Aminopeptidase B from the rat testis is a bifunctional enzyme structurally related to leukotriene-A4 hydrolase

The aminopeptidase B (Ap-B) is phosphorylated in HEK293 cells

Proteolysis is a post-translational modification (PTM) that affects the whole proteome. First regarded as only destructive, it is more precise than expected. It is finely regulated by other PTMs like phosphorylation. Aminopeptidase B (Ap-B), a M1 metallopeptidase, hydrolyses the peptide bond on the carbonyl side of basic residues at the NH₂-terminus of peptides. 2D electrophoresis (2DE) was used to show that Ap-B is modified by phosphorylation. Detection of Ap-B by western blot after 2DE reveals several isoforms with different isoelectric points. Using alkaline phosphatase, Pro-Q Diamond phosphorylation-specific dye and kinase-specific inhibitors, we confirmed that Ap-B is phosphorylated. Phosphorylation can alter the structure of proteins leading to changes in their activity, localization, stability and association with other interacting molecules. We showed that Ap-B phosphorylation might delay its turnover. Our study illustrates the central role of the crosstalk between kinases and proteases in the regulation of many biological processes.

Variation in the Evolution and Sequences of Proglucagon and the Receptors for Proglucagon-Derived Peptides in Mammals

The mammalian proglucagon gene (Gcg) encodes three glucagon like sequences, glucagon, glucagon-like peptide-1 (GLP-1), and glucagon-like peptide-2 that are of similar length and share sequence similarity, with these hormones having cell surface receptors, glucagon receptor (Gcgr), GLP-1 receptor (Glp1r), and GLP-2 receptor (Glp2r), respectively. Gcgr, Glp1r, and Glp2r are all class B1 G protein-coupled receptors (GPCRs). Despite their sequence and structural similarity, analyses of sequences from rodents have found differences in patterns of sequence conservation and evolution. To determine whether these were rodent-specific traits or general features of these genes in mammals I analyzed coding and protein sequences for proglucagon and the receptors for proglucagon-derived peptides from the genomes of 168 mammalian species. Single copy genes for each gene were found in almost all genomes. In addition to glucagon sequences within Hystricognath rodents (e.g., guinea pig), glucagon sequences from a few other groups (e.g., pangolins and some bats) as well as changes in the proteolytic processing of GLP-1 in some bats are suggested to have functional effects. GLP-2 sequences display increased variability but accepted few substitutions that are predicted to have functional consequences. In parallel, Glp2r sequences display the most rapid protein sequence evolution, and show greater variability in amino acids at sites involved in ligand interaction, however most were not predicted to have a functional consequence. These observations suggest that a greater diversity in biological functions for proglucagon-derived peptides might exist in mammals.

Obesity significantly alters the human sperm proteome, with potential implications for fertility

PURPOSE

In men, obesity may lead to poor semen parameters and reduced fertility. However, the causative links between obesity and male infertility are not totally clear, particularly on a molecular level. As such, we investigated how obesity modifies the human sperm proteome, to elucidate any important implications for fertility.

METHODS

Sperm protein lysates from 5 men per treatment, classified as a healthy weight (body mass index (BMI) ≤ 25 kg/m2) or obese (BMI ≥ 30 kg/m2), were FASP digested, submitted to liquid chromatography tandem mass spectrometry, and compared by label-free quantification. Findings were confirmed for several proteins by qualitative immunofluorescence and a quantitative protein immunoassay.

RESULTS

A total of 2034 proteins were confidently identified, with 24 proteins being significantly (p < 0.05) less abundant (fold change < 0.05) in the spermatozoa of obese men and 3 being more abundant (fold change > 1.5) compared with healthy weight controls. Proteins with altered abundance were involved in a variety of biological processes, including oxidative stress (GSS, NDUFS2, JAGN1, USP14, ADH5), inflammation (SUGT1, LTA4H), translation (EIF3F, EIF4A2, CSNK1G1), DNA damage repair (UBEA4), and sperm function (NAPA, RNPEP, BANF2).

CONCLUSION

These results suggest that oxidative stress and inflammation are closely tied to reproductive dysfunction in obese men. These processes likely impact protein translation and folding during spermatogenesis, leading to poor sperm function and subfertility. The observation of these changes in obese men with no overt andrological diagnosis further suggests that traditional clinical semen assessments fail to detect important biochemical changes in spermatozoa which may compromise fertility.

The effects of curcumin, mangiferin, resveratrol and other natural plant products on aminopeptidase B activity

Aminopeptidase B (Ap-B) is a Zn²⁺-aminopeptidase of the M1 family which is implicated, in conjunction with the nardilysin endoprotease, in the generation of miniglucagon, a peptide involved in the maintenance of glucose homeostasis. Other in vivo physiological roles have been established for this vertebrate enzyme, such as the processing of Arg-extended forms of human insulin and cholecystokinin 9 and the degradation of viral epitopes in the cytoplasm. Among M1 family members, Ap-B is phylogenetically close to leukotriene A₄ hydrolase (LTA₄H), a bi-functional aminopeptidase also able to transform LTA₄ in LTB₄ (a lipid mediator of inflammation). As the activities of LTA₄H are reported to be inhibited by resveratrol, a polyphenolic molecule from red wine, the effect of this molecule was investigated on the Ap-B activity. Several other active phenolic compounds produced in plants were also tested. Among them, curcumin and mangiferin are the most effective inhibitors. Dixon analysis indicates that curcumin is a non-competitive inhibitor with a Ki value of 46 μmol.L^-1. Dixon and Lineweaver-Burk representations with mangiferin show a mixed non-competitive inhibition with Ki' and Ki values of 194 μmol.L^-1 and 105 μmol.L^-1, respectively. At 200 μmol.L^-1, no significant effect was observed with caffeic, chlorogenic, ferulic, salicylic and sinapic acids as well as with resveratrol. Analyses on the 3D-structure of LTA₄H with resveratrol (pdb: 3FTS) and the Ap-B 3D-model allow hypothesis to explain theses results.

Dgat1 and Dgat2 regulate enterocyte triacylglycerol distribution and alter proteins associated with cytoplasmic lipid droplets in response to dietary fat

Enterocytes, the absorptive cells of the small intestine, mediate efficient absorption of dietary fat (triacylglycerol, TAG). The digestive products of dietary fat are taken up by enterocytes, re-esterified into TAG, and packaged on chylomicrons (CMs) for secretion into blood or temporarily stored within cytoplasmic lipid droplets (CLDs). Altered enterocyte TAG distribution impacts susceptibility to high fat diet associated diseases, but molecular mechanisms directing TAG toward these fates are unclear. Two enzymes, acyl CoA: diacylglycerol acyltransferase 1 (Dgat1) and Dgat2, catalyze the final, committed step of TAG synthesis within enterocytes. Mice with intestine-specific overexpression of Dgat1 (Dgat1Int) or Dgat2 (Dgat2Int), or lack of Dgat1 (Dgat1-/-), were previously found to have altered intestinal TAG secretion and storage. We hypothesized that varying intestinal Dgat1 and Dgat2 levels alters TAG distribution in subcellular pools for CM synthesis as well as the morphology and proteome of CLDs. To test this we used ultrastructural and proteomic methods to investigate intracellular TAG distribution and CLD-associated proteins in enterocytes from Dgat1Int, Dgat2Int, and Dgat1-/- mice 2h after a 200μl oral olive oil gavage. We found that varying levels of intestinal Dgat1 and Dgat2 altered TAG pools involved in CM assembly and secretion, the number or size of CLDs present in enterocytes, and the enterocyte CLD proteome. Overall, these results support a model where Dgat1 and Dgat2 function coordinately to regulate the process of dietary fat absorption by preferentially synthesizing TAG for incorporation into distinct subcellular TAG pools in enterocytes.

