Antibodies to calnexin and mutated calreticulin are common in human sera

PURPOSE OF THE STUDY

Calreticulin is an endoplasmic reticulum chaperone protein, which is involved in protein folding and in peptide loading of major histocompatibility complex class I molecules together with its homolog calnexin. Mutated calreticulin is associated with a group of hemopoietic disorders, especially myeloproliferative neoplasms. Currently only the cellular immune response to mutated calreticulin has been described, although preliminary findings have indicated that antibodies to mutated calreticulin are not specific for myeloproliferative disorders. These findings have prompted us to characterize the humoral immune response to mutated calreticulin and its chaperone homologue calnexin.

PATIENTS AND METHODS

We analyzed sera from myeloproliferative neoplasm patients, healthy donors and relapsing-remitting multiple sclerosis patients for the occurrence of autoantibodies to wild type and mutated calreticulin forms and to calnexin by enzyme-linked immunosorbent assay.

RESULTS

Antibodies to mutated calreticulin and calnexin were present at similar levels in serum samples of myeloproliferative neoplasm and multiple sclerosis patients as well as healthy donors. Moreover, a high correlation between antibodies to mutated calreticulin and calnexin was seen for all patient and control groups. Epitope binding studies indicated that cross-reactive antibodies bound to a three-dimensional epitope encompassing a short linear sequence in the C-terminal of mutated calreticulin and calnexin.

CONCLUSION

Collectively, these findings indicate that calreticulin mutations may be common and not necessarily lead to onset of myeloproliferative neoplasm, possibly due to elimination of cells with mutations. This, in turn, may suggest that additional molecular changes may be required for development of myeloproliferative neoplasm.

Incidence of neuromyelitis optica spectrum disorder in the Central Denmark Region

OBJECTIVES

Neuromyelitis optica (NMO)/NMO spectrum disorder (NMOSD) may be misdiagnosed as multiple sclerosis. The aim of this study was to (i) to measure AQP4-IgG in patients who fulfilled the clinical and radiological criteria of NMOSD in the Central Denmark Region and (ii) to estimate the incidence of NMOSD in the region, according to both the 2006 Wingerchuk criteria and the 2015 International Panel for NMO Diagnosis criteria.

MATERIALS AND METHODS

Medical records of all patients diagnosed with a demyelinating disorder in the region from 1 January 2012 to 31 December 2013 were reviewed. Patients were classified as having (i) "NMO" if the 2006 criteria were met, (ii) "NMOSD with AQP4-IgG" or (iii) "NMOSD without/unknown AQP-IgG" if the new 2015 NMOSD criteria were met. Patients with core symptoms were invited to provide a blood sample for AQP4-IgG analysis with an enzyme-linked immunosorbent assay and a cell-based indirect immunofluorescence assay.

RESULTS

In 191 patients with core symptoms, one met the 2015 NMOSD with AQP4-IgG criteria. Two patients met the 2006 NMO and 2015 NMOSD without/unknown AQP4-IgG criteria. Among 108 patients providing a blood sample, all were seronegative. The estimated incidence of NMO (2006 criteria) and NMOSD (2015 criteria) was 0.08 and 0.12 per 100 000 person-years, respectively.

CONCLUSION

NMO/NMOSD is a rare disease in the Central Denmark Region, with a considerably lower incidence rate than previously estimated in a neighbouring region.

Humoral markers of active Epstein-Barr virus infection associate with anti-extractable nuclear antigen autoantibodies and plasma galectin-3 binding protein in systemic lupus erythematosus

We investigated if signs of active Epstein-Barr virus and cytomegalovirus infections associate with certain autoantibodies and a marker of type I interferon activity in patients with systemic lupus erythematosus. IgM and IgG plasma levels against Epstein-Barr virus early antigen diffuse and cytomegalovirus pp52 were applied as humoral markers of ongoing/recently active Epstein-Barr virus and cytomegalovirus infections, respectively. Plasma galectin-3 binding protein served as a surrogate marker of type I interferon activity. The measurements were conducted in 57 systemic lupus erythematosus patients and 29 healthy controls using ELISAs. Regression analyses and univariate comparisons were performed for associative evaluation between virus serology, plasma galectin-3 binding protein and autoantibodies, along with other clinical and demographic parameters. Plasma galectin-3 binding protein concentrations were significantly higher in systemic lupus erythematosus patients (P = 0.009) and associated positively with Epstein-Barr virus early antigen diffuse-directed antibodies and the presence of autoantibodies against extractable nuclear antigens in adjusted linear regressions (B = 2.02 and 2.02, P = 0.02 and P = 0.002, respectively). Furthermore, systemic lupus erythematosus patients with anti-extractable nuclear antigens had significantly higher antibody levels against Epstein-Barr virus early antigen diffuse (P = 0.02). Our study supports a link between active Epstein-Barr virus infections, positivity for anti-extractable nuclear antigens and increased plasma galectin-3 binding protein concentrations/type I interferon activity in systemic lupus erythematosus patients.

Antibodies to early EBV, CMV, and HHV6 antigens in systemic lupus erythematosus patients

OBJECTIVES

We investigated the antibody levels against early antigens of Epstein-Barr virus (EBV), cytomegalovirus (CMV), and human herpesvirus 6 (HHV6) in systemic lupus erythematosus (SLE) patients and healthy controls, and further correlated these antibodies to haematology/biochemistry, serology, and disease activity measures.

METHOD

Immunoglobulin (Ig)M, IgG, and IgA levels against the DNA polymerase processivity factors of EBV, CMV, and HHV6, termed early antigen diffuse (EA/D), pp52, and p41, respectively, were determined in plasma samples from 77 SLE patients and 29 healthy controls by using enzyme-linked immunosorbent assays (ELISAs).

