Comparison of the enzymatic and biochemical properties of human insulin-degrading enzyme and Escherichia coli protease III

Strategies to Enhance Periplasmic Recombinant Protein Production Yields in Escherichia coli

Main reasons to produce recombinant proteins in the periplasm of E. coli rather than in its cytoplasm are to -i- enable disulfide bond formation, -ii- facilitate protein isolation, -iii- control the nature of the N-terminus of the mature protein, and -iv- minimize exposure to cytoplasmic proteases. However, hampered protein targeting, translocation and folding as well as protein instability can all negatively affect periplasmic protein production yields. Strategies to enhance periplasmic protein production yields have focused on harmonizing secretory recombinant protein production rates with the capacity of the secretory apparatus by transcriptional and translational tuning, signal peptide selection and engineering, increasing the targeting, translocation and periplasmic folding capacity of the production host, preventing proteolysis, and, finally, the natural and engineered adaptation of the production host to periplasmic protein production. Here, we discuss these strategies using notable examples as a thread.

Identification of Essential Genes Associated With Prodigiosin Production in Serratia marcescens FZSF02

Prodigiosin is a promising secondary metabolite produced mainly by Serratia strains. To study the global regulatory mechanism of prodigiosin biosynthesis, a mutagenesis library containing 23,000 mutant clones was constructed with the EZ-Tn5 transposon, and 114 clones in the library showed altered prodigiosin production ability. For 37 of the 114 clones, transposon insertion occurred on the prodigiosin biosynthetic cluster genes; transposon inserted genes of the 77 clones belonged to 33 different outside prodigiosin biosynthetic cluster genes. These 33 genes can be divided into transcription-regulating genes, membrane protein-encoding genes, and metabolism enzyme-encoding genes. Most of the genes were newly reported to be involved in prodigiosin production. Transcriptional levels of the pigA gene were significantly downregulated in 22 mutants with different inserted genes, which was in accordance with the phenotype of decreased prodigiosin production. Functional confirmation of the mutant genes involved in the pyrimidine nucleotide biosynthesis pathway was carried out by adding orotate and uridylate (UMP) into the medium. Gene complementation confirmed the regulatory function of the EnvZ/OmpR two-component regulatory system genes envZ and ompR in prodigiosin production.

Modulation of Insulin Sensitivity by Insulin-Degrading Enzyme

Insulin-degrading enzyme (IDE) is a highly conserved and ubiquitously expressed metalloprotease that degrades insulin and several other intermediate-size peptides. For many decades, IDE had been assumed to be involved primarily in hepatic insulin clearance, a key process that regulates availability of circulating insulin levels for peripheral tissues. Emerging evidence, however, suggests that IDE has several other important physiological functions relevant to glucose and insulin homeostasis, including the regulation of insulin secretion from pancreatic β-cells. Investigation of mice with tissue-specific genetic deletion of Ide in the liver and pancreatic β-cells (L-IDE-KO and B-IDE-KO mice, respectively) has revealed additional roles for IDE in the regulation of hepatic insulin action and sensitivity. In this review, we discuss current knowledge about IDE’s function as a regulator of insulin secretion and hepatic insulin sensitivity, both evaluating the classical view of IDE as an insulin protease and also exploring evidence for several non-proteolytic functions. Insulin proteostasis and insulin sensitivity have both been highlighted as targets controlling blood sugar levels in type 2 diabetes, so a clearer understanding the physiological functions of IDE in pancreas and liver could led to the development of novel therapeutics for the treatment of this disease.

Type 3 Diabetes: Cross Talk between Differentially Regulated Proteins of Type 2 Diabetes Mellitus and Alzheimer's Disease

Type 3 Diabetes (T3D) is a neuroendocrine disorder that represents the progression of Type 2 Diabetes Mellitus (T2DM) to Alzheimer’s disease (AD). T3D contributes in the increase of the total load of Alzheimer’s patients worldwide. The protein network based strategies were used for the analysis of protein interactions and hypothesis was derived describing the possible routes of communications among proteins. The hypothesis provides the insight on the probable mechanism of the disease progression for T3D. The current study also suggests that insulin degrading enzyme (IDE) could be the major player which holds the capacity to shift T2DM to T3D by altering metabolic pathways like regulation of beta-cell development, negative regulation of PI3K/AKT pathways and amyloid beta degradation.

