Comprehensive glyco-proteomic analysis of human alpha1-antitrypsin and its charge isoforms

Human alpha1-antitrypsin (A1PI) is a well-known glycoprotein in human plasma important for the protection of tissues from proteolytic enzymes. The three N-glycosylation sites of A1PI contain diantennary N-glycans but also triantennary and even traces of tetraantennary structures leading to the typical IEF pattern observed for A1PI. Here we present an approach to characterize A1PI isoforms from human plasma and its PTMs by LC-ESI-MS and LC-ESI-MS/MS of peptides obtained by proteolytic digestion. The single cysteine residue of A1PI formed a disulfide bridge with free cysteine. The variability of the number of antennae and hence sialic acids on glycosylation site N107, which even contained minute amounts of tetraantennary structures, emerged as a major cause for the IEF pattern of A1PI. Only negligible amounts of triantennary structures were identified attached to N70, and exclusively diantennary structures were present on site N271 in each of the isoforms analyzed. Exoglycosidase digests revealed alpha2,6-linked neuraminic acids on diantennary N-glycans, and triantennary contained additionally one single alpha2,3-neuraminic acid per N-glycan, which, together with a fucose, formed a sialyl Lewis X determinant on the beta1,4-linked N-acetylglucosamine, as shown by 2-D-HPLC of pyridylaminated asialoglycans. Fucosylation of diantennary structures was marginal and of the core alpha1,6 type.

Identification of a 4-mer peptide inhibitor that effectively blocks the polymerization of pathogenic Z alpha1-antitrypsin

alpha(1)-Antitrypsin (AT) is a major proteinase inhibitor within the lung. The Z variant of AT (E342K) polymerizes within the liver and lung, resulting in hepatic aggregation of AT and tissue deficiency, predisposing to early onset of cirrhosis and emphysema, respectively. Polymerization of the aberrant protein can be prevented in vitro by specific peptides such as FLEAIG. This peptide serves as a lead molecule to design a shorter peptide that may be effective as a therapeutic agent. In this study we employed a systematic chemical approach using alanine scanning of Ac-FLEAIG-OH and subsequent peptide shortening to study the binding of shorter peptides to Z-AT. While two additional 6-mer peptides Ac-FLAAIG-OH and Ac-FLEAAG-OH were found to bind to Z-AT, their daughter peptides Ac-FLEAA-NH(2) and Ac-FLAA-NH(2) also bound avidly to Z-AT and prevented polymerization of the protein. Further comparative studies revealed that the binding of Ac-FLAA-NH(2) was more specific for Z-AT. The peptide-AT complex formation was enhanced by the presence of C-terminal amide group on the peptide, and circular dichroism analysis demonstrated that a random coil rather than a beta-helical conformation favored binding of the peptide to AT. In summary, this study has identified novel small peptides that inhibit Z-AT polymerization, and are a significant advance towards the treatment of Z-AT-related cirrhosis and emphysema.

Human mesotrypsin exhibits restricted S1' subsite specificity with a strong preference for small polar side chains

Mesotrypsin, an inhibitor-resistant human trypsin isoform, does not activate or degrade pancreatic protease zymogens at a significant rate. These observations led to the proposal that mesotrypsin is a defective digestive protease on protein substrates. Surprisingly, the studies reported here with alpha1-antitrypsin (alpha1AT) revealed that, even though mesotrypsin was completely resistant to this serpin-type inhibitor, it selectively cleaved the Lys10-Thr11 peptide bond at the N-terminus. Analyzing a library of alpha1AT mutants in which Thr11 was mutated to various amino acids, we found that mesotrypsin hydrolyzed lysyl peptide bonds containing Thr or Ser at the P1' position with relatively high specificity (kcat/KM approximately 10(5) m(-1) x s(-1)). Compared with Thr or Ser, P1' Gly or Met inhibited cleavage 13- and 25-fold, respectively, whereas P1' Asn, Asp, Ile, Phe or Tyr resulted in 100-200-fold diminished rates of proteolysis, and Pro abolished cleavage completely. Consistent with the Ser/Thr P1' preference, mesotrypsin cleaved the Arg358-Ser359 reactive-site peptide bond of alpha1AT Pittsburgh and was rapidly inactivated by the serpin mechanism (ka approximately 10(6) m(-1) s(-1)). Taken together, the results indicate that mesotrypsin is not a defective protease on polypeptide substrates in general, but exhibits a relatively high specificity for Lys/Arg-Ser/Thr peptide bonds. This restricted, thrombin-like subsite specificity explains why mesotrypsin cannot activate pancreatic zymogens, but might activate certain proteinase-activated receptors. The observations also identify alpha1AT Pittsburgh as an effective mesotrypsin inhibitor and the serpin mechanism as a viable stratagem to overcome the inhibitor-resistance of mesotrypsin.