RESULTS

IgM, IgG, and IgA levels against EBV EA/D, and IgG and IgA levels against CMV pp52, were significantly higher in SLE patients compared with healthy controls. Furthermore, EBV EA/D- and CMV pp52-directed IgG levels were inversely and positively associated, respectively, with lymphocyte counts in SLE patients. None of the findings seemed to be associated with use of immunosuppressive medication.

CONCLUSIONS

Our results suggest strong, but opposite, associations of lytic EBV and CMV infections with SLE. The amplified humoral responses to EBV EA/D and CMV pp52 in our SLE patient cohort probably reflect aberrant control of EBV and CMV reactivation. However, reactivation of EBV appeared to correlate with lymphopenic manifestations in SLE patients whereas CMV reactivation seemed to correlate with increments in lymphocyte levels.

Epstein-Barr virus early antigen diffuse (EBV-EA/D)-directed immunoglobulin A antibodies in systemic lupus erythematosus patients

OBJECTIVE

We sought to determine whether the serological response towards lytic cycle antigens of Epstein-Barr virus (EBV) is altered in systemic lupus erythematosus (SLE) patients.

METHOD

We used enzyme-linked immunosorbent assay (ELISA) to investigate the prevalence of EBV early antigen diffuse (EBV-EA/D) antibodies in sera from 60 patients with SLE, 40 with scleroderma (SSc), 20 with primary Sjögren's syndrome (pSS), 20 with rheumatoid arthritis (RA), 20 healthy controls, and also subjects with various circulating autoantibodies. Samples from patients were obtained from clinics specialized within the diseases in Denmark and Sweden and samples from healthy controls were obtained from volunteers.

RESULTS

A significant elevated titre of immunoglobulin (Ig)A, IgG, and IgM EBV-EA/D antibodies was found in SLE patients compared to healthy controls, a finding not explained by immunosuppressive treatment or disease activity. The largest difference was observed for IgA EBV-EA/D antibodies (p = 0.0013) with a seropositive rate of 58% in SLE patients and 0% in healthy controls. RA and SSc patients and individuals seropositive for anti-Scl-70 were additionally found to have elevated titres of IgA EBV-EA/D antibodies (40%, p = 0.014; 60%, p = 0.015; and 38.5%, p = 0.045, respectively). However, the titres were generally lower than in SLE patients.

CONCLUSION

Our findings support an association between EBV and SLE. The elevated titre of EBV-EA/D-directed IgA antibodies found in SLE patients could suggest reactivation of EBV in epithelial cells or reinfection of epithelial cells after reactivation in B cells, indicating lack of control of the latent infection.

Synthesis and anti-angiogenic effect of conjugates between serum albumin and non-steroidal anti-inflammatory drugs

Non-steroidal anti-inflammatory drugs (NSAIDs) inhibit tumor growth and angiogenesis. Covalent linkage of naproxen to human serum albumin (HSA) has been shown to target it efficiently to the liver and this may potentially be exploited for liver-selective inhibition of angiogenesis. With the aim of investigating the anti-angiogenic efficiency of NSAID-HSA conjugates in vitro, three NSAIDs, aspirin, ibuprofen, and naproxen were conjugated to HSA using different concentrations of their N-hydroxysuccinimide esters. Conjugation ratios from 10 to 50 were achieved and the conjugates retained a growth inhibitory effect on endothelial cells at or above the level of the non-conjugated NSAIDs in an in vitro angiogenesis assay.

Interaction of the chaperone calreticulin with proteins and peptides of different structural classes

The interaction of calreticulin with native and denatured forms and polypeptides in proteolytic digests of proteins representing structural classes of all-alpha-helix (hemoglobin, serum albumin), all-beta-sheet (IgG) and alpha-helix + beta-sheets (lysozyme, ovalbumin) was investigated. The binding of calreticulin to denatured proteins was found to depend on conformation and structural class of the protein. No interaction was observed with the native proteins, whereas binding was seen for the denatured proteins, the order of interaction being lysozyme = IgG > ovalbumin >> hemoglobin = serum albumin. Moreover, the interaction between calreticulin and the heat-denatured proteins depended on the temperature and time used for denaturation and the degree of proteolytic fragmentation. Calreticulin bound well to peptides in proteolytic digests from protease K or chymotrypsin treatment of lysozyme, IgG and ovalbumin but weakly or not at all to peptides in proteolytic digests of hemoglobin and serum albumin. Synthetic peptides from lysozyme and ovalbumin confirmed binding to hydrophobic peptides from these proteins. These results show that calreticulin has the ability to interact with denatured and fragmented forms of proteins with a preference for beta-strand structure and hydrophobicity.

Absence of high‐affinity calreticulin autoantibodies in patients with systemic rheumatic diseases and coeliac disease

Calreticulin has been reported to be an autoantigen in various autoimmune connective tissue diseases and in coeliac disease. Previous studies have used incubation buffers with low salt and low detergent concentrations (low stringency conditions) with serum albumin or other proteins as a blocking agent. Using these conditions we found a relatively high level of non-specific binding in many sera. Antibodies to proteins that are used as blocking reagents in ELISA (bovine serum albumin (BSA), ovalbumin, skimmed milk powder) are frequently present in sera, and these may cause false-positive results. Moreover, the low isoelectric point of calreticulin and its chaperone properties may give rise to false-positive results under low stringency conditions. We report that the use of a simple buffer without protein (50 mM Tris, pH 7.5, 1% Tween 20, 0.3 M NaCl) removes most of the problems with unwanted binding (high stringency conditions). Using the high stringency conditions, we screened sera from 107 patients with systemic lupus erythematosus, sera from patients with other systemic autoimmune diseases and from children with coeliac disease for the presence of high-affinity calreticulin autoantibodies by immunoblotting and ELISA. None of the sera contained high-affinity calreticulin antibodies. It is concluded that calreticulin is not a common autoantigen in patients with autoimmune connective tissue diseases or coeliac disease.