Proteolytically inactive insulin-degrading enzyme inhibits amyloid formation yielding non-neurotoxic aβ peptide aggregates

Insulin-degrading enzyme (IDE) is a neutral Zn²⁺ peptidase that degrades short peptides based on substrate conformation, size and charge. Some of these substrates, including amyloid β (Aβ) are capable of self-assembling into cytotoxic oligomers. Based on IDE recognition mechanism and our previous report of the formation of a stable complex between IDE and intact Aβ in vitro and in vivo, we analyzed the possibility of a chaperone-like function of IDE. A proteolytically inactive recombinant IDE with Glu111 replaced by Gln (IDEQ) was used. IDEQ blocked the amyloidogenic pathway of Aβ yielding non-fibrillar structures as assessed by electron microscopy. Measurements of the kinetics of Aβ aggregation by light scattering showed that 1) IDEQ effect was promoted by ATP independent of its hydrolysis, 2) end products of Aβ-IDEQ co-incubation were incapable of “seeding” the assembly of monomeric Aβ and 3) IDEQ was ineffective in reversing Aβ aggregation. Moreover, Aβ aggregates formed in the presence of IDEQ were non-neurotoxic. IDEQ had no conformational effects upon insulin (a non-amyloidogenic protein under physiological conditions) and did not disturb insulin receptor activation in cultured cells. Our results suggest that IDE has a chaperone-like activity upon amyloid-forming peptides. It remains to be explored whether other highly conserved metallopeptidases have a dual protease-chaperone function to prevent the formation of toxic peptide oligomers from bacteria to mammals.

A mammalian insulysin homolog is regulated by enzyme IIA(Glc) of the glucose transport system in Vibrio vulnificus

Vibrio vulnificus is an opportunistic human pathogen that causes severe infections in susceptible individuals. While the components of the Escherichia coli phosphoenolpyruvate: sugar phosphotransferase system (PTS) have been shown to regulate numerous targets, little such information is available for the V. vulnificus PTS. Here we show that enzyme IIA(Glc) of the PTS regulates the peptidase activity of a mammalian insulysin homolog in V. vulnificus. While interaction of IIA(Glc) with the insulysin homolog is independent of the phosphorylation state of IIA(Glc), only unphosphorylated IIA(Glc) activates the insulysin homolog. Taken together, our results suggest that the V. vulnificus insulysin-IIA(Glc) complex plays a role in survival in the host by sensing glucose.

Yeast Ste23p shares functional similarities with mammalian insulin-degrading enzymes

The S. cerevisiae genome encodes two M16A enzymes: Axl1p and Ste23p. Of the two, Ste23p shares significantly higher sequence identity with M16A enzymes from other species, including mammalian insulin-degrading enzymes (IDEs). In this study, recombinant Ste23p and R. norvegicus IDE (RnIDE) were isolated from E. coli, and their enzymatic properties compared. Ste23p was found to cleave established RnIDE substrates, including the amyloid-beta peptide (Abeta1-40) and insulin B-chain. A novel internally quenched fluorogenic substrate (Abz-SEKKDNYIIKGV-nitroY-OH) based on the polypeptide sequence of the yeast P2 a-factor mating propheromone was determined to be a suitable substrate for both Ste23p and RnIDE, and was used to conduct comparative enzymological studies. Both enzymes were most active at 37 degrees C, in alkaline buffers and in high salt environments. In addition, the proteolytic activities of both enzymes towards the fluorogenic substrate were inhibited by metal chelators, thiol modifiers, inhibitors of cysteine protease activity and insulin. Characteristics of STE23 expression were also evaluated. Our analysis indicates that the 5' terminus of the STE23 gene has been mischaracterized, with the physiologically relevant initiator corresponding to residue M53 of the publicly annotated protein sequence. Finally, we demonstrate that, unlike haploid-specific Axl1p, Ste23p is expressed in both haploid and diploid cell types. Our study presents the first comprehensive biochemical analysis of a yeast M16A enzyme, and provides evidence that S. cerevisiae Ste23p has enzymatic properties that are highly consistent with mammalian IDEs and other M16A enzymes.