Structural analyses on intermediates in serine protease catalysis

Although the subject of many studies, detailed structural information on aspects of the catalytic cycle of serine proteases is lacking. Crystallographic analyses were performed in which an acyl-enzyme complex, formed from elastase and a peptide, was reacted with a series of nucleophilic dipeptides. Multiple analyses led to electron density maps consistent with the formation of a tetrahedral species. In certain cases, apparent peptide bond formation at the active site was observed, and the electron density maps suggested production of a cis-amide rather than a trans-amide. Evidence for a cis-amide configuration was also observed in the noncovalent complex between elastase and an alpha1-antitrypsin-derived tetrapeptide. Although there are caveats on the relevance of the crystallographic data to solution catalysis, the results enable detailed proposals for the pathway of the acylation step to be made. At least in some cases, it is proposed that the alcohol of Ser-195 may preferentially attack the carbonyl of the cis-amide form of the substrate, in a stereoelectronically favored manner, to give a tetrahedral oxyanion intermediate, which undergoes N-inversion and/or C-N bond rotation to enable protonation of the leaving group nitrogen. The mechanistic proposals may have consequences for protease inhibition, in particular for the design of high energy intermediate analogues.

Sugar and alcohol molecules provide a therapeutic strategy for the serpinopathies that cause dementia and cirrhosis

Mutations in neuroserpin and alpha1-antitrypsin cause these proteins to form ordered polymers that are retained within the endoplasmic reticulum of neurones and hepatocytes, respectively. The resulting inclusions underlie the dementia familial encephalopathy with neuroserpin inclusion bodies (FENIB) and Z alpha1-antitrypsin-associated cirrhosis. Polymers form by a sequential linkage between the reactive centre loop of one molecule and beta-sheet A of another, and strategies that block polymer formation are likely to be successful in treating the associated disease. We show here that glycerol, the sugar alcohol erythritol, the disaccharide trehalose and its breakdown product glucose reduce the rate of polymerization of wild-type neuroserpin and the Ser49Pro mutant that causes dementia. They also attenuate the polymerization of the Z variant of alpha1-antitrypsin. The effect on polymerization was apparent even when these agents had been removed from the buffer. None of these agents had any detectable effect on the structure or inhibitory activity of neuroserpin or alpha1-antitrypsin. These data demonstrate that sugar and alcohol molecules can reduce the polymerization of serpin mutants that cause disease, possibly by binding to and stabilizing beta-sheet A.

A proteomic approach reveals transient association of reticulocalbin-3, a novel member of the CREC family, with the precursor of subtilisin-like proprotein convertase, PACE4

SPCs (subtilisin-like proprotein convertases) are a family of seven structurally related serine endoproteases that are involved in the proteolytic activation of proproteins. In an effort to examine the substrate protein for PACE4 (paired basic amino-acid-cleaving enzyme-4), an SPC, a potent protein inhibitor of PACE4, an alpha1-antitrypsin RVRR (Arg-Val-Arg-Arg) variant, was expressed in GH4C1 cells. Ectopic expression of the RVRR variant caused accumulation of the 48 kDa protein in cells. Sequence analysis indicates that the 48 kDa protein is a putative Ca2+-binding protein, RCN-3 (reticulocalbin-3), which had previously been predicted by bioinformatic analysis of cDNA from the human hypothalamus. RCN-3 is a member of the CREC (Cab45/reticulocalbin/ERC45/calumenin) family of multiple EF-hand Ca2+-binding proteins localized to the secretory pathway. The most interesting feature of the RCN-3 sequence is the presence of five Arg-Xaa-Xaa-Arg motifs, which represents the target sequence of SPCs. Biosynthetic studies showed that RCN-3 is transiently associated with proPACE4, but not with mature PACE4. Inhibition of PACE4 maturation by a Ca2+ ionophore resulted in accumulation of the proPACE4-RCN-3 complex in cells. Furthermore, autoactivation and secretion of PACE4 was increased upon co-expression with RCN-3. Our findings suggest that selective and transient association of RCN-3 with the precursor of PACE4 plays an important role in the biosynthesis of PACE4.

