The bifunctional enzyme leukotriene-A4 hydrolase is an arginine aminopeptidase of high efficiency and specificity

Activation of leukotriene B4 receptor 1 is a prerequisite for complement receptor 3-mediated antifungal responses of neutrophils

Invasive fungal infections are life-threatening, and neutrophils are vital cells of the innate immune system that defend against them. The role of LTA4H-LTB4-BLT1 axis in regulation of neutrophil responses to fungal infection remains poorly understood. Here, we demonstrated that the LTA4H-LTB4-BLT1 axis protects the host against Candida albicans and Aspergillus fumigatus, but not Cryptococcus neoformans infection, by regulating the antifungal activity of neutrophils. Our results show that deleting Lta4h or Blt1 substantially impairs the fungal-specific phagocytic capacity of neutrophils. Moreover, defective activation of the spleen tyrosine kinase (Syk) and extracellular signal-related kinase (ERK1/2) pathways in neutrophils accompanies this impairment. Mechanistically, BLT1 regulates CR3-mediated, β-1,3-glucan-induced neutrophil phagocytosis, while a physical interaction with CR3 with slight influence on its dynamics is observed. Our findings thus demonstrate that the LTA4H-LTB4-BLT1 axis is essential for the phagocytic function of neutrophils in host antifungal immune response against Candida albicans and Aspergillus fumigatus.

Leukotriene A4 hydrolase inhibition improves age-related cognitive decline via modulation of synaptic function

Leukotrienes, a class of inflammatory bioactive lipids, are well studied in the periphery, but less is known of their importance in the brain. We identified that the enzyme leukotriene A4 hydrolase (LTA4H) is expressed in healthy mouse neurons, and inhibition of LTA4H in aged mice improves hippocampal dependent memory. Single-cell nuclear RNA sequencing of hippocampal neurons after inhibition reveals major changes to genes important for synaptic organization, structure, and activity. We propose that LTA4H inhibition may act to improve cognition by directly inhibiting the enzymatic activity in neurons, leading to improved synaptic function. In addition, LTA4H plasma levels are increased in both aging and Alzheimer's disease and correlated with cognitive impairment. These results identify a role for LTA4H in the brain, and we propose that LTA4H inhibition may be a promising therapeutic strategy to treat cognitive decline in aging related diseases.

Modulation of the 5-Lipoxygenase Pathway by Chalcogen-Containing Inhibitors of Leukotriene A4 Hydrolase

The 5-lipoxygenase (5-LOX) pathway gives rise to bioactive inflammatory lipid mediators, such as leukotrienes (LTs). 5-LOX carries out the oxygenation of arachidonic acid to the 5-hydroperoxy derivative and then to the leukotriene A₄ epoxide which is converted to a chemotactic leukotriene B₄ (LTB₄) by leukotriene A₄ hydrolase (LTA₄H). In addition, LTA₄H possesses aminopeptidase activity to cleave the N-terminal proline of a pro-inflammatory tripeptide, prolyl-glycyl-proline (PGP). Based on the structural characteristics of LTA₄H, it is possible to selectively inhibit the epoxide hydrolase activity while sparing the inactivating, peptidolytic, cleavage of PGP. In the current study, chalcogen-containing compounds, 4-(4-benzylphenyl) thiazol-2-amine (ARM1) and its selenazole (TTSe) and oxazole (TTO) derivatives were characterized regarding their inhibitory and binding properties. All three compounds selectively inhibit the epoxide hydrolase activity of LTA₄H at low micromolar concentrations, while sparing the aminopeptidase activity. These inhibitors also block the 5-LOX activity in leukocytes and have distinct inhibition constants with recombinant 5-LOX. Furthermore, high-resolution structures of LTA₄H with inhibitors were determined and potential binding sites to 5-LOX were proposed. In conclusion, we present chalcogen-containing inhibitors which differentially target essential steps in the biosynthetic route for LTB₄ and can potentially be used as modulators of inflammatory response by the 5-LOX pathway.

Parasite Metalo-aminopeptidases as Targets in Human Infectious Diseases

BACKGROUND

Parasitic human infectious diseases are a worldwide health problem due to the increased resistance to conventional drugs. For this reason, the identification of novel molecular targets and the discovery of new chemotherapeutic agents are urgently required. Metalo- aminopeptidases are promising targets in parasitic infections. They participate in crucial processes for parasite growth and pathogenesis.

OBJECTIVE

In this review, we describe the structural, functional and kinetic properties, and inhibitors, of several parasite metalo-aminopeptidases, for their use as targets in parasitic diseases.

CONCLUSION

Plasmodium falciparum M1 and M17 aminopeptidases are essential enzymes for parasite development, and M18 aminopeptidase could be involved in hemoglobin digestion and erythrocyte invasion and egression. Trypanosoma cruzi, T. brucei and Leishmania major acidic M17 aminopeptidases can play a nutritional role. T. brucei basic M17 aminopeptidase down-regulation delays the cytokinesis. The inhibition of Leishmania basic M17 aminopeptidase could affect parasite viability. L. donovani methionyl aminopeptidase inhibition prevents apoptosis but not the parasite death. Decrease in Acanthamoeba castellanii M17 aminopeptidase activity produces cell wall structural modifications and encystation inhibition. Inhibition of Babesia bovis growth is probably related to the inhibition of the parasite M17 aminopeptidase, probably involved in host hemoglobin degradation. Schistosoma mansoni M17 aminopeptidases inhibition may affect parasite development, since they could participate in hemoglobin degradation, surface membrane remodeling and eggs hatching. Toxoplasma gondii M17 aminopeptidase inhibition could attenuate parasite virulence, since it is apparently involved in the hydrolysis of cathepsin Cs- or proteasome-produced dipeptides and/or cell attachment/invasion processes. These data are relevant to validate these enzymes as targets.

