has been discovered recently, by several groups working on data mining of genomic banks. The molecular pharmacology of these two receptors is only described on the basis of the action of peptides derived from MCH. The present review tentatively summarizes the knowledge on these two receptors and presents the first attempts to discover new classes of antagonists that might have major roles in the control of obesity and feeding behaviour.

Analysis of multiple samples using multiplex sample NMR: selective excitation and chemical shift imaging approaches

Two improved approaches for the rapid analysis of multiple samples using multiplex sample NMR are described. In the first approach, frequency-selective 90 degrees radio frequency pulses and large pulsed field gradients are applied to excite and detect multiple samples in rapid succession. This method is advantageous for samples with relatively long longitudinal (T1) relaxation times. In the second approach, chemical shift imaging is applied to acquire both the spectral and spatial information of multiple samples simultaneously. Chemical shift imaging is more time-consuming than selective excitation; however, it is advantageous for detecting samples with short T1's and for signal averaging. Both approaches demonstrate the potential of multiplex sample NMR for carrying out high-throughput NMR detection.

Structure-activity relationship studies of melanin-concentrating hormone (MCH)-related peptide ligands at SLC-1, the human MCH receptor

Melanin-concentrating hormone (MCH) is a cyclic nonadecapeptide involved in the regulation of feeding behavior, which acts through a G protein-coupled receptor (SLC-1) inhibiting adenylcyclase activity. In this study, 57 analogues of MCH were investigated on the recently cloned human MCH receptor stably expressed in HEK293 cells, on both the inhibition of forskolin-stimulated cAMP production and guanosine-5'-O-(3-[(35)S]thiotriphosphate ([(35)S]- GTPgammaS) binding. The dodecapeptide MCH-(6-17) (MCH ring between Cys(7) and Cys(16), with a single extra amino acid at the N terminus (Arg(6)) and at the C terminus (Trp(17))) was found to be the minimal sequence required for a full and potent agonistic response on cAMP formation and [(35)S]- GTPgammaS binding. We Ala-scanned this dodecapeptide and found that only 3 of 8 amino acids of the ring, namely Met(8), Arg(11), and Tyr(13), were essential to elicit full and potent responses in both tests. Deletions inside the ring led either to inactivity or to poor antagonists with potencies in the micromolar range. Cys(7) and Cys(16) were substituted by Asp and Lys or one of their analogues, in an attempt to replace the disulfide bridge by an amide bond. However, those modifications were deleterious for agonistic activity. In [(35)S]- GTPgammaS binding, these compounds behaved as weak antagonists (K(B) 1-4 microm). Finally, substitution in MCH-(6-17) of 6 out of 12 amino acids by non-natural residues and concomitant replacement of the disulfide bond by an amide bond led to three compounds with potent antagonistic properties (K(B) = 0.1-0.2 microm). Exploitation of these structure-activity relationships should open the way to the design of short and stable MCH peptide antagonists.

From peptide libraries to optimized nonpeptide ligands in the search for S-farnesyltransferase inhibitors

A complete 331,776-member library of tetrapeptides made of 24 amino acid building blocks was synthesized robotically on solid phase and subjected to a deconvolution based on the inhibitory potency of the sublibraries in a HPLC assay of the S-farnesyltransferase activity in vitro. One of the non-natural peptide and noncysteine-containing leads Nip-Trp-Phe-His (Nip=p-nitrophenyl-L-alanine) was optimized chemically to give a proteolytically stable pseudopeptide with a 200-fold potency compared with the original lead. The final compound was converted to the C-terminal ethyl ester: p-F-C6H4-CO(CH2)2-CO-Bta-D-Phepsi[CH2NH]His-OEt (Bta = benzothienyl-L-alanine) and shown to behave as a prodrug which was hydrolyzed back to the C-terminal acid following cell penetration. The method confirmed that several structurally original leads can be discovered in large libraries when deconvolution relies upon a highly specific assay and that these leads can be optimized by chemical modification to impart the final compound the desired pharmacological and pharmacokinetic properties.

Applications and mechanisms of charge-remote fragmentation

Studies on the applications, energetics, and mechanisms of charge-remote fragmentations are reviewed, with emphasis given to those articles published after 1992. Independent of the charge status, charge-remote fragmentations are analogous to gas-phase thermolysis. Under collisional activation and with a fixed charge, ions containing long-chain or poly-ring structures undergo charge-remote fragmentations, generating productions that are structurally informative. Interpretation of the production spectra enables one to elucidate molecular structures. Although charge-remote fragmentations have been successfully used in the structural determination of fatty acids, phospholipids, glycolipids, triacylglycerols, steroids, peptides, ceramides, and other systems, the energetics and mechanisms of these reactions are still debated because none of the existing mechanisms can explain all the experimental data.