The Conformational Dynamics of a Metastable Serpin Studied by Hydrogen Exchange and Mass Spectrometry

Serpins are a class of protease inhibitors that initially fold to a metastable structure and subsequently undergo a large conformational change to a stable structure when they inhibit their target proteases. How serpins are able to achieve this remarkable conformational rearrangement is still not understood. To address the question of how the dynamic properties of the metastable form may facilitate the conformational change, hydrogen/deuterium exchange and mass spectrometry were employed to probe the conformational dynamics of the serpin human alpha(1)-antitrypsin (alpha(1)AT). It was found that the F helix, which in the crystal structure appears to physically block the conformational change, is highly dynamic in the metastable form. In particular, the C-terminal half of the F helix appears to spend a substantial fraction of time in a partially unfolded state. In contrast, beta-strands 3A and 5A, which must separate to accommodate insertion of the reactive center loop (RCL), are not conformationally flexible in the metastable state but are rigid and stable. The conformational lability required for loop insertion must therefore be triggered during the inhibition reaction. Beta-strand 1C, which anchors the distal end of the RCL and thus prevents transition to the so-called latent form, is also stable, consistent with the observation that alpha(1)AT does not spontaneously adopt the latent form. A surprising degree of flexibility is seen in beta-strand 6A, and it is speculated that this flexibility may deter the formation of edge-edge polymers.

EDEM accelerates ERAD by preventing aberrant dimer formation of misfolded α1-antitrypsin

Misfolded glycoproteins are degraded by a mechanism known as ERAD (ER-associated degradation) after retrotranslocation out of the endoplasmic reticulum (ER). This mechanism plays an important role in ER quality control. We previously reported that an ER membrane protein, EDEM, accelerates ERAD of a misfolded alpha1-antitrypsin variant, null (Hong Kong) (NHK), suggesting that EDEM may function as an acceptor of terminally misfolded glycoproteins. In this study, we constructed several genetically manipulated cell lines to test this hypothesis. EDEM expression did not alter the secretion rate of properly folded molecules and the forced retention of wild-type alpha1-antitrypsin in the ER did not cause its association with EDEM, suggesting that EDEM may function as a molecular chaperone. To examine this possibility, we analyzed the effect of EDEM over-expression on the structure of NHK, and found that the accumulation of covalent NHK dimers was selectively prevented by the over-expression of EDEM. Co-expression of NHK with two other ER membrane proteins, calnexin and H(+)/K(+)-ATPase (beta subunit), did not inhibit NHK dimer formation or accelerate NHK ERAD. These results indicate that EDEM may maintain the retrotranslocation competence of NHK by inhibiting aggregation so that unstable misfolded proteins can be accommodated by the dislocon for ERAD.

Viscous drag as the source of active site perturbation during protease translocation: insights into how inhibitory processes are controlled by serpin metastability

The native form of serine protease inhibitors (serpins) is kinetically trapped in a metastable state, which is thought to play a central role in the inhibitory mechanism. The initial binding complex between a serpin and a target protease undergoes a conformational change that forces the protease to translocate toward the opposite pole. Although structural determination of the final stable complex revealed a detailed mechanism of keeping the bound protease in an inactive conformation, it has remained unknown how the serpin exquisitely translocates a target protease with an acyl-linkage unhydrolyzed. We previously suggested that the acyl-linkage hydrolysis is strongly suppressed by active site perturbation during the protease translocation. Here, we address what induces the transient perturbation and how the serpin metastability contributes to the perturbation. Inhibitory activity of alpha1-antitrypsin (alpha1AT) toward elastase showed negative correlations with medium viscosity and Stokes radius of elastase moiety, indicating that viscous drag directly affects the protease translocation. Stopped-flow measurements revealed that the change in the inhibitory activity is primarily caused by the change in the translocation rate. The native stability of alpha1AT cavity mutants showed a negative correlation with the translocation rate but a positive correlation with the acyl-linkage hydrolysis rate, suggesting that the two kinetic steps are not independent but closely related. The degree of active site perturbation was probed by amino acid nucleophiles, supporting the view that the changes in the acyl-linkage hydrolysis rate are due to different perturbation states. These results suggest that the active site perturbation is caused by local imbalance between a pulling force driving protease translocation and a counteracting viscous drag force. The structural architecture of serpin metastability seems to be designed to ensure the active site perturbation by providing a sufficient pulling force, so the undesirable hydrolytic activity of protease is strongly suppressed during the translocation.

Changes of endoplasmic reticulum chaperone complexes, redox state, and impaired protein disulfide reductase activity in misfolding alpha1-antitrypsin transgenic mice

Alpha1-antitrypsin (AAT) deficiency is characterized by the accumulation of the misfolded mutant, Z form of alpha1-antitrypsin (PiZ) inside the lumen of the hepatic endoplasmic reticulum (ER). Both human patients and PiZ transgenic mice have similar symptoms of hepatic failure culminating in cirrhosis and hepatocellular carcinoma. The involvement of molecular chaperones, as well as the relevance of oxidative stress in this disease is not characterized well yet. Here, we show that, in the PiZ transgenic mice, the 58-kDa protein disulfide isomerase (PDI), the most important oxidoreductase and chaperone of the endoplasmic reticulum, is in a complex with PiZ, which is accompanied by a decrease of protein disulfide reductase activity of the ER. PiZ transgenic mice have a shift toward a more reduced ER environment and an elevation of cytoplasmic chaperones and antioxidant enzymes. Our data suggest that lower availability of PDI and a decreased protein disulfide reductase activity of the ER along with a cytoplasmic stress may contribute to the toxic effects of PiZ aggregation.

Characterization and suppression of dysfunctional human alpha1-antitrypsin variants

Human alpha1-antitrypsin-deficient variants may aggregate in the liver, with subsequent deficiency in the plasma, which can lead to emphysema. The structural and functional characteristics of 10 dysfunctional alpha1-antitrypsin variants (R39C, S53F, V55P, I92N, G115S, N158K, E264V, A336T, P369S, and P369L) were analyzed in detail. Most of them were unstable, as compared to the wild-type molecule, and many of the variants folded into an intermediate form. When five thermostable mutations (T68A, A70G, M374I, S381A, and K387R) were introduced into dysfunctional alpha1-antitrypsin variants, the stabilities and inhibitory activities of most of the variants were restored to levels comparable to those of the wild-type molecule. However, the extremely unstable S53F variant was not stabilized sufficiently by these mutations so as to exhibit function. N158K variant, which carries a mutation in the region critical for the reactive site loop insertion into beta-sheet A, exhibited a reduced level of inhibitory activity, despite conformational stabilization. These results show that aberrant folding caused by conformational destabilization due to mutations can be compensated for by increasing the overall stability of the alpha1-antitrypsin molecule, with exception of a mutation in the highly localized region critical for functional execution.

Misfolding-assisted selection of stable protein variants using phage displays

We describe a phage display strategy, based on the differential resistance of proteins to denaturant-induced unfolding, that can be used to select protein variants with improved conformational stability. To test the efficiency of this strategy, wild-type and two stable variants of alpha1-antitrypsin (alpha1AT) were fused to the gene III protein of M13 phage. These phages were incubated in unfolding solution containing denaturant (urea or guanidinium chloride), and then subjected to an unfavorable refolding procedure (dialysis at 37 degrees C). Once the alpha1AT moiety of the fusion protein had unfolded in the unfolding solution, in which the denaturant concentration was higher than the unfolding transition midpoint (Cm) of the alpha1AT variant, around 20% of the phage retained binding affinity to anti-alpha1AT antibody due to a low refolding efficiency. Moreover, this affinity reduced to less than 5% when 10 mg/mL skimmed milk (a misfolding-promoting additive) was included during the unfolding/refolding procedure. In contrast, most binding affinity (>95%) remained if the alpha1AT variant was stable enough to resist unfolding. Because this selection procedure does not affect the infectivity of M13, the method is expected to be generally applicable to the high-throughput screening of stable protein variants, when activity-based screening is not possible.