Substrate-dependent modulation of the leukotriene A4 hydrolase aminopeptidase activity and effect in a murine model of acute lung inflammation

The aminopeptidase activity (AP) of the leukotriene A4 hydrolase (LTA4H) enzyme has emerged as a therapeutic target to modulate host immunity. Initial reports focused on the benefits of augmenting the LTA4H AP activity and clearing its putative pro-inflammatory substrate Pro-Gly-Pro (PGP). However, recent reports have introduced substantial complexity disconnecting the LTA4H modulator 4-methoxydiphenylmethane (4MDM) from PGP as follows: (1) 4MDM inhibits PGP hydrolysis and subsequently inhibition of LTA4H AP activity, and (2) 4MDM activates the same enzyme target in the presence of alternative substrates. Differential modulation of LTA4H by 4MDM was probed in a murine model of acute lung inflammation, which showed that 4MDM modulates the host neutrophilic response independent of clearing PGP. X-ray crystallography showed that 4MDM and PGP bind at the zinc binding pocket and no allosteric binding was observed. We then determined that 4MDM modulation is not dependent on the allosteric binding of the ligand, but on the N-terminal side chain of the peptide. In conclusion, our study revealed that a peptidase therapeutic target can interact with its substrate and ligand in complex biochemical mechanisms. This raises an important consideration when ligands are designed to explain some of the unpredictable outcomes observed in therapeutic discovery targeting LTA4H.

Design and synthesis of Leukotriene A4 hydrolase inhibitors to alleviate idiopathic pulmonary fibrosis and acute lung injury

Idiopathic pulmonary fibrosis (IPF) and acute lung injury (ALI) are considered two severe public health issues, attributed to malfunctions of neutrophils. They can cause chronic inflammation and have association with subsequent tissue damages. There have been rare drugs applying to the efficient treatment in clinical practice. Existing research revealed that Leukotriene B4 (LTB4) is the critical endogenous molecule to induce neutrophil inflammatory response. LTB4 blocking biosynthesis is the potential strategy treating IPF and ALI. In the present study, 45 hydroxamic acid derivatives were produced, and compound 26 was screened out as a highly selective Lead compound of Leukotriene A4 Hydrolase (LTA4H), i.e., an enzyme critical to the biosynthesis of LTB4. This compound is capable of relieving neutrophilic inflammation in an IPF mouse model at early stage, as well as mitigating LPS-induced acute lung injury via a mechanism of LTB4 blocking biosynthesis in vivo. Whether this compound acts as the potential lead compound for the treatment of IPF and ALI requires further verification.

Leukotriene A4 hydrolase deficiency protects mice from diet-induced obesity by increasing energy expenditure through neuroendocrine axis

Obesity is a health problem worldwide, and brown adipose tissue (BAT) is important for energy expenditure. Here, we explored the role of leukotriene A₄ hydrolase (LTA₄ H), a key enzyme in the synthesis of the lipid mediator leukotriene B₄ (LTB₄ ), in diet-induced obesity. LTA₄ H-deficient (LTA₄ H-KO) mice fed a high-fat diet (HFD) showed a lean phenotype, and bone-marrow transplantation studies revealed that LTA₄ H-deficiency in non-hematopoietic cells was responsible for this lean phenotype. LTA₄ H-KO mice exhibited greater energy expenditure, but similar food intake and fecal energy loss. LTA₄ H-KO BAT showed higher expression of thermogenesis-related genes. In addition, the plasma thyroid-stimulating hormone and thyroid hormone concentrations, as well as HFD-induced catecholamine secretion, were higher in LTA₄ H-KO mice. In contrast, LTB₄ receptor (BLT1)-deficient mice did not show a lean phenotype, implying that the phenotype of LTA₄ H-KO mice is independent of the LTB₄ /BLT1 axis. These results indicate that LTA₄ H mediates the diet-induced obesity by reducing catecholamine and thyroid hormone secretion.

The enzymology of human eicosanoid pathways: the lipoxygenase branches

Eicosanoids are short-lived derivatives of polyunsaturated fatty acids that serve as autocrine and paracrine signaling molecules. They are involved numerous biological processes of both the well state and disease states. A thorough understanding of the progression the disease state and homeostasis of the well state requires a complete evaluation of the systems involved. This review examines the enzymology for the enzymes involved in the production of eicosanoids along the lipoxygenase branches of the eicosanoid pathways with particular emphasis on those derived from arachidonic acid. The enzymatic parameters, protocols to measure them, and proposed catalytic mechanisms are presented in detail.

Two-domain aminopeptidase of M1 family: Structural features for substrate binding and gating in absence of C-terminal domain

Zinc metallopeptidases of the M1 family (M1 peptidases) with unique metal binding motif HEXXH(X)₁₈E regulate many important biological processes such as tumor growth, angiogenesis, hormone regulation, and immune cell development. Typically, these enzymes exist in three-domain [N-terminal domain (N-domain), catalytic domain, and C-terminal domain (C-domain)] or four-domain (N-domain, catalytic domain, middle domain, and C-domain) format in which N-domain and catalytic domain are more conserved. The C-domain plays important roles in substrate binding and gating. In this study we report the first structure of a two-domain (N-domain and catalytic domain) M1 peptidase at 2.05 Å resolution. Despite the lack of C-domain, the enzyme is active and prefers peptide substrates with large hydrophobic N-terminal residues. Its substrate-bound structure was determined at 1.9 Å resolution. Structural analyses supported by site directed mutagenesis and molecular dynamics simulations reveal structural features that could compensate for the lack of C-domain. A unique loop insertion (loop A) in the N-domain has important roles in gating and desolvation of active site. Three Arg residues of the catalytic domain are involved in substrate-binding roles typically played by positively charged residues of C-domain in other M1 peptidases. Further, its unique exopeptidase sequence motif, LALET, creates a more hydrophobic environment at the S1 subsite (which binds N-terminal residue of the substrate in aminopeptidases) than the more common GXMEN motif in the family. This leads to high affinity for large hydrophobic residues in the S1 subsite, which contributes towards efficient substrate binding in absence of C-domain.

LTA4H regulates cell cycle and skin carcinogenesis

Leukotriene A4 hydrolase (LTA4H), a bifunctional zinc metallo-enzyme, is reportedly overexpressed in several human cancers. Our group has focused on LTA4H as a potential target for cancer prevention and/or therapy. In the present study, we report that LTA4H is a key regulator of cell cycle at the G0/G1 phase acting by negatively regulating p27 expression in skin cancer. We found that LTA4H is overexpressed in human skin cancer tissue. Knocking out LTA4H significantly reduced skin cancer development in the 7,12-dimethylbenz(a)anthracene (DMBA)-initiated/12-O-tetradecanoylphorbol-13-acetate (TPA)-promoted two-stage skin cancer mouse model. LTA4H depletion dramatically decreased anchorage-dependent and -independent skin cancer cell growth by inducing cell cycle arrest at the G0/G1 phase. Moreover, our findings showed that depletion of LTA4H enhanced p27 protein stability, which was associated with decreased phosphorylation of CDK2 at Thr160 and inhibition of the CDK2/cyclin E complex, resulting in down-regulated p27 ubiquitination. These findings indicate that LTA4H is critical for skin carcinogenesis and is an important mediator of cell cycle and the data begin to clarify the mechanisms of LTA4H's role in cancer development.

Protective effects of bestatin in the retina of streptozotocin-induced diabetic mice

CD13/APN (aminopeptidase N) was first identified as a selective angiogenic marker expressed in tumor vasculature and is considered a target for anti-cancer therapy. CD13 was also reported to express in non-diabetic, hypoxia-induced retinal neovascularization. Whether diabetes induces upregulation of CD13 expression in the retina is unknown. We hypothesize that at an early stage of non-proliferative diabetic retinopathy (NPDR) characterized by disruption of blood-retinal barrier (BRB) permeability is related to upregulated expression of CD13 because of its known role in extracellular matrix (ECM) degradation. The purpose of this study is to evaluate the role of CD13/APN and the therapeutic efficacy of a CD13/APN inhibitor in a mouse model of streptozotocin-induced NPDR. Hyperglycemic C57Bl/6 mice 26 weeks after streptozotocin (STZ) injection were intravitreally injected with a sustained release formulation of CD13/APN inhibitor bestatin. At 15th day of post-bestatin treatment, mouse retinas were evaluated for vascular permeability by Evans blue dye extravasation assay, fluorescent angiography of retinal vascular permeability and leukostasis. Retinal protein extracts were analyzed by Western blot to determine the effects of bestatin treatment on the expression of CD13/APN related inflammatory mediators of ECM degradation and angiogenesis. Intravitreal bestatin treatment significantly inhibited retinal vascular permeability and leukostasis. This treatment also significantly inhibited retinal expression of CD13, ECM degrading proteases (heparanase and MMP9 and angiogenic molecules (HIF-1α and VEGF). Intravitreal CD13 inhibition may relate to furthering our knowledge on the protective effect of bestatin against diabetic retinal vasculature abnormalities through inhibition of retinal permeability, leukostasis, inflammatory molecules of ECM degradation and angiogenesis.

Binding of Pro-Gly-Pro at the active site of leukotriene A4 hydrolase/aminopeptidase and development of an epoxide hydrolase selective inhibitor

Leukotriene (LT) A4 hydrolase/aminopeptidase (LTA4H) is a bifunctional zinc metalloenzyme that catalyzes the committed step in the formation of the proinflammatory mediator LTB4. Recently, the chemotactic tripeptide Pro-Gly-Pro was identified as an endogenous aminopeptidase substrate for LTA4 hydrolase. Here, we determined the crystal structure of LTA4 hydrolase in complex with a Pro-Gly-Pro analog at 1.72 Å. From the structure, which includes the catalytic water, and mass spectrometric analysis of enzymatic hydrolysis products of Pro-Gly-Pro, it could be inferred that LTA4 hydrolase cleaves at the N terminus of the palindromic tripeptide. Furthermore, we designed a small molecule, 4-(4-benzylphenyl)thiazol-2-amine, denoted ARM1, that inhibits LTB4 synthesis in human neutrophils (IC50 of ∼0.5 μM) and conversion of LTA4 into LTB4 by purified LTA4H with a Ki of 2.3 μM. In contrast, 50- to 100-fold higher concentrations of ARM1 did not significantly affect hydrolysis of Pro-Gly-Pro. A 1.62-Å crystal structure of LTA4 hydrolase in a dual complex with ARM1 and the Pro-Gly-Pro analog revealed that ARM1 binds in the hydrophobic pocket that accommodates the ω-end of LTA4, distant from the aminopeptidase active site, thus providing a molecular basis for its inhibitory profile. Hence, ARM1 selectively blocks conversion of LTA4 into LTB4, although sparing the enzyme's anti-inflammatory aminopeptidase activity (i.e., degradation and inactivation of Pro-Gly-Pro). ARM1 represents a new class of LTA4 hydrolase inhibitor that holds promise for improved anti-inflammatory properties.

A remarkable activity of human leukotriene A4 hydrolase (LTA4H) toward unnatural amino acids

Leukotriene A₄ hydrolase (LTA4H––EC 3.3.2.6) is a bifunctional zinc metalloenzyme, which processes LTA₄ through an epoxide hydrolase activity and is also able to trim one amino acid at a time from N-terminal peptidic substrates via its aminopeptidase activity. In this report, we have utilized a library of 130 individual proteinogenic and unnatural amino acid fluorogenic substrates to determine the aminopeptidase specificity of this enzyme. We have found that the best proteinogenic amino acid recognized by LTA4H is arginine. However, we have also observed several unnatural amino acids, which were significantly better in terms of cleavage rate (k_cat/K_m values). Among them, the benzyl ester of aspartic acid exhibited a k_cat/K_m value that was more than two orders of magnitude higher (1.75 × 10⁵ M⁻¹ s⁻¹) as compared to l-Arg (1.5 × 10³ M⁻¹ s⁻¹). This information can be used for design of potent inhibitors of this enzyme, but may also suggest yet undiscovered functions or specificities of LTA4H.

A sensitive fluorigenic substrate for selective in vitro and in vivo assay of leukotriene A4 hydrolase activity

Leukotriene A4 hydrolase (LTA4H) is a bifunctional zinc-dependent metalloprotease bearing both an epoxide hydrolase, producing the pro-inflammatory LTB4 leukotriene, and an aminopeptidase activity, whose physiological relevance has long been ignored. Distinct substrates are commonly used for each activity, although none is completely satisfactory; LTA4, substrate for the hydrolase activity, is unstable and inactivates the enzyme, whereas aminoacids β-naphthylamide and para-nitroanilide, used as aminopeptidase substrates, are poor and nonselective. Based on the three-dimensional structure of LTA4H, we describe a new, specific, and high-affinity fluorigenic substrate, PL553 [L-(4-benzoyl)phenylalanyl-β-naphthylamide], with both in vitro and in vivo applications. PL553 possesses a catalytic efficiency (k(cat)/K(m)) of 3.8±0.5×10⁴ M⁻¹ s⁻¹ using human recombinant LTA4H and a limit of detection and quantification of less than 1 to 2 ng. The PL553 assay was validated by measuring the inhibitory potency of known LTA4H inhibitors and used to characterize new specific amino-phosphinic inhibitors. The LTA4H inhibition measured with PL553 in mouse tissues, after intravenous administration of inhibitors, was also correlated with a reduction in LTB4 levels. This authenticates the assay as the first allowing the easy measurement of endogenous LTA4H activity and in vitro specific screening of new LTA4H inhibitors.

Structural origins for the loss of catalytic activities of bifunctional human LTA4H revealed through molecular dynamics simulations

Human leukotriene A4 hydrolase (hLTA4H), which is the final and rate-limiting enzyme of arachidonic acid pathway, converts the unstable epoxide LTA4 to a proinflammatory lipid mediator LTB4 through its hydrolase function. The LTA4H is a bi-functional enzyme that also exhibits aminopeptidase activity with a preference over arginyl tripeptides. Various mutations including E271Q, R563A, and K565A have completely or partially abolished both the functions of this enzyme. The crystal structures with these mutations have not shown any structural changes to address the loss of functions. Molecular dynamics simulations of LTA4 and tripeptide complex structures with functional mutations were performed to investigate the structural and conformation changes that scripts the observed differences in catalytic functions. The observed protein-ligand hydrogen bonds and distances between the important catalytic components have correlated well with the experimental results. This study also confirms based on the structural observation that E271 is very important for both the functions as it holds the catalytic metal ion at its location for the catalysis and it also acts as N-terminal recognition residue during peptide binding. The comparison of binding modes of substrates revealed the structural changes explaining the importance of R563 and K565 residues and the required alignment of substrate at the active site. The results of this study provide valuable information to be utilized in designing potent hLTA4H inhibitors as anti-inflammatory agents.

Glu121-Lys319 salt bridge between catalytic and N-terminal domains is pivotal for the activity and stability of Escherichia coli aminopeptidase N

Escherichia coli aminopeptidase N (ePepN) belongs to the gluzincin family of M1 class metalloproteases that share a common primary structure with consensus zinc binding motif (HEXXH-(X18)-E) and an exopeptidase motif (GXMEN) in the active site. There is one amino acid, E121 in Domain I that blocks the extended active site grove of the thermolysin like catalytic domain (Domain II) limiting the substrate to S1 pocket. E121 forms a part of the S1 pocket, while making critical contact with the amino-terminus of the substrate. In addition, the carboxylate of E121 forms a salt bridge with K319 in Domain II. Both these residues are absolutely conserved in ePepN homologs. Analogous Glu-Asn pair in tricon interacting factor F3 (F3) and Gln-Asn pair in human leukotriene A(4) hydrolase (LTA(4) H) are also conserved in respective homologs. Mutation of either of these residues individually or together substantially reduced or entirely eliminated enzymatic activity. In addition, thermal denaturation studies suggest that the mutation at K319 destabilizes the protein as much as by 3.7 °C, while E121 mutants were insensitive. Crystal structure of E121Q mutant reveals that the enzyme is inactive due to the reduced S1 subsite volume. Together, data presented here suggests that ePepN, F3, and LTA(4) H homologs adopted a divergent evolution that includes E121-K319 or its analogous pairs, and these cannot be interchanged.

Engagement of the S1, S1' and S2' subsites drives efficient catalysis of peptide bond hydrolysis by the M1-family aminopeptidase from Plasmodium falciparum

The M1-family aminopeptidase PfA-M1 catalyzes the last step in the catabolism of human hemoglobin to amino acids in the Plasmodium falciparum food vacuole. In this study, the structural features of the substrate that promote efficient PfA-M1-catalyzed peptide bond hydrolysis were analyzed. X-Ala and Ala-X dipeptide substrates were employed to characterize the specificities of the enzyme's S1 and S1' subsites. Both subsites exhibited a preference for basic and hydrophobic sidechains over polar and acidic sidechains. The relative specificity of the S1 subsite was similar over the pH range 5.5-7.5. Substrate P1 and P1' residues affected both K(m) and k(cat), revealing that sidechain-subsite interactions not only drive the formation of the Michaelis complex but also influence the rates of ensuing chemical steps. Only a small fraction of the available binding energy was exploited in interactions between substrate sidechains and the S1 and S1' subsites, which indicates a modest level of complementarity. There was no correlation between S1 and S1' specificities and amino acid abundance in hemoglobin. Interactions between PfA-M1 and the backbone atoms of the P1' and P2' residues as well as the P2' sidechain further contributed to the catalytic efficiency of substrate hydrolysis. By demonstrating the engagement of multiple, broad-specificity subsites in PfA-M1, these studies provide insight into how this enzyme is able to efficiently generate amino acids from highly sequence-diverse di- and oligopeptides in the food vacuole.

A glutathione peroxidase, intracellular peptidases and the TOR complexes regulate peptide transporter PEPT-1 in C. elegans

The intestinal peptide transporter PEPT-1 in Caenorhabditis elegans is a rheogenic H⁺-dependent carrier responsible for the absorption of di- and tripeptides. Transporter-deficient pept-1(lg601) worms are characterized by impairments in growth, development and reproduction and develop a severe obesity like phenotype. The transport function of PEPT-1 as well as the influx of free fatty acids was shown to be dependent on the membrane potential and on the intracellular pH homeostasis, both of which are regulated by the sodium-proton exchanger NHX-2. Since many membrane proteins commonly function as complexes, there could be proteins that possibly modulate PEPT-1 expression and function. A systematic RNAi screening of 162 genes that are exclusively expressed in the intestine combined with a functional transport assay revealed four genes with homologues existing in mammals as predicted PEPT-1 modulators. While silencing of a glutathione peroxidase surprisingly caused an increase in PEPT-1 transport function, silencing of the ER to Golgi cargo transport protein and of two cytosolic peptidases reduced PEPT-1 transport activity and this even corresponded with lower PEPT-1 protein levels. These modifications of PEPT-1 function by gene silencing of homologous genes were also found to be conserved in the human epithelial cell line Caco-2/TC7 cells. Peptidase inhibition, amino acid supplementation and RNAi silencing of targets of rapamycin (TOR) components in C. elegans supports evidence that intracellular peptide hydrolysis and amino acid concentration are a part of a sensing system that controls PEPT-1 expression and function and that involves the TOR complexes TORC1 and TORC2.

Effect of the leukotriene A4 hydrolase aminopeptidase augmentor 4-methoxydiphenylmethane in a pre-clinical model of pulmonary emphysema

The leukotriene A(4) hydrolase enzyme is a dual functioning enzyme with the following two catalytic activities: an epoxide hydrolase function that transforms the lipid metabolite leukotriene A(4) to leukotriene B(4) and an aminopeptidase function that hydrolyzes short peptides. To date, all drug discovery efforts have focused on the epoxide hydrolase activity of the enzyme, because of extensive biological characterization of the pro-inflammatory properties of its metabolite, leukotriene B(4). Herein, we have designed a small molecule, 4-methoxydiphenylmethane, as a pharmacological agent that is bioavailable and augments the aminopeptidase activity of the leukotriene A(4) hydrolase enzyme. Pre-clinical evaluation of our drug showed protection against intranasal elastase-induced pulmonary emphysema in murine models.

A leukotriene A4 hydrolase-related aminopeptidase from yeast undergoes induced fit upon inhibitor binding

Vertebrate leukotriene A(4) hydrolases are bifunctional zinc metalloenzymes with an epoxide hydrolase and an aminopeptidase activity. In contrast, highly homologous enzymes from lower organisms only have the aminopeptidase activity. From sequence comparisons, it is not clear why this difference occurs. In order to obtain more information on the evolutionary relationship between these enzymes and their activities, the structure of a closely related leucine aminopeptidase from Saccharomyces cerevisiae that only shows a very low epoxide hydrolase activity was determined. To investigate the molecular architecture of the active site, the structures of both the native protein and the protein in complex with the aminopeptidase inhibitor bestatin were solved. These structures show a more spacious active site, and the protected cavity in which the labile substrate leukotriene A(4) is bound in the human enzyme is partially obstructed and in other parts is more solvent accessible. Furthermore, the enzyme undergoes induced fit upon binding of the inhibitor bestatin, leading to a movement of the C-terminal domain. The main triggers for the domain movement are a conformational change of Tyr312 and a subtle change in backbone conformation of the PYGAMEN fingerprint region for peptide substrate recognition. This leads to a change in the hydrogen-bonding network pulling the C-terminal domain into a different position. Inasmuch as bestatin is a structural analogue of a leucyl dipeptide and may be regarded as a transition state mimic, our results imply that the enzyme undergoes induced fit during substrate binding and turnover.

A critical role for LTA4H in limiting chronic pulmonary neutrophilic inflammation

Leukotriene A(4) hydrolase (LTA(4)H) is a proinflammatory enzyme that generates the inflammatory mediator leukotriene B(4) (LTB(4)). LTA(4)H also possesses aminopeptidase activity with unknown substrate and physiological importance; we identified the neutrophil chemoattractant proline-glycine-proline (PGP) as this physiological substrate. PGP is a biomarker for chronic obstructive pulmonary disease (COPD) and is implicated in neutrophil persistence in the lung. In acute neutrophil-driven inflammation, PGP was degraded by LTA(4)H, which facilitated the resolution of inflammation. In contrast, cigarette smoke, a major risk factor for the development of COPD, selectively inhibited LTA(4)H aminopeptidase activity, which led to the accumulation of PGP and neutrophils. These studies imply that therapeutic strategies inhibiting LTA(4)H to prevent LTB(4) generation may not reduce neutrophil recruitment because of elevated levels of PGP.

Asparagine362 is essential for zinc binding and catalysis in the peptidase reaction of Saccharomyces cerevisiae leukotriene A₄ hydrolase

Zinc metallopeptidases are ubiquitous enzymes with diverse cellular functions that can be found in most organisms. Leukotriene A₄ hydrolase (LTA4H; E.C. 3.3.2.6) is an unusual zinc metallopeptidase of the M1 family that also possesses an epoxide hydrolase activity; however, the role of its peptidase activity remains unknown. To further characterize the peptidase activity of LTA4H and other closely related metallopeptidases, a multiple sequence alignment and predicted structure were used to target three amino acid residues of yeast LTA4H for mutagenesis: Asn362, Trp365, and Asp399. Although mutating Trp365 and Asp399 had little effect on catalysis, altering Asn362 had varying effects on catalysis, depending on the replacement residue. Mutation of Asn362 to glutamine (N362Q) caused minor catalytic defects, while mutation to leucine (N362L) or glutamate (N362E) caused large reductions in activity. Both N362L and N362E also exhibited an altered pH dependence of catalysis, reduced chloride activation, and reduced zinc affinity and content, indicating that Asn362 may interact with the nearby zinc coordinating residue His344, and possibly with Glu363 as well, to polarize and/or orient these residues.

Structure-based dissection of the active site chemistry of leukotriene A4 hydrolase: implications for M1 aminopeptidases and inhibitor design

M1 aminopeptidases comprise a large family of biologically important zinc enzymes. We show that peptide turnover by the M1 prototype, leukotriene A4 hydrolase/aminopeptidase, involves a shift in substrate position associated with exchange of zinc coordinating groups, while maintaining the overall coordination geometry. The transition state is stabilized by residues conserved among M1 members and in the final reaction step, Glu-296 of the canonical zinc binding HEXXH motif shuffles a proton from the hydrolytic water to the leaving group. Tripeptide substrates bind along the conserved GXMEN motif, precisely occupying the distance between Glu-271 and Arg-563, whereas the Arg specificity is governed by a narrow S1 pocket capped with Asp-375. Our data provide detailed insights to the active site chemistry of M1 aminopeptidases and will aid in the development of novel enzyme inhibitors.

Structural basis for the unusual specificity of Escherichia coli aminopeptidase N

Aminopeptidase N from Escherichia coli is a M1 class aminopeptidase with the active-site region related to that of thermolysin. The enzyme has unusual specificity, cleaving adjacent to the large, nonpolar amino acids Phe and Tyr but also cleaving next to the polar residues Lys and Arg. To try to understand the structural basis for this pattern of hydrolysis, the structure of the enzyme was determined in complex with the amino acids L-arginine, L-lysine, L-phenylalanine, L-tryptophan, and L-tyrosine. These amino acids all bind with their backbone atoms close to the active-site zinc ion and their side chain occupying the S1 subsite. This subsite is in the form of a cylinder, about 10 A in cross-section and 12 A in length. The bottom of the cylinder includes the zinc ion and a number of polar side chains that make multiple hydrogen-bonding and other interactions with the alpha-amino group and the alpha-carboxylate of the bound amino acid. The walls of the S1 cylinder are hydrophobic and accommodate the nonpolar or largely nonpolar side chains of Phe and Tyr. The top of the cylinder is polar in character and includes bound water molecules. The epsilon-amino group of the bound lysine side chain and the guanidinium group of arginine both make multiple hydrogen bonds to this part of the S1 site. At the same time, the hydrocarbon part of the lysine and arginine side chains is accommodated within the nonpolar walls of the S1 cylinder. This combination of hydrophobic and hydrophilic binding surfaces explains the ability of ePepN to cleave Lys, Arg, Phe, and Tyr. Another favored substrate has Ala at the P1 position. The short, nonpolar side chain of this residue can clearly be bound within the hydrophobic part of the S1 cylinder, but the reason for its facile hydrolysis remains uncertain.

Identification of modulating residues defining the catalytic cleft of insulin-regulated aminopeptidase

Inhibition of insulin-regulated aminopeptidase (IRAP) has been demonstrated to facilitate memory in rodents, making IRAP a potential target for the development of cognitive enhancing therapies. In this study, we generated a 3-D model of the catalytic domain of IRAP based on the crystal structure of leukotriene A4 hydrolase (LTA4H). This model identified two key residues at the ‘entrance’ of the catalytic cleft of IRAP, Ala427 and Leu483, which present a more open arrangement of the S1 subsite compared with LTA4H. These residues may define the size and 3-D structure of the catalytic pocket, thereby conferring substrate and inhibitor specificity. Alteration of the S1 subsite by the mutation A427Y in IRAP markedly increased the rate of substrate cleavage V of the enzyme for a synthetic substrate, although a corresponding increase in the rate of cleavage of peptide substrates Leu-enkephalin and vasopressin was was not apparent. In contrast, [L483F]IRAP demonstrated a 30-fold decrease in activity due to changes in both substrate affinity and rate of substrate cleavage. [L483F]IRAP, although capable of efficiently cleaving the N-terminal cysteine from vasopressin, was unable to cleave the tyrosine residue from either Leu-enkephalin or Cyt6-desCys1-vasopressin (2–9), both substrates of IRAP. An 11-fold reduction in the affinity of the peptide inhibitor norleucine1-angiotensin IV was observed, whereas the affinity of angiotensin IV remained unaltered. In additionm we predict that the peptide inhibitors bind to the catalytic site, with the NH2-terminal P1 residue occupying the catalytic cleft (S1 subsite) in a manner similar to that proposed for peptide substrates.

Synthesis of glutamic acid analogs as potent inhibitors of leukotriene A4 hydrolase

Leukotriene B(4) (LTB(4)) is a potent pro-inflammatory mediator that has been implicated in the pathogenesis of multiple diseases, including psoriasis, inflammatory bowel disease, multiple sclerosis and asthma. As a method to decrease the level of LTB(4) and possibly identify novel treatments, inhibitors of the LTB(4) biosynthetic enzyme, leukotriene A(4) hydrolase (LTA(4)-h), have been explored. Here we describe the discovery of a potent inhibitor of LTA(4)-h, arylamide of glutamic acid 4f, starting from the corresponding glycinamide 2. Analogs of 4f are then described, focusing on compounds that are both active and stable in whole blood. This effort culminated in the identification of amino alcohol 12a and amino ester 6b which meet these criteria.

The significance of brain aminopeptidases in the regulation of the actions of angiotensin peptides in the brain

From the outset, the concept of a brain renin-angiotensin system (RAS) has been controversial and this controversy continues to this day. In addition to the unresolved questions as to the means by which, and location(s) where brain Ang II is synthesized, and the uncertainties regarding the functionality of the different subtypes of Ang II receptors in the brain, a new controversy has arisen with respect to the identity of the angiotensin peptide(s) that activate brain AT(1) receptors. While it has been known for some time that Ang III can activate Ang II receptors with equivalent or near-equivalent efficacy to Ang II, it has been proposed that in the brain, only Ang III is active. This proposal, which we have named "The Angiotensin III Hypothesis" states that Ang II must be converted to Ang III in order to activate brain AT(1) receptors. This review examines several aspects of the controversies regarding the brain RAS with a special focus on brain aminopeptidases, studies that either support or refute The Angiotensin III Hypothesis, and the implications of The Angiotensin III Hypothesis for the activity of the brain RAS. It also addresses the need for further research that can test The Angiotensin III Hypothesis and definitively identify the angiotensin peptide(s) that activate brain AT(1) receptor-mediated effects.

Assay for rapid analysis of the tri-peptidase activity of LTA4 hydrolase

Leukotriene A4 hydrolase is a bifunctional zinc metalloenzyme with an epoxide hydrolase activity as well as an arginyl tri-peptidase activity. Detailed enzymological and mechanistic investigations of the latter activity have been hampered by the lack of a rapid and convenient enzyme assay. Here we have developed a new method allowing direct spectrophotometric assessment of the tri-peptide cleaving activity of leukotriene A4 hydrolase, as well as other peptidases. The method utilizes two competing substrates, one chromogenic reference substrate together with the tri-peptide substrate of interest, and relies on computer-assisted analysis of progress curves. The chromogenic reference substrate serves to disclose the "invisible" tri-peptide substrate for kinetic analysis. The method is fast and simple and will allow detailed kinetic studies and screening for natural peptide substrates of leukotriene A4 hydrolase as well as other members of the M1 family of aminopeptidases.

Structure and catalytic mechanisms of leukotriene A4 hydrolase

Leukotriene A4 hydrolase catalyzes the final and committed step in the biosynthesis of leukotriene B4, a potent chemotactic agent for neutrophils, eosinophils, monocytes, and T-cells that play key roles in the innate immune response. Recent data strongly implicates leukotriene B4 in the pathogenesis of cardiovascular diseases, in particular arteriosclerosis, myocardial infarction and stroke. Here, we highlight the most salient features of leukotriene A4 hydrolase with emphasis on its biochemistry and structure biology.

Leukotriene A4 hydrolase expression in PEL cells is regulated at the transcriptional level and leads to increased leukotriene B4 production

Primary effusion lymphoma (PEL) is a herpesvirus-8-associated lymphoproliferative disease characterized by migration of tumor cells to serous body cavities. PEL cells originate from postgerminal center B cells and share a remarkable alteration in B cell transcription factor expression and/or activation with classical Hodgkin's disease cells. Comparative analysis of gene expression by cDNA microarray of BCBL-1 cells (PEL), L-428 (classical Hodgkin's disease), and BJAB cells revealed a subset of genes that were differentially expressed in BCBL-1 cells. Among these, four genes involved in cell migration and chemotaxis were strongly up-regulated in PEL cells: leukotriene A4 (LTA4) hydrolase (LTA4H), IL-16, thrombospondin-1 (TSP-1), and selectin-P ligand (PSGL-1). Up-regulation of LTA4H was investigated at the transcriptional level. Full-length LTA4H promoter exhibited 50% higher activity in BCBL-1 cells than in BJAB or L-428 cells. Deletion analysis of the LTA4H promoter revealed a positive cis-regulatory element active only in BCBL-1 cells in the promoter proximal region located between -76 and -40 bp. Formation of a specific DNA-protein complex in this region was confirmed by EMSA. Coculture of ionophore-stimulated primary neutrophils with BCBL-1 cells leads to an increased production of LTB4 compared with coculture with BJAB and L-428 cells as measured by enzyme immunoassay, demonstrating the functional significance of LTA4H up-regulation.

A conserved tyrosine residue of Saccharomyces cerevisiae leukotriene A4 hydrolase stabilizes the transition state of the peptidase activity

Saccharomyces cerevisiae leukotriene A4 hydrolase (LTA4H) is a bifunctional aminopeptidase/epoxide hydrolase and a member of the M1 family of metallopeptidases. In order to obtain a more thorough understanding of the aminopeptidase activity of the enzyme, two conserved tyrosine residues, Tyr244 and Tyr456, were altered to phenylalanine and the mutant proteins characterized by determining KM and kcat for various amino acid beta-naphthylamide substrates. While mutation of Tyr456 exhibited minimal effect on catalysis, mutation of Tyr244 caused an overall 25-100-fold reduction in catalytic activity for all substrates tested. Furthermore, LTA4H Y244F exhibited a 40-fold decrease in affinity for RB-3014, a transition state analog inhibitor, implicating Tyr244 in transition state stabilization.

The ER aminopeptidase, ERAP1, trims precursors to lengths of MHC class I peptides by a "molecular ruler" mechanism

Endoplasmic reticulum aminopeptidase 1 (ERAP1) is an IFN-gamma-induced aminopeptidase in the endoplasmic reticulum that trims longer precursors to the antigenic peptides presented on MHC class I molecules. We recently reported that purified ERAP1 trimmed N-extended precursors but spared peptides of 8-9 residues, the length required for binding to MHC class I molecules. Here, we show another remarkable property of ERAP1: that it strongly prefers substrates 9-16 residues long, the lengths of peptides transported efficiently into the ER by the transporter associated with antigen processing (TAP) transporter. This aminopeptidase rapidly degraded a model 13-mer to a 9-mer and then stopped, even though the substrate and the product had identical N- and C-terminal sequences. No other aminopeptidase, including the closely related ER-aminopeptidase ERAP2, showed a similar length preference. Unlike other aminopeptidases, the activity of ERAP1 depended on the C-terminal residue of the substrate. ERAP1, like most MHC class I molecules, prefers peptides with hydrophobic C termini and shows low affinity for peptides with charged C termini. Thus, ERAP1 is specialized to process precursors transported by TAP to peptides that can serve as MHC class I epitopes. Its "molecular ruler" mechanism involves binding the hydrophobic C terminus of the substrate 9-16 residues away from the active site.

Identification of Human Aminopeptidase O, a Novel Metalloprotease with Structural Similarity to Aminopeptidase B and Leukotriene A4 Hydrolase

We have cloned and characterized a human brain cDNA encoding a new metalloprotease that has been called aminopeptidase O (AP-O). AP-O exhibits a series of structural features characteristic of aminopeptidases, including a conserved catalytic domain with a zinc-binding site (HEXXHX18E) that allows its classification in the M1 family of metallopeptidases or gluzincins. The structural complexity of AP-O is further increased by the presence of an additional C-terminal domain 170 residues long, which is predicted to have an ARM repeat fold originally identified in the Drosophila segment polarity gene product Armadillo. This ARM repeat domain is also present in aminopeptidase B, aminopeptidase B-like, and leukotriene A4 hydrolase and defines a novel subfamily of aminopeptidases that we have called ARM aminopeptidases. Northern blot analysis revealed that AP-O is mainly expressed in the pancreas, placenta, liver, testis, and heart. Human AP-O was produced in Escherichia coli, and the purified recombinant protein hydrolyzed synthetic substrates used for assaying aminopeptidase activity. This activity was abolished by general inhibitors of metalloproteases and specific inhibitors of aminopeptidases. Recombinant AP-O also cleaved angiotensin III to generate angiotensin IV, a bioactive peptide of the renin-angiotensin pathway with multiple actions on diverse tissues, including brain, testis, and heart. On the basis of these results we suggest that AP-O could play a role in the proteolytic processing of bioactive peptides in those tissues where it is expressed.

Nuclear localization of leukotriene A4 hydrolase in type II alveolar epithelial cells in normal and fibrotic lung

Leukotriene A4 (LTA4) hydrolase catalyzes the final step in leukotriene B4 (LTB4) synthesis. In addition to its role in LTB4 synthesis, the enzyme possesses aminopeptidase activity. In this study, we sought to define the subcellular distribution of LTA4 hydrolase in alveolar epithelial cells, which lack 5-lipoxygenase and do not synthesize LTA4. Immunohistochemical staining localized LTA4 hydrolase in the nucleus of type II but not type I alveolar epithelial cells of normal mouse, human, and rat lungs. Nuclear localization of LTA4 hydrolase was also demonstrated in proliferating type II-like A549 cells. The apparent redistribution of LTA4 hydrolase from the nucleus to the cytoplasm during type II-to-type I cell differentiation in vivo was recapitulated in vitro. Surprisingly, this change in localization of LTA4 hydrolase did not affect the capacity of isolated cells to convert LTA4 to LTB4. However, proliferation of A549 cells was inhibited by the aminopeptidase inhibitor bestatin. Nuclear accumulation of LTA4 hydrolase was also conspicuous in epithelial cells during alveolar repair following bleomycin-induced acute lung injury in mice, as well as in hyperplastic type II cells associated with fibrotic lung tissues from patients with idiopathic pulmonary fibrosis. These results show for the first time that LTA4 hydrolase can be accumulated in the nucleus of type II alveolar epithelial cells and that redistribution of the enzyme to the cytoplasm occurs with differentiation to the type I phenotype. Furthermore, the aminopeptidase activity of LTA4 hydrolase within the nucleus may play a role in promoting epithelial cell growth.

Epoxide hydrolases: their roles and interactions with lipid metabolism

The epoxide hydrolases (EHs) are enzymes present in all living organisms, which transform epoxide containing lipids by the addition of water. In plants and animals, many of these lipid substrates have potent biologically activities, such as host defenses, control of development, regulation of inflammation and blood pressure. Thus the EHs have important and diverse biological roles with profound effects on the physiological state of the host organisms. Currently, seven distinct epoxide hydrolase sub-types are recognized in higher organisms. These include the plant soluble EHs, the mammalian soluble epoxide hydrolase, the hepoxilin hydrolase, leukotriene A4 hydrolase, the microsomal epoxide hydrolase, and the insect juvenile hormone epoxide hydrolase. While our understanding of these enzymes has progressed at different rates, here we discuss the current state of knowledge for each of these enzymes, along with a distillation of our current understanding of their endogenous roles. By reviewing the entire enzyme class together, both commonalities and discrepancies in our understanding are highlighted and important directions for future research pertaining to these enzymes are indicated.

Leukotriene A4 Hydrolase

Leukotriene (LT) A(4) hydrolase is a bifunctional zinc metalloenzyme, which converts LTA(4) into the neutrophil chemoattractant LTB(4) and also exhibits an anion-dependent aminopeptidase activity. In the x-ray crystal structure of LTA(4) hydrolase, Arg(563) and Lys(565) are found at the entrance of the active center. Here we report that replacement of Arg(563), but not Lys(565), leads to complete abrogation of the epoxide hydrolase activity. However, mutations of Arg(563) do not seem to affect substrate binding strength, because values of K(i) for LTA(4) are almost identical for wild type and (R563K)LTA(4) hydrolase. These results are supported by the 2.3-A crystal structure of (R563A)LTA(4) hydrolase, which does not reveal structural changes that can explain the complete loss of enzyme function. For the aminopeptidase reaction, mutations of Arg(563) reduce the catalytic activity (V(max) = 0.3-20%), whereas mutations of Lys(565) have limited effect on catalysis (V(max) = 58-108%). However, in (K565A)- and (K565M)LTA(4) hydrolase, i.e. mutants lacking a positive charge, values of the Michaelis constant for alanine-p-nitroanilide increase significantly (K(m) = 480-640%). Together, our data indicate that Arg(563) plays an unexpected, critical role in the epoxide hydrolase reaction, presumably in the positioning of the carboxylate tail to ensure perfect substrate alignment along the catalytic elements of the active site. In the aminopeptidase reaction, Arg(563) and Lys(565) seem to cooperate to provide sufficient binding strength and productive alignment of the substrate. In conclusion, Arg(563) and Lys(565) possess distinct roles as carboxylate recognition sites for two chemically different substrates, each of which is turned over in separate enzymatic reactions catalyzed by LTA(4) hydrolase.

Fluorometric assay using naphthylamide substrates for assessing novel venom peptidase activities

In the present study we examined the feasibility of using the fluorometry of naphthylamine derivatives for revealing peptidase activities in venoms of the snakes Bothrops jararaca, Bothrops alternatus, Bothrops atrox, Bothrops moojeni, Bothrops insularis, Crotalus durissus terrificus and Bitis arietans, of the scorpions Tityus serrulatus and Tityus bahiensis, and of the spiders Phoneutria nigriventer and Loxosceles intermedia. Neutral aminopeptidase (APN) and prolyl-dipeptidyl aminopeptidase IV (DPP IV) activities were presented in all snake venoms, with the highest levels in B. alternatus. Although all examined peptidase activities showed relatively low levels in arthropod venoms, basic aminopeptidase (APB) activity from P. nigriventer venom was the exception. Compared to the other peptidase activities, relatively high levels of acid aminopeptidase (APA) activity were restricted to B. arietans venom. B. arietans also exhibited a prominent content of APB activity which was lower in other venoms. Relatively low prolyl endopeptidase and proline iminopeptidase activities were, respectively, detectable only in T. bahiensis and B. insularis. Pyroglutamate aminopeptidase activity was undetectable in all venoms. All examined peptidase activities were undetectable in T. serrulatus venom. In this study, the specificities of a diverse array of peptidase activities from representative venoms were demonstrated for the first time, with a description of their distribution which may contribute to guiding further investigations. The expressive difference between snake and arthropod venoms was indicated by APN and DPP IV activities while APA and APB activities distinguished the venom of B. arietans from those of Brazilian snakes. The data reflected the relatively uniform qualitative distribution of the peptidase activities investigated, together with their unequal quantitative distribution, indicating the evolutionary divergence in the processing of peptides in these different venoms and/or the different abilities of the venoms examined to hydrolyze different peptides during envenomation.

Crystal structures of leukotriene A4 hydrolase in complex with captopril and two competitive tight-binding inhibitors

Leukotriene (LT) A4 hydrolase/aminopeptidase is a bifunctional zinc enzyme that catalyzes the final step in the biosynthesis of LTB4, a potent chemoattractant and immune modulating lipid mediator. Here, we report a high-resolution crystal structure of LTA4 hydrolase in complex with captopril, a classical inhibitor of the zinc peptidase angiotensin-converting enzyme. Captopril makes few interactions with the protein, but its free thiol group is bound to the zinc, apparently accounting for most of its inhibitory action on LTA4 hydrolase. In addition, we have determined the structures of LTA4 hydrolase in complex with two selective tight-binding inhibitors, a thioamine and a hydroxamic acid. Their common benzyloxyphenyl tail, designed to mimic the carbon backbone of LTA4, binds into a narrow hydrophobic cavity in the protein. The free hydroxyl group of the hydroxamic acid makes a suboptimal, monodentate complex with the zinc, and strategies for improved inhibitor design can be deduced from the structure. Taken together, the three crystal structures provide the molecular basis for the divergent pharmacological profiles of LTA4 hydrolase inhibitors. Moreover, they help define the binding pocket for the fatty acid-derived epoxide LTA4 as well as the subsites for a tripeptide substrate, which in turn have important implications for the molecular mechanisms of enzyme catalyses.

Leukotriene A4 hydrolase

The leukotrienes (LTs) are a family of lipid mediators involved in inflammation and allergy. Leukotriene B4 is a classical chemoattractant, which triggers adherence and aggregation of leukocytes to the endothelium at only nanomolar concentrations. In addition, leukotriene B4 modulates immune responses, participates in the host-defense against infections, and is a key mediator of PAF-induced lethal shock. Because of these powerful biological effects, leukotriene B4 is implicated in a variety of acute and chronic inflammatory diseases, e.g. nephritis, arthritis, dermatitis, and chronic obstructive pulmonary disease. The final step in the biosynthesis of leukotriene B4 is catalyzed by leukotriene A4 hydrolase, a unique bi-functional zinc metalloenzyme with an anion-dependent aminopeptidase activity. Here we describe the most recent developments regarding our understanding of the structure, function, and catalytic mechanisms of leukotriene A4 hydrolase.