Degradation of relaxin family peptides by insulin-degrading enzyme

Insulin-degrading enzyme (IDE) is a ubiquitously expressed metalloproteinase responsible for the intracellular degradation of insulin. IDE also interacts with other members of the insulin superfamily, including relaxin, but no studies have been reported regarding the interaction of other relaxin-like peptides with IDE. In this study, we determined that relaxin, relaxin-3, and InsL3 all competitively inhibited the degradation of insulin by IDE to different degrees, and all inhibited covalent cross-linking of insulin to IDE. Each of the peptides was degraded by IDE to various degrees (insulin > relaxin > InsL3 = relaxin-3). In summary, relaxin, InsL3, and relaxin-3 all bound to IDE, competed for the binding and degradation of insulin, and were all substrates for the proteolytic activity of IDE. Therefore, it is possible that in addition to insulin, IDE may be important for the cellular proteolysis of relaxin, InsL3, and relaxin-3.

Molecular bases for the recognition of short peptide substrates and cysteine-directed modifications of human insulin-degrading enzyme

Insulin degrading enzyme (IDE) utilizes a large catalytic chamber to selectively bind and degrade peptide substrates such as insulin and amyloid beta (Abeta). Tight interactions with substrates occur at an exosite located approximately 30 A away from the catalytic center that anchors the N-terminus of substrates to facilitate binding and subsequent cleavages at the catalytic site. However, IDE also degrades peptide substrates that are too short to occupy both the catalytic site and the exosite simultaneously. Here, we use kinins as a model system to address the kinetics and regulation of human IDE with short peptides. IDE specifically degrades bradykinin and kallidin at the Pro/Phe site. A 1.9 A crystal structure of bradykinin-bound IDE reveals the binding of bradykinin to the exosite and not to the catalytic site. In agreement with observed high K(m) values, this suggests low affinity of bradykinin for IDE. This structure also provides the molecular basis on how the binding of short peptides at the exosite could regulate substrate recognition. We also found that human IDE is potently inhibited by physiologically relevant concentrations of S-nitrosylation and oxidation agents. Cysteine-directed modifications play a key role, since an IDE mutant devoid of all 13 cysteines is insensitive to the inhibition by S-nitrosoglutathione, hydrogen peroxide, or N-ethylmaleimide. Specifically, cysteine 819 of human IDE is located inside the catalytic chamber pointing toward an extended hydrophobic pocket and is critical for the inactivation. Thiol-directed modification of this residue likely causes local structural perturbation to reduce substrate binding and catalysis.

Molecular basis for the thiol sensitivity of insulin-degrading enzyme

Insulin-degrading enzyme (IDE) is a ubiquitous zinc-metalloprotease that hydrolyzes several pathophysiologically relevant peptides, including insulin and the amyloid beta-protein (Abeta). IDE is inhibited irreversibly by compounds that covalently modify cysteine residues, a mechanism that could be operative in the etiology of type 2 diabetes mellitus (DM2) or Alzheimer's disease (AD). However, despite prior investigation, the molecular basis underlying the sensitivity of IDE to thiol-alkylating agents has not been elucidated. To address this topic, we conducted a comprehensive mutational analysis of the 13 cysteine residues within IDE. Our analysis implicates C178, C812, and C819 as the principal residues conferring thiol sensitivity. The involvement of C812 and C819, residues quite distant from the catalytic zinc atom, provides functional evidence that the active site of IDE comprises two separate domains that are operational only in close apposition. Structural analysis and other evidence predict that alkylation of C812 and C819 disrupts substrate binding, whereas alkylation of C178 interferes with the apposition of active-site domains and subtly repositions zinc-binding residues. Unexpectedly, alkylation of C590 was found to activate hydrolysis of Abeta significantly, while having no effect on insulin, demonstrating that chemical modulation of IDE can be both bidirectional and highly substrate selective. Our findings resolve a long-standing riddle about the basic enzymology of IDE with important implications for the etiology of DM2 and AD. Moreover, this work uncovers key details about the mechanistic basis of the unusual substrate selectivity of IDE that may aid the development of pharmacological agents or IDE mutants with therapeutic value.

Cloning, expression and characterization of insulin-degrading enzyme from tomato (Solanum lycopersicum)

A cDNA encoding insulin-degrading enzyme (IDE) was cloned from tomato (Solanum lycopersicum) and expressed in Escherichia coli in N-terminal fusion with glutathione S-transferase. GST-SlIDE was characterized as a neutral thiol-dependent metallopeptidase with insulinase activity: the recombinant enzyme cleaved the oxidized insulin B chain at eight peptide bonds, six of which are also targets of human IDE. Despite a certain preference for proline in the vicinity of the cleavage site, synthetic peptides were cleaved at apparently stochastic positions indicating that SlIDE, similar to IDEs from other organisms, does not recognize any particular amino acid motif in the primary structure of its substrates. Under steady-state conditions, an apparent K(m) of 62+/-7 microm and a catalytic efficiency (k(cat)/K(m)) of 62+/-15 mm(-1) s(-1) were determined for Abz-SKRDPPKMQTDLY(NO(3))-NH(2) as the substrate. GST-SlIDE was effectively inhibited by ATP at physiological concentrations, suggesting regulation of its activity in response to the energy status of the cell. While mammalian and plant IDEs share many of their biochemical properties, this similarity does not extend to their function in vivo, because insulin and the beta-amyloid peptide, well-established substrates of mammalian IDEs, as well as insulin-related signaling appear to be absent from plant systems.

The C-terminal domain of human insulin degrading enzyme is required for dimerization and substrate recognition

Insulin degrading enzyme (IDE), a zinc metalloprotease, can specifically recognize and degrade insulin, as well as several amyloidogenic peptides such as amyloid beta (Abeta) and amylin. The disruption of IDE function in rodents leads to glucose intolerance and cerebral Abeta accumulation, hallmarks of type 2 diabetes and Alzheimer's disease, respectively. Using limited proteolysis, we found that human IDE (113kDa) can be subdivided into two roughly equal sized domains, IDE-N and IDE-C. Oligomerization plays a key role in the activity of IDE. Size-exclusion chromatography and sedimentation velocity experiments indicate that IDE-N is a monomer and IDE-C serves to oligomerize IDE-N. IDE-C alone does not have catalytic activity. It is IDE-N that contains the crucial catalytic residues, however IDE-N alone has only 2% of the catalytic activity of wild type IDE. By complexing IDE-C with IDE-N, the activity of IDE-N can be restored to approximately 30% that of wild type IDE. Fluorescence polarization assays using labeled insulin reveal that IDE-N has reduced affinity to insulin relative to wild type IDE. Together, our data reveal the modular nature of IDE. IDE-N is the catalytic domain and IDE-C facilitates substrate recognition as well as plays a key role in the oligomerization of IDE.

Amyloidogenecity and pitrilysin sensitivity of a lysine-free derivative of amyloid beta-peptide cleaved from a recombinant fusion protein

The progressive cerebral deposition of a 40-42 residues amyloid beta-peptide (Abeta) is regarded as a major factor in the onset of the Alzheimer's disease. It has recently been shown that Abeta(1-40) is cleaved by Escherichia coli pitrilysin, a homologue of insulysin, at a specific site. To facilitate the studies on a recognition mechanism of Abeta by pitrilysin, an overproduction system of Abeta(1-40) as a fusion protein with E. coli RNase HI was constructed. This fusion protein was designed such that an Abeta(1-40) derivative, Abeta(1-40)*, in which Lys16 and Lys28 of Abeta(1-40) are simultaneously replaced by Ala, is attached to the C-terminus of E. coli RNase HI and Abeta(1-40)* is separated from RNase HI upon cleavage with lysyl endopeptidase. The fusion protein was overproduced in E. coli in inclusion bodies, solubilized and purified in the presence of guanidine hydrochloride, and cleaved by lysyl endopeptidase. Abeta(1-40)* was purified from the resultant peptide fragments by reverse-phase HPLC. Measurement of the far-UV CD spectra suggests that Abeta(1-40)* is conformationally similar to Abeta(1-40). However, the thioflavin T binding assay suggests that Abeta(1-40)* is more amyloidogenic than Abeta(1-40). Nevertheless, Abeta(1-40)* was cleaved by pitrilysin at the site identical to that in Abeta(1-40).

Yeast as a tractable genetic system for functional studies of the insulin-degrading enzyme

We have developed yeast as an expression and genetic system for functional studies of the insulin-degrading enzyme (IDE), which cleaves and inactivates certain small peptide molecules, including insulin and the neurotoxic A beta peptide. We show that heterologously expressed rat IDE is enzymatically active, as judged by the ability of IDE-containing yeast extracts to cleave insulin in vitro. We also show that IDE can promote the in vivo production of the yeast a-factor mating pheromone, a function normally attributed to the yeast enzymes Axl1p and Ste23p. However, IDE cannot substitute for the function of Axl1p in promoting haploid axial budding and repressing haploid invasive growth, activities that require an uncharacterized activity of Axl1p. Particulate fractions enriched for Axl1p or Ste23p are incapable of cleaving insulin, suggesting that the functional conservation of these enzymes may not be bidirectionally conserved. We have made practical use of our genetic system to confirm that residues composing the extended zinc metalloprotease motif of M16A family enzymes are required for the enzymatic activity of IDE, Ste23p, and Axl1p. We have determined that IDE and Axl1p both require an intact C terminus for optimal activity. We expect that the tractable genetic system that we have developed will be useful for investigating the enzymatic and structure/function properties of IDE and possibly for the identification of novel IDE alleles having altered substrate specificity.

Mutation of Active Site Residues of Insulin-degrading Enzyme Alters Allosteric Interactions

The active site glutamate (Glu(111)) and the active site histidine (His(112)) of insulin-degrading enzyme (IDE) were mutated. These mutant enzymes exhibit, in addition to a large decrease in catalytic activity, a change in the substrate-velocity response from a sigmoidal one seen with the native enzyme (Hill coefficient > 2), to a hyperbolic response. With 2-aminobenzoyl-GGFLRKHGQ-N-(2,4-dinitrophenyl)ethylenediamine as substrate, ATP and triphosphate increase the reaction rate of the wild type enzyme some 50-80-fold. This effect is dampened with glutamate mutants to no effect or less than a 3-fold increase in activity and changed to inhibition with the histidine mutants. Sedimentation equilibrium shows the IDE mutants exhibit a similar oligomeric distribution as the wild type enzyme, being predominantly monomeric, with triphosphate having little if any effect on the oligomeric state. Triphosphate did induce aggregation of many of the IDE mutants. Thus, the oligomeric state of IDE does not correlate with kinetic properties. The His(112) mutants were shown to bind zinc, but with a lower affinity than the wild type enzyme. The glutamate mutants displayed an altered cleavage profile for the peptide beta-endorphin. Wild type IDE cleaved beta-endorphin at Leu(17)-Phe(18) and Phe(18)-Lys(19), whereas the glutamate mutants cleaved at these sites, but in addition at Lys(19)-Asn(20) and at Met(5)-Thr(6). Thus, active site mutations of IDE are suggested to not only reduce catalytic activity but also cause local conformational changes that affect the allosteric properties of the enzyme.

Cloning, expression and characterization of a 46.5-kDa metallopeptidase from Bacillus halodurans H4 sharing properties with the pitrilysin family

A 1242 base pair DNA fragment from Bacillus halodurans H4 isolated from alkaline sediments of Lake Bogoria (Kenya) coding for a potential protease was cloned and sequenced. The hexa-histidine-tagged enzyme was overexpressed in Escherichia coli and was purified in one step by immobilized-metal affinity chromatography (IMAC) on Ni-NTA resin. The protease (ppBH4) presents an inverted zincin motif, HXXEH, which defines the inverzincin family. It shares several biochemical and molecular properties with the clan ME family M16 metallopeptidases (pitrilysins), as well as with database hypothetical proteins that are potential M16 family enzymes. Thus, like insulysin and nardilysin, but contrary to bacterial pitrilysin, ppBH4 is inactivated by sulfhydryl alkylating agents. On the other hand, like bacterial pitrilysin, ppBH4 is sensitive to reducing agents. The enzymatic activity of ppBH4 is limited to substrates smaller than proteins. In contrast to insulin, dynorphin and insulin B-chain are very good substrates for ppBH4 and several cleavage sites are common with those observed with well-characterized pitrilysins. As deduced from amino acid sequence, as well as determined by gel-filtration and SDS-polyacrylamide gel electrophoresis, ppBH4 is an active monomer of 46.5 kDa. This feature distinguishes ppBH4 from all other enzymes of the pitrilysin family so far described whose molecular masses range from 100 to 140 kDa.

Non-covalent interaction of ubiquitin with insulin-degrading enzyme

Insulin-degrading enzyme (IDE) is a metalloprotease implicated in insulin degradation and suggested to have a variety of additional functions, including the clearance of amyloid beta peptides of Alzheimer's disease. Little is known about endogenous proteins that may interact with and modulate IDE's activity in the cell. We purified and characterized two proteins from mouse leukemic splenocytes that interact with IDE and inhibit its insulin-degrading activity. A protein of 14 kDa was similar to a competitive IDE inhibitor reported previously. The major inhibitor was identified by amino acid sequencing as ubiquitin, a protein that is post-translationally covalently attached to other intracellular proteins and regulates diverse cellular processes. Ubiquitin inhibited insulin-degrading activity of IDE and diminished crosslinking of 125I-insulin to IDE in a specific, concentration-dependent, reversible, and ATP-independent manner. Ubiquitin did not affect the crosslinking of 125I-insulin to insulin receptors or of 125I-atrial natriuretic peptide (ANP) to its receptor guanylate cyclase-A. These findings suggest a novel role for ubiquitin or perhaps proteins with ubiquitin-like domains in regulating the function of IDE.

Mechanisms of nuclear translocation of insulin

Insulin (Ins) and various other hormones and growth factors have been shown to be rapidly internalized and translocated to the cell nucleus. This review summarizes the mechanisms that are involved in the translocation of Ins to the nucleus, and discusses its possible role in Ins action, based on observations by the authors and others. Ins is internalized to endosomes by both receptor-mediated and fluid-phase endocytosis, the latter occurring only at high Ins concentrations. The authors recently demonstrated the caveolae are the primary cell membrane locations responsible for initiating the signal transduction cascade induced by Ins. Once Ins is internalized, Ins dissociates from the Ins receptor in the endosome, and is translocated to the cytoplasm, where most Ins is degraded by Ins-degrading enzyme (IDE), although how the polypeptides cross the lipid bilayer is unknown. Some Ins escapes the degradation and binds to cytosolic Ins-binding proteins (CIBPs), in addition to IDE. IDE and some CIBPs are known to be binding proteins for other hormones or their receptors, and are involved in gene regulation, suggesting physiological relevance of CIBPs in the signaling of Ins and other hormones. Ins is eventually translocated through the nuclear pore to the nucleus, where Ins tightly associates with nuclear matrix. The role of Ins internalization and translocation to the nucleus is still controversial, although there is substantial evidence to support its role in cellular responses caused by Ins. Many studies indicate that nuclear translocation of various growth factors and hormones plays an important role in cell proliferation or DNA synthesis. It would be reasonable to suggest that Ins internalization, its association with CIBPs, and its translocation to the nucleus may be essential for the regulation of nuclear events by Ins.

Insulin degradation: progress and potential

Insulin degradation is a regulated process that plays a role in controlling insulin action by removing and inactivating the hormone. Abnormalities in insulin clearance and degradation are present in various pathological conditions including type 2 diabetes and obesity and may be important in producing clinical problems. The uptake, processing, and degradation of insulin by cells is a complex process with multiple intracellular pathways. Most evidence supports IDE as the primary degradative mechanism, but other systems (PDI, lysosomes, and other enzymes) undoubtedly contribute to insulin metabolism. Recent studies support a multifunctional role for IDE, as an intracellular binding, regulatory, and degradative protein. IDE increases proteasome and steroid hormone receptor activity, and this activation is reversed by insulin. This raises the possibility of a direct intracellular interaction of insulin with IDE that could modulate protein and fat metabolism. The recent findings would place intracellular insulin-IDE interaction into the insulin signal transduction pathway for mediating the intermediate effects of insulin on fat and protein turnover.

The mitochondrial processing peptidase behaves as a zinc-metallopeptidase

The yeast mitochondrial processing peptidase (MPP) and its subunits were purified in Escherichia coli under conditions for which the enzyme retains most of its processing activity in the absence of externally added divalent cation. The holoenzyme exhibited a Km value of 1.35 microM and a Vmax value of 0.25 microM/min and was inhibited by metal chelators in a time-dependent manner. Measurement of the metal content showed that both, MPP and beta-MPP, contained 0.86 and 1.05 atoms of Zn2+ per molecule, respectively. An enzymatically inactive MPP mutant carrying a mutation of the first histidine of the putative metal-ion binding HXXEH motif in beta-MPP retained less than 0.2 atom of Zn2+ per molecule. A metal-free enzyme (apoenzyme) was prepared from the holoenzyme and shown to be devoid of any processing activity. Incubation of the apoenzyme with 50 nM and 500 nM Zn2+ restored 50% and 80% of the processing activity, respectively. However, no reactivation occurred at concentrations of Zn2+ higher than 1 microM. Addition of 500 nM Mn2+ or higher concentrations (up to 50 microM) reactivated only 50% of the processing activity. The holoenzyme was competitively inhibited by molar excess of Zn2+ (Ki of 3.1 microM) but not by molar excess of Mn2+. Taken together, our data suggest that the authentic MPP is a Zn2+ rather than a Mn2+ metallopeptidase.

Insulin is degraded extracellularly in wounds by insulin-degrading enzyme (EC 3.4.24.56)

The exact mechanism by which insulin reverses impaired wound healing is unknown. Previous investigators have shown that insulin is degraded in experimental wounds, suggesting that the action of insulin may be locally modified. The following study corroborates these findings and identifies the major proteinase responsible for insulin degradation in wound fluid (WF). Adult male Fisher rats were wounded by subcutaneous implantation of polyvinyl alcohol sponges while under pentobarbital sodium anesthesia. WF and serum were collected on 1, 5, 10, and 14 days postinjury. Decreased insulin concentration in late WF correlated with an increased insulin-degrading activity. Multiple proteinases appear to participate in the overall degradation of insulin in WF. However, the primary enzyme responsible for insulin degradation in WF was characterized by immunoprecipitation and immunoblotting and identified as the neutral thiol-dependent metalloproteinase, insulin-degrading enzyme (EC 3.4.24.56). Exogenous steroid administration caused a decrease in WF insulin-degrading activity. Glucagon and adrenocorticotrophin degradation was also observed, whereas minimal degradation of insulin-like growth factors I and II and epidermal growth factor was detected in WF. The ability to extracellularly degrade insulin may represent a unique mechanism for the regulation of this hormone's role in healing wounds.

Immunohistochemical localization of insulin-degrading enzyme along the rat intestine, in the human colon adenocarcinoma cell line (Caco-2), and in human ileum

Insulin-degrading enzyme (IDE) has been implicated in the intracellular degradation of insulin in insulin target cells. Knowledge of the existence of this enzyme in the intestine will be beneficial to the achievement of clinical oral efficacy of insulin. A comparative study was conducted with rat intestine, human colon adenocarcinoma (Caco-2) cells, and human ileum. Confocal microscopy analysis using the anti-IDE antibody showed that IDE was localized in the mucosal cells of rat and human intestines, as well as in Caco-2 cells. Immunostaining of this enzyme was homogeneous throughout the cell excluding nucleus, indicating a typical cytosolic distribution in rat and human enterocytes and in Caco-2 cells.

The mitochondrial processing peptidase: function and specificity

Targeting signals of mitochondrial precursors are cleaved in the matrix during or after import by the mitochondrial processing peptidase (MPP). This enzyme consists of two nonidentical alpha- and beta-subunits each of molecular weight of about 50 kDa. In mammals and fungi, MPP is soluble in the matrix, whereas in plants the enzyme is part of the cytochrome bc1 complex. MPP is a metalloendopeptidase which has been classified as a member of the pitrilysin family on the basis of the HXXEHX76E zinc-binding motif present in beta-MPP. Both subunits of MPP are required for processing activity. The alpha-subunit of MPP, which probably recognizes a three-dimensional motif adopted by the presequence, presents the presequence to beta-MPP, which carries the catalytic active site. MPP acts as an endoprotease on chemically synthesized peptides corresponding to mitochondrial presequences. Matrix-targeting signals and MPP cleavage signals seem to be distinct, although the two signals may overlap within a given presequence. The structural element helix-turn-helix, that cleavable presequences adopt in a membrane mimetic environment, may be required for processing but is not sufficient for proteolysis. Binding of the presequence by alpha-MPP tolerates a high degree of mutations of the presequence. alpha-MPP may present a degenerated cleavage site motif to beta-MPP in an accessible conformation for processing. The conformation of mitochondrial presequences bound to MPP remains largely unknown.

Role of yeast insulin-degrading enzyme homologs in propheromone processing and bud site selection

The Saccharomyces cerevisiae AXL1 gene product Axl1p shares homology with the insulin-degrading enzyme family of endoproteases. Yeast axl1 mutants showed a defect in a-factor pheromone secretion, and a probable site of processing by Axl1p was identified within the a-factor precursor. In addition, Axl1p appears to function as a morphogenetic determinant for axial bud site selection. Amino acid substitutions within the presumptive active site of Axl1p caused defects in propheromone processing but failed to perturb bud site selection. Thus, Axl1p has been shown to participate in the dual regulation of distinct signaling pathways, and a member of the insulinase family has been implicated in propeptide processing.