Characterization of proteins in human pancreatic cancer serum using differential gel electrophoresis and tandem mass spectrometry

The purpose of this study was to develop techniques for identifying cancer biomarkers in human serum using differential in-gel electrophoresis (DIGE), and characterizing the protein biomarkers using tandem mass spectrometry (MS/MS). A major problem in profiling protein expression by DIGE comes from the presence of high concentrations of a small number of proteins. Therefore, serum samples were first chromatographed using an immunoaffinity HPLC column (Agilent Technologies), to selectively remove albumin, immunoglobulins, transferrin, haptoglobin, and antitrypsin. Serum samples from three individuals with pancreatic cancer and three individuals without cancer were compared. Serum samples were processed using the immunoaffinity column. Differential protein analysis was performed using DIGE. A total of 56 protein spot-features were found to be significantly increased and 43 significantly decreased in cancer serum samples. These spot features were excised, trypsin digested, and analyzed by MALDI/TOF/TOF (4700 Proteomics Analyzer, Applied Biosystems). We identified 24 unique proteins that were increased and 17 unique proteins that were decreased in cancer serum samples. Western blot analysis confirmed increased levels of several of these proteins in the pancreatic cancer serum samples. In an independent series of serum samples from 20 patients with pancreatic cancer and 14 controls, increased levels of apolipoprotein E, alpha-1-antichymotrypsin, and inter-alpha-trypsin inhibitor were found to be associated with pancreatic cancer. These results suggest that affinity column enrichment and 2-D DIGE can be used to identify numerous proteins differentially expressed in serum from individuals with pancreatic cancer.

Diverse range of small peptides associated with high-density lipoprotein

High-density lipoproteins (HDL) were examined as potential carriers of small peptides in plasma. HDL purified by density gradient centrifugation was delipidated and fractionated by size-exclusion chromatography under denaturing conditions. By HPLC and mass spectrometry, more than 100 peptide components were found in the size range from 1000 to 5000 Da. By sequence analysis, peptides were identified as fragments of proteins such as apolipoproteins, fibrinogen, alpha1-proteinase inhibitor, and transthyretin. The results indicate that purified HDL bears a complex range of small peptides. It is unclear whether the peptides have any significant functional role as apolipopeptides, but they may represent a pathway for peptide delivery or scavenging and a significant reservoir of plasma peptides for diagnostic evaluation.

SP-A binds alpha1-antitrypsin in vitro and reduces the association rate constant for neutrophil elastase

Background

α1-antitrypsin and surfactant protein-A (SP-A) are major lung defense proteins. With the hypothesis that SP-A could bind α1-antitrypsin, we designed a series of in vitro experiments aimed at investigating the nature and consequences of such an interaction.

Methods and results

At an α1-antitrypsin:SP-A molar ratio of 1:1, the interaction resulted in a calcium-dependent decrease of 84.6% in the association rate constant of α1-antitrypsin for neutrophil elastase. The findings were similar when SP-A was coupled with the Z variant of α1-antitrypsin. The carbohydrate recognition domain of SP-A appeared to be a major determinant of the interaction, by recognizing α1-antitrypsin carbohydrate chains. However, binding of SP-A carbohydrate chains to the α1-antitrypsin amino acid backbone and interaction between carbohydrates of both proteins are also possible. Gel filtration chromatography and turnover per inactivation experiments indicated that one part of SP-A binds several molar parts of α1-antitrypsin.

Conclusion

We conclude that the binding of SP-A to α1-antitrypsin results in a decrease of the inhibition of neutrophil elastase. This interaction could have potential implications in the physiologic regulation of α1-antitrypsin activity, in the pathogenesis of pulmonary emphysema, and in the defense against infectious agents.

Alpha-1-antitrypsin deficiency in children: liver disease is not reflected by low serum levels of alpha-1-antitrypsin - a study on 48 pediatric patients

BACKGROUND

Alpha-1-antitrypsin (alpha1-AT) is an important protease inhibitor. The phenotypes are characterized by a low total serum alpha1-AT or by an abnormal protein accumulating in the hepatocytes. The aim of our study was to examine a correlation of total serum alpha1-AT, phenotype, and liver involvement in pediatric patients.

METHODS

48 patients, deficient for alpha1-AT were included. The phenotypes for alpha1-AT were determined by isoelectric focusing. Liver disease was defined either as elevated transaminases or/and as elevated conjugated bilirubin and gammaGT. Patients were reexamined after a mean interval of 2 years.

RESULTS

Homozygous alpha1-AD was found in 12 patients, heterozygous in 24 patients. In 12 children rare variants of alpha1-AD were diagnosed. Serum alpha1-AT levels less than 60% of normal were found in all patients with homozygous, in 37% of patients with heterozygous alpha1-antitrypsin deficiency (alpha1-AD), and in patients with the homozygous variant PiM(palermo). Liver disease was found in 8/12 patients with the phenotype PiZZ and in 15/24 patients with heterozygous alpha1-AD. Three of 4 patients with the phenotype PiMQ0 had severe liver disease despite normal serum levels for alpha1-AT. In 11 patients with heterozygous alpha1-AD liver disease was apparent despite normal serum alpha1-AT levels. In two patients with the variant type Mpalermo serum levels were as low as 11% of normal without any signs of liver disease.

CONCLUSIONS

Our data clearly show that in the diagnostic workup of neonatal cholestasis measurement of total serum alpha1-AT does not exclude liver disease due to abnormal alpha1-AT variants. We suggest analysis of alpha1-AT-phenotype by isoelectric focussing in patients with unknown liver disease. Heterozygous or rare variant types might remain undiagnosed by measuring total alpha1-AT only.

Mechanism of uptake of C105Y, a novel cell-penetrating peptide

C105Y, a synthetic peptide (CSIPPEVKFNKPFVYLI) based on the amino acid sequence corresponding to residues 359-374 of alpha1-antitrypsin, enhances gene expression from DNA nanoparticles. To investigate how this enhancement occurs, C105Y was fluorescently labeled to study its uptake and intracellular trafficking. When human hepatoma cells (HuH7) were incubated with fluorescently labeled C105Y for as little as 3 min, C105Y displayed nuclear and cytoplasmic staining with enrichment of fluorescent signal in the nucleus and nucleolus. Uptake and nucleolar localization were observed with the short sequence PFVYLI, but not with SIPPEVKFNK, and the D-isomer was readily taken up into cells but not into the nucleus. We found that the C105Y peptide is routed to the nucleolus very rapidly in an energy-dependent fashion, whereas membrane translocation and nuclear localization are energy-independent. When we tested the involvement of known endocytosis pathways in uptake and trafficking of this peptide, we demonstrated that C105Y peptide is internalized by a clathrin- and caveolin-independent pathway, although lipid raft-mediated endocytosis may play a role in peptide intracellular trafficking. Efficient energy-independent cell entry with rapid nuclear localization probably accounts for enhancement of gene expression from inclusion of C105Y into DNA nanoparticles.

Cell toxicity and conformational disease

Numerous disorders, including Alzheimer's, Parkinson's and other late-onset neurodegenerative diseases, arise from the conformationally driven aggregation of individual proteins. Previous focus on just one end-product of such aggregation - extracellular deposits of amyloid - has diverted attention from what is now recognized as being primarily intracellular disease processes. Recent structural findings show how cytotoxicity can result from even minor changes in conformation that do not lead to amyloid formation, as with the accumulation within the endoplasmic reticulum of intact mutant alpha-1-antitrypsin in hepatocytes and of neuroserpin in neurons. Studies in Alzheimer's and other dementias also indicate that the damage occurs at the stage of the initial intermolecular linkages that precede amyloid formation. The challenge now is to determine the detailed mechanisms of this cytotoxicity.

Aggrecanase-1 (ADAMTS-4) interacts with alpha1-antitrypsin

In degradative articular diseases such as rheumatoid arthritis and osteoarthritis, loss of the extracellular matrix occurs, resulting in the destruction of joint cartilage. Proteolysis of aggrecan is one of the early events that leads to breakdown of the extracellular matrix. Aggrecanase-1 (ADAMTS--4) is considered to play a pivotal role in the abrasion of cartilage aggrecan in rheumatoid arthritis and osteoarthritis. To identify an endogenous inhibitor of aggrecanase-1, we performed a yeast two-hybrid screen using the catalytic domain of human aggrecanase-1 as a bait and transformed an EGY 48 yeast strain carrying the bait plasmid with a human liver cDNA library plasmid. This screen identified alpha1-antitrypsin, a member of the family of plasma serine proteinase inhibitors, as a prey. Recombinant aggrecanase-1 and alpha1-antitrypsin were expressed in mammalian cells and used in co-immunoprecipitation experiments, which showed that full-length aggrecanase-1 and alpha1-antitrypsin are also associated in vivo. However, aggrecanase-1 did not interfere with the inhibitory activity of alpha1-antitrypsin against elastase, and alpha1-antitrypsin had no effect on the proteolytic activity of aggrecanase-1. Taken together, these data suggest that aggrecanase-1 and alpha1-antitrypsin bind in vivo, although the physiological significance of the interaction between aggrecanase-1 and alpha1-antitrypsin remains unclear.

Application of 2-D free-flow electrophoresis/RP-HPLC for proteomic analysis of human plasma depleted of multi high-abundance proteins

Free-flow electrophoresis (FFE) and rapid (6 min) RP-HPLC was used to fractionate human citrate-treated plasma. Prior to analysis, the six most abundant proteins in plasma were removed by immunoaffinity chromatography; both depleted plasma and the fraction containing the six abundant proteins depleted were taken for MS-based analysis. Fractionated proteins were digested with trypsin and the generated peptides were subjected to MS-based peptide sequencing. To date, 78 plasma proteins have been unambiguously identified by manual validation from 16% (15/96 FFE total fractions) of the collected FFE pools; 55 identifications were based on > or = 2 tryptic peptides and 23 using single peptides. The molecular weight range of proteins and peptides isolated by this method ranged from approximately 190 K (e.g., Complement C3 and C4) to approximately 4-6 K (e.g., CRISPP and Apolipoprotein C1). This FFE/RP-HPLC approach reveals low-abundance proteins and peptides (e.g., L-Selectin approximately 17 ng/mL and the cancer-associated SCM-recognition, immunodefense suppression, and serine protease protection peptide (CRISPP) at approximately 0.5-1 ng/mL), where CRISPP was found in association with alpha-1-antitrypsin as a non-covalent complex, in the fraction containing the depleted high-abundance proteins. In contrast to shotgun proteomic approaches, the FFE/RP-HPLC method described here allows the identification of potentially interesting peptides to be traced back to their protein of origin, and for the first time, has confirmed the "protein sponge" hypothesis where the 35 residue CRISPP polypeptide is non-covalently complexed with the major circulating plasma protein alpha-1-antitrypsin.

Examination of the molecular diversity of alpha1 antitrypsin in urine: deficit of an alpha1 globulin fraction on cellulose acetate membrane electrophoresis

In the clinical field of nephrology, a noninvasive approach employing the analysis of electrophoretic patterns in urinary protein has been established. In this study a total of 52 urine samples with IgA nephropathy (IgAN), anti-neutrophil cytoplasmic antigen-associated crescentic glomerulonephritis (GN), and other types of GN were analyzed. Patients with high alpha1 globulin (alpha1G) fractions, which contained alpha1AT in cellulose acetate membrane electrophoresis (CAE), tended to have alpha1 antitrypsin (alpha1AT) of normal molecular weight (57 kDa and 49 kDa), while patients with a deficit alpha1G fraction tended to have alpha1AT of low molecular weight (<49 kDa) (P < 0.01). The alpha1G fraction was significantly higher in patients with IgAN, and there were significantly more patients with normal molecular weight alpha1AT compared to patients with other diseases (P < 0.01). The isoelectric point of alpha1AT with lower-weight molecules was more on the alkali side compared to higher-weight molecules in two-dimensional electrophoresis. Detecting changes in alpha1G fractions in CAE may support the differential diagnosis of IgAN from other types of GN.

Modulating furin activity with designed mini-PDX peptides: Synthesis and in vitro kinetic evaluation

A peptide was designed from reactive site loop structure of alpha1 Antitrypsin Portland known as alpha1 PDX as a novel mini-PDX inhibitor of furin. The sequence was derived from (367-394) that contains the crucial furin cleavage motif RIPR382. A P3 mutant replacing Ile380 by Leu was prepared as a first model peptide. A Cys residue was inserted at each terminal of the peptide for purpose of cyclisation which was accomplished by air or iodine-induced oxidation. This mini-PDX peptide both cyclic and acyclic form inhibited in vitro furin activity (IC50 in nM) when measured against either substrates Boc-RVRRdown double arrow MCA or QVEGF-C [Abz-QVHSIIRRdown double arrow SLP-Y(NO2)-A-CONH2, Abz=2-amino benzoic acid and Y(NO2)=3-nitro tyrosine], latter being derived from vascular endothelial growth factor-C (VEGF-C) processing site. The geometrically constrained structure mimicking PDX reactive loop is crucial for enzyme inhibition. Our study further revealed that both mini-PDX peptides inactivate furin in a slow tight binding manner, with disulfide-bridged cyclic form being slightly more potent. Unlike PDX, these peptides inhibit furin via a different mechanistic pathway. The study provides an alternate strategy for development of efficient peptide-based inhibitors of Proprotein Convertases including furin.