Comparison of rocket and crossed immuno‐electrophoresis assays for determination of the level of actin complexing of Gc globulin

OBJECTIVE

Gc globulin (vitamin D-binding protein) is a component of the extracellular actin scavenger system. The level of Gc globulin is reduced in patients with fulminant hepatic failure, septic shock and trauma. Furthermore, low levels of Gc globulin in patients with fulminant hepatic failure and multiple trauma have been found to correlate with the morbidity and mortality of patients. Owing to a large increase in the turnover of Gc globulin upon complex formation with actin, it may be important to determine both the total Gc globulin concentration and the degree of complexing with actin for estimating the clinical prognosis of a patient. For this reason, we have compared a crossed immuno-electrophoresis method (CIE), suitable for visualizing the degree of complexing with actin, with a rocket immuno-electrophoresis method (RIE), previously used for determination of the complex degree.

MATERIAL AND METHODS

Sera from healthy donors and from patients with acetaminophen-induced liver disease or trauma were investigated using CIE, RIE and enzyme-linked immunosorbent assay (ELISA).

RESULTS

Using the CIE, no Gc globulin-actin complexes were detected among healthy donors. Complexes were present in 21 of 39 patients with liver disease and 3 of 37 trauma patients. High complex ratios (> 20 %) were found in 6 of 7 patients with hepatic encephalopathy. Using the RIE, complexes were detected in most samples.

CONCLUSION

The results show that the CIE method may be used for determining the degree of actin complexing in conjunction with ELISA or RIE in determining the levels of total Gc globulin.

Gc globulin (vitamin D‐binding protein) levels: an inhibition ELISA assay for determination of the total concentration of Gc globulin in plasma and serum

Gc globulin, also called vitamin D-binding protein, is a plasma protein involved in the actin-scavenger system. In this study, the total Gc globulin concentration in serum or plasma samples was determined using a new, fast, solid-phase inhibition assay. Included in the study were 228 healthy volunteers (131 M, 97 F), 22 pregnant women, 90 cancer patients and 9 patients with chronic liver disease. Moreover, the degree of complexing with actin was determined in selected samples using crossed immunoelectrophoresis. The Gc globulin level in healthy controls was in the range 176-623 mg/L, showing no age dependency. The median level was found to be significantly higher in women than in men. Gc globulin concentrations were raised during pregnancy, showing a median value of 541 mg/L in the first trimester, and slightly raised to 574 mg/L in the second trimester. Cancer patients showed no changes in Gc globulin level, and there was no sign of increased amounts of complexing with actin. Chronic liver patients showed increased levels of Gc globulin following transplantation, but no signs of complexing with actin. This new solid-phase inhibition assay is fast, it is a good complement to the existing quantification methods, and it is especially suitable for determination of the Gc globulin status in acute liver patients before and during treatment.

Autoantibodies to the proteasome in monosymptomatic optic neuritis may predict progression to multiple sclerosis

OBJECTIVE

Proteasome autoantibodies (PAB) have been found in multiple sclerosis (MS) patient sera and cerebrospinal fluid (CSF). Presence of PAB could thus be a possible diagnostic marker for MS. We investigated whether PAB serum status in acute monosymptomatic optic neuritis (ON) and MS differed significantly from that of healthy controls, and whether or not PAB status is predictive of later MS development in patients with ON.

MATERIAL AND METHODS

Sera from ON patients, MS patients and healthy donors were analysed retrospectively using ELISA. Subsequently, a small group of PAB-positive samples were subjected to SDS-PAGE, immunoblotting and 2-D PAGE.

RESULTS

We found that 20 % (6/30) ON patients, 47 % (22/47) MS patients and 9 % (7/81) controls tested PAB positive using ELISA analysis. High PAB levels were found in 2 (4 %) MS patients, 1 (3 %) ON patient and 2 (3 %) controls. PAB positivity in ELISA was confirmed by immunoblotting. Separation of proteasome subunits by 2D PAGE followed by immunoblotting revealed no particular PAB subunit preference.

CONCLUSIONS

A retrospective search in available patient files revealed that 6 of 6 (100.0 %) PAB-positive ON patients developed MS over time. Eight of 24 (33 %) PAB-negative ON patients developed MS over time and 47 % (14/30) of all ON patients developed MS. A series of patient CSF was analysed by ELISA to assess the possible correlation between PAB status of concurrent serum and CSF samples, but no correlation was found. However, the results from the six PAB-positive ON patients could potentially be of prognostic value.

Interaction of calreticulin with amyloid beta peptide 1-42

The interaction of calreticulin with amyloid beta (Abeta) was investigated using solid phase and solution binding assays. Calreticulin bound Abeta 1-42 in a time and concentration dependent fashion. The binding was optimal at pH 5 and was stimulated by Ca2+ and inhibited by Zn2+ at pH 7. Interaction took place through the hydrophobic C-terminus of Abeta 1-42 and the polypeptide binding site of calreticulin. The results are discussed in the light of a reported role of calreticulin as a cell surface scavenger receptor.

Second-generation nanofiltered plasma-derived mannan-binding lectin product: process and characteristics

BACKGROUND AND OBJECTIVES

Mannan-binding lectin (MBL) is an important component of the innate immune defence; it binds to carbohydrate structures on pathogenic micro-organisms resulting in complement activation and opsonization. Individuals with low MBL levels are at risk of recurrent and severe infections. Substitution therapy with plasma-derived MBL is a promising treatment of diseases associated with MBL deficiency. A first-generation MBL product has been shown to be safe and well tolerated, and patients have benefited from MBL treatment. Following is a description of the development of a nanofiltered second-generation MBL product from Cohn fraction III, with the use of a new affinity matrix for MBL purification and the characteristics of this improved product.

MATERIALS AND METHODS

Carbohydrate-based gels were comparatively screened as affinity matrices. MBL was extracted from fraction III, and affinity purified on a Superdex 200 pg column. The eluted material underwent two virus reduction steps: filtration through Planova 20N and solvent/detergent treatment. It was further purified by anion-exchange and gel-filtration chromatography. The affinity eluate and the final MBL fraction were characterized by protein chemical, immunological, and functional assays.

RESULTS

In production scale, Superdex 200 pg was found to be superior to other carbohydrate-based matrices, and MBL was affinity purified from fraction III with a yield of 70%. The viral safety was increased by performing a nanofiltration of the affinity eluate through Planova 20N with a minimal loss of MBL. The purity of the final MBL fraction was 53% excluding the MBL-associated serine proteases (MASP). The product consisted of high-oligomeric MBL, with two dominating forms, and with MASP-1, -2, -3 and 19 kDa MBL-associated protein (MAp19). Only a few protein impurities were present, the major being alpha2-macroglobulin. MBL formed complexes with alpha2-macroglobulin bridged by MASP-1 covalently attached to the latter. The functional activity, assessed by mannan-binding activity and opsonic function, was intact, whereas half of the C4 activating capacity was lost during the production process.

CONCLUSION

A second-generation MBL process was developed with an average yield of 50%. It was possible to nanofilter the MBL-MASP complexes through Planova 20N with only a minor loss resulting in an increased safety profile of this MBL product.

Interaction of C1q with the receptor calreticulin requires a conformational change in C1q

The interaction between C1q and the chaperone calreticulin was studied under various conditions. When both proteins were present in equal amounts in solution, no interaction could be demonstrated. However, C1q immobilized on a hydrophobic surface, exposed to heat-treatment or bound to immunoglobulins (Igs) showed a strong, rapid and specific binding of calreticulin. The interaction appeared to be a two-step process, and the initial phase of interaction was sensitive to high concentrations of salt but not to a physiological salt concentration. The following strong binding was insensitive to salt and extremes of pH but sensitive to strongly denaturing agents (urea and guanidine). The sensitivity to salt during the initial phase of interaction was practically identical to that observed when calreticulin was bound to type V collagen. Binding between C1q and calreticulin could be inhibited by serum amyloid P component and by proteinase K-digested ovalbumin, and the binding of calreticulin to proteinase K-digested ovalbumin was shown to be inhibited by C1q. The data indicate that C1q binds stably to the peptide-binding site of calreticulin and that the initial binding of calreticulin to C1q involves the collagen-like domain of the C1q molecule. In conclusion, our results suggest calreticulin as a potential receptor for an altered conformation of C1q as occurs during binding to Igs. Thus, the chaperone and protein-scavenging function of calreticulin may extend from the endoplasmic reticulum to the topologically equivalent cell surface, where it may contribute to the elimination of immune complexes and apoptotic cells.

Preparation of bioconjugates by solid-phase conjugation to ion exchange matrix-adsorbed carrier proteins

A solid-phase conjugation method utilizing carrier protein bound to an ion exchange matrix was developed. Ovalbumin was adsorbed to an anion exchange matrix using a batch procedure, and the immobilized protein was then derivatized with iodoacetic acid N-hydroxysuccinimid ester. The activated protein was conjugated with glutathione, the conjugation ratio determined by acid hydrolysis, and amino acid analysis performed with quantification of carboxymethyl cysteine. Elution of conjugates from the resin by a salt gradient revealed considerable heterogeneity in the degree of derivatization, and immunization experiments with the eluted conjugates showed that the more substituted conjugates gave rise to the highest titers of glutathione antibodies. Direct immunization with the conjugates adsorbed to the ion exchange matrix was possible and gave rise to high titers of glutathione antibodies. Conjugates of ovalbumin and various peptides were prepared in a similar manner and used for production of peptide antisera by direct immunization with the conjugates bound to the ion exchanger. Advantages of the method are its solid-phase nature, allowing fast and efficient reactions and intermediate washings, and the ability to release conjugates from the solid phase under mild conditions.

Aminoalkyl affinity matrices

Aminoalkyl matrices are used in affinity chromatography of amine oxidases and other proteins with affinity for amino groups. Under appropriate circumstances chromatography on aminoalkyl matrices may yield purification factors around 100 to 1000, and they have been used in affinity purification of many members of the amine oxidase family. Other proteins with affinity for aminoalkyl matrices include thiol ester proteins, lactoferrin, and proteins with lysine-binding kringles (plasminogen, plasminogen activator, apolipoprotein A). The affinity of thiol ester proteins for aminoalkyl matrices is abolished after inactivation of the thiol ester group by reaction with low molecular weight amines including ammonia. Due to this, an ammonium sulphate precipitation step should be included in purification schemes for amine oxidases. The affinity of lactoferrin for aminoalkyl matrices stems from an affinity for the repeating amino groups in glycosaminoglycans, and this explains why lactoferrin requires diamines for efficient elution. The affinity of plasminogen for aminoalkyl groups is exploited in a one-step purification from plasma, and is also utilised in purification schemes for angiostatin, an angiogenesis-inhibiting fragment of plasminogen. Apolipoprotein A is homologous to plasminogen, and also has affinity for aminohexyl columns. The common binding motif for these proteins are lysine-binding kringles. Due to the properties of the amino group itself, aminoalkyl matrices will inevitably also function as anion exchangers, and this must be taken into consideration in the choice of conditions for sample loading, column washing and elution of bound proteins. Depending on the length of the alkyl chain, the matrices also have a potential for hydrophobic interactions. This property has been exploited in the purification of several proteins but must be minimized during affinity chromatography of amine oxidases. In conclusion, aminoalkyl matrices are valuable tools for affinity chromatography of several different proteins, and simple variations of sample pretreatment, sample loading, and column washing and elution conditions allow efficient selective purification of proteins with different affinities for the matrices.

Human placental calreticulin characterization of domain structure and post-translational modifications

The domain organization and the post-translational modifications of human placenta calreticulin were analysed by MS in combination with proteolytic digestion. Prolonged treatment with trypsin, chymotrypsin, elastase, Staphylococcus aureus V8 protease, or proteinase K all led to a 6- to 7-kDa decrease in the molecular mass of calreticulin. The decrease was found to be due to cleavages in the region around residue 340. In addition, minor fragments resulting from secondary cleavages close to the N-terminus were observed, but no stable fragments of intermediate size were found. These results show that the C-domain of calreticulin is susceptible to proteolytic cleavage and that the N- and P-domains form a proteolytically stable tight association. A disulfide bridge between the first two cysteines was mapped in the N-domain, and the third cysteine was found in the reduced form. No post-translational modifications in the form of glycosylation or phosphorylation were found. A modified form of calreticulin lacking the C-terminal hexapeptide including the KDEL endoplasmic reticulum retention sequon was isolated. Such a truncation may point to a mechanism that allows escape of calreticulin from the endoplasmic reticulum.

Determination of autoantibodies to annexin XI in systemic autoimmune diseases

Annexin XI, a calcyclin-associated protein, has been shown to be identical to a 56,000 Da antigen recognized by antibodies found in sera from patients suffering from systemic autoimmune diseases. In this work hexahistidine-tagged recombinant annexin XI (His6- rAnn XI) was used as antigen in ELISA experiments for determination of autoantibodies to annexin XI in sera of patients with systemic rheumatic autoimmune diseases. Immunoblotting with HeLa cell extract and with His6-rAnn XI as antigen was used for confirmation of positive ELISA results. We found eleven anti-annexin XI positive sera (3.9%) out of 282 sera from patients with systemic rheumatic diseases. The highest number of annexin XI positive sera were found in primary antiphospholipid syndrome (3/17), and in subacute lupus erythematosus (1/6), while lower frequencies of positive sera were found in patients with systemic sclerosis (5/137), rheumatoid arthritis (1/21), and systemic lupus erythematosus (1/58). Sera from healthy donors and patients with chronic infections were negative, except for one Salmonella typhimurium antibody positive serum. Autoantibodies to annexin XI were found to relate to thrombosis, but not to other clinical or laboratory features. A relation between antibodies to annexins and thrombosis has so far only been known for annexin V.

Polypeptide binding properties of the chaperone calreticulin

Calreticulin is a highly conserved eukaryotic ubiquitious protein located mainly in the endoplasmic reticulum. Two major characteristics of calreticulin are its chaperone activity and its lectin properties, but its precise function in intracellular protein and peptide processing remains to be elucidated. We have investigated the interactions of human calreticulin with denatured ovalbumin, proteolytic digests of ovalbumin, and different available peptides by solid phase assays, size-exclusion chromatography, capillary electrophoresis, and MS. The results show that calreticulin interacts better with unfolded ovalbumin than with native ovalbumin, that calreticulin strongly binds components in proteolytic digests of denatured ovalbumin, and that calreticulin interacts strongly with certain synthetic peptides.

Mammalian Cu-containing amine oxidases (CAOs): new methods of analysis, structural relationships, and possible functions

This thesis describes new and original experimental results on Cu-dependent amine oxidases (CAOs), which show that these enzymes can be conveniently and specifically detected in situ using a peroxidase-coupled activity staining method with 4-Cl-1-naphtole as hydrogen donor substrate. Even more sensitive in situ detection can be achieved using a chemiluminescence-based coupled peroxidase assay which was applied to show that human placenta CAO activity is confined to maternal vessels. A general purification scheme for CAOs is described, and applied to purification of different CAOs. Peptide maps and immunological crossreactivity studies with monoclonal antibodies raised against the purified enzymes showed that they were closely related. Amino acid sequence data for the bovine serum CAO showed that they form a separate group (E.C. 1.4.3.6) with no homology to other enzymes. A cDNA sequence was obtained on the basis of the amino acid sequence data, and this was found to encode a bovine lung CAO, related to bovine serum CAO. The genes for bovine lung and bovine serum CAO are characterized, and Southern blotting analysis of bovine chromosomal DNA shows the existence of a least one more bovine CAO. The purification of human neutrophil CAO is attempted, but it is described how lactoferrin, a protein with many properties in common with CAOs, and with a low degree of sequence identity can account for many observations on human neutrophil CAO. The products of bovine serum CAO oxidation of polyamines are characterised, and 3-aminopropanal is found to be the principal aminoaldehyde produced. Finally, a polyamine-stimulated binding of human placenta CAO to single-stranded DNA is described, and it is reported that the DNA-bound CAO is enzymically active and that the oxidation of DNA-bound polyamines leads to degradation of DNA. In addition to the experimental results, the properties of polyamines and Cu-dependent amine oxidases are reviewed. The polyamines spermidine and spermine interact specifically with nucleic acids and several other molecules. They are synthesised from putrescine, which is a key regulatory molecule formed from ornithine by ornithine decarboxylase, a highly inducible and regulated enzyme. The polyamines can be converted to putrescine by CAOs or spermidine/spermine acetyltransferase and polyamine oxidase. Putrescine is degraded by CAOs, which are also involved in degradation of histamine, a mediator of inflammatory processes. CAOs catalyse the general reaction: R1CH2NHR2 + O2 + H2O-->R1CHO + R2NH2 + H2O2 and in addition to the catabolism of putrescine and histamine CAOs are involved in regulation of growth and apoptosis by to the generation of aminoaldehydes and hydrogen peroxide which have growth inhibitory properties. Several homologous CAOs have been purified and characterized and they form a family with two subgroups. They are homodimers with a relative molecular weight of 180,000 and contain Cu2+ and a modified tyrosine, topaquinone, in the active site. CAOs are present in most tissues with highest amounts in intestine, kidneys, liver and placenta, but the cellular distributions and functions of CAOs are still poorly described, partly due to the use of many different assays and partly due to a broad substrate specificity of the enzymes. However, polyamines and CAOs seem to form a universal system contributing to regulation of growth, differentiation, and apoptosis.

The solubilities of denatured proteins in different organic solvents

The solubilities of heat-denatured and reduced, S-carboxymethylated proteins have been investigated in various organic solvents. Polar, protic solvents (formic acid, trifluoroacetic acid, 3-mercaptopropionic acid) were found to be good solvents for the denatured proteins (20-40 mg ml-1), and the solubilities of the reduced, S-carboxymethylated proteins were generally higher than those of the heat-denatured forms. Most other organic solvents were less effective in solubilizing the denatured proteins. Apolar solvents did not solubilise denatured proteins, but low solubilizing powers were observed for polar, aprotic solvents. Heat-denaturation was observed to result in the formation of large intermolecular aggregates, which, for ovalbumin and lysozyme, were formed by intermolecular S-S bonds, but for bovine serum albumin involved intermolecular isopeptide bonds.

Evolution of the genetic code: the nonsense, antisense, and antinonsense codes make no sense

According to the molecular recognition theory, the complementarity of the sense and nonsense DNA strands is reflected in a complementarity of polypeptides and the corresponding nonsense polypeptides. A comparison of the sense and nonsense code matrices, and of the antisense and antinonsense code matrices, either by visual inspection or by comparing the corresponding hydrophobicity matrices (e.g. by simply adding them together), revealed no complementarity of these pairs of matrices in terms of possible attractive physical forces. Instead, it was evident that the codes divide the amino acids into two major groups: hydrophilic and hydrophobic, a division which is directly correlated with the folding property of proteins. A simple primordial genetic code distinguishing between these two types of amino acids would have been capable of generating three-dimensionally folded peptides, which could stabilize coding RNAs by forming ribonucleoprotein complexes. This evolutionary scheme is reflected in the present organisation of information processing and storage in essentially all organisms. RNAs are processed and translated into proteins by ribonucleoproteins, while other steps in information retrieval and processing, such as DNA replication, transcription, protein folding and posttranslational processing, are catalyzed by proteins. This shows that the evolution of DNA as an information storage medium was a secondary event, unrelated to the evolution of the genetic code. From the primordial hydrophilic/hydrophobic (f.ex. Leu/Arg) code, evolution proceeded by introduction of a catalytic amino acid (Ser). The further evolution of the code has mainly served to increase the number of functional hydrophilic amino acids, since there has not been a great advantage in increasing the number of structural, hydrophobic amino acids. At some stage during the evolution of the genetic code, double-stranded DNA was introduced as a maximally safe genetic copy of RNA. This required the action of highly specific enzymes, and was therefore preceded by the refinement of the genetic code. As a conclusion of this evolutionary scheme, it can be inferred that, in general only the sense strand encodes proteins.

Reactions of N-N and N-O compounds with horseradish peroxidase and peroxidases from Mycobacterium tuberculosis

Several N-N-and N-O-containing compounds were analysed for their ability to act as substrates for horseradish peroxidase and peroxidases in Mycobacterium tuberculosis extracts. Aminoguanidine, diaminoguanidine, isoniazid, hydroxylamine and hydrazine were found to be weak substrates for horseradish peroxidase in reaction I and to inhibit the reaction of horseradish peroxidase with hydrogen peroxide. The same compounds inhibited the reaction of Mycobacterium tuberculosis peroxidase-catalase with hydrogen peroxide, and hydroxylamine was found to be a weak substrate for this enzyme. In growth inhibition experiments, diaminoguanidine inhibited the growth of M. tuberculosis H37Rv at 50 microg/mL, but not the growth of two isoniazid-resistant strains. Isonicotinic acid hydroxamate inhibited the reaction of the peroxidases with hydrogen peroxide, but was not itself a substrate and had no growth-inhibitory effects. On the basis of these results we suggest that the effect of isoniazid on growth of M. tuberculosis results from increased oxidative stress due to inhibition of catalase-peroxidase as well as from generation of toxic radicals with the structure [structure in text].

Affinity chromatography of serine proteases on the triazine dye ligand Cibacron Blue F3G-A

The interaction between complement component factor B and the triazine dye ligand Cibacron Blue F3G-A coupled to a cross-linked agarose matrix (Blue Sepharose) was found to involve the Bb part of the molecule, and to be inhibited by benzamidine. Human, chicken and rainbow trout factor B which had bound to Blue Sepharose could, subsequently be eluted with benzamidine. Other serine proteases (C2, factor II, factor IX, trypsin, chymotrypsin, proteinase 3) also bound to Blue Sepharose but only those belonging to the trypsin family could be eluted with benzamidine. Trypsin treated with the active-site inhibitor phenylmethylsulfonyl fluoride did not bind to Blue Sepharose and pretreatment of Blue Sepharose with benzamidine did not influence binding of proteases. We conclude that trypsin-like serine proteases can be purified on Blue Sepharose and that the interaction of these serine proteases with Blue Sepharose involves the active site of the enzyme.

Chaperone properties of calreticulin

Calreticulin is a highly conserved protein with a relative molecular weight of 46,000, and is mainly located in the endoplasmic reticulum. Calreticulin was first characterized as a calcium-binding protein in the endoplasmic reticulum, but since then other functions of calreticulin have been characterized, including chaperone and lectin properties, and regulation of integrin and nuclear hormone receptor activity. We have investigated the interactions between purified human placental calreticulin and native and denatured proteins. Our results show that calreticulin binds to denatured proteins in a time- and pH-dependent manner, which at physiological pH is dependent on divalent cations. The binding was dependent on the state of the denatured protein, and was highly sensitive to the ionic composition of the environment, being strongly inhibited by phosphate-containing compounds. Calreticulin did not seem to distinguish between denatured glycosylated and non-glycosylated proteins, and was found to bind to native basic proteins, presumably by sheer electrostatic forces.

Polyamine-stimulated binding of diamine oxidase to DNA

Diamine oxidase is a Cu-containing enzyme which intracellularly participates in the regulation of the levels of putrescine, spermidine and spermine and in this process produces growth inhibitory amino aldehydes and hydrogen peroxide. Extracellularly, the enzyme participates in the inactivation of biogenic amines, notably histamine. Here we present evidence that in the presence of polyamines, diamine oxidase has the ability to bind DNA and to oxidise DNA-bound polyamines. The enzyme associates with chromosomal DNA since it can be released from human placental DNA by treatment with DNase I and it may be involved in the degradation of DNA. Thus, diamine oxidase may belong to a new class of DNA-binding proteins.

Affinity chromatography of thiol ester-containing proteins

A method is described for the affinity chromatographic purification of thiol ester proteins. These comprise the complement proteins C3 and C4 and the protease inhibitor alpha 2-macroglobulin (alpha 2M) and are known to contain an internal beta-cysteinyl-gamma-glutamyl thiol ester. The method employs aminoalkyl ligands coupled to a divinylsulfonyl-derivatized agarose matrix, and the length of the aminoalkyl spacer arm was found to be important for the effectiveness of the matrix. Optimal results were obtained with diaminododecyldivinylsulfonyl-agarose. Employing this matrix the thiol ester proteins C3, C4 and alpha 2M were isolated from human pregnancy serum. Application of the method to chicken and rainbow trout serum gave rise to isolation of several proteins including chicken and rainbow trout alpha 2M.

Structure and tissue-specific expression of genes encoding bovine copper amine oxidases

A cDNA coding for an enzyme belonging to the family of copper amine oxidases was cloned from a bovine lung cDNA library using a PCR approach. The nucleotide sequence of this cDNA was found to be different from that of the previously published liver cDNA encoding bovine serum amine oxidase, another copper amine oxidase. Analyses using reverse transcription followed by PCR of RNA extracted from different bovine tissues confirmed that the copper amine oxidase gene expressed in bovine liver is closely related to, but different from, the copper amine oxidase gene expressed in bovine lung, kidney, spleen and heart. Northern blotting data showed that the level of copper amine oxidase expression in liver is considerably higher than in the other tissues tested. Southern blotting analyses of bovine chromosomal DNA suggested the existence of at least three copper amine oxidase genes. Two of these genes are apparently expressed in a tissue-specific manner as outlined above. A fragment of a third copper amine oxidase gene is identified. The exon-intron organization of the bovine copper amine oxidase genes analyzed is similar to that of the related human diamine oxidase gene, except that no intron in the position equivalent to that of the third intron in the human gene is found. In the third gene, a complete replacement of the third intron of the bovine copper amine oxidase gene (equivalent to the fourth intron of the human gene) has occurred.

Interference of sugars with the binding of biotin to streptavidin and avidin

Streptavidin and avidin have found widespread use as detection reagents in immunology, biochemistry and cell biology due to their high affinity binding to biotin, but the cellular functions of these proteins are not known. We have found that various sugars interfere with the binding of streptavidin and avidin to biotin. Mannose was most effective in inhibiting the binding to biotin followed by other saccharides. The inhibitory effect is most probably due to interactions of the sugars with residues in the binding pocket of streptavidin and avidin for biotin. These results show that great caution has to be exercised in the evaluation of experiments conducted with these detection reagents in the presence of sugars.

Conjugation to preadsorbed preactivated proteins and efficient generation of anti peptide antibodies

A solid phase conjugation method is described based on the preadsorption of proteins to aluminium hydroxide adjuvant followed by activation of the adsorbed carrier proteins with iodoacetic acid N-hydroxysuccinimidester or other conjugation reagents. Cysteine-containing peptides were coupled to the iodoacetic acid-activated carrier-adjuvant particles through their SH groups. No dialysis is required since the reaction product is isolated at each step of the procedure by a simple centrifugation and can easily be extensively washed between individual manipulations. The method generates peptide-carrier-adjuvant particles with sterically defined presentation of the peptides at the surface of the particles. When used for immunization of mice and rabbits the conjugates elicited high-titered specific anti-peptide sera, which reacted well with the parent protein in ELISA. The strongest reactions were with the denatured form of the parent protein. On immunoblots antisera to the N- and C-terminus of calreticulin recognized the same M, 52,000 protein.

Combined immunostaining and Coomassie Brilliant Blue staining of polyvinylidene difluoride membranes without organic solvent

A method for staining proteins on polyvinylidene difluoride membranes without using organic solvent is described. The method uses preblocking of the membrane with either Tween 20 or polyethylene glycol followed by staining with 0.01% Coomassie Brilliant Blue. No destaining of the membrane is needed afterwards. Preblocking with polyethylene glycol is compatible with microsequencing while Tween 20 leads to very low initial yields. Preblocking with Tween 20 has the additional advantage of allowing immunostaining followed by Coomassie Brilliant Blue staining for total protein on the same membrane.

A non-denaturing enzyme linked immunosorbent assay with protein preadsorbed onto aluminum hydroxide

A method has been developed which prevents denaturation of proteins used for coating of plastic surfaces in enzyme linked immunosorbent assays (ELISA). The system takes advantage of the use of aluminum hydroxide (Al(OH)3) as an adsorbent for proteins. A model protein has been analyzed. and monoclonal antibodies specific for either the native form or the denatured form of the protein were used to monitor the extent of denaturation. Adsorption of the proteins to Al(OH)3 in carbonate buffer, pH 9.3, before coating the ELISA plate abolished the denaturation otherwise observed after direct adsorption of protein to plastic surfaces. The protection against denaturation was dependent on the buffer system and was not observed when phosphate buffers were used, due to elution of protein from Al(OH)3 or lack of binding to Al(OH)3 in the presence of phosphate. There is evidence that protein adsorbed onto the Al(OH)3 is required for binding of Al(OH)3 onto the plastic surface. This system may be useful in assay systems where discrimination between the native and denatured forms of proteins is important.

Lactoferrin: similarity to diamine oxidase and purification by aminohexyl affinity chromatography

A protein reacting with a monoclonal antibody against human placental diamine oxidase was purified from the specific granules of human neutrofil granulocytes using affinity chromatography on aminohexyl-divinylsulfonyl-agarose. The protein had an M(r) determined by SDS/PAGE, corresponding to diamine oxidase, but had other properties which indicated that it might be a different protein. A combination of protein chemical techniques, including N-terminal sequencing, identified the protein as lactoferrin, an iron-containing protein with an M(r) of approximately 800000, a high isoelectric point and ferroxidase activity. Purified commercial lactoferrin was shown to bind to aminohexyl-divinylsulfonyl-agarose, and to be eluted in a heterogenous way from the matrix by amines and salt. Alignment of the sequences of diamine oxidase and lactoferrin showed that they are similar, indicating a common ancestry for these two different classes of metallo-oxidases.

Cation-dependent interactions of calreticulin with denatured and native proteins

Calreticulin has sequence homology with the molecular chaperone calnexin, which is known to control folding and assembly of nascent proteins in the endoplasmic reticulum in a calcium-dependent manner. We have investigated the interaction between human placental calreticulin and denatured placental and serum proteins under various incubation conditions. The interactions with denatured proteins differed significantly from the interactions with native proteins. The interactions were highly dependent on divalent metal ions or polyamines, but were not influenced by detergent and sulfhydryl agents. Our results indicate that calreticulin might have a similar role in protein folding as the chaperone calnexin.

Structural requirements for alpha-mating factor activity

The sexual hormone of S. cerevisiae, alpha-mating factor (alpha-MF, WHWLQLKPGQPMY) has structural homology with mammalian luteinizing hormone releasing hormone (LHRH, pEHWSYGLRPG-NH2) and has been shown to exhibit LHRH activity [Loumaye et al. (1982) Science 218, 1323-1325]. We have tested whether LHRH has alpha-MF activity in yeast and found that it does not. We therefore synthesized a series of hybrid peptides of alpha-MF and LHRH to study the structural features which determine alpha-MF and LHRH activities. A hybrid peptide consisting of the LHRH sequence with the C-terminal tetrapeptide (QPMY) of alpha-MF did not exhibit alpha-MF activity. Thus, the lack of alpha-MF activity of LHRH is not due solely to the absence of the C-terminal residues. Substitution of Lys7 in alpha-MF with Arg, as is found in LHRH, did not affect the alpha-MF activity, nor did an additional substitution of Trp1 with pGlu. However, the C-terminal four amino acids of alpha-MF were necessary for alpha-MF activity. Our results indicate that insertion of a Ser residue in position 4 as found in LHRH abolishes alpha-MF activity. These results suggest that, in addition to an intact C-terminus, correct spacing of the N-terminal His2 and the C-terminus is required for alpha-MF activity. The hybrid peptides all exhibited less LHRH activity than either LHRH or alpha-MF. These structure-function studies indicate that the structural homology between these two reproductive hormones may not reflect an evolutionary relationship between them.

The primary structure and enzymic properties of porcine prochymosin and chymosin

Preliminary investigations by N-terminal sequence analysis showed that pig and calf chymosin possessed 80% amino acid sequence identity but showed considerable differences in their enzymatic properties. A comparison of their structures may therefore contribute to an understanding of the significance of the amino acid residues responsible for the differences in these properties. Pig chymosis was extracted from the stomachs of pigs of less than 3 weeks of age, and was purified by ion exchange chromatography. Half of the primary structure was determined by amino acid sequencing and the complete structure was deduced from a cloned chymosin cDNA. Results showed that the zymogen showed 81% sequence identity with calf prochymosin and 57% identity with pig pepsinogen A. The size of the propart and location of the residue which becomes the N-terminus in the active molecule were the same in the prochymosins. The maximum general proteolytic activity at pH 3.5 of pig chymosin was 2-3% of that of the activity of pig pepsin A at pH 2, whereas the milk clotting activity relative to the general proteolytic activity of pig chymosin was much higher than that of calf chymosin. Agar gel electrophoresis at pH 5.3 of stomach extracts of individual pigs showed the existence of two predominant genetic variants of zymogen and enzyme. The two variants could not be distinguished by amino acid composition or N-terminal sequencing, and no differences in the enzymatic properties of the genetic variants were observed. It was concluded that of the residues that participate in the substrate binding, calf and pig chymosin differ in the following positions (pig pepsin numbering, subsites in parentheses): Ser 12 Thr (S4), Leu 30 Val (S1/S3), His 74 Gln (S'2), Val 111 Ile (S1/S3), Lys 220 Met (S4). With regard to the low general proteolytic activity of pig chymosin, the substitution Asp 303 Val relative to calf chymosin may contribute to an explanation of this.

A comparison of the immunogenicity of the native and denatