Development of a mechanism-based assay for tissue transglutaminase—results of a high-throughput screen and discovery of inhibitors

High-Content C. elegans Screen Identifies Natural Compounds Impacting Mitochondria-Lipid Homeostasis and Promoting Healthspan

The aging process is concurrently shaped by genetic and extrinsic factors. In this work, we screened a small library of natural compounds, many of marine origin, to identify novel possible anti-aging interventions in Caenorhabditis elegans, a powerful model organism for aging studies. To this aim, we exploited a high-content microscopy platform to search for interventions able to induce phenotypes associated with mild mitochondrial stress, which is known to promote animal's health- and lifespan. Worms were initially exposed to three different concentrations of the drugs in liquid culture, in search of those affecting animal size and expression of mitochondrial stress response genes. This was followed by a validation step with nine compounds on solid media to refine compounds concentration, which led to the identification of four compounds (namely isobavachalcone, manzamine A, kahalalide F and lutein) consistently affecting development, fertility, size and lipid content of the nematodes. Treatment of Drosophila cells with the four hits confirmed their effects on mitochondria activity and lipid content. Out of these four, two were specifically chosen for analysis of age-related parameters, kahalalide F and lutein, which conferred increased resistance to heat and oxidative stress and extended animals' healthspan. We also found that, out of different mitochondrial stress response genes, only the C. elegans ortholog of the synaptic regulatory proteins neuroligins, nlg-1, was consistently induced by the two compounds and mediated lutein healthspan effects.

Transglutaminase 2: Development of therapeutic antibodies reveals four inhibitory epitopes and confirms extracellular function in fibrotic remodelling

BACKGROUND AND PURPOSE

Transglutaminase type 2 (TG2) catalyses formation of ε-(γ-glutamyl)-lysine bonds between proteins, including those of the extracellular matrix (ECM). Elevated extracellular TG2 leads to accelerated ECM deposition and reduced clearance that underlies tissue scarring and fibrosis. Many transglutaminase inhibitors exist and allowed for proof-of-concept studies in disease models, but their lack of specificity for the TG2 isoform, and/or poor pharmacokinetic/pharmacodynamic properties have limited their clinical application. We hypothesised that a high affinity TG2-specific antibody could be developed to specifically inhibit extracellular TG2 activity, with characteristics suitable for therapeutic development.

EXPERIMENTAL APPROACH

Individual human TG2 domains were used to immunise mice and generate hybridomas. Supernatants were screened for inhibition of recombinant human TG2 activity, with TG2 specificity determined by ELISA.

KEY RESULTS

Thirteen TG2-specific supernatants inhibited human transamidation activity. Each hybridoma was cloned and antibody mapped to an epitope in the TG2 core domain, using phage display panning of a TG2 fragment library. Four distinct inhibitory epitopes were determined. The most effective antibodies (AB1, DC1 and BB7) bound to amino acids 313-327 (catalytic core), with an IC₅₀ of approximately 10 nM. The antibodies inhibit TG2 in human cells and block ECM accumulation in a primary human proximal tubular epithelial cell model of fibrosis, only 7 antibodies inhibited rat TG2, and all with higher IC₅₀ values.

CONCLUSIONS AND IMPLICATIONS

We identified a preferred inhibitory epitope in human TG2, developed antibodies with required characteristics for clinical development, and established that targeted inhibition of extracellular TG2 transamidation activity is sufficient to modify fibrotic remodelling.

Development of an 18F-Labeled Irreversible Inhibitor of Transglutaminase 2 as Radiometric Tool for Quantitative Expression Profiling in Cells and Tissues

The transamidase activity of transglutaminase 2 (TGase 2) is considered to be important for several pathophysiological processes including fibrotic and neoplastic tissue growth, whereas in healthy cells this enzymatic function is predominantly latent. Methods that enable the highly sensitive detection of TGase 2, such as application of radiolabeled activity-based probes, will support the exploration of the enzyme's function in various diseases. In this context, the radiosynthesis and detailed in vitro radiopharmacological evaluation of an ¹⁸F-labeled N^ε-acryloyllysine piperazide are reported. Robust and facile detection of the radiotracer-TGase 2 complex by autoradiography of thin layer plates and polyacrylamide gels after chromatographic and electrophoretic separation owing to irreversible covalent bond formation was demonstrated for the isolated protein, cell lysates, and living cells. By use of this radiotracer, quantitative data on the expression profile of activatable TGase 2 in mouse organs and selected tumors were obtained for the first time by autoradiography of tissue sections.