Stability of mutant serpin/furin complexes: dependence on pH and regulation at the deacylation step

Furin proteolytically cleaves a wide variety of proprotein substrates mainly within the trans-Golgi network (TGN) but also at the cell membrane and in endosomal compartments where pH is more acidic. Incorporation of furin recognition sequences within the reactive site loop (RSL) of alpha(1)-antitrypsin (AT) leads to the production of furin inhibitors. In an attempt to design more stable, potent, and specific serpin-based inhibitors, we constructed a series of AT and alpha(1)-antichymotrypsin (ACT) mutants by modifying the P(7)-P(1) region of their RSLs. The biochemical properties of these variants were assessed by evaluating their propensity to establish SDS-resistant complexes with furin in a variety of conditions (pH 6.0-9.0) and by measuring their association rate constants. The effect of pH during the initial steps of complex formation was minimal, suggesting that the acylation step is not rate-limiting. The decrease in stoichiometry of inhibition (SI) values observed in AT variants at high pHs was a result of the reduced pH-dependent deacylation rate, which is rate-limiting in this mechanism and which suggests increased complex stability. Conversely, the SI values for ACT mutants had a tendency to be lower at acidic pH. Transiently transfecting HEK293 cells with these mutants abolished processing of the pro-von Willebrand factor precursor but, interestingly, only the ACT variants were secreted in the media as uncleaved forms. Our results suggest that reengineering the reactive site loops of serpins to accommodate and target furin or other serine proteases must take into account the intrinsic physicochemical properties of the serpin.

A pilot study comparing the purity, functionality and isoform composition of alpha-1-proteinase inhibitor (human) products

OBJECTIVE

Alpha-1-proteinase deficiency predisposes affected individuals to early onset pulmonary emphysema, and is treated with an alpha-1-proteinase inhibitor (A1-PI) from pooled human plasma. The objective of this pilot study was to assess analytical parameters of the three A1-PI products (Aralast, Prolastin, Zemaira) that may impact on clinical efficacy, safety, and convenience. These included: purity of the preparation; nature of impurities; functionality; and isoform composition.

METHODS

Purity was evaluated using reverse phase and size exclusion chromatography high performance liquid chromatography (RP-HPLC and SEC-HPLC), capillary zone electrophoresis (CZE), sodium dodecyl sulfate polyacrylamide gel electrophoresis, sodium dodecyl sulfate capillary gel electrophoresis and Western blot analysis. The identity of protein impurities was determined by immunonephelometry; functionality by calculating the ratio of mg active A1-P1 present (by anti-neutrophil elastase activity assay) to the mg antigenic A1-PI (by immunonephelometry); and normality of the A1-PI isoform pattern by isoelectric focusing (IEF). Three samples of Zemaira and one sample each of Aralast and Prolastin were available for analysis.

RESULTS

Zemaira had the highest specific activity. Using RP-HPLC analysis Zemaira averaged 99% purity, Aralast 70% and Prolastin less than 62%. Using SEC-HPLC Zemaira was 95.98% monomeric, Prolastin 79.00% and Aralast 63.55%. Prolastin had lower activity/mg antigenic A1-PI than the other two products. A shift in isoforms in Aralast was suggested by the results of CZE, and was confirmed by IEF.

CONCLUSIONS

Zemaira demonstrated greater purity compared with Aralast and Prolastin. Prolastin had more inactive A1-PI than Zemaira or Aralast. Isoform ratios appeared to be altered in Aralast. The results from this pilot study warrant further investigation.

Alpha 1-antitrypsin polymerization: a fluorescence correlation spectroscopic study

Alpha(1)-antitrypsin (AT) is the most abundantly circulating human proteinase inhibitor in the serpin family. The polymerization of AT, leading to alpha(1)-antitrypsin deficiency, has been studied extensively in vitro by a variety of ensemble methods. Here we report the use of fluorescence correlation spectroscopy to gain further insight into this process. Measurements of the distributions of diffusion times of polymerizing AT, carried out at 45, 50, and 55 degrees C, clearly show the existence of a kinetic lag phase, during which short oligomers are formed, prior to the formation of heterogeneous mixtures of longer polymers, and suggest that long polymers, which appear to be metastable, are produced through the condensation of shorter oligomers.