Mapping of the binding sites of human histamine N-methyltransferase (HNMT) monoclonal antibodies.

OBJECTIVE: Recently, we characterized mouse monoclonal antibodies that allow the specific and sensitive detection of human histamine N-methyltransferase (HNMT). To understand differences in binding characteristics and recognition of enzyme variants we mapped the antibody binding sites. METHODS: Fragments of human HNMT were expressed as glutathione S-transferase fusion proteins that were used for testing antibody binding on immunoblots. Combined information from species cross-reactivity, sequence comparison, protein structure, and binding site prediction software were used to localize the epitope recognized by each antibody. RESULTS: All eight monoclonal HNMT antibodies bound to linear epitopes in the C-terminal domain of the 292 amino acid protein. Of the five antibodies cross-reacting with HNMT from other species, one bound region L182-T223, three region M224-E261, and one region L262-A292. All three antibodies recognising only human HNMT bound the C-terminal region L262-A292 that contains residues present only in the human protein. CONCLUSIONS: Our HNMT monoclonal antibodies bind in three different regions of the protein and those binding the same putative epitope exhibit similar binding characteristics and species cross-reactivity. Antibodies binding non-overlapping epitopes will facilitate analyses of all clinically relevant variants described for HNMT.

Leucine 208 in human histamine N-methyltransferase emerges as a hotspot for protein stability rationalizing the role of the L208P variant in intellectual disability.

The degradation of histamine catalyzed by the SAM-dependent histamine N-methyltransferase (HNMT) is critically important for the maintenance of neurological processes. Recently, two mutations in the encoding human gene were reported to give rise to dysfunctional protein variants (G60D and L208P) leading to intellectual disability. In the present study, we have expressed eight L208 variants with either apolar (L208F and L208V), polar (L208N and L208T) or charged (L208D, L208H, L208K and L208R) amino acids to define the impact of side chain variations on protein structure and function. We found that the variants L208N, L208T, L208D and L208H were severely compromised in their stability. The other four variants were obtained in lower amounts in the order wild-type HNMT>L208F=L208V>L208K=L208R. Biochemical characterization of the two variants L208F and L208V exhibited similar Michaelis-Menten parameters for SAM and histamine while the enzymatic activity was reduced to 21% and 48%, respectively. A substantial loss of enzymatic activity and binding affinity for histamine was seen for the L208K and L208R variants. Similarly the thermal stability for the latter variants was reduced by 8 and 13°C, respectively. These findings demonstrate that position 208 is extremely sensitive to side chain variations and even conservative replacements affect enzymatic function. Molecular dynamics simulations showed that amino acid replacements in position 208 perturb the helical character and disrupt interactions with the adjacent ß-strand, which is involved in the binding and correct positioning of histamine. This finding rationalizes the gradual loss of enzymatic activity observed in the L208 variants.

Predicting targets of compounds against neurological diseases using cheminformatic methodology.

Recently developed multi-targeted ligands are novel drug candidates able to interact with monoamine oxidase A and B; acetylcholinesterase and butyrylcholinesterase; or with histamine N-methyltransferase and histamine H3-receptor (H3R). These proteins are drug targets in the treatment of depression, Alzheimer's disease, obsessive disorders, and Parkinson's disease. A probabilistic method, the Parzen-Rosenblatt window approach, was used to build a "predictor" model using data collected from the ChEMBL database. The model can be used to predict both the primary pharmaceutical target and off-targets of a compound based on its structure. Molecular structures were represented based on the circular fingerprint methodology. The same approach was used to build a "predictor" model from the DrugBank dataset to determine the main pharmacological groups of the compound. The study of off-target interactions is now recognised as crucial to the understanding of both drug action and toxicology. Primary pharmaceutical targets and off-targets for the novel multi-target ligands were examined by use of the developed cheminformatic method. Several multi-target ligands were selected for further study, as compounds with possible additional beneficial pharmacological activities. The cheminformatic targets identifications were in agreement with four 3D-QSAR (H3R/D1R/D2R/5-HT2aR) models and by in vitro assays for serotonin 5-HT1a and 5-HT2a receptor binding of the most promising ligand (71/MBA-VEG8).

Molecular identification of carnosine N-methyltransferase as chicken histamine N-methyltransferase-like protein (hnmt-like).

Anserine (beta-alanyl-N(Pi)-methyl-L-histidine), a naturally occurring derivative of carnosine (beta-alanyl-L-histidine), is an abundant constituent of skeletal muscles and brain of many vertebrates. Although it has long been proposed to serve as a proton buffer, radicals scavenger and transglycating agent, its physiological function remains obscure. The formation of anserine is catalyzed by carnosine N-methyltransferase which exhibits unknown molecular identity. In the present investigation, we have purified carnosine N-methyltransferase from chicken pectoral muscle about 640-fold until three major polypeptides of about 23, 26 and 37 kDa coeluting with the enzyme were identified in the preparation. Mass spectrometry analysis of these polypeptides resulted in an identification of histamine N-methyltransferase-like (HNMT-like) protein as the only meaningful candidate. Analysis of GenBank database records indicated that the hnmt-like gene might be a paralogue of histamine N-methyltransferase gene, while comparison of their protein sequences suggested that HNMT-like protein might have acquired a new activity. Chicken HNMT-like protein was expressed in COS-7 cells, purified to homogeneity, and shown to catalyze the formation of anserine as confirmed by both chromatographic and mass spectrometry analysis. Both specificity and kinetic studies carried out on the native and recombinant enzyme were in agreement with published data. Particularly, several compounds structurally related to carnosine, including histamine and L-histidine, were tested as potential substrates for the enzyme, and carnosine was the only methyl group acceptor. The identification of the gene encoding carnosine N-methyltransferase might be beneficial for estimation of the biological functions of anserine.

The nonsynonymous Thr105Ile polymorphism of the histamine N-methyltransferase is associated to the risk of developing essential tremor.

OBJECTIVE: We analyzed in patients with essential tremor (ET) the Thr105Ile polymorphism of the Histamine N-methyltransferase (HNMT) enzyme that is associated to Parkinson's disease (PD) risk. METHODS: Leukocytary DNA from 204 ET patients and a control group of 295 unrelated healthy individuals was studied for the nonsynonymous HNMT Thr105Ile polymorphism by using amplification-restriction analyses. RESULTS: Patients with ET showed a higher frequency of homozygous HNMT 105Thr genotypes leading to high metabolic activity (p < 0.015) with a statistically significant gene-dose effect, as compared to healthy subjects. These findings were independent of gender, and of tremor localization, but the association of the HNMT polymorphism is more prominent among patients with late-onset ET (p < 0.007). CONCLUSION: These results, combined with previous findings indicating alterations in the frequency for the HNMT Thr105Ile polymorphism in patients with PD, suggest that alterations of histamine homeostasis in the SNC are associated with the risk of movement disorders.

The histamine N-methyltransferase T105I polymorphism affects active site structure and dynamics.

Histamine N-methyltransferase (HNMT) is the primary enzyme responsible for inactivating histamine in the mammalian brain. The human HNMT gene contains a common threonine-isoleucine polymorphism at residue 105, distal from the active site. The 105I variant has decreased activity and lower protein levels than the 105T protein. Crystal structures of both variants have been determined but reveal little regarding how the T105I polymorphism affects activity. We performed molecular dynamics simulations for both 105T and 105I at 37 degrees C to explore the structural and dynamic consequences of the polymorphism. The simulations indicate that replacing Thr with the larger Ile residue leads to greater burial of residue 105 and heightened intramolecular interactions between residue 105 and residues within helix alpha3 and strand beta3. This altered, tighter packing is translated to the active site, resulting in the reorientation of several cosubstrate-binding residues. The simulations also show that the hydrophobic histamine-binding domain in both proteins undergoes a large-scale breathing motion that exposes key catalytic residues and lowers the hydrophobicity of the substrate-binding site.

In-silico pharmacodynamics: correlation of adverse effects of H2-antihistamines with histamine N-methyl transferase binding potential.

Adverse effects are exhibited by most drugs in current clinical practice, the causes for which are often not known. In this post genomic era, bioinformatics has the potential to address several issues in understanding the mechanism of drug action and in designing improved drugs. This study describes the analysis of the possible pharmacodynamic behaviour of antihistamines blocking the histamine H(2) receptor (H(2)-antihistamines), by adopting the basic tenets of a systems biology approach. The different components that could form an appropriate sub-system are identified, thus providing a system landscape. Docking and analysis of the chosen antihistamines into each of these components resulted in identifying histamine N-methyl transferase (HNMT) as a potential unintended target for H(2)-antihistamines. Correlation with experimental data available from the literature indicates the inhibition of HNMT to be a possible cause for the adverse effects exhibited by these drugs. Implications for design of safer H(2)-antihistamines are discussed. The method reported here has the potential for application as a general strategy in understanding drug effects.

Structural basis for inhibition of histamine N-methyltransferase by diverse drugs.