New insight into transglutaminase 2 and link to neurodegenerative diseases

Formation of toxic protein aggregates is a common feature and mainly contributes to the pathogenesis of neurodegenerative diseases (NDDs), which include amyotrophic lateral sclerosis (ALS), Alzheimer’s, Parkinson’s, Huntington’s, and prion diseases. The transglutaminase 2 (TG2) gene encodes a multifunctional enzyme, displaying four types of activity, such as transamidation, GTPase, protein disulfide isomerase, and protein kinase activities. Many studies demonstrated that the calcium-dependent transamidation activity of TG2 affects the formation of insoluble and toxic amyloid aggregates that mainly consisted of NDD-related proteins. So far, many important and NDD-related substrates of TG2 have been identified, including amlyoid-β, tau, α-synuclein, mutant huntingtin, and ALS-linked trans-activation response (TAR) DNA-binding protein 43. Recently, the formation of toxic inclusions mediated by several TG2 substrates were efficiently inhibited by TG2 inhibitors. Therefore, the development of highly specific TG2 inhibitors would be an important tool in alleviating the progression of TG2-related brain disorders. In this review, the authors discuss recent advances in TG2 biochemistry, several mechanisms of molecular regulation and pleotropic signaling functions, and the presumed role of TG2 in the progression of many NDDs.

Recent Progress in the Development of Transglutaminase 2 (TGase2) Inhibitors

Transglutaminase 2 (TGase2, TG2) activity has been implicated in the pathogenesis of a number of unrelated disorders, including celiac, neurological, and renal diseases, and various forms of cancer. It has been suggested that TGase2 activity, such as cross-linking, deamidation, and GTP-related activity, is associated with each disease. Continuing efforts to develop small molecule TG2 inhibitors are ongoing. To develop a new class of TG2 inhibitors, the factors impeding the development of TG2 inhibitors have been identified. Additionally, the conformational effect of TG2 enzyme in regard to its pathological roles, in vitro screening methods, recently discovered TG2 inhibitors, and preclinical evaluations are discussed with a brief summary of current TG2 inhibitor pipelines under the clinical setting.

A fluorescence anisotropy-based assay for determining the activity of tissue transglutaminase

Tissue transglutaminase (TGase 2) is the most abundantly expressed enzyme of the transglutaminase family and involved in a large variety of pathological processes, such as neurodegenerative diseases, disorders related to autoimmunity and inflammation as well as tumor growth, progression and metastasis. As a result, TGase 2 represents an attractive target for drug discovery and development, which requires assays that allow for the characterization of modulating agents and are appropriate for high-throughput screening. Herein, we report a fluorescence anisotropy-based approach for the determination of TGase 2's transamidase activity, following the time-dependent increase in fluorescence anisotropy due to the enzyme-catalyzed incorporation of fluorescein- and rhodamine B-conjugated cadaverines 1-3 (acyl acceptor substrates) into N,N-dimethylated casein (acyl donor substrate). These cadaverine derivatives 1-3 were obtained by solid-phase synthesis. To allow efficient conjugation of the rhodamine B moiety, different linkers providing secondary amine functions, such as sarcosyl and isonipecotyl, were introduced between the cadaverine and xanthenyl entities in compounds 2 and 3, respectively, with acyl acceptor 3 showing the most optimal substrate properties of the compounds investigated. The assay was validated for the search of both irreversible and reversible TGase 2 inhibitors using the inactivators iodoacetamide and a recently published L-lysine-derived acrylamide and the allosteric binder GTP, respectively. In addition, the fluorescence anisotropy-based method was proven to be suitable for high-throughput screening (Z' factor of 0.86) and represents a non-radioactive and highly sensitive assay for determining the active TGase 2 concentration.

Tissue transglutaminase: an emerging target for therapy and imaging

Tissue transglutaminase (transglutaminase 2) is a multifunctional enzyme with many interesting properties resulting in versatile roles in both physiology and pathophysiology. Herein, the particular involvement of the enzyme in human diseases will be outlined with special emphasis on its role in cancer and in tissue interactions with biomaterials. Despite recent progress in unraveling the different cellular functions of transglutaminase 2, several questions remain. Transglutaminase 2 features in both confirmed and some still ambiguous roles within pathological conditions, raising interest in developing inhibitors and imaging probes which target this enzyme. One important prerequisite for identifying and characterizing such molecular tools are reliable assay methods to measure the enzymatic activity. This digest Letter will provide clarification about the various assay methods described to date, accompanied by a discussion of recent progress in the development of inhibitors and imaging probes targeting transglutaminase 2.

Transglutaminase 2: biology, relevance to neurodegenerative diseases and therapeutic implications

Neurodegenerative disorders are characterized by progressive neuronal loss and the aggregation of disease-specific pathogenic proteins in hallmark neuropathologic lesions. Many of these proteins, including amyloid Αβ, tau, α-synuclein and huntingtin, are cross-linked by the enzymatic activity of transglutaminase 2 (TG2). Additionally, the expression and activity of TG2 is increased in affected brain regions in these disorders. These observations along with experimental evidence in cellular and mouse models suggest that TG2 can contribute to the abnormal aggregation of disease causing proteins and consequently to neuronal damage. This accumulating evidence has provided the impetus to develop inhibitors of TG2 as possible neuroprotective agents. However, TG2 has other enzymatic activities in addition to its cross-linking function and can modulate multiple cellular processes including apoptosis, autophagy, energy production, synaptic function, signal transduction and transcription regulation. These diverse properties must be taken into consideration in designing TG2 inhibitors. In this review, we discuss the biochemistry of TG2, its various physiologic functions and our current understanding about its role in degenerative diseases of the brain. We also describe the different approaches to designing TG2 inhibitors that could be developed as potential disease-modifying therapies.

The mechanism of transglutaminase 2 inhibition with glucosamine: implications of a possible anti-inflammatory effect through transglutaminase inhibition

PURPOSE

Although many efforts on revealing mechanism of the constitutive activation of NF-κB in cancer cells contributed to understanding canonical pathways, largely it remains to be determined for therapeutic approaches. Recently, we found that increased expression of transglutaminase 2 (TGase 2) appears to be responsible for constitutive activation of NF-κB in certain types of cancer cells. In previous studies, we demonstrated that TGase 2 inhibition markedly increases anti-cancer drug sensitivity in drug resistance cancer cells. Therefore, we develop safe and effective TGase 2 inhibitors for therapeutic approach.

METHODS

We screened a chemical library of natural compounds using in vitro TGase 2 activity assay. The salient discovery was that glucosamine (GlcN), a known anti-inflammatory substance, inhibited the cross-linking activity of TGase 2. We tested, through a biochemical analysis including kinetics, whether the GlcN and GlcN analogs specifically inhibit TGase 2. We also determined the inhibitory mechanism using conformational change of TGase 2.

RESULTS

We found that the primary amine of GlcN plays a key role in TGase 2 inhibition. We also demonstrated that GlcN reversed TGase 2-mediated I-κBα polymerization in vitro. Interestingly, the metabolite of GlcN, glucosamine-6-phosphate (GlcN6P), inhibited TGase 2 activity via binding to the GTP-binding site with better efficiency than GlcN. In the native gel electrophoresis, it was clearly observed that GlcN6P binds to TGase 2 directly as an allosteric inhibitor.

CONCLUSIONS

We concluded that GlcN inhibits TGase 2 activity by direct contact. GlcN and its metabolite GlcN6P can down-regulate constitutive activation of NF-κB in vivo via inhibition of TGase 2.

Dihydroisoxazole analogs for labeling and visualization of catalytically active transglutaminase 2

We report the synthesis and preliminary characterization of "clickable" inhibitors of human transglutaminase 2 (TG2). These inhibitors possess the 3-halo-4,5-dihydroisoxazole warhead along with bioorthogonal groups such as azide or alkyne moieties that enable subsequent covalent modification with fluorophores. Their mechanism for inhibition of TG2 is based on halide displacement, resulting in the formation of a stable imino thioether. Inhibition assays against recombinant human TG2 revealed that some of the clickable inhibitors prepared in this study have comparable specificity as benchmark dihydroisoxazole inhibitors reported earlier. At low micromolar concentrations they completely inhibited transiently activated TG2 in a WI-38 fibroblast scratch assay and could subsequently be used to visualize the active enzyme in situ. The potential use of these inhibitors to probe the role of TG2 in celiac sprue as well as other diseases is discussed.

A High-Throughput Screen for Receptor Protein Tyrosine Phosphatase–γ Selective Inhibitors

Protein tyrosine phosphatase–γ (PTP-γ) is a receptor-like PTP whose biological function is poorly understood. A recent mouse PTP-γ genetic deletion model associated the loss of PTP-γ gene expression with a potential antidepressant phenotype. This led the authors to screen a subset of the Bristol-Myers Squibb (BMS) compound collection to identify selective small-molecule inhibitors of receptor-like PTP-γ (RPTP-γ) for use in evaluating enzyme function in vivo. Here, they report the design of a high-throughput fluorescence resonance energy transfer (FRET) assay based on the Z′-LYTE technology to screen for inhibitors of RPTP-γ. A subset of the BMS diverse compound collection was screened and several compounds identified as RPTP-γ inhibitors in the assay. After chemical triage and clustering, compounds were assessed for potency and selectivity by IC50 determination with RPTP-γ and two other phosphatases, PTP-1B and CD45. One hundred twenty-nine RPTP-γ selective (defined as IC50 value greater than 5- to 10-fold over PTP-1B and CD45) inhibitors were identified and prioritized for evaluation. One of these hits, 3-(3, 4-dichlorobenzylthio) thiophene-2-carboxylic acid, was the primary chemotype for the initiation of a medicinal chemistry program.

Amine functionalization of collagen matrices with multifunctional polyethylene glycol systems

A method to functionalize collagen-based biomaterials with free amine groups was established in an attempt to improve their potential for tethering of bioactive molecules. Collagen sponges were incorporated with amine-terminated multifunctional polyethylene glycol (PEG) derivatives after N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide and N-hydroxysuccinimide (EDC/NHS) cross-linking. The extent of the incorporation of different amounts and different numbers of active moieties of amine-terminated PEG systems into the collagen scaffolds was evaluated using ninhydrin assay, Fourier transform infrared spectrophotometry (FTIR), collagenase degradation assay, denaturation temperature measurements, and in vitro cell studies. A 3% 8-arm amine-terminated PEG was found to be the minimum required effective concentration to functionalize EDC/NHS stabilized collagen scaffolds. EDC/NHS stabilized scaffolds treated with 3% 8-arm amine-terminated PEG exhibited significantly improved denaturation temperature and resistance to collagenase degradation over non-cross-linked scaffolds (p < 0.002). Biological evaluation using 3T3 cells demonstrated that the produced scaffolds facilitated maintenance of the cells' morphology, metabolic activity, and ability to proliferate in vitro. Overall, our results indicate that amine-terminated PEG systems can be used as means to enhance the functionality of collagenous structures.

An in situ and in vitro investigation for the transglutaminase potential in tissue engineering

Transglutaminases (TGases) constitute a family of enzymes that stabilize protein assemblies by gamma-glutamyl-epsilon-lysine crosslinks. The role of tissue transglutaminase (TGase 2) in several pathophysiologies, wound healing applications, biomaterials functionalization, and drug delivery systems provides grounds for its use in tissue engineering. Herein, we initially studied the endogenous TGase activity and expression under normal (skin, duodenum, colon, and small bowel) and pathophysiological (keloid scar) conditions on cadaveric human tissues. Successful inhibition was achieved using low concentrations of BOC-DON-QIV-OMe (0.1 mM and 1 mM for normal skin and keloid scar, respectively), iodoacetamide (0.1 mM and 1 mM for normal skin and keloid scar, respectively), and cystamine dihydrochloride (1 mM and 10 mM for normal skin and keloid scar, respectively), whilst di-BOC-cystamine was found ineffective even at 100 mM concentration. Secondly, the addition of exogenous guinea pig liver transglutaminase (gpTGase) onto the inhibited tissues and collagen scaffolds was studied, and results presented advocate its use as potential tissue adhesive and drug delivery tool. However, the investigation of its crosslinking extent using second harmonic generation microscopy and differentially scanning calorimetry revealed rather poor stabilization function. Overall, our study indicates that TGase 2 has a role as a biological glue to consolidate various micro-structural components of tissues and biomaterials.

Transglutaminase activation in neurodegenerative diseases

The following review examines the role of calcium in promoting the in vitro and in vivo activation of transglutaminases in neurodegenerative disorders. Diseases such as Alzheimer's disease, Parkinson's disease and Huntington's disease exhibit increased transglutaminase activity and rises in intracellular calcium concentrations, which may be related. The aberrant activation of transglutaminase by calcium is thought to give rise to a variety of pathological moieties in these diseases, and the inhibition has been shown to have therapeutic benefit in animal and cellular models of neurodegeneration. Given the potential clinical relevance of transglutaminase inhibitors, we have also reviewed the recent development of such compounds.

Defining Balanced Conditions for Inhibitor Screening Assays That Target Bisubstrate Enzymes

High-throughput screening (HTS) is a common mechanism for identifying lead compounds for drug discovery efforts. Small molecules can inhibit enzymes by a variety of mechanisms, such as competitive, noncompetitive, and uncompetitive with respect to the substrate(s) of the catalytic reaction. To optimize the chances of finding the broadest diversity of inhibitor modalities during screening, one must run assays under ``balanced'' conditions where the potency of inhibitors with various modes of action falls within a similar range. When an enzyme reaction involves more than one substrate, the definition and assessment of the apparent potency of inhibitors (IC50), in relation to their true potency (Ki), can be nontrivial. This article provides a theoretical analysis, on the basis of the Cheng-Prusoff derivation, of the IC50/K i relationship of bisubstrate enzyme reactions following various sequential kinetic mechanisms, as well as the application and limitations of this information for defining optimal screening conditions for such enzymes. (Journal of Biomolecular Screening 2009:111-120)

Identification of small molecule inhibitors of the mitotic kinase haspin by high-throughput screening using a homogeneous time-resolved fluorescence resonance energy transfer assay

Haspin/Gsg2 is a kinase that phosphorylates histone H3 at Thr-3 (H3T3ph) during mitosis. Its depletion by RNA interference results in failure of chromosome alignment and a block in mitosis. Haspin, therefore, is a novel target for development of antimitotic agents. We report the development of a high-throughput time-resolved fluorescence resonance energy transfer (TR-FRET) kinase assay for haspin. Histone H3 peptide was used as a substrate, and a europium-labeled H3T3ph phosphospecific monoclonal antibody was used to detect phosphorylation. A library of 137632 small molecules was screened at K(m) concentrations of ATP and peptide to allow identification of diverse inhibitor types. Reconfirmation of hits and IC( 50) determinations were carried out with the TR-FRET assay and by a radiometric assay using recombinant histone H3 as the substrate. A preliminary assessment of specificity was made by testing inhibition of 2 unrelated kinases. EC( 50) values in cells were determined using a cell-based ELISA of H3T3ph. Five compounds were selected as leads based on potency and chemical structure considerations. These leads form the basis for the development of specific inhibitors of haspin that will have clear utility in basic research and possible use as starting points for development of antimitotic anticancer therapeutics.

Identifying druglike inhibitors of myelin-reactive T cells by phenotypic high-throughput screening of a small-molecule library

Inflammatory T cells that are reactive to myelin protein components of the CNS play a critical role in the pathogenesis of multiple sclerosis (MS) and its animal model, experimental autoimmune encephalomyelitis (EAE). The authors have previously generated mice that predominantly harbor T cells transgenic for a T-cell receptor (TCR) that is specific to the myelin proteolipid protein (PLP) 139-151 and that spontaneously develop MS-like paralysis. T cells from healthy transgenic mice respond to stimulation with PLP139-151 in a highly specific manner by proliferation and secretion of proinflammatory cytokines such as interleukin (IL)-2 and interferon (INF)-gamma in vitro. To identify druglike compounds that may inhibit inflammatory T-cell responses, the authors have developed a high-throughput screening assay with primary T cells from PLP TCR transgenic mice. They have screened 41,184 small-molecule compounds that follow Lipinski's rules for their inhibitory activity on the proliferation and secretion of proinflammatory cytokines in PLP-reactive T cells. To this end, the screen identified 6 nontoxic compounds with a molecular weight <500 that inhibited inflammatory responses in PLP-reactive T cells in a concentration-dependent fashion. The identified compounds represent valid leads that may be developed into novel therapeutics for MS that could be administered orally.

A Rapid Transglutaminase Assay for High-Throughput Screening Applications

Transglutaminases (TGs) are widely distributed enzymes that catalyze posttranslational modification of proteins by Ca2+-dependent cross-linking reactions. The family members of TGs participate in many significant processes of biological functions such as tissue regeneration, cell differentiation, apoptosis, and certain pathologies. A novel technique for TG activity assay was developed in this study. It was based on the rapid capturing, fluorescence quenching, and fast separation of the unreacted fluorescent molecules from the macromolecular product with magnetic dextran-coated charcoal. As few as 3 ng of guinea pig liver transglutaminase (gpTG) could be detected by the method; activities of 96 TG samples could be measured within an hour. The Km of gpTG determined by this method for monodansylcadaverine (dansyl-CAD) and N, N-dimethylcasein was 14 and 5 μM, respectively. A typical competitive inhibition pattern of cystamine on dansyl-CAD for gpTG activity was also demonstrated. The application of this technique is not limited to the use of dansyl-CAD as the fluorescent substrate of TG; other small fluor-labeled TG substrates may substitute dansyl-CAD. Finally, this method is rapid, highly sensitive, and inexpensive. It is suitable not only for high-throughput screening of enzymes or enzyme inhibitors but also for enzyme kinetic analysis.

A high-throughput drug screen targeted to the 5'untranslated region of Alzheimer amyloid precursor protein mRNA

The authors employed a novel approach to identify therapeutics effective in Alzheimer disease (AD). The 5'untranslated region (5'UTR) of the mRNA of AD amyloid precursor protein (APP) is a significant regulator of the levels of the APP holoprotein and amyloid beta (Abeta) peptide in the central nervous system. The authors generated stable neuroblastoma SH-SY5Y transfectants that express luciferase under the translational control of the 146-nucleotide APP mRNA 5'UTR and green fluorescent protein (GFP) driven by a viral internal ribosomal entry site. Using a high-throughput screen (HTS), they screened for the effect of 110,000 compounds obtained from the library of the Laboratory for Drug Discovery on Neurodegeneration (LDDN) on the APP mRNA 5'UTR-controlled translation of the luciferase reporter. This screening yielded several nontoxic specific inhibitors of APP mRNA 5'UTR-driven luciferase that had no effect on the GFP expression in the stable SH-SY5Y transfectants. Moreover, these compounds either did not inhibit or inhibited to a much lower extent the expression of the luciferase reporter regulated by a prion protein (PrP) mRNA 5'UTR, used as an alternative mRNA structure to counterscreen APP mRNA 5'UTR in stably transfected SH-SY5Y cell lines. The hits obtained from this robust, specific, and highly quantitative HTS will be characterized to identify agents that may be developed into useful future therapeutic agents to limit APP translation and Abeta production for AD.

Reduced transglutaminase-catalyzed protein aggregation is observed in the presence of creatine using sedimentation velocity

Transglutaminases (TGases) are enzymes that catalyze covalent isopeptide crosslinks between reactive lysine and glutamine residues in proteins. Higher than normal local concentrations of TGase have been correlated with increased protein aggregation in vivo. These insoluble protein aggregates are the hallmark of several neurodegenerative diseases, including Alzheimer's, Parkinson's, and Huntington's diseases, although each aggregating protein involved is disease specific. Because TGase is implicated in protein aggregation, there is evidence that its regulation may retard disease progression. Here we report on a laser light transmission technique as an in vitro tool to gauge the efficacy of creatine, a candidate inhibitor, to regulate aggregation. Sedimentation velocities of protein-coated particles in TGase-containing water-glycerol solutions were tracked with different levels of creatine. Sedimentation velocities were converted to apparent aggregate sizes using Stoke's law of sedimentation. The results indicated that creatine promoted up to a 20% reduction in protein aggregation in vitro. This technique may prove to be useful in identifying other functional TGase inhibitors.

Structure–activity relationship study of novel tissue transglutaminase inhibitors

Thieno[2,3-d]pyrimidin-4-one acylhydrazide derivatives were discovered as moderately potent inhibitors of TGase 2 (tissue transglutaminase) utilizing a fluorescence-based assay that measured TGase 2 catalyzed incorporation of the dansylated Lys derivative alpha-N-Boc-Lys-CH(2)-CH(2)-dansyl into the protein substrate N,N-dimethylated-casein. A SAR study revealed that the acylhydrazide thioether side-chain and the thiophene ring were critical to inhibitory activity.