Synthesis and pharmacological characterization of novel fluorescent histamine H2-receptor ligands derived from aminopotentidine

In an effort to develop a non-radioactive alternative to the [3H]tiotidine and [125I]iodoaminopotentidine binding assays for the histamine H2-receptor (H2R), primary amines related to aminopotentidine were prepared and coupled with the succinimidyl esters of the bulky fluorescent dyes S0536 and BODIPY 650/665-X. The primary amines exhibited different degrees of antagonistic potency at the human and guinea pig H2R. Surprisingly, one compound (5) coupled to the cyanine dye S0536 acted as potent partial agonist/antagonist at the H2R (KB approximately 50 nM; EC50 approximately 100-150 nM). Compounds coupled to the BODIPY dye exhibited moderately high H2R-affinity, too. Thus, the H2R accommodates bulky fluorophores, probably through interaction with extracellular receptor domains. The compounds presented herein provide a starting point for the optimization of fluorescent H2R ligands with respect to affinity and fluorescence as valuable tools to analyze the molecular mechanisms of H2R activation.

Structural basis of catalysis by monometalated methionine aminopeptidase

Methionine aminopeptidase (MetAP) removes the amino-terminal methionine residue from newly synthesized proteins, and it is a target for the development of antibacterial and anticancer agents. Available x-ray structures of MetAP, as well as other metalloaminopeptidases, show an active site containing two adjacent divalent metal ions bridged by a water molecule or hydroxide ion. The predominance of dimetalated structures leads naturally to proposed mechanisms of catalysis involving both metal ions. However, kinetic studies indicate that in many cases, only a single metal ion is required for full activity. By limiting the amount of metal ion present during crystal growth, we have now obtained a crystal structure for a complex of Escherichia coli MetAP with norleucine phosphonate, a transition-state analog, and only a single Mn(II) ion bound at the active site in the position designated M1, and three related structures of the same complex that show the transition from the mono-Mn(II) form to the di-Mn(II) form. An unliganded structure was also solved. In view of the full kinetic competence of the monometalated MetAP, the much weaker binding constant for occupancy of the M2 site compared with the M1 site, and the newly determined structures, we propose a revised mechanism of peptide bond hydrolysis by E. coli MetAP. We also suggest that the crystallization of dimetalated forms of metallohydrolases may, in some cases, be a misleading experimental artifact, and caution must be taken when structures are generated to aid in elucidation of reaction mechanisms or to support structure-aided drug design efforts.

High-throughput screening for Hsp90 ATPase inhibitors

Recently, we reported a useful assay for the determination of yeast Hsp90 ATPase activity. Using this assay, high-throughput screening of approximately 10,000 compounds was performed to determine the feasibility of this assay on large scale. Results from high-throughput screening indicated that the assay was reproducible (av Z-factor = 0.80) and identified 0.57% of the compounds as Hsp90 inhibitors that exhibited IC50s less than 20 microM. The structures of several of these inhibitory scaffolds are reported along with their IC50 values.

N1-(3-Cyclohexylbutanoyl)-N2-[3-(1H-imidazol-4-yl)propyl]guanidine (UR-AK57), a Potent Partial Agonist for the Human Histamine H1- and H2-Receptors

Both the histamine H1-receptor (H1R) and H2-receptor (H2R) exhibit pronounced species selectivity in their pharmacological properties; i.e., bulky agonists possess higher potencies and efficacies at guinea pig (gp) than at the corresponding human (h) receptor isoforms. In this study, we examined the effects of NG-acylated imidazolylpropylguanidines substituted with a single phenyl or cyclohexyl substituent on H1R and H2R species isoforms expressed in Sf9 insect cells. N1-(3-Cyclohexylbutanoyl)-N2-[3-(1H-imidazol-4-yl)propyl]guanidine (UR-AK57) turned out to be the most potent hH2R agonist identified so far (EC50 of 23 nM in the GTPase assay at the hH2R-Gsalpha fusion protein expressed in Sf9 insect cells). UR-AK57 was almost a full-hH2R agonist and only slightly less potent and efficacious than at gpH2R-Gsalpha. Several NG-acylated imidazolylpropylguanidines showed similar potency at hH2R and gpH2R. Most unexpectedly, UR-AK57 exhibited moderately strong partial hH1R agonism with a potency similar to that of histamine, whereas at gpH1R, UR-AK57 was only a very weak partial agonist. Structure/activity relationship studies revealed that both the alkanoyl chain connecting the aromatic or alicyclic substituent with the guanidine moiety and the nature of the carbocycle (cyclohexyl versus phenyl ring) critically determine the pharmacological properties of this class of compounds. Collectively, our data show that gpH1R and gpH R do not necessarily exhibit preference for bulky agonists (2) compared with hH1R and hH2R, respectively, and that UR-AK57 is a promising starting point for the development of both potent and efficacious hH1R and hH2R agonists.

Probing Ligand-Specific Histamine H1- and H2-Receptor Conformations withNG-Acylated Imidazolylpropylguanidines

Impromidine (IMP) and arpromidine (ARP)-derived guanidines are more potent and efficacious guinea pig (gp) histamine H(2)-receptor (gpH(2)R) than human (h) H(2)R agonists and histamine H(1)-receptor (H(1)R) antagonists with preference for hH(1)R relative to gpH(1)R. We examined N(G)-acylated imidazolylpropylguanidines (AIPGs), which are less basic than guanidines, at hH(2)R, gpH(2)R, rat H(2)R (rH(2)R), hH(1)R, and gpH(1)R expressed in Sf9 cells as probes for ligand-specific receptor conformations. AIPGs were similarly potent H(2)R agonists as the corresponding guanidines IMP and ARP, respectively. Exchange of pyridyl in ARP against phenyl increased AIPG potency 10-fold, yielding the most potent agonists at the hH(2)R-G(salpha) fusion protein and gpH(2)R-G(salpha) identified so far. Some AIPGs were similarly potent and efficacious at hH(2)R-G(salpha) and gpH(2)R-G(salpha). AIPGs stabilized the ternary complex in hH(2)R-G(salpha) and gpH(2)R-G(salpha) differently than the corresponding guanidines. Guanidines, AIPGs, and small H(2)R agonists exhibited distinct agonist properties at hH(2)R, gpH(2)R, and rH(2)R measuring adenylyl cyclase activity. In contrast to ARP and IMP, AIPGs were partial H(1)R agonists exhibiting higher efficacies at hH(1)R than at gpH(1)R. This is remarkable because, so far, all bulky H(1)R agonists exhibited higher efficacies at gpH(1)R than at hH(1)R. Collectively, our data suggest that AIPGs stabilize different active conformations in hH(2)R, gpH(2)R, and rH(2)R than guanidines and that, in contrast to guanidines, AIPGs are capable of stabilizing a partially active state of hH(1)R.

Support Vector Machines in HTS Data Mining: Type I MetAPs Inhibition Study

This article reports a successful application of support vector machines (SVMs) in mining high-throughput screening (HTS) data of a type I methionine aminopeptidases (MetAPs) inhibition study. A library with 43,736 small organic molecules was used in the study, and 1355 compounds in the library with 40% or higher inhibition activity were considered as active. The data set was randomly split into a training set and a test set (3:1 ratio). The authors were able to rank compounds in the test set using their decision values predicted by SVM models that were built on the training set. They defined a novel score PT50, the percentage of the test set needed to be screened to recover 50% of the actives, to measure the performance of the models. With carefully selected parameters, SVM models increased the hit rates significantly, and 50% of the active compounds could be recovered by screening just 7% of the test set. The authors found that the size of the training set played a significant role in the performance of the models. A training set with 10,000 member compounds is likely the minimum size required to build a model with reasonable predictive power.

Identification of potent type I MetAPs inhibitors by simple bioisosteric replacement. Part 2: SAR studies of 5-heteroalkyl substituted TCAT derivatives

Systematic SAR studies on the thiazole ring 5-substituent of TCAT derivatives revealed that the introduction of a beta-alkoxy or an amino group enhanced the inhibitory activity significantly. The present compounds are representative of specific Co(II)-MetAP1 inhibitors. Before the physiologically relevant metal ions for MetAPs are established, these small molecular compounds could be used as tools for detailed biological studies.

Metal mediated inhibition of methionine aminopeptidase by quinolinyl sulfonamides

Quinolinyl sulfonamides, such as N-(quinolin-8-yl)methanesulfonamide (10) and N-(5-chloroquinolin-8-yl)methanesulfonamide (11), were identified as potent methionine aminopeptidase (MetAP) inhibitors by high throughput screening of a diverse chemical library of small organic compounds. They showed different inhibitory potencies on Co(II)-, Ni(II)-, Fe(II)-, Mn(II)-, and Zn(II)-forms of Escherichia coli MetAP, and their inhibition is dependent on metal concentration. X-ray structures of E. coli MetAP complexed with 10 revealed that the inhibitor forms a metal complex with the residue H79 at the enzyme active site; the complex is further stabilized by an extended H-bond and metal interaction network. Analysis of the inhibition of MetAP by these inhibitors indicates that this is a typical mechanism of inhibition for many non-peptidic MetAP inhibitors and emphasizes the importance of defining in vitro conditions for identifying and evaluating MetAP inhibitors that will be capable of giving information relevant to the in vivo situation.

Metalloform-selective inhibition: synthesis and structure-activity analysis of Mn(II)-form-selective inhibitors of Escherichia coli methionine aminopeptidase

Methionine aminopeptidase (MetAP) is a promising target for development of novel antibacterial, antifungal and anticancer agents. However, its physiologically relevant metal ion remains to be defined, and its inhibitors need to inhibit the in vivo metalloform. Based on the Mn(II)-form-selective inhibitors discovered by high throughput screening as leads, a series of analogs of 5-phenylfuran-2-carboxylic acid was prepared and subsequently evaluated on Co(II)-, Mn(II)-, Ni(II)-, and Fe(II)-forms of Escherichia coli MetAP, in order to define the structural elements responsible for their inhibitory potency and metalloform selectivity. Various substitutions on the phenyl ring changed their potency on the Mn(II)-form but not their metalloform selectivity. We conclude that the preferential coordination of the carboxyl group to Mn(II) ions is the major determinant for their superb selectivity toward the Mn(II)-form. Changing the carboxylate to hydroxamate alters its ability to bind and discriminate different metal ions, and the hydroxamate derivative becomes non-selective among the metalloforms tested.

Identification of potent type I MetAP inhibitors by simple bioisosteric replacement. Part 1: Synthesis and preliminary SAR studies of thiazole-4-carboxylic acid thiazol-2-ylamide derivatives

A series of thiazole-4-carboxylic acid thiazol-2-ylamide (TCAT, 4) derivatives were designed and synthesized according to simple bioisosteric replacement from previously reported pyridine-2-carboxylic acid thiazol-2-ylamide (PCAT) MetAP inhibitors. The preliminary SAR studies demonstrated that these TCAT series of compounds showed different activity and selectivity compared with those of the corresponding PCAT compounds. These findings provide useful information for the design and discovery of more potent inhibitors of type I MetAPs.

Inhibitors of type I MetAPs containing pyridine-2-carboxylic acid thiazol-2-ylamide. Part 2: SAR studies on the pyridine ring 3-substituent

Systematic SAR studies on the pyridine ring 3-substituent of PCAT, an inhibitor of EcMetAP1 and ScMetAP1, revealed that 3-substituents have different selectivity for EcMetAP1 and ScMetAP1. The selective inhibitors of type I MetAP are useful tools for investigating the detailed interactions between the enzymes and their inhibitors. In addition, these findings provide useful information for the design and discovery of more potent inhibitors of type I MetAPs.

Inhibitors of type I MetAPs containing pyridine-2-carboxylic acid thiazol-2-ylamide. Part 1: SAR studies on the determination of the key scaffold

Systematic SAR studies on the HTS hit pyridine-2-carboxylic acid thiazol-2-ylamide (PACT) analogues revealed that the scaffold of PCAT is indispensable for the inhibition of type I MetAP. For effective inhibition of the enzyme, the most suitable position to modify is the 3-position of the pyridine ring of PCAT, and the best substituents are those containing O or N atoms connected directly with the pyridine ring. These findings provide useful information for the design and discovery of more potent inhibitors of type I MetAPs.

Estimating the number of viable animal cells in multiwell culture--a tetrazolium-based assay

A reliable, indirect method (GPD/INT assay) for estimating the number of live animal cells in multiwell culture has been devised. It is based on the glucose-6-phosphate dehydrogenase (Gpdh) and 6-phosphogluconate dehydrogenase activities present in the cytoplasm of viable eukaryotic cells but not in their bathing medium nor in nonviable cells. A single reagent mixture, buffered at pH 7.8 and containing Tris, Triton X-100, glucose-6-phosphate, nicotinamide adenine dinucleotide phosphate (NADP), phenazine methosulfate, and iodonitrotetrazolium violet, is added to the cultures. The Triton X-100 releases the cytoplasmic contents into the medium, facilitating enzyme-catalyzed oxidation of the glucose-6-phosphate and 6-phosphogluconate by NADP. The resulting reduced nicotinamide adenine dinucleotide phosphate, NADPH, reduces tetrazolium violet to its formazan, the color of which reflects the number of living cells that were in the culture. The assay was tested on recombinant Gpdh and the several types of animal and insect cell lines to verify the premise that there is proportionality between the amount of GPdh and number of viable cells in the cultures. The method has been used to quantitate the effects of growth inhibitors on cells in 96-well cultures.

A high-throughput screen utilizing the fluorescence of riboflavin for identification of lumazine synthase inhibitors

A high-throughput screening method based on the competitive binding of a lumazine synthase inhibitor and riboflavin to the active site of Schizosaccharomyces pombe lumazine synthase was developed. This assay is sensitive, simple, and robust. During assay development, all of the known active inhibitors tested were positively identified. Preliminary high-throughput screening in 384-well format resulted in a Z factor of 0.7. The approach utilizes a thermodynamic assay to bypass the problems associated with the instabilities of both lumazine synthase substrates that complicate the use of a kinetic assay in a high-throughput format, and it removes the time element from the assay, thus simplifying the procedure.

Metalloform-selective inhibitors of escherichia coli methionine aminopeptidase and X-ray structure of a Mn(II)-form enzyme complexed with an inhibitor

Methionine aminopeptidase (MetAP) enzymes require a divalent metal ion such as Mn(II), Fe(II), Co(II), Ni(II), or Zn(II) for its removal of the N-terminal methionine from newly synthesized proteins, but it is not certain which of these ions is most important in vivo. Metalloform-selective MetAP inhibitors could be valuable for defining which metals are physiologically relevant for MetAP activation and could serve as leads for development of new therapeutic agents. We have screened a library of 43 736 small drug-like molecules against Escherichia coli MetAP and identified two groups of potent and highly metalloform-selective inhibitors of the Co(II)-form, and of the Mn(II)-form, of this enzyme. Compound 1 is 790-fold more selective for the Co(II)-form, while compound 4 is over 640-fold more potent toward the Mn(II)-form. The X-ray structure of a di-Mn(II) form of E. coli MetAP complexed with the Mn(II)-form-selective compound 4 was obtained, and it shows that the inhibitor interacts with both Mn(II) ions through the two oxygen atoms of its free carboxylate group. The preferential coordination of the hard (oxygen) donors to Mn(II) may contribute to its superb selectivity toward the Mn(II)-form.

Characterization of full length and truncated type I human methionine aminopeptidases expressed from Escherichia coli

Methionine aminopeptidase (MetAP) carries out an essential posttranslational modification of nascent proteins by removing the initiator methionine and is recognized as a potential target for developing antibacterial, antifungal, and anticancer agents. We have established an Escherichia coli expression system for human type I MetAP (HsMetAP1) and characterized the full length HsMetAP1 and its N-terminal-truncated mutants HsMetAP1(Delta1-66) and HsMetAP1(Delta1-135) for hydrolysis of several thiopeptolide and peptide substrates and inhibition by a series of nonpeptidic inhibitors. Although the N-terminal extension with zinc finger motifs in HsMetAP1 is not required for enzyme activity, it has a significant impact on the interaction of the enzyme with substrates and inhibitors. In hydrolysis of the thiopeptolide substrates, a relaxation of stringent specificity for the terminal methionine was observed in the truncated mutants. However, this relaxation of specificity was not detectable in hydrolysis of tripeptide or tetrapeptide substrates. Several nonpeptidic inhibitors showed potent inhibition of the mutant HsMetAP1(Delta1-66) but exhibited only weak or no inhibition of the full length enzyme. With the recombinant HsMetAP1 available, we have identified several MetAP inhibitors with submicromolar inhibitory potencies against E. coli MetAP (EcMetAP1) that do not affect HsMetAP1. These results have demonstrated the possibility of developing MetAP inhibitors as antibacterial agents with minimum human toxicity. In addition, micromolar inhibitors of HsMetAP1 identified in this study can serve as tools for investigating the functions of HsMetAP1 in physiological and pathological processes.

Design and synthesis of chromogenic thiopeptolide substrates as MetAPs active site probes

Twenty one chromogenic thiopeptolide substrates were designed and synthesized as the active site probes and analyzed with each S1 site of mutant residues and enzymes of wild-type MetAP1s. The preliminary enzymatic experiments indicate that cysteine 70 or 202, at either Escherichia coli or human MetAP1, played a crucial role in the methionine hydrolysis.