In mammals, histamine action is terminated through metabolic inactivation by histamine N-methyltransferase (HNMT) and diamine oxidase. In addition to three well-studied pharmacological functions, smooth muscle contraction, increased vascular permeability, and stimulation of gastric acid secretion, histamine plays important roles in neurotransmission, immunomodulation, and regulation of cell proliferation. The histamine receptor H1 antagonist diphenhydramine, the antimalarial drug amodiaquine, the antifolate drug metoprine, and the anticholinesterase drug tacrine (an early drug for Alzheimer's disease) are surprisingly all potent HNMT inhibitors, having inhibition constants in the range of 10-100nM. We have determined the structural mode of interaction of these four inhibitors with HNMT. Despite their structural diversity, they all occupy the histamine-binding site, thus blocking access to the enzyme's active site. Near the N terminus of HNMT, several aromatic residues (Phe9, Tyr15, and Phe19) adopt different rotamer conformations or become disordered in the enzyme-inhibitor complexes, accommodating the diverse, rigid hydrophobic groups of the inhibitors. The maximized shape complementarity between the protein aromatic side-chains and aromatic ring(s) of the inhibitors are responsible for the tight binding of these varied inhibitors.

Search for histamine H(3) receptor ligands with combined inhibitory potency at histamine N-methyltransferase: omega-piperidinoalkanamine derivatives.

In an effort to design new hybrid compounds with dual properties, i.e. binding affinity at histamine H(3) receptors and inhibitory potency at the catabolic enzyme histamine N(tau)-methyltransferase (HMT), a novel series of 1-substituted piperidine derivatives was synthesized. This alicyclic heterocycle is structurally linked via aminoalkyl spacers of variable lengths to additional aromatic carbo- or hetero-cycles. These new hybrid drugs were pharmacologically evaluated regarding their binding affinities at recombinant human H(3) receptors, stably expressed in CHO cells, and in a functional assay for their inhibitory potencies at rat kidney HMT. All compounds investigated proved to be H(3) receptor ligands with binding affinities in the micro- to nanomolar concentration range despite significant differences in the type of the aromatic moiety introduced. The most potent compound in this series was the quinoline derivative 20 (K(i) = 5.6 nM). Likewise, all new ligands studied showed impressive HMT inhibitory activities. Here, compounds 5, 10, 14 and 18-20 exhibited submicromolar potencies (IC(50) = 0.061-0.56 microM). The aminomethylated quinoline 19 showed almost the same, well balanced nanomolar activities on both targets. In this study, new hybrid compounds with a dual mode biological action were developed. These pharmacological agents are valuable leads for further development and candidates for treatment of histamine-dependent disorders.

Genotype-phenotype correlation for histamine N-methyltransferase in a Chinese Han population.

BACKGROUND: Two potential single-nucleotide polymorphisms (SNP) (C314T and A595G) exist in the gene for human histamine N-methyltransferase (HNMT). METHODS: A radiochemical microassay was used to measure the erythrocyte HNMT activities, whereas the polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) was employed to perform the genetic analysis in 247 unrelated Chinese Han subjects. RESULTS: All subjects had detectable HNMT activity. The activity of HNMT was gender related (males>females, p<0.0001), with a 5.5-fold individual variation. The distribution of HNMT activity was compatible with a normal distribution. There were 28 heterozygotes for the variant T314 allele among the 247 subjects, whereas no A595G transition was observed. All heterozygotes for the T314 allele displayed an intermediate or low HNMT activity, with an average HNMT activity being 34.0% lower than those with wild-type genotype (623.1+/-136.0 vs. 944.8+/-249.3 U/ml red blood cells [RBC], p<0.0001). CONCLUSION: The C314T polymorphism was functionally important and contributes in part to phenotypic variance of HNMT activity in Chinese Han population. Additional unknown genetic or epigenetic factors should also play important roles in the regulation of HNMT activity.

Theoretical 3D model of histamine N-methyltransferase: insights into the effects of a genetic polymorphism on enzymatic activity and thermal stability.

Histamine N-methyltransferase (HNMT) catalyzes the N-methylation of histamine in mammals. The experimentally determined HNMT three-dimensional (3D) structure is not available. However, there is a common genetic polymorphism for human HNMT (Thr105Ile) that reduces enzymatic activity and is a risk factor for asthma. To obtain insights into mechanisms responsible for the effects of that polymorphism on enzymatic activity and thermal stability, we predicted the 3D structure of HNMT using the threading method and molecular dynamics simulations in water. Herein, we report a theoretical 3D model of human HNMT which reveals that polymorphic residue Thr105Ile is located in the turn between a beta strand and an alpha helix on the protein surface away from the active site of HNMT. Ile105 energetically destabilizes folded HNMT because of its low Chou-Fasman score for forming a turn conformation and the exposure of its hydrophobic side chain to aqueous solution. It thus promotes the formation of misfolded proteins that are prone to the clearance by proteasomes. This information explains, for the first time, how genetic polymorphisms can cause enhanced protein degradation and why the thermal stability of allozyme Ile105 is lower than that of Thr105. It also supports the hypothesis that the experimental observation of a significantly lower level of HNMT enzymatic activity for allozyme Ile105 than that with Thr105 is due to a decreased concentration of allozyme Ile105, but not an alternation of the active-site topology of HNMT caused by the difference at residue 105.

Two polymorphic forms of human histamine methyltransferase: structural, thermal, and kinetic comparisons.

BACKGROUND: Histamine plays important biological roles in cell-to-cell communication; it is a mediator in allergic responses, a regulator of gastric acid secretion, a messenger in bronchial asthma, and a neurotransmitter in the central nervous system. Histamine acts by binding to histamine receptors, and its local action is terminated primarily by methylation. Human histamine N-methyltransferase (HNMT) has a common polymorphism at residue 105 that correlates with the high- (Thr) and low- (Ile) activity phenotypes. RESULTS: Two ternary structures of human HNMT have been determined: the Thr105 variant complexed with its substrate histamine and reaction product AdoHcy and the Ile105 variant complexed with an inhibitor (quinacrine) and AdoHcy. Our steady-state kinetic data indicate that the recombinant Ile105 variant shows 1.8- and 1.3-fold increases in the apparent K(M) for AdoMet and histamine, respectively, and slightly (16%) but consistently lower specific activity as compared to that of the Thr105 variant. These differences hold over a temperature range of 25 degrees C-45 degrees C in vitro. Only at a temperature of 50 degrees C or higher is the Ile105 variant more thermolabile than the Thr105 enzyme. CONCLUSIONS: HNMT has a 2 domain structure including a consensus AdoMet binding domain, where the residue 105 is located on the surface, consistent with the kinetic data that the polymorphism does not affect overall protein stability at physiological temperatures but lowers K(M) values for AdoMet and histamine. The interactions between HNMT and quinacrine provide the first structural insights into a large group of pharmacologic HNMT inhibitors and their mechanisms of inhibition.

Guinea pig histamine N-methyltransferase: cDNA cloning and mRNA distribution.

We report here the isolation of histamine N-methyltransferase (HMT) cDNA from the guinea pig brain by the polymerase chain reaction on the basis of nucleotide sequences of rat and human counterparts. Guinea pig HMT consists of 292 amino acids, with homologies of 75.6% and 79.1% to rat and human HMT, respectively. Northern blotting analysis indicated that the 1.6-kb guinea pig HMT transcript was expressed at various levels in different tissues at the following relative abundance: jejunum, brain > lung, spleen, stomach > liver, kidney. HMT mRNA localized throughout the jejunum, and it was mainly expressed in epithelial cells and in Auerbach's plexus.

Human histamine N-methyltransferase pharmacogenetics: common genetic polymorphisms that alter activity.

Histamine N-methyltransferase (HNMT) catalyzes a major pathway in histamine metabolism. Levels of HNMT activity in humans are regulated by inheritance. We set out to study the molecular basis for this genetic regulation. Northern blot analysis showed that HNMT is highly expressed in the kidney, so we determined levels of enzyme activity and thermal stability in 127 human renal biopsy samples. DNA was isolated from 12 kidney samples with widely different HNMT phenotypes, and exons of the HNMT gene were amplified with the polymerase chain reaction. In these 12 samples, we observed a C314T transition that resulted in a Thr105Ile change in encoded amino acid, as well as an A939G transition within the 3'-untranslated region. All remaining renal biopsy samples then were genotyped for these two variant sequences. Frequencies of the alleles encoding Thr105 and Ile105 in the 114 samples studied were 0.90 and 0.10, respectively, whereas frequencies for the nucleotide A939 and G alleles were 0.79 and 0.21, respectively. Kidney samples with the allele encoding Ile105 had significantly lower levels of HNMT activity and thermal stability than did those with the allele that encoded Thr105. These observations were confirmed by transient expression in COS-1 cells of constructs that contained all four alleles for these two polymorphisms. COS-1 cells transfected with the Ile105 allele had significantly lower HNMT activity and immunoreactive HNMT protein than did those transfected with the Thr105 allele. These observations will make it possible to test the hypothesis that genetic polymorphisms for HNMT may play a role in the pathophysiology of human disease.

Structural analysis of histamine N-methyltransferase gene.

A clone encoding a part of rat histamine N-tele-methyltransferase gene of 11 kb was isolated. The clone contained 4 exons, encoding from 191 to the 3' end of cDNA. The last exon was 692 bases long and specified more than half of the HMT cDNA. A comparison of the sequences of rat and human cDNAs shows that more than one-third of the human 3' untranslated region does not correspond to the rat counterpart, but a homology was found between this region of human cDNA and the 3' franking region of the rat gene. It was found that an exon was interrupted at 4 residues after a glycine residue, which putatively corresponds to the conserved residue among methyltransferases.