Development of a capillary zone electrophoresis method for the separation of a furan combinatorial library

Nine component mixtures of a furan library were simultaneously separated by capillary zone electrophoresis (CZE) using a phosphate buffer as a background electrolyte at low pH. The effects of buffer concentration, buffer pH, type and concentration of organic solvents on the electrophoretic mobility, resolution, and analysis time were systematically investigated. Resolution and efficiency of furan library components were further improved using cyclodextrin (CD)-modified CZE. Under optimum conditions, eight of the nine furans were baseline-resolved in less than 10 min at 30 kV using 50 mM phosphate buffer, 10% v/v acetonitrile (ACN), pH 2.0, with 5 mM gamma-CD.

Mass spectrometry in combinatorial chemistry

In the fast expanding field of combinatorial chemistry, profiling libraries has always been a matter of concern--as illustrated by the buoyant literature over the past seven years. Spectroscopic methods, including especially mass spectrometry and to a lesser extent IR and NMR, have been applied at different levels of combinatorial library synthesis: in the rehearsal phase to optimize the chemistry prior to library generation, to confirm library composition, and to characterize after screening each structure that exhibits positive response. Most of the efforts have been concentrated on library composition assessment. The difficulties of such analyses have evolved from the infancy of the combinatorial concept, where large mixtures were prepared, to the recent parallel syntheses of collections of discrete compounds. Whereas the complexity of the analyses has diminished, an increased degree of automation was simultaneously required to achieve efficient library component identification and quantification. In this respect, mass spectrometry has been found to be the method of choice, providing rapid, sensitive, and informative analyses, especially when coupled to chromatographic separation. Fully automated workstations able to cope with several hundreds of compounds per day have been designed. After a brief introduction to describe the combinatorial approach, library characterization will be discussed in detail, considering first the solution-based methodologies and secondly the support-bound material analyses.

Substrate specificity and inhibition studies of human serotonin N-acetyltransferase

Arylalkylamine N-acetyltransferase (AANAT) catalyzes the reaction of serotonin with acetyl-CoA to form N-acetylserotonin and plays a major role in the regulation of the melatonin circadian rhythm in vertebrates. In the present study, the human cloned enzyme has been expressed in bacteria, purified, cleaved, and characterized. The specificity of the human enzyme toward substrates (natural as well as synthetic arylethylamines) and cosubstrates (essentially acyl homologs of acetyl-CoA) has been investigated. Peptide combinatorial libraries of tri-, tetra-, and pentapeptides with various amino acid compositions were also screened as potential sources of inhibitors. We report the findings of several peptides with low micromolar inhibitory potency. For activity measurement as well as for specificity studies, an original and rapid method of analysis was developed. The assay was based on the separation and detection of N-[(3)H]acetylarylethylamine formed from various arylethylamines and tritiated acetyl-CoA, by means of high performance liquid chromatography with radiochemical detection. The assay proved to be robust and flexible, could accommodate the use of numerous synthetic substrates, and was successfully used throughout this study. We also screened a large number of pharmacological bioamines among which only one, tranylcypromine, behaved as a substrate. The synthesis and survey of simple arylethylamines also showed that AANAT has a large recognition pattern, including compounds as different as phenyl-, naphthyl-, benzothienyl-, or benzofuranyl-ethylamine derivatives. An extensive enzymatic study allowed us to pinpoint the amino acid residue of the pentapeptide inhibitor, S 34461, which interacts with the cosubstrate-binding site area, in agreement with an in silico study based on the available coordinates of the hAANAT crystal.

Fast, generic gradient high performance liquid chromatography coupled to fourier transform ion cyclotron resonance mass spectrometry for the accurate mass analysis of mixtures

Fast gradient high performance liquid chromatography (HPLC) has been combined with a commercially available Fourier transform ion cyclotron resonance (FTICR) mass spectrometer for the routine and high performance analysis of mixtures. With this combination we were able to separate and detect, under high mass accuracy conditions, a six-component drug mixture in less than 5 minutes. The fast gradients described are now possible due to the development of mechanically robust, ultra pure silica packing materials, which allow relatively high flow rates (ca. 1 mL/min for a 2 mm diameter column). For the six compounds present in the model mixture, relative mass errors of less than 1 ppm were obtained (based on an external calibration) providing sufficient mass accuracy to make unequivocal assignments of empirical formulae. Preliminary results of fast gradient HPLC/FTICR-MS/MS are also shown for the same six-component mixture. Copyright 2000 John Wiley & Sons, Ltd.