Mutations at the S1 sites of methionine aminopeptidases from Escherichia coli and Homo sapiens reveal the residues critical for substrate specificity

Methionine aminopeptidase (MetAP) catalyzes the removal of methionine from newly synthesized polypeptides. MetAP carries out this cleavage with high precision, and Met is the only natural amino acid residue at the N terminus that is accepted, although type I and type II MetAPs use two different sets of residues to form the hydrophobic S1 site. Characteristics of the S1 binding pocket in type I MetAP were investigated by systematic mutation of each of the seven S1 residues in Escherichia coli MetAP type I (EcMetAP1) and human MetAP type I (HsMetAP1). We found that Tyr-65 and Trp-221 in EcMetAP1, as well as the corresponding residues Phe-197 and Trp-352 in HsMetAP1, were essential for the hydrolysis of a thiopeptolide substrate, Met-S-Gly-Phe. Mutation of Phe-191 to Ala in HsMetAP1 caused inactivity in contrast to the full activity of EcMetAP1(Y62A), which may suggest a subtle difference between the two type I enzymes. The more striking finding is that mutation of Cys-70 in EcMetAP1 or Cys-202 in HsMetAP1 opens up the S1 pocket. The thiopeptolides Leu-S-Gly-Phe and Phe-S-Gly-Phe, with previously unacceptable Leu or Phe as the N-terminal residue, became efficient substrates of EcMetAP1(C70A) and HsMetAP1(C202A). The relaxed specificity shown in these S1 site mutants for the N-terminal residues was confirmed by hydrolysis of peptide substrates and inhibition by reaction products. The structural features at the enzyme active site will be useful information for designing specific MetAP inhibitors for therapeutic applications.

Specificity for inhibitors of metal-substituted methionine aminopeptidase

Methionine aminopeptidases (MetAPs) have been studied in vitro as Co(II) enzymes, but their in vivo metal remains to be defined. While activation of Escherichia coli MetAP (EcMetAP1) by Co(II), Mn(II), and Zn(II) was detectable by a colorimetric Met-S-Gly-Phe assay, significant activation by Ni(II) was shown in a fluorescence Met-AMC assay, in addition to Co(II) and Mn(II) activation. When tested on the metal-substituted EcMetAP1s, a few inhibitors that we obtained recently from a random screening on Co-EcMetAP1 either became much weak or lost activity on Mn- or Zn-EcMetAP1, although they kept inhibitory activity on Ni-EcMetAP1. A couple of peptidic inhibitors and the methionine mimetic (3R)-amino-(2S)-hydroxyheptanoic acid (AHHpA, 6) maintained moderate activities on Co-, Mn-, Zn-, and Ni-EcMetAP1s. Our results clearly demonstrate that the metal-substitution has changed the enzyme specificity for substrates and inhibitors. Therapeutic applications call for inhibitors specific for MetAP with a physiologically relevant metal at its active site.

Discovery and structural modification of inhibitors of methionine aminopeptidases from Escherichia coli and Saccharomyces cerevisiae

A series of pyridine-2-carboxylic acid derivatives were synthesized according to the leads from the screening, and potent inhibitors have been obtained by structural modification. They have shown submicromolar inhibition of the enzymes (for example, for 9n, IC(50) = 130 nM for EcMetAP1 and IC(50) = 380 nM for ScMetAP1). They represent small-molecule MetAP inhibitors with novel structures different from alkylating fumagillin derivatives and peptidic bestatin-based MetAP inhibitor.

Purification and characterization of catalytic domains of gelatinase A with or without fibronectin insert for high-throughput inhibitor screening

Gelatinase A represents an attractive therapeutic target for cancer invasion and metastasis. In order to screen for gelatinase A inhibitors, we have cloned, overexpressed in a bacterial system, and purified the catalytic domain of human gelatinase A with (GaCDfn) or without (GaCD) fibronectin-like insert. GaCDfn and GaCD were purified to homogeneity and refolded in vitro. GaCDfn was refolded to a stable and active form in the presence of calcium and zinc ions. GaCD was refolded through direct dialysis against Tris-HCl (pH 7.5) buffer without calcium and zinc ions. GaCD is unstable in the presence of calcium and zinc ions. The enzymatic activities of GaCDfn and GaCD require calcium and zinc ions, but high concentration of zinc and calcium ions inhibited the activities. The GaCDfn and GaCD cleaved several synthetic substrates including a chromogenic thiopeptolide (TPL) and fluorogenic peptides with optimal activity around pH 7.5. Moreover, GaCDfn and GaCD cleave gelatin and collagen VII and display similar cleavage patterns on the gel, but the digestion rate of these protein substrates by GaCD is apparently slower than GaCDfn. EDTA, 1,10-phenanthroline, and reference inhibitors potently blocked GaCDfn and GaCD enzymatic activities. A set of 3596 compounds from our center collection were screened by using GaCDfn and GaCD to cleave TPL. Further analysis by using MMP inhibitors indicated there is a correlation between IC(50) values on GaCDfn and GaCD. A few compounds with selectivity toward gelatinase A catalytic domain were identified for structure modification.

Crassicaulisine, a new sulphonoglycolipid from the red alga Chondria crassicaulis Harv

A new sulphonoglycolipid, crassicaulisine, has been isolated from the red alga Chondria crassicaulis Harv.. Four known compounds were also found from the title plant. The structure of the new compound was elucidated on the basis of chemical reactions and spectroscopic analysis.

Comparison of the Catalytic Domains of Collagenase-1 and Stromelysin-1

The catalytic domains of two matrix metalloproteinases--collagenase-1 and stromelysin-1 have been studied by means of fluorescence spectroscopy and high hydrostatic pressure. The hydrophobic fluorescence probe ANS could bind to stromelysin-1, with a dissociation constant of 26.3 &mgr;mol/L, but could not bind to collagenase-1, indicating that there exists a hydrophobic site on the surface of stromelysin-1. Further study suggests that the hydrophobic site may not be the catalytic site. The biological activity of catalytic domains of collagenase-1 and stromelysin-1 showed obvious difference under high pressure the activity of collagenase-1 increased with elevating pressure, with an activation volume of D18.9 ml/mol however, the activity of stromelysin-1 did not change under high pressure. The results indicate that there are some obvious differences between the catalytic domain conformations of these two enzymes, though the crystal analysis indicated that they were very similar as reported before.

Expression and purification of catalytic domain of human macrophage elastase for high throughput inhibitor screening

AIM

To obtain a catalytically active human macrophage elastase catalytic domain (hMECD) and to establish an efficient high-throughput method for screening macrophage elastase inhibitors.

METHODS

Catalytic domain of human macrophage elastase was expressed in E coli and characterized to establish a high-throughput screening assay using a colorimetric method. A set of 8560 pure compounds and mixtures were screened.

RESULTS

We have constructed an efficient E coli system for this human protein expression, and the recombinant hMECD protein was purified to homogeneity using anion-exchange chromatography after in vitro refolding from inclusion bodies. The yield of active hMECD protein was 23 mg from one liter of E coli culture after purification. Calcium and zinc ions were required both in refolding and enzymatic activity, but high concentration of zinc inhibited the refolding and activity. The hMECD cleaved several synthetic substrates including a chromogenic thiopeptolide and fluorogenic peptides with optimal activity around pH 8.0. Screening of 8560 compounds and mixtures led to identify 27 pure compounds and 14 natural products with inhibitory activity higher than 80 % at 20 mg/L.

CONCLUSION

An efficient expression and purification method for hMECD protein has been established, and the assay is effective, reliable, and fast in identifying the recombinant protein inhibitors.

A new model for random screening inhibitors of vascular endothelial growth factor receptor 1 kinase

AIM

To establish a 96-well plate based kinase assay using a recombinant vascular endothelial growth factor (VEGF) receptor 1 kinase domain protein.

METHODS

A human VEGF receptor 1 kinase domain protein was expressed in E coli, and its activity was monitored by its ability of phosphorylating the polyE4Y substrate coated on the walls of 96-well plates with antibody recognition and a colorimetric readout. A random screening of a sample organic compound library was carried out, and the hits were characterized with a transformed cell line stably expressing VEGF receptor 1 protein.

RESULTS

An efficient E coli expression system for human VEGF receptor 1 kinase domain protein was constructed, and the purified recombinant protein was used to establish a practical screening assay for kinase inhibitors in vitro. Two thousand eight hundred organic compounds were screened, and two disubstituted furans (A1 and A5) with new structure showed inhibition of VEGF receptor 1 kinase. Compound A1 inhibited only phosphorylation of substrate, while compound A5 inhibited both autophosphorylation and substrate phosphorylation. Both inhibitors affected phosphorylation in the transformed cells.

CONCLUSION

The recombinant receptor kinase based assay is simple and effective in identifying kinase inhibitors.