The M1 family of vertebrate aminopeptidases: role of evolutionarily conserved tyrosines in the enzymatic mechanism of aminopeptidase B

Aminopeptidase B (Ap-B), a member of the M1 family of Zn(2+)-aminopeptidases, removes basic residues at the NH2-terminus of peptides and is involved in the in vivo proteolytic processing of miniglucagon and cholecystokinin-8. M1 enzymes hydrolyze numerous different peptides and are implicated in many physiological functions. As these enzymes have similar catalytic mechanisms, their respective substrate specificity and/or catalytic efficiency must be based on subtle structural differences at or near the catalytic site. This leads to the hypothesis that each primary structure contains a consensus structural template, strictly necessary for aminopeptidase activity, and a specific amino acid environment localized in or outside the catalytic pocket that finely tunes the substrate specificity and catalytic efficiency of each enzyme. A multiple sequence alignment of M1 peptidases from vertebrates allowed to identify conserved tyrosine amino acids, which are members of this catalytic backbone. In the present work, site-directed mutagenesis and 3D molecular modeling of Ap-B were used to specify the role of four fully (Y281, Y229, Y414, and Y441) and one partially (Y409) conserved residues. Tyrosine to phenylalanine mutations allowed confirming the influence of the hydroxyl groups on the enzyme activity. These groups are implicated in the reaction mechanism (Y414), in substrate specificity and/or catalytic efficiency (Y409), in stabilization of essential amino acids of the active site (Y229, Y409) and potentially in the maintenance of its structural integrity (Y281, Y441). The importance of hydrogen bonds is verified by the Y229H substitution, which preserves the enzyme activity. These data provide new insights into the catalytic mechanism of Ap-B in the M1 family of aminopeptidases.

The forgotten members of the glucagon family

From proglucagon, at least six final biologically active peptides are produced by tissue-specific post-translational processing. While glucagon and GLP-1 are the subject of permanent studies, the four others are usually left in the shadow, in spite of their large biological interest. The present review is devoted to oxyntomodulin and miniglucagon, not forgetting glicentin, although much less is known about it. Oxyntomodulin (OXM) and glicentin are regulators of gastric acid and hydromineral intestinal secretions. OXM is also deeply involved in the control of food intake and energy expenditure, properties that make this peptide a credible treatment of obesity if the question of administration is solved, as for any peptide. Miniglucagon, the C-terminal undecapeptide of glucagon which results from a secondary processing of original nature, displays properties antagonistic to that of the mother-hormone glucagon: (a) it inhibits glucose-, glucagon- and GLP-1-stimulated insulin release at sub-picomolar concentrations, (b) it reduces the in vivo insulin response to glucose with no change in glycemia, (c) it displays insulin-like properties at the cellular level using only a part of the pathway used by insulin, making it a good basis for developing a pharmacological workaround of insulin resistance.

The interrelationship between leukotriene B4 and leukotriene-A4-hydrolase in collagen/adjuvant-induced arthritis in rats

This study aimed to check the involvement of lipid mediator leukotriene (LT) B4 and the activity of LTA4 hydrolase (LTA4H) in the development of arthritis induced in rats by collagen and adjuvant (CIA). High-performance liquid chromatography (HPLC) and enzyme immunoassay (EIA) were used for measurements of LTB4 and LTA4H in plasma, synovial fluid (SF), soluble (SO), and solubilized membrane-bound fraction (MB) from synovial tissue (ST) and peripheral blood mononuclear cells (PBMCs) of CIA-arthritic and CIA-resistant. EIA process is simple, clean, and rapid and offered advantages over HPLC, showing that in SF and MB-PBMCs of CIA-arthritic and CIA-resistant, and in MB-ST of CIA-resistant, LTB4 and LTA4H were altered in parallel and were positively related. In the plasma and SO-ST and SO-PBMCs of CIA-arthritic and CIA-resistant, and in MB-ST of CIA-arthritic, this pattern was not found. The primordial role played by LTA4H in the biosynthesis of LTB4 was confirmed together with the existence of alternative steps that regulate LTB4 without participation of LTA4H. The involvement of compartmentalized and coupled changes of LTB4 and LTA4H in the resistance and development of arthritis in CIA model was demonstrated for the first time.

Role of glutamine-169 in the substrate recognition of human aminopeptidase B

BACKGROUND

Aminopeptidase B (EC 3.4.11.6, APB) preferentially hydrolyzes N-terminal basic amino acids of synthetic and peptide substrates. APB is involved in the production and maturation of peptide hormones and neurotransmitters such as miniglucagon, cholecystokinin and enkephalin by cleaving N-terminal basic amino acids in extended precursor proteins. Therefore, the specificity for basic amino acids is crucial for the biological function of APB.

METHODS

Site-directed mutagenesis and molecular modeling of the S1 site were used to identify amino acid residues of the human APB responsible for the basic amino acid preference and enzymatic efficiency.

RESULTS

Substitution of Gln169 with Asn caused a significant decrease in hydrolytic activity toward the fluorescent substrate Lys-4-methylcoumaryl-7-amide (MCA). Substantial retardation of enzyme activity was observed toward Arg-MCA and substitution with Glu caused complete loss of enzymatic activity of APB. Substitution with Asn led to an increase in IC50 values of inhibitors that interact with the catalytic pocket of APB. The EC50 value of chloride ion binding was also found to increase with the Asn mutant. Gln169 was required for maximal cleavage of the peptide substrates. Molecular modeling suggested that interaction of Gln169 with the N-terminal Arg residue of the substrate could be bridged by a chloride anion.

CONCLUSION

Gln169 is crucial for obtaining optimal enzymatic activity and the unique basic amino acid preference of APB via maintaining the appropriate catalytic pocket structure and thus for its function as a processing enzyme of peptide hormones and neurotransmitters.

The efficiency of human cytomegalovirus pp65(495-503) CD8+ T cell epitope generation is determined by the balanced activities of cytosolic and endoplasmic reticulum-resident peptidases

Control of human CMV (HCMV) infection depends on the cytotoxic activity of CD8(+) CTLs. The HCMV phosphoprotein (pp)65 is a major CTL target Ag and pp65(495-503) is an immunodominant CTL epitope in infected HLA-A*0201 individuals. As immunodominance is strongly determined by the surface abundance of the specific epitope, we asked for the components of the cellular Ag processing machinery determining the efficacy of pp65(495-503) generation, in particular, for the proteasome, cytosolic peptidases, and endoplasmic reticulum (ER)-resident peptidases. In vitro Ag processing experiments revealed that standard proteasomes and immunoproteasomes generate the minimal 9-mer peptide epitope as well as N-terminal elongated epitope precursors of different lengths. These peptides are largely degraded by the cytosolic peptidases leucine aminopeptidase and tripeptidyl peptidase II, as evidenced by increased pp65(495-503) epitope presentation after leucine aminopeptidase and tripeptidyl peptidase II knockdown. Additionally, with prolyl oligopeptidase and aminopeptidase B we identified two new Ag processing machinery components, which by destroying the pp65(495-503) epitope limit the availability of the specific peptide pool. In contrast to cytosolic peptidases, silencing of ER aminopeptidases 1 and 2 strongly impaired pp65(495-503)-specific T cell activation, indicating the importance of ER aminopeptidases in pp65(495-503) generation. Thus, cytosolic peptidases primarily interfere with the generation of the pp65(495-503) epitope, whereas ER-resident aminopeptidases enhance such generation. As a consequence, our experiments reveal that the combination of cytosolic and ER-resident peptidase activities strongly shape the pool of specific antigenic peptides and thus modulate MHC class I epitope presentation efficiency.

The role of multifunctional M1 metallopeptidases in cell cycle progression

BACKGROUND

Metallopeptidases of the M1 family are found in all phyla (except viruses) and are important in the cell cycle and normal growth and development. M1s often have spatiotemporal expression patterns which allow for strict regulation of activity. Mutations in the genes encoding M1s result in disease and are often lethal. This family of zinc metallopeptidases all share the catalytic region containing a signature amino acid exopeptidase (GXMXN) and a zinc binding (HEXXH[18X]E) motif. In addition, M1 aminopeptidases often also contain additional membrane association and/or protein interaction motifs. These protein interaction domains may function independently of M1 enzymatic activity and can contribute to multifunctionality of the proteins.

SCOPE

A brief review of M1 metalloproteases in plants and animals and their roles in the cell cycle is presented. In animals, human puromycin-sensitive aminopeptidase (PSA) acts during mitosis and perhaps meiosis, while the insect homologue puromycin-sensitive aminopeptidase (PAM-1) is required for meiotic and mitotic exit; the remaining human M1 family members appear to play a direct or indirect role in mitosis/cell proliferation. In plants, meiotic prophase aminopeptidase 1 (MPA1) is essential for the first steps in meiosis, and aminopeptidase M1 (APM1) appears to be important in mitosis and cell division.

CONCLUSIONS

M1 metalloprotease activity in the cell cycle is conserved across phyla. The activities of the multifunctional M1s, processing small peptides and peptide hormones and contributing to protein trafficking and signal transduction processes, either directly or indirectly impact on the cell cycle. Identification of peptide substrates and interacting protein partners is required to understand M1 function in fertility and normal growth and development in plants.

Molecular characterization of three genes encoding aminopeptidases N in the poplar leaf beetle Chrysomela tremulae

Three genes encoding proteins showing sequence similarity and features typical of insect APNs were characterized in C. tremulae and designed as CtAPN1, CtAPN2 and CtAPN3. Expression analysis of the three C. tremulae APN genes showed that CtAPN2 transcript is more abundant in the fat body, whereas both CtAPN1 and CtAPN3 are specifically expressed in the midgut. Despite a similar genomic organization, lepidopteran and coleopteran APNs are phylogenetically distant, suggesting that APN gene duplication events occurred after these two insect orders split. Sequence and expression comparisons of CtAPN1, CtAPN2 and CtAPN3 cDNAs in a C. tremulae Bacillus thuringiensis (Bt)-susceptible and in a Bt-resistant strain did not show any polymorphism at the amino acid level or difference at the transcription level.

The catalytic and protein-protein interaction domains are required for APM1 function

Aminopeptidase M1 (APM1) is essential for embryonic, vegetative, and reproductive development in Arabidopsis (Arabidopsis thaliana). Here, we show that, like mammalian M1 proteases, APM1 appears to have distinct enzymatic and protein-protein interaction domains and functions as a homodimer. Arabidopsis seedlings treated with ezetimibe, an inhibitor of M1 protein-protein interactions, mimicked a subset of apm1 phenotypes distinct from those resulting from treatment with PAQ-22, an inhibitor of M1 catalytic activity, suggesting that the APM1 catalytic and interaction domains can function independently. apm1-1 knockdown mutants transformed with catalytically inactive APM1 did not prevent seedling lethality. However, apm1-2 has a functional enzymatic domain but lacks the carboxyl (C) terminus, and transformation with catalytically inactive APM1 rescued the mutant. Overexpression of human insulin-responsive aminopeptidase/oxytocinase rescued all apm1 phenotypes, suggesting that the catalytic activity was sufficient to compensate for loss of APM1 function, while overexpression of catalytically inactive insulin-responsive aminopeptidase/oxytocinase only rescued apm1-2. Increased catalytic activity alone is not sufficient to compensate for loss of APM1 function, as overexpression of another Arabidopsis M1 family member lacking an extended C terminus did not rescue apm1-1. The protein interactions facilitating enzymatic activity appear to be dependent on the C terminus of APM1, as transformation with an open reading frame containing an internal deletion of a portion of the C terminus or a point mutation in a dileucine motif did not rescue the mutant. These results suggest that both the catalytic and interaction domains are necessary for APM1 function but that APM1 function and dimerization do not require these domains to be present in the same linear molecule.

Calpain-truncated CRMP-3 and -4 contribute to potassium deprivation-induced apoptosis of cerebellar granule neurons

Increasing evidence shows that calpain-mediated proteolytic processing of a selective number of proteins plays an important role in neuronal apoptosis. Study of calpain-mediated cleavage events and related functions may contribute to a better understanding of neuronal apoptosis and neurodegenerative diseases. We, therefore, investigated the role of calpain substrates in potassium deprivation-induced apoptosis of cerebellar granule neurons (CGNs). Twelve previously known and seven novel candidates of calpain substrates were identified by 2-D DIGE and MALDI-TOF/TOF MS analysis. Further, the identified novel calpain substrates were validated by Western blot analysis. Moreover, we focused on the collapsin response mediator proteins (CRMP-1, -2, -3 and -4 isoforms) and found that CRMPs were proteolytically processed by calpain but not by caspase, both in vivo and in vitro. To clarify the properties of the calpain-mediated proteolysis of CRMPs, we constructed the deletion mutants of CRMPs for additional biochemical studies. In vitro cleavage assays revealed that CRMP-1, -2 and -4 were truncated by calpain at the C-terminus, whereas CRMP-3 was cleaved at the N-terminus. Finally, we assessed the role of CRMPs in the process of potassium deprivation-triggered neuronal apoptosis by overexpressing the truncated CRMPs in CGNs. Our data clearly showed that the truncated CRMP-3 and -4, but not CRMP-1 and -2, significantly induced neuronal apoptosis. These findings demonstrated that calpain-truncated CRMP-3 and -4 act as pro-apoptotic players when CGNs undergo apoptosis.

Cathepsin L plays a major role in cholecystokinin production in mouse brain cortex and in pituitary AtT-20 cells: protease gene knockout and inhibitor studies

Cholecystokinin (CCK) is a peptide neurotransmitter whose production requires proteolytic processing of the proCCK precursor to generate active CCK8 neuropeptide in brain. This study demonstrates the significant role of the cysteine protease cathepsin L for CCK8 production. In cathepsin L knockout (KO) mice, CCK8 levels were substantially reduced in brain cortex by an average of 75%. To evaluate the role of cathepsin L in producing CCK in the regulated secretory pathway of neuroendocrine cells, pituitary AtT-20 cells that stably produce CCK were treated with the specific cathepsin L inhibitor, CLIK-148. CLIK-148 inhibitor treatment resulted in decreased amounts of CCK secreted from the regulated secretory pathway of AtT-20 cells. CLIK-148 also reduced cellular levels of CCK9 (Arg-CCK8), consistent with CCK9 as an intermediate product of cathepsin L, shown by the decreased ratio of CCK9/CCK8. The decreased CCK9/CCK8 ratio also suggests a shift in the production to CCK8 over CCK9 during inhibition of cathepsin L. During reduction of the PC1/3 processing enzyme by siRNA, the ratio of CCK9/CCK8 was increased, suggesting a shift to the cathepsin L pathway for the production of CCK9. The changes in ratios of CCK9 compared to CCK8 are consistent with dual roles of the cathepsin L protease pathway that includes aminopeptidase B to remove NH2-terminal Arg or Lys, and the PC1/3 protease pathway. These results suggest that cathepsin L functions as a major protease responsible for CCK8 production in mouse brain cortex, and participates with PC1/3 for CCK8 production in pituitary cells.

Zinc regulation of aminopeptidase B involved in neuropeptide production

Aminopeptidase B (AP-B) is a metallopeptidase that removes basic residues from the N-termini of neuropeptide substrates in secretory vesicles. This study assessed zinc regulation of AP-B activity, since secretory vesicles contain endogenous zinc. AP-B was inhibited by zinc at concentrations typically present in secretory vesicles. Zinc effects were dependent on concentration, incubation time, and the molar ratio of zinc to enzyme. AP-B activity was recovered upon removal of zinc. AP-B with zinc became susceptible to degradation by trypsin, suggesting that zinc alters enzyme conformation. Zinc regulation demonstrates the metallopeptidase property of AP-B.

Proteases for processing proneuropeptides into peptide neurotransmitters and hormones

Peptide neurotransmitters and peptide hormones, collectively known as neuropeptides, are required for cell-cell communication in neurotransmission and for regulation of endocrine functions. Neuropeptides are synthesized from protein precursors (termed proneuropeptides or prohormones) that require proteolytic processing primarily within secretory vesicles that store and secrete the mature neuropeptides to control target cellular and organ systems. This review describes interdisciplinary strategies that have elucidated two primary protease pathways for prohormone processing consisting of the cysteine protease pathway mediated by secretory vesicle cathepsin L and the well-known subtilisin-like proprotein convertase pathway that together support neuropeptide biosynthesis. Importantly, this review discusses important areas of current and future biomedical neuropeptide research with respect to biological regulation, inhibitors, structural features of proneuropeptide and protease interactions, and peptidomics combined with proteomics for systems biological approaches. Future studies that gain in-depth understanding of protease mechanisms for generating active neuropeptides will be instrumental for translational research to develop pharmacological strategies for regulation of neuropeptide functions. Pharmacological applications for neuropeptide research may provide valuable therapeutics in health and disease.

Aminopeptidase B, a glucagon-processing enzyme: site directed mutagenesis of the Zn2+-binding motif and molecular modelling

BACKGROUND

Aminopeptidase B (Ap-B; EC 3.4.11.6) catalyzes the cleavage of basic residues at the N-terminus of peptides and processes glucagon into miniglucagon. The enzyme exhibits, in vitro, a residual ability to hydrolyze leukotriene A4 into the pro-inflammatory lipid mediator leukotriene B4. The potential bi-functional nature of Ap-B is supported by close structural relationships with LTA4 hydrolase (LTA4H ; EC 3.3.2.6). A structure-function analysis is necessary for the detailed understanding of the enzymatic mechanisms of Ap-B and to design inhibitors, which could be used to determine the complete in vivo functions of the enzyme.

RESULTS

The rat Ap-B cDNA was expressed in E. coli and the purified recombinant enzyme was characterized. 18 mutants of the H325EXXHX18E348 Zn2+-binding motif were constructed and expressed. All mutations were found to abolish the aminopeptidase activity. A multiple alignment of 500 sequences of the M1 family of aminopeptidases was performed to identify 3 sub-families of exopeptidases and to build a structural model of Ap-B using the x-ray structure of LTA4H as a template. Although the 3D structures of the two enzymes resemble each other, they differ in certain details. The role that a loop, delimiting the active center of Ap-B, plays in discriminating basic substrates, as well as the function of consensus motifs, such as RNP1 and Armadillo domain are discussed. Examination of electrostatic potentials and hydrophobic patches revealed important differences between Ap-B and LTA4H and suggests that Ap-B is involved in protein-protein interactions.

CONCLUSION

Alignment of the primary structures of the M1 family members clearly demonstrates the existence of different sub-families and highlights crucial residues in the enzymatic activity of the whole family. E. coli recombinant enzyme and Ap-B structural model constitute powerful tools for investigating the importance and possible roles of these conserved residues in Ap-B, LTA4H and M1 aminopeptidase catalytic sites and to gain new insight into their physiological functions. Analysis of Ap-B structural model indicates that several interactions between Ap-B and proteins can occur and suggests that endopeptidases might form a complex with Ap-B during hormone processing.

Molecular portrait of high productivity in recombinant NS0 cells

Many important therapeutic proteins are produced in recombinant mammalian cells. Upon the introduction of the product gene, the isolated clones typically exhibit a wide range of productivity and high producers are subsequently selected for use in production. Using DNA microarray, two-dimensional gel electrophoresis (2DE), and iTRAQ as global surveying tools, we examined the transcriptome and proteome profiles of 11 lines of NS0 cells producing the same antibody molecule. Genes that are significantly differentially expressed between high and low producer groups statistically fall into a number of functional classes. Their distribution among the functional classes differs somewhat between transcriptomic and proteomic results. Overall, a high degree of consistency between transcriptome and proteome analysis are seen, although some genes exhibiting inconsistent trends between transcript and protein levels were observed as expected. In a novel approach, functional gene networks were retrieved using computational pathway analysis tools and their association with productivity was tested by physiological comprehension of the possible pathways involved in high recombinant protein production. Network analysis indicates that protein synthesis pathways were altered in high producers at both transcriptome and proteome levels, whereas the effect on cell growth/death pathways was more prominent only at the transcript level. The results suggest a common mechanism entailing the alteration of protein synthesis and cell growth control networks leading to high productivity. However, alternate routes with different sets of genes may be invoked to give rise to the same mechanistic outcomes. Such systematic approaches, combining transcriptomic and proteomic tools to examine high and low producers of recombinant mammalian cells will greatly enhance our capability to rationally design high producer cells. This work is a first step towards shedding a new light on the global physiological landscape of hyper productivity of recombinant cells.

Pro-protein convertases in intermediary metabolism: islet hormones, brain/gut hormones and integrated physiology

Many peptide hormones implicated in the regulation of intermediary metabolism arise from larger precursors called prohormones. These precursors are cut into pieces by proprotein convertases, more precisely those called prohormone convertases (PCs) that cleave at the C terminus of basic doublets. The remaining basic amino acids are eliminated by a specialized carboxypeptidase, leading to the active hormone. This processing may provide, from a single precursor, several peptides with different biological activities depending on the site(s) of cleavage on the precursor. When the processing is tissue-specific, this mechanism allows to produce, from a single protein, different sets of hormones depending on the tissue considered, leading to novel regulatory processes. The archetype of such a pluripotent prohormone in the field of intermediary metabolism is pro-glucagon that, when cut by PC1 in intestinal L cells, produces four different peptides with different specificities [glicentin, oxyntomodulin (OXM), glucagon-like peptide-1, and glucagon-like peptide-2], whereas, when cut by PC2 in the alpha cells of the endocrine pancreas, glucagon is produced and, through the supplementary action of NRD convertase, a fragment of glucagon (miniglucagon) with original properties.

Cathepsin L expression is directed to secretory vesicles for enkephalin neuropeptide biosynthesis and secretion

Proteases within secretory vesicles are required for conversion of neuropeptide precursors into active peptide neurotransmitters and hormones. This study demonstrates the novel cellular role of the cysteine protease cathepsin L for producing the (Met)enkephalin peptide neurotransmitter from proenkephalin (PE) in the regulated secretory pathway of neuroendocrine PC12 cells. These findings were achieved by coexpression of PE and cathepsin L cDNAs in PC12 cells with analyses of PE-derived peptide products. Expression of cathepsin L resulted in highly increased cellular levels of (Met)enkephalin, resulting from the conversion of PE to enkephalin-containing intermediates of 23, 18-19, 8-9, and 4.5 kDa that were similar to those present in vivo. Furthermore, expression of cathepsin L with PE resulted in increased amounts of nicotine-induced secretion of (Met)enkephalin. These results indicate increased levels of (Met)enkephalin within secretory vesicles of the regulated secretory pathway. Importantly, cathespin L expression was directed to secretory vesicles, demonstrated by colocalization of cathepsin L-DsRed fusion protein with enkephalin and chromogranin A neuropeptides that are present in secretory vesicles. In vivo studies also showed that cathepsin L in vivo was colocalized with enkephalin. The newly defined secretory vesicle function of cathepsin L for biosynthesis of active enkephalin opioid peptide contrasts with its function in lysosomes for protein degradation. These findings demonstrate cathepsin L as a distinct cysteine protease pathway for producing the enkephalin member of neuropeptides.

Insulin-regulated aminopeptidase: analysis of peptide substrate and inhibitor binding to the catalytic domain

Peptide inhibitors of insulin-regulated aminopeptidase (IRAP) accelerate spatial learning and facilitate memory retention and retrieval by binding competitively to the catalytic site of the enzyme and inhibiting its catalytic activity. IRAP belongs to the M1 family of Zn2+-dependent aminopeptidases characterized by a catalytic domain that contains two conserved motifs, the HEXXH(X)18E Zn2+-binding motif and the GXMEN exopeptidase motif. To elucidate the role of GXMEN in binding peptide substrates and competitive inhibitors, site-directed mutagenesis was performed on the motif. Non-conserved mutations of residues G428, A429 and N432 resulted in mutant enzymes with altered catalytic activity, as well as divergent changes in kinetic properties towards the synthetic substrate leucine beta-naphthylamide. The affinities of the IRAP inhibitors angiotensin IV, Nle1-angiotensin IV, and LVV-hemorphin-7 were selectively decreased. Substrate degradation studies using the in vitro substrates vasopressin and Leu-enkephalin showed that replacement of G428 by either D, E or Q selectively abolished the catalysis of Leu-enkephalin, while [A429G]IRAP and [N432A]IRAP mutants were incapable of cleaving both substrates. These mutational studies indicate that G428, A429 and N432 are important for binding of both peptide substrates and inhibitors, and confirm previous results demonstrating that peptide IRAP inhibitors competitively bind to its catalytic site.

Secretory vesicle aminopeptidase B related to neuropeptide processing: molecular identification and subcellular localization to enkephalin- and NPY-containing chromaffin granules

Biosynthesis of peptide hormones and neurotransmittters involves proteolysis of proprotein precursors by secretory vesicle cathepsin L. Cathepsin L generates peptide intermediates with basic residues at their NH(2)-termini, indicating that Arg/Lys aminopeptidase is needed to generate the smaller biologically active peptide. Therefore, this study identified the Arg/Lys aminopeptidase that is present in secretory vesicles of adrenal medulla and neuroendocrine tissues, achieved by molecular cloning and localization in 'model' neuropeptide-containing secretory vesicles (bovine). Molecular cloning of the bovine aminopeptidase B (AP-B) cDNA defined its primary sequence that allowed selection of antisera for immunolocalization studies. AP-B was present in secretory vesicles that contain cathepsin L with the neuropeptides enkephalin and neuropeptide Y. The AP-B in several neuroendocrine tissues was detected by western blots. Recombinant bovine AP-B showed preference for Arg-methylcoumarinamide substrate. AP-B was inhibited by arphamenine, an inhibitor of aminopeptidases. Bovine AP-B showed similar activities for Arg-(Met)enkephalin (ME) and Lys-ME neuropeptide substrates to generate ME, while rat AP-B preferred Arg-ME. Furthermore, AP-B possesses an acidic pH optimum of 5.5-6.5 that is similar to the internal pH of secretory vesicles. The significant finding of the secretory vesicle localization of AP-B with neuropeptides and cathepsin L suggests a role for this exopeptidase in the biosynthesis of neuropeptides.

Identification of a Bacillus thuringiensis Cry11Ba toxin-binding aminopeptidase from the mosquito, Anopheles quadrimaculatus

BACKGROUND

Aminopeptidase N (APN) type proteins isolated from several species of lepidopteran insects have been implicated as Bacillus thuringiensis (Bt) toxin-binding proteins (receptors) for Cry toxins. We examined brush border membrane vesicle (BBMV) proteins from the mosquito Anopheles quadrimaculatus to determine if APNs from this organism would bind mosquitocidal Cry toxins that are active to it.

RESULTS

A 100-kDa protein with APN activity (APNAnq 100) was isolated from the brush border membrane of Anopheles quadrimaculatus. Native state binding analysis by surface plasmon resonance shows that APNAnq 100 forms tight binding to a mosquitocidal Bt toxin, Cry11Ba, but not to Cry2Aa, Cry4Ba or Cry11Aa.

CONCLUSION

An aminopeptidase from Anopheles quadrimaculatus mosquitoes is a specific binding protein for Bacillus thuringiensis Cry11Ba.

Isolation and characterization of aminopeptidase from Capnocytophaga granulosa ATCC 51502

There is evidence that enzymes from the genus Capnocytophaga play a role in dental calculus formation. Although most of the species in the genus produce aminopeptidases, there is a paucity of data on the purification and characterization of the enzyme, except in the case of Capnocytophaga gingivalis. The aim of this study was to purify aminopeptidase from culture supernatant of Capnocytophaga granulosa ATCC 51502, a new species of the genus. Purification was performed using ammonium sulfate fractionation and two chromatographic steps. The aminopeptidase was purified 158,433-fold with a yield of 12.0%. The enzyme appeared to be a trimer with a molecular mass of 270 kDa. The optimal pH of the aminopeptidase was 6.5 and its activity was completely inhibited by incubation at 50 degrees C for 10 min. The enzyme showed maximum specificity for basic amino acids (Arg and Lys) and also hydrolyzed noncharged amino acids (Met, Leu and Ala). Ca(2+), Zn(2+) and Fe(3+) activated the enzyme, while EDTA, Ag(+), Hg(+) and Cu(2+) inhibited it. These results suggest that aminopeptidase of C. granulosa is different from that of C. gingivalis but similar to aminopeptidase B.

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.

Cathepsin L and Arg/Lys aminopeptidase: a distinct prohormone processing pathway for the biosynthesis of peptide neurotransmitters and hormones

Peptide neurotransmitters and hormones are synthesized as protein precursors that require proteolytic processing to generate smaller, biologically active peptides that are secreted to mediate neurotransmission and hormone actions. Neuropeptides within their precursors are typically flanked by pairs of basic residues, as well as by monobasic residues. In this review, evidence for secretory vesicle cathepsin L and Arg/Lys aminopeptidase as a distinct proteolytic pathway for processing the prohormone proenkephalin is presented. Cleavage of prohormone processing sites by secretory vesicle cathepsin L occurs at the NH2-terminal side of dibasic residues, as well as between the dibasic residues, resulting in peptide intermediates with Arg or Lys extensions at their NH2-termini. A subsequent Arg/Lys aminopeptidase step is then required to remove NH2-terminal basic residues to generate the final enkephalin neuropeptide. The cathepsin L and Arg/Lys aminopeptidase prohormone processing pathway is distinct from the proteolytic pathway mediated by the subtilisin-like prohormone convertases 1/3 and 2 (PC1/3 and PC2) with carboxypeptidase E/H. Differences in specific cleavage sites at paired basic residue sites distinguish these two pathways. These two proteolytic pathways demonstrate the increasing complexity of regulatory mechanisms for the production of peptide neurotransmitters and hormones.

Expression of aminopeptidase B in the developing and adult rat retina

Aminopeptidase B (Ap-B), a ubiquitous enzyme, catalyses the amino-terminal cleavage of basic residues of peptide or protein substrates, indicating a role in precursor processing. The physiological function of Ap-B still remains an open question, even though its activity suggests that it could be involved in inflammatory processes and proliferation of tumor cells. This study was conducted to determine the expression of Ap-B in the developing and adult retina as a path to envisage physiological roles of Ap-B. RT-PCR and in situ hybridization were used to detect expression of Ap-B mRNA and activity tests, Western blotting and immunofluorescence microscopy were performed to identify and localize the enzyme in the rat retina. These biochemical and morphological methods show that Ap-B is expressed in the retina from embryo to adult. Expression level is restricted to specific layers (pigmented epithelium, outer and inner plexiform layers and ganglion cell layer) and is developmentally regulated. Moreover, a preliminary analysis indicates that Ap-B, the glucose transporter GLUT3 and choline acetyltransferase (ChAT) share a similar expression pattern in retina. Altogether, Ap-B appears predominantly expressed in neuronal cells lying in retinal layers containing neuritic extensions and synaptic junctions. Such expression is up-regulated during ontogenesis allowing to hypothesized that Ap-B participates in processes accompanying retinal neuronal cell differentiation.

Expression and purification of rat recombinant aminopeptidase B secreted from baculovirus-infected insect cells

Aminopeptidase B (Ap-B) is a ubiquitous enzyme and its physiological function still remains an open question. This Zn2+ -exopeptidase catalyzes the amino-terminal cleavage of basic residues of peptide or protein substrates, indicating a role in precursor processing. In addition, the enzyme exhibits a residual capacity to hydrolyze leukotriene A4 (LTA4) into the pro-inflammatory lipid mediator leukotriene B4 (LTB4) in vitro. This potential bi-functional nature of Ap-B is supported by a close structural relationship with LTA4 hydrolase, which hydrolyzes LTA4 into LTB4, in vivo, and exhibits an aminopeptidase activity, in vitro. Structural studies are necessary for the detailed understanding of the bi-functional enzymatic mechanism of Ap-B. In this study, we report cDNA cloning, baculovirus expression, and purification of the rat Ap-B (rAp-B). The Ap-B cDNA was constructed from extracted rat testes total RNA and introduced into the pBAC1 baculovirus transfer vector to generate recombinant baculoviruses. rAp-B expression, with or without COOH-hexahistidine tag, was tested in two different insect cell hosts (Sf9 and H5). The enzyme is secreted into the insect cell culture medium, which allowed a rapid purification of the protein. The His-tagged rAp-B was purified using metal affinity resin while the native recombinant rAp-B was partially purified using a single step DEAE Trisacryl ion exchange column. Although the recombinant rAp-B exhibits biochemical properties equivalent to those of the rat testes purified protein, the presence of the histidine-tag seems to partially inhibit the exopeptidase activity. However, this report shows that baculovirus-infected cells are a useful system to produce rat Ap-B for use in studying enzymatic mechanisms in vitro and 3D structure.

The Drosophila slamdance gene: a mutation in an aminopeptidase can cause seizure, paralysis and neuronal failure

We report here the characterization of slamdance (sda), a Drosophila melanogaster "bang-sensitive" (BS) paralytic mutant. This mutant exhibits hyperactive behavior and paralysis following a mechanical "bang" or electrical shock. Electrophysiological analyses have shown that this mutant is much more prone to seizure episodes than normal flies because it has a drastically lowered seizure threshold. Through genetic mapping, molecular cloning, and RNA interference, we have demonstrated that the sda phenotype can be attributed to a mutation in the Drosophila homolog of the human aminopeptidase N (APN) gene. Furthermore, using mRNA in situ hybridization and LacZ staining, we have found that the sda gene is expressed specifically in the central nervous system at particular developmental stages. Together, these results suggest that the bang sensitivity in sda mutants is caused by a defective APN gene that somehow increases seizure susceptibility. Finally, by using the sda mutation as a sensitized background, we have been able to identify a rich variety of sda enhancers and other independent BS mutations.

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.

Human aminopeptidase B (rnpep) on chromosome 1q32.2: complementary DNA, genomic structure and expression

Aminopeptidase B (APB) is a Zn(2+)-metalloexopeptidase, which selectively removes Arg and/or Lys residues from the N-terminus of several peptide substrates. Several data strongly support the hypothesis that this enzyme could participate in the final stages of precursor processing mechanisms and/or in particular inflammatory processes and tumor developments. Therefore, we have cloned the complementary DNA encoding the human APB, a 658-residues protein, containing the canonical "HEXXH(X(18))E", a signature allowing its classification in the M1 family of metallopeptidases. The genomic structure of the human APB gene (rnpep; 1q32.1-q32.2) was also determined. rnpep is bracketed by pre-protein translocase of the inner mitochondrial membrane gene and ETS family transcription factor ELF3 gene. It spans more than 24 kbp and contains 11 exons ranging from 109 to 574 bp. Finally, expression of the human APB messenger RNA (mRNA) was investigated using a pre-made dot-blot. This mRNA seems to be ubiquitous although its expression level varies depending of the cells or tissues considered.

Leukotriene A4 hydrolase: selective abrogation of leukotriene B4 formation by mutation of aspartic acid 375

Leukotriene A4 (LTA4, 5S-trans-5,6-oxido-7,9-trans-11,14-cis-eicosatetraenoic acid) hydrolase (LTA4H)/aminopeptidase is a bifunctional zinc metalloenzyme that catalyzes the final and rate-limiting step in the biosynthesis of leukotriene B4 (LTB4, 5S,12R-dihydroxy-6,14-cis-8,10-trans-eicosatetraenoic acid), a classical chemoattractant and immune modulating lipid mediator. Two chemical features are key to the bioactivity of LTB4, namely, the chirality of the 12R-hydroxyl group and the cis-trans-trans geometry of the conjugated triene structure. From the crystal structure of LTA4H, a hydrophilic patch composed of Gln-134, Tyr-267, and Asp-375 was identified in a narrow and otherwise hydrophobic pocket, believed to bind LTA4. In addition, Asp-375 belongs to peptide K21, a previously characterized 21-residue active site-peptide to which LTA4 binds during suicide inactivation. In the present report we used site-directed mutagenesis and x-ray crystallography to show that Asp-375, but none of the other candidate residues, is specifically required for the epoxide hydrolase activity of LTA4H. Thus, mutation of Asp-375 leads to a selective loss of the enzyme's ability to generate LTB4 whereas the aminopeptidase activity is preserved. We propose that Asp-375, possibly assisted by Gln-134, acts as a critical determinant for the stereoselective introduction of the 12R-hydroxyl group and thus the biological activity of LTB4.

Identification and characterization of leptin-containing intracellular compartment in rat adipose cells

The major leptin-containing membrane compartment was identified and characterized in rat adipose cells by means of equilibrium density and velocity sucrose gradient centrifugation. This compartment appears to be different from peptide-containing secretory granules present in neuronal, endocrine, and exocrine cells, as well as from insulin-sensitive GLUT-4-containing vesicles abundant in adipocytes. Exocytosis of both leptin- and GLUT-4-containing vesicles can be induced by insulin; however, only leptin secretion is responsive to serum stimulation. This latter effect is resistant to cycloheximide, suggesting that serum triggers the release of a stored pool of presynthesized leptin molecules. We conclude that regulated secretion of leptin and insulin-dependent translocation of GLUT-4 represent different pathways of membrane trafficking in rat adipose cells. NIH 3T3 cells ectopically expressing CAAT box enhancer binding protein-alpha and Swiss 3T3 cells expressing peroxisome proliferator-activated receptor-gamma undergo differentiation in vitro and acquire adipocyte morphology and insulin-responsive glucose uptake. Only the former cell line, however, is capable of leptin secretion. Thus different transcriptional mechanisms control the developmental onset of these two major and independent physiological functions in adipose cells.

Genetic variation in the gene encoding calpain-10 is associated with type 2 diabetes mellitus

Type 2 or non-insulin-dependent diabetes mellitus (NIDDM) is the most common form of diabetes worldwide, affecting approximately 4% of the world's adult population. It is multifactorial in origin with both genetic and environmental factors contributing to its development. A genome-wide screen for type 2 diabetes genes carried out in Mexican Americans localized a susceptibility gene, designated NIDDM1, to chromosome 2. Here we describe the positional cloning of a gene located in the NIDDM1 region that shows association with type 2 diabetes in Mexican Americans and a Northern European population from the Botnia region of Finland. This putative diabetes-susceptibility gene encodes a ubiquitously expressed member of the calpain-like cysteine protease family, calpain-10 (CAPN10). This finding suggests a novel pathway that may contribute to the development of type 2 diabetes.

Distribution of thimet oligopeptidase (E.C. 3.4.24.15) in human and rat testes

Thimet oligopeptidase (TOP:E.C. 3.4.24.15) is a thiol sensitive metalloendopeptidase which is widely distributed and active in most tissues including testis, brain and pituitary. In the median eminence it is postulated to play a role in the degradation of GnRH released from the hypothalamus and thus to modulate LH levels. In the rat and human, the testis is the richest source of TOP activity with levels 3- to 5-fold higher than that of the brain. In order to define the exact localisation of this enzyme within the rat and human testis, the distribution of TOP in the developing and adult gonad was examined in situ and in isolated cells by immunohistochemistry, western blotting and northern blotting analysis. Ontogeny studies have demonstrated that TOP is detectable by western blotting from 9 days with levels of expression increasing with the age of the animal. Immunolocalisation of the protein in the interstitium was positive from 9 days onwards but was negative within the seminiferous tubules before 35 days of age, whereas TOP mRNA was not detected within the testis until 35 days of age with subsequent stable expression levels up to 90 days. In the adult rat testis, a strong TOP immunoreactivity was observed within seminiferous tubules, in elongating and elongated spermatids and residual bodies. In the interstitial compartment, immunoreactivity was also observed in Leydig cells and throughout the interstitial space. Western blot analyses confirmed the distribution of expression observed using immunochemistry, however Leydig cells display a lower signal than expected from the immunohistochemical data. Northern hybridization showed that the transcript is present in pachytene spermatocytes, early spermatids, and residual bodies, whereas its presence was not observed in Leydig cells probably due to very low levels of expression of the message. Analyses of various human tissue extracts showed that the testis displays the highest levels of TOP mRNA, with immunohistochemical experiments revealing that, as in the rat, the protein is principally expressed in elongated spermatids/residual bodies, and in Leydig cells. It is concluded that in the human and rat testes, TOP is highly expressed, in particular in post-meiotic germ cells and Leydig cells. The possible involvement of TOP in proteolytic events associated with the process of spermiogenesis and Leydig cell function is currently under investigation.

Modification of leukotriene A(4) hydrolase/aminopeptidase by sulfhydryl-blocking reagents: differential effects on dual enzyme activities by methyl-methane thiosulfonate

The presence of a cysteine residue at or near the active site of leukotriene A(4) hydrolase (EC 3.3.2.6) was suggested by inactivation of the enzyme with sulfhydryl-blocking reagents and by protection against inactivation afforded by substrates and competitive inhibitors. The aminopeptidase activity was more susceptible to inactivation than the epoxide hydrolase activity. The sulfhydryl-modifying reagent methyl-methane thiosulfonate reacted with one thiol as judged by kinetic data and titration with 5, 5'-dithiobis-2-nitrobenzoate. Inactivation was a time- and dose-dependent process of apparent pseudo-first-order and maximal at 80-85%. The inactivation rate was nonsaturable and strongly influenced by ion strength. The second-order rate constant increased from 0.9 to 4.3 M(-1) s(-1) in the presence of 0.2 M NaCl. Albumin, a stimulator of the aminopeptidase activity, increased apparent inactivation rates by shifting pK(a) for the modification from 8.2 to 7.8. The inactivated enzyme partially regained activity upon treatment with beta-mercaptoethanol. Peptide substrates and competitive inhibitors protected against inactivation. Bestatin, a competitive inhibitor, afforded complete protection with a K(D) = 0.15 microM, similar to K(i) = 0.17 microM for inhibition of peptidase activity. Treated enzyme had an unchanged K(m) but a reduced V(max). The epoxide hydrolase activity was only weakly affected by methyl-methane thiosulfonate with a maximal inactivation of 15-20% after prolonged treatment. Pretreatment of leukotriene A(4) hydrolase with the reagent did not protect against mechanism-based inactivation by its lipid substrate, leukotriene A(4). On the other hand, leukotriene B(4) was a competitive inhibitor of aminopeptidase activity and protected against modification by methyl-methane thiosulfonate. Our results suggest the presence of a cysteine at or close to subsite S'(1) of the active site of leukotriene A(4) hydrolase and that modification of this residue interferes with the function of the aminopeptidase activity, but not the epoxide hydrolase activity. This is the first report to distinguish the two catalytic activities of leukotriene A(4) hydrolase by chemical means.

Aminopeptidase B (EC 3.4.11.6)

Aminopeptidase B (EC 3.4.11.6) is a Zn(2+)-dependent exopeptidase which selectively removes arginine and/or lysine residues from the NH2-terminus of several peptide substrates including Arg0-Leu-enkephalin, Arg0-Met-enkephalin and Arg-1-Lys0-somatostatin-14. Analysis of its primary structure showed that aminopeptidase-B is structurally related to leukotriene A4 hydrolase, an important enzyme of the arachidonic acid pathway. This structural relationship is further supported by the capacity of aminopeptidase-B to hydrolyse leukotriene A4. Aminopeptidase-B is widely distributed in a number of tissues, including endocrine and non-endocrine cells. Moreover, in rat PC12 pheochromocytoma cells, the enzyme is secreted and associated with the external face of the plasma membrane. Together these data strongly argue in favour of a role of this bi-functional enzyme in the final stages of precursor processing mechanisms occurring either in the secretory pathway, at the plasma membrane, or at both locations.

Glucose transporter Glut3 is targeted to secretory vesicles in neurons and PC12 cells

In rat brain and cultured neuroendocrine PC12 cells, Glut3 is localized at the cell surface and, also, in a distinct population of homogenous synaptic-like vesicles. Glut3-containing vesicles co-purify with "classical" synaptic vesicles, but can be separated from the latter by sucrose gradient centrifugation. Unlike classical synaptic vesicles, Glut3-containing vesicles possess a high level of aminopeptidase activity, which has been identified as aminopeptidase B. This enzyme has recently been shown to be a marker of the secretory pathway in PC12 cells (Balogh, A., Cadel, S., Foulon, T., Picart, R., Der Garabedian, A., Rousselet, A., Tougard, C., and Cohen, P. (1998) J. Cell Sci. 111, 161-169). We, therefore, conclude that Glut3 is targeted to secretory vesicles in both neurons and PC12 cells.

Aminopeptidase B is structurally related to leukotriene-A4 hydrolase but is not a bifunctional enzyme with epoxide hydrolase activity

Aminopeptidase B (Ap B; EC 3.4.11.6) is a zinc-binding protein that contains the consensus sequence HEXXHX18E (324-347), conserved among the M1 family of metallopeptidases. To determine if these putative zinc-binding residues (His324, His328 and Glu347) and the active-site Glu325 are essential for the enzyme activity, we replaced the histidines with tyrosines and the glutamic acid residues with alanines using site-directed mutagenesis. The cDNAs were expressed in Escherichia coli, and the resulting recombinant proteins, named H324Y, E325A, H328Y and E347A, were purified to apparent homogeneity. None of the expressed mutated proteins showed aminopeptidase activity. The E325A enzyme contained 1 mol of zinc per mol of protein, and the other three mutants, H324Y, H328Y and E347A, did not contain significant amounts of zinc, as determined by atomic absorption spectrometry. From sequence-homology searches, Ap B is known to be closely related to leukotriene (LT)-A4 hydrolase (EC 3.3.2.6). We examined human placental Ap B and recombinant rat Ap B, both of which had been purified previously [Fukasawa, Fukasawa, Kanai, Fujii and Harada (1996) J. Biol. Chem. 271, 30731-30735], to determine whether or not they had epoxide hydrolase activities. However, neither enzyme hydrolysed LTA4 into LTB4. We then replaced some amino acids in the domain of the rat enzyme similar to the LTA4-binding site of LTA4 hydrolase. However, these mutants, Y408F, N409S and NE409-410SS also did not possess any epoxide hydrolase activity. We concluded that Ap B is an M1-family zinc metallopeptidase without epoxide hydrolase activity.

PC18, a specific aminopeptidase N inhibitor, induces vasopressin release by increasing the half-life of brain angiotensin III

Angiotensin III (AngIII), which is metabolized in vivo by aminopeptidase N (APN), was previously shown to be one of the main effector peptides of the brain renin-angiotensin system (RAS) in the control of vasopressin release. Recently, a potent APN inhibitor, PC18 (2-amino-4-methylsulfonyl butane thiol, methionine thiol), has been developed. In this study, we first checked the in vitro selectivity of PC18 towards APN, aminopeptidase A (APA) and aminopeptidase B (APB), three zinc metalloproteases with significant identity between their amino acid sequences. The Ki values of this compound on APN were found to be in the nanomolar range (Ki = 8.0 +/- 1.7 nM) but it was 2,150 and 125 times less active on APA and APB, respectively. Secondly, we evaluated in vivo the effect of brain APN inhibition with PC18 on the inactivation of brain AngIII and on vasopressin secretion in mice. For this purpose, mice received [3H]AngII intracerebroventricularly in the presence or absence of the APN inhibitor PC18 (30 microg). At different times after the injection, [3H]AngIII levels were evaluated from hypothalamus homogenates after separation by cation-exchange chromatography. PC18 induced an accumulation of [3H]AngIII, increasing its half-life 3.9 times as compared with control values. In addition, the effect of PC18 on vasopressin release was studied in mice. PC18 (10-100 microgram) was injected intracerebroventricularly, and plasma vasopressin levels were estimated by radioimmunoassay. PC18 increased vasopressin levels in a dose-dependent manner. The maximal increase in vasopressin release (+220%) is observed for a dose of PC18 of 100 microgram and was inhibited 75% by the coadministration of the AngII receptor antagonist (Sar1-Ala8)-AngII (0.5 microgram). These results indicate that in vivo, in the mouse brain, APN inhibition by PC18 increases the half-life of endogenous AngIII, resulting in an enhanced vasopressin release.

Molecular cloning and functional expression of a Caenorhabditis elegans aminopeptidase structurally related to mammalian leukotriene A4 hydrolases

In a search of the Caenorhabditis elegans DNA data base, an expressed sequence tag of 327 base pairs (termed cm01c7) with strong homology to the human leukotriene A4 (LTA4) hydrolase was found. The use of cm01c7 as a probe, together with conventional hybridization screening and anchored polymerase chain reaction techniques resulted in the cloning of the full-length 2.1 kilobase pair C. elegans LTA4 hydrolase-like homologue, termed aminopeptidase-1 (AP-1). The AP-1 cDNA was expressed transiently as an epitope-tagged recombinant protein in COS-7 mammalian cells, purified using an anti-epitope antibody affinity resin, and tested for LTA4 hydrolase and aminopeptidase activities. Despite the strong homology between the human LTA4 hydrolase and C. elegans AP-1(63% similarity and 45% identity at the amino acid level), reverse-phase high pressure liquid chromatography and radioimmunoassay for LTB4 production revealed the inability of the C. elegans AP-1 to use LTA4 as a substrate. In contrast, the C. elegans AP-1 was an efficient aminopeptidase, as demonstrated by its ability to hydrolyze a variety of amino acid p-nitroanilide derivatives. The aminopeptidase activity of C. elegans AP-1 resembled that of the human LTA4 hydrolase/aminopeptidase enzyme with a preference for arginyl-p-nitroanilide as a substrate. Hydrolysis of the amide bond of arginyl-p-nitroanilide was inhibited by bestatin with an IC50 of 2.6 +/- 1.2 microM. The bifunctionality of the mammalian LTA4 hydrolase is still poorly understood, as the physiological substrate for its aminopeptidase activity is yet to be discovered. Our results support the idea that the enzyme originally functioned as an aminopeptidase in lower metazoa and then developed LTA4 hydrolase activity in more evolved organisms.

A glutamate residue contributes to the exopeptidase specificity in aminopeptidase A

Aminopeptidase A (EC 3.4.11.7, APA) is a 130 kDa membrane-bound aminopeptidase that contains the consensus sequence HEXXH (385-389) found in the zinc metalloprotease family, the zincins. Sequence alignment of the mouse APA with other monozinc-aminopeptidases indicates the presence of a highly conserved glutamate residue (Glu352 in APA) found in the conserved motif GAMEN (349-353). In monozinc-aminopeptidases, the negative charge of the glutamate side-chain carboxylate may constitute the anionic binding site involved in the recognition of the free amino group of substrates or inhibitors. The functional role of Glu352 in APA was investigated by substituting this residue with an aspartate (Asp352), a glycine (Gly352), a glutamine (Gln352) or an arginine (Arg352) residue by site-directed mutagenesis. Kinetic studies showed that the Km values of the mutant enzymes were unaffected, whereas kcat values were decreased 10-250-fold, resulting in a 10-, 30- 260- and 400-fold reduction in cleavage efficiencies for the mutants Asp352, Gly352, Gln352 and Arg352 respectively. The inhibitory potency of two different classes of inhibitors, a thiol and a phosphonate compound, was significantly (P<0.05) decreased by 10- and 4-fold respectively in the mutated enzymes. Moreover, the inhibitory potency of angiotensin I, used as a competitor of the synthetic substrate alpha-l-glutamyl beta-naphthylamide, displayed a 4-fold reduction (P<0.01) in the mutated enzymes, whereas the Ki values of its N-acetyl derivative were unchanged. These data strongly suggest that Glu352 is involved in the catalytic process of APA and contributes to the exopeptidase activity of this enzyme through interaction with the N-terminal part of substrates or inhibitors.

Purification and characterization of leukotriene A4 hydrolase from Xenopus laevis oocytes

In mammals, leukotriene A4 hydrolase converts leukotriene A4 into the proinflammatory mediator leukotriene B4. We have purified and characterized a non-mammalian leukotriene A4 hydrolase from Xenopus laevis oocytes. This enzyme contains one zinc atom and catalyzes an anion-dependent peptidase activity, two key features of the mammalian enzymes. The amino acid sequence of an internal segment is 60% identical with human leukotriene A4 hydrolase but only 27% identical with rat aminopeptidase B. The Xenopus laevis enzyme is catalytically very efficient and, unlike the human enzyme, converts leukotriene A4 into two enzymatic metabolites, viz. leukotriene B4 and delta6-trans-delta8-cis-leukotriene B4.

Evidence for a leukotriene A4 hydrolase in Xenopus laevis skin exudate

Leukotriene A4 hydrolase is a cytosolic metalloenzyme of the arachidonic acid biosynthetic pathway responsible for leukotriene A4 conversion into leukotriene B4. In addition to its epoxide hydrolase properties, this enzyme exhibits an aminopeptidase activity which was used as an assay to monitor the purification of a novel form of leukotriene A4 hydrolase from Xenopus laevis skin exudate. This 70 kDa, secreted, form of leukotriene A4 hydrolase was identified by immunochemical cross-reactivity with anti-human leukotriene A4 hydrolase antibodies and by its capacity to convert leukotriene A4 into leukotriene B4. Moreover this enzyme produced a second metabolite which could be the leukotriene B4 isomer 5S,12R-dihydroxy-6,10-trans-8,14-4-cis-eicosatetraenoic acid, previously shown by Strömberg et al. (Eur.J. Biochem. 238 (1996) 599-605) to be formed by incubation of the leukotriene A4 with amphibian tissue extracts. Partial amino acid sequencing of peptides generated by endolysin C fragmentation of the purified enzyme confirmed the presence, in X. laevis skin secretions, of a related but distinct form of leukotriene A4 hydrolase which is likely to be responsible for the production of these eicosanoid metabolites of leukotriene A4.