Monitoring solution-phase combinatorial library synthesis by capillary electrophoresis

Capillary electrophoresis has been applied to monitor model reactions in solution-phase combinatorial chemistry. In particular, the simultaneous alkylation reactions of secondary amines with a series of benzyl halides has been investigated. Reactant and product concentrations were monitored using capillary electrophoresis in a non-aqueous buffer system. The simplified sample preparation was a key feature making this an attractive method of analysis. The results demonstrate that capillary electrophoresis is a useful tool for monitoring reactions to determine initial rates, rate constants, and extinction correlation coefficients for quantitative analysis in combinatorial chemistry, and is a broadly applicable technique for the analysis of a variety of organic and bioorganic transformations.

Physico-chemical and biological analysis of true combinatorial libraries

Combinatorial libraries offer new sources of compounds for the research of pharmacological agents such as receptor ligands, enzyme inhibitors or substrates and antibody-binding epitopes. The present review stresses the main roles played by both physico-chemical analysis, particularly when complex mixture of compounds are synthesized as libraries, and biological analysis from which active compounds are identified. After a brief discussion of semantic problems related to the designation of the product mixtures, the physico-chemical analysis of mixtures is reviewed with special emphasis on mass spectrometric techniques. These methods are able both to give a representative view of a library composition and to identify single critical compounds in large libraries. Then the biological screening of such combinatorial libraries is critically discussed with respect to the power and limitations of the methods used for the identification of the active components. Special attention is given to the complex process of library deconvolution. It is pointed out that while combinatorial techniques have evolved towards sophisticated high-tech methods, simple and robust biochemical tests should be used to deconvolute. From a large panel of published examples, a set of trends are identified which should help investigators to choose the most appropriate assay for the discovery of new entities.

Analyses of compound libraries obtained by high-throughput parallel synthesis: strategy of quality control by high-performance liquid chromatography, mass spectrometry and nuclear magnetic resonance techniques

The growing interest in combinatorial chemistry has led to a new source of compounds from which a large number of leads has emerged over recent years. Parallel synthesis, in particular, allows a quick production of a wide number of individual compounds. A rapid analytical control is needed to determine their quality. A strategy using automated, fast reversed-phase C18 high-performance liquid chromatography with diode-array detection (LC-DAD-MS) followed by atmospheric pressure chemical ionisation mass spectrometry (APCI-MS) and NMR has been developed for their characterisation and purity control. Complementary NMR analyses are done on selected compounds to provide a better structural characterisation of the expected compounds and their potential side-products. Validated libraries are then registered in ISIS databases using automated procedures.

Investigation of S-farnesyl transferase substrate specificity with combinatorial tetrapeptide libraries

Using biased tetrapeptide libraries made up of proteinogenic amino acids of the general formula Cys-O2-X3-X4, we searched for new substrates of partly purified rat brain S-farnesyl transferase (FTase). To achieve this task, an assay was developed in which the consumption of the co-substrate (farnesyl pyrophosphate) was measured. After three steps of deconvolution including each synthesis and enzymatic assay, the most efficient substrates found under these particular conditions were Cys-Lys-Gln-Gln (peptide I) and Cys-Lys-Gln-Met (peptide II). As a control, we used another tetrapeptide library (Cys-Val-O3-X4) in which the valine position was arbitrarily fixed, corresponding to Cys-Val-Ile-Met in the CAAX box of K-RasB, although this sublibrary was only marginally active compared with Cys-Lys-X3-X4 in the first round of deconvolution. The best substrate sublibrary was Cys-Val-Thr-X4, threonine being more favourable than the aliphatic amino acids (Val, Ile, Leu, Ala) in this position. Deconvolution finally led to Cys-Val-Thr-Gln, -Met, -Thr and -Ser as the most efficient substrates of FTase. Those tetrapeptides were not substrates of a partly purified geranylgeranyl transferase 1 (GGTase1). We also investigated the influence of the -1 position (at the N-terminus of cysteine) on the specificity of the enzyme, by using a series of pentapeptides constructed on the basis of the best tetrapeptide core (peptide 1). Among this family of analogues, only His-Cys-Lys-Gln-Gln did not behave as a substrate, whereas all the other pentapeptides were measurable substrates, with Gly-, Asn- and Thr-Cys-Lys-Gln-Gln displaying kinetic constants similar to that of Cys-Lys-Gln-Gln. The present work provides strong evidence that the best tetrapeptide substrates of FTase do not necessarily belong to the classical CAAX box, in which A's are lipophilic residues, but rather contain hydrophilic amino acids in the middle of their sequences. Among them, peptides I and II are potent FTase in vitro substrates that are not recognised by GGTase1 and might be new starting points for the design of FTase inhibitors.

Chromatographic assay and peptide substrate characterization of partially purified farnesyl- and geranylgeranyltransferases from rat brain cytosol

A simple method for partially purifying both farnesyltransferase and geranylgeranyltransferase from rat brain cytosol is presented. Each of the final protein preparations contains one single transferase activity. A common method of measurement of both activities is described. The assay, which follows substrate prenylation, is also convenient for the measurement of the concomitant decrease in cosubstrates during the two transfer reactions. The quantitative HPLC detection of the prenylated substrates and of the cosubstrate consumption is used here to follow the purification processes. The same method is also used for substrate-specificity studies of the two enzymes performed on 18 synthetic hexapeptides derived from the C-terminus of proteins known to be prenylated in vivo. These studies partially confirm the reported differences in the substrate specificities of the two prenyltransferases. However, the observed recognition of overlapping sequences by the two enzymes might have important consequences for the inhibition of either of the enzymes in vivo and for the design of specific inhibitors.

Peptide and peptidomimetic libraries. Molecular diversity and drug design

Various techniques for generation of peptide and peptidomimetic libraries are summarized in this article. Multipin, tea bag, and split-couple-mix techniques represent the major methods used to make peptides and peptidomimetics libraries. The synthesis of these libraries were made in either discrete or mixture format. Peptides and peptidomimetics combinatorial libraries were screened to discover leads against a variety of targets. These targets, including bacteria, fungus, virus, receptors, and enzymes were used in the screening of the libraries. Discovered leads can be further optimized by combinatorial approaches.

Limitations of the coupling of amino acid mixtures for the preparation of equimolar peptide libraries

The standard method of peptide library synthesis involves coupling steps in which a single amino acid is reacted with a mixture of resin-bound amino acids. The more recently described positional scanning strategy (in which each position in the peptide sequence is occupied in turn by a single residue) is different since it involves the coupling of mixtures of amino acids to mixtures of resin-bound amino acids. In the present study, we analyze the compounds produced under these conditions measuring coupling rates and amounts of formed products, using mainly UV, HPLC, LC/MS and MS/MS techniques. Our data do not permit to conclude that the resulting libraries are complete. Indeed, our analytical data indicate that a large part of the di-, tri- and tetrapeptides synthesized with this method are not present in the final mixture. Although chemical compensation (in which poor coupling kinetics is compensated by a larger excess of the incoming amino acid) has been thought to counterbalance these biases, our experiments show that the compensation method does not take into account the crucial influence of the resin-bound amino acid and that even the dipeptide libraries obtained in this way are far from completeness. The present work provides strong evidence that the coupling of mixtures of amino acids to resin-bound residues, which is required by the positional scanning strategy, results in incomplete and/or non-equimolar libraries. It also clearly confirms that coupling rates in solid-phase peptide synthesis are dependent on the nature of both the incoming and the immobilized amino acid.

Peptide analysis by capillary (zone) electrophoresis

In this review various aspects concerning the application of capillary (zone) electrophoresis for peptide analysis will be critically examined. First, the basic instrumental requirements of CE apparatus and the strategies employed to enhance sensitivity in the analysis of underivatized sample are described. Multidimensional separative techniques of complex peptide mixtures that use CE as final step and the coupling of CE with mass spectrometry are subsequently discussed. A theoretical section describes the relationships existing between peptide mobility and the pH of the separation buffer. These relationships evidence that proton dissociation constants and Stokes radius at different protonation stages can be calculated by measuring the electrophoretic mobility at different pH values. Investigation of peptide mobility dependence on pH allows us to establish the optimum conditions, in terms of resolution, for peptide separation. Subsequently, a critical discussion about semiempirical models predicting peptide mobility as a function of chemico-physical peptide properties is presented. A section is devoted to the description of principles of peptide affinity capillary electrophoresis, underlining the similarity with peptide-proton interaction. CE separations performed in aquo-organic solvents are also critically discussed, showing the good performance obtained by using water-2,2,2-trifluoroethanol solutions. Finally, selected CE applications for the determination of peptide chemico-physical properties and conventional analysis, like peptide mapping, are reported.

Quantitative electrospray mass spectrometry for the rapid assay of enzyme inhibitors

BACKGROUND

Combinatorial chemistry has become an important method for identifying effective ligand-receptor binding, new catalysts and enzyme inhibitors. In order to distinguish the most active component of a library or to obtain structure-activity relationships of compounds in a library, an efficient quantitative assay is crucial. Electrospray mass spectrometry has become an indispensable tool for qualitatively screening combinatorial libraries and its use for quantitative analysis has recently been demonstrated.

RESULTS

This paper describes the use of quantitative electrospray mass spectrometry for screening libraries of inhibitors of enzymatic reactions, specifically the enzymatic glycosylation by beta-1,4-galactosyltransferase, which catalyzes the transfer of galactose from uridine-5'-diphosphogalactose to the 4-position of N-acetylglucosamine beta OBn (Bn: benzene) to form N-acetyllactosamine beta OBn. Our mass spectrometric screening approach showed that both nucleoside diphosphates and triphosphates inhibited galactosyltransferase while none of the nucleoside monophosphates, including uridine-5'-monophosphate, showed any inhibition. Additional libraries were generated in which the concentrations of the inhibitors were varied and, using mass spectrometry, uridine-5'-diphosphate-2-deoxy-2-fluorogalactose was identified as the best inhibitor.

CONCLUSIONS

This report introduces quantitative electrospray mass spectrometry as a rapid, sensitive and accurate quantitative assaying tool for inhibitor libraries that does not require a chromophore or radiolabeling. A viable alternative to existing analytical techniques is thus provided. The new technique will greatly facilitate the discovery of novel inhibitors against galactosyltransferase, an enzyme for which there are few potent inhibitors.

Micellar electrokinetic chromatography as a generalized alternative to high-performance liquid chromatography for purity determination of a class of investigational antibacterial drugs

Micellar electrokinetic chromatography (MEKC) was successfully used to provide purity data for a number of oxazolidinone antibacterials. A run buffer of 100 mM SDS and 40 mM HEPES (pH 7.5, NaOH) separated fifteen different materials of neutral and cationic, anionic or zwitterionic character, usually with efficiencies ten-fold of those observed for HPLC. Different HPLC conditions were required for compounds with different structural characteristics. While the high efficiency and finite migration window of MEKC may allow observation of impurities not seen by HPLC, general use of this method for purity screening of combinatorial compounds will require micellar solutions tolerant of high amounts of organic, in order to accommodate materials of low aqueous solubility.

Selection of a histidine-containing inhibitor of gelatinases through deconvolution of combinatorial tetrapeptide libraries

A fully automated peptide synthesizer was used to generate tetrapeptide sublibraries from 24 natural and nonnatural amino acids, from which new inhibitors of gelatinases (matrix metalloproteinases MMP-2 and MMP-9) were selected as potential anticancer drugs. MMP-2 and MMP-9 from mouse Balbc/3T3 fibroblasts conditioned media were assayed in their linear range response by zymography to quantify inhibition at each step of the tetrapeptide library deconvolution. The histidine-epsilon-amino caproic acid-beta-alanine-histidine (His-epsilon Ahx-beta Ala-His) sequence was found to yield optimal inhibition of both MMP-2 and MMP-9. Inhibition by selected tetrapeptides was also evaluated with two other techniques, a native type IV collagen degradation assay and a fluorogenic enzymatic assay, confirming the tetrapeptide potency. The His-epsilon Ahx-beta Ala-His tetrapeptide also inhibited purified human MMP-2 and MMP-9 and the corresponding enzymes present in conditioned media from human tumour cells. Finally, the length of the spacer between the two terminal histidines was found to be crucial to the inhibitory potential. This approach may thus be considered as a-successful strategy to yield specific peptide or pseudopeptide inhibitors, although their potency remains moderate, since it was measured before any chemical optimization was undertaken.

Spectrometrically monitored selection experiments: quantitative laser desorption mass spectrometry of small chemical libraries

BACKGROUND

Selection experiments involving chemical libraries are routinely used in the pharmaceutical industry for finding and optimizing lead compounds. In principle, almost any process involving a binding event or a reaction could be probed systematically with chemical libraries prepared by combinatorial synthesis. Traditionally, however, the vast majority of library members cannot be monitored during the selection, making a systematic correlation of structure and activity difficult. To interpret selection experiments on the level of all library components, monitoring technologies are required that give a unique and quantitative spectroscopic signal for every compound in a mixture.

RESULTS

Quantitative matrix-assisted laser desorption mass spectrometry of libraries of porphyrins and peptide-DNA hybrids consisting of 2-35 compounds is described. Porphyrin libraries were subjected to in vitro selections for liposome incorporation and binding to a protein pocket. It was shown that mesohydroxyphenyl substituted porphyrins, known high activity photosensitizers of tumors, are preferentially incorporated in liposome membranes. A mixture of peptide-DNA hybrids was assayed for the nuclease stability of its components.

CONCLUSIONS

Small libraries of non-isobaric compounds can be exhaustively or near-exhaustively monitored by mass spectrometry. Monitored selection experiments can yield detailed structure-activity maps in a single experiment, speeding up drug discovery and the probing of biochemically relevant recognition events. It is proposed that monitored assays for target binding, membrane partitioning, and biostability could be run in parallel, to select drug candidates combining several favorable properties in 'multidimensional' selection experiments.

Myristoylation

N-myristoylation is an acylation process absolutely specific to the N-terminal amino acid glycine in proteins. This maturation process concerns about a hundred proteins in lower and higher eukaryotes involved in oncogenesis, in secondary cellular signalling, in infectivity of retroviruses and, marginally, of other virus types. Thy cytosolic enzyme responsible for this activity, N-myristoyltransferase (NMT), studied since 1987, has been purified from different sources. However, the studies of the specificities of the various NMTs have not progressed in detail except for those relating to the yeast cytosolic enzyme. Still to be explained are differences in species specificity and between various putative isoenzymes, also whether the data obtained from the yeast enzyme can be transposed to other NMTs. The present review discusses data on the various addressing processes subsequent to myristoylation, a patchwork of pathways that suggests myristoylation is only the first step of the mechanisms by which a protein associates with the membrane. Concerning the enzyme itself, there are evidences that NMT is also present in the endoplasmic reticulum and that its substrate specificity is different from that of the cytosolic enzyme(s). These differences have major implications for their differential inhibition and for their respective roles in several pathologies. For instance, the NMTs from mammalians are clearly different from those found in several microorganisms, which raises the question whether the NMT may be a new targets for fungicides. Finally, since myristoylation has a central role in virus maturation and oncogenesis, specific NMT inhibitors might lead to potent antivirus and anticancer agents.

Tyrosine protein kinase assays

Protein kinases form a large family of enzymes that play a major role in a number of live processes. The study of their action is important for the understanding of the transformation mechanisms and of the normal and pathological growth events. The quality of an enzyme assay is often the key point of an enzymatic study. It must be flexible and compatible with various experimental conditions, such as those for the purification process, the screening of inhibitors and the substrate specificity studies. As will be shown in the present review, two categories of substrates, peptidic and proteic, should be distinguished. The use of peptide substrates facilitates the determination of the recognition requirements of the enzyme and of the kinetic effects of even minute variations in their sequence. These linear peptide structures are assumed to mimic a complex interaction between the enzyme and a protein substrate in which distant amino acids in the sequence are vicinal in the folded substrate. Less amenable to a systematic study, but probably more adequate to investigate the natural substrate of a given kinase, are the proteic substrates. Obviously the tools to measure protein kinase activities are not the same in these two cases. The main difficulty in assaying protein kinases is the use of labelled gamma-ATP, mostly at large excess concentration, since the final product of the reaction has to be separated from the non-reacted labelled ATP. In the case of peptide substrates, the difficulty is to separate them from ATP basing on differences of molecular mass. Despite the efforts of many investigators to rely upon differences in solubility, in charges or in "affinity", this separation, which is crucial for the assay, is still an unsolved experimental problem. Chromatographic, as well as electrophoretic assays appeared relatively late in this domain, and more work in assessing new methodologies might bring new breakthroughs in the next few years. Specific, simple and reliable kinase assays are still a major challenge. Their improvement will help to conduct specificity studies, to elucidate complex growth mechanisms in which they are involved and to discover more selective potent inhibitors.