Are the 'core' proteins of the mitochondrial bc1 complex evolutionary relics of a processing protease?

The so-called 'core' proteins of the respiratory cytochrome bc1 complex and the two subunits of the mitochondrial processing peptidase (MPP) are structurally similar but their evolutionary relationship remains a mystery. Here, we present a model suggesting that the core proteins originated from an ancient proteolytic enzyme that was integrated into the bc1 complex during early stages of endosymbiosis.

A periplasmic insulin-cleaving proteinase (ICP) from Acinetobacter calcoaceticus sharing properties with protease III from Escherichia coli and IDE from eucaryotes

A periplasmic insulin-cleaving proteinase (ICP), purified to its electrophoretic homogeneity in the SDS-PAGE from the Gram-negative bacterium Acinetobacter calcoaceticus, was examined and compared in its properties with the protease III (protease Pi, pitrilysin, EC 3.4.99.44) of Escherichia coli and the insulin-destroying proteinase (IDE, insulinase, EC 3.4.99.45) from eucaryotes. The enzyme was proven to be a metalloprotease like protease III and IDE, as was shown by the inhibitory effects exerted by EDTA and o-phenanthroline. Furthermore, dialysis against EDTA and o-phenanthroline led to a complete loss of activity, which could be restored by addition of Co2+, and, to a lesser extent, but at a lower metal ion concentration by Zn2+. Similar to protease III and IDE, ICP prefers the cleavage of small polypeptides (insulin, insulin B-chain, glucagon) to the cleavage of proteins (casein, human serum albumin, globin) and was inactive against synthetic amino acid derivates (esters, p-nitranilides, and furoylacroleyl substrates) of subtilisin, thermolysin, trypsin, and chymotrypsin. The peptide-bond-specificity of the ICP in the cleavage of the oxidized insulin B-chain was investigated and the results were compared to the specificity of protease III of E. coli, IDE, protease-24,11, and thermolysin. Cleavage sites in the oxidized insulin B-chain generated by ICP are Asn3-Gln4, His10-Leu11, Ala14-Leu15, Leu17-Val18, Gly23-Phe24, Phe24-Phe25, and Phe25-Tyr26. Principally, ICP cleaves between hydrophobic amino acids and amides. The ICP shares one of the only two cleavage sites with the protease III and four sites with the IDE.

1,10-Phenanthroline increases nuclear accumulation of insulin in response to inhibiting insulin degradation but has a biphasic effect on insulin's ability to increase mRNA levels

Previous reports demonstrated that insulin is translocated through the cytoplasm to the nucleus of H35 hepatoma cells and suggested that nuclear insulin may be involved in stimulating transcription of immediate-early genes. In a recent study, inhibition of insulin-degrading enzyme with 1,10-phenanthroline, a Zn2+ chelator, caused a significant increase in the nuclear accumulation of insulin. The present study characterized the effects of 1,10-phenanthroline and its nonchelating isomer, 1,7-phenanthroline, on insulin degradation, nuclear accumulation, and stimulation of immediate-early gene expression. 1,10- but not 1,7-phenanthroline inhibited insulin degradation and increased nuclear accumulation of insulin in a dose-dependent manner. 1,7-phenanthroline caused a dose-dependent decrease in the expression of insulin-stimulated immediate-early genes, but had no significant effect on alpha-tubulin mRNA levels. In the presence of insulin, Northern analysis revealed that 1,10-phenanthroline at all concentrations tested increased alpha-tubulin mRNA levels, but had a biphasic effect on insulin-stimulated immediate-early gene expression. At low concentrations (5-200 microM), 1,10-phenanthroline increased the expression of insulin-stimulated g33, c-fos, and Egr-1 mRNA. At concentrations greater than 1 mM, insulin-stimulated immediate-early gene expression was decreased similar to the effect seen with 1,7-phenanthroline. Nuclear run-on analysis demonstrated that high concentrations of 1,10-phenanthroline decreased insulin-stimulated immediate-early gene transcription but had no effect on transcription of alpha-tubulin. However, low concentrations of 1,10-phenanthroline did not increase transcription of any genes.(ABSTRACT TRUNCATED AT 250 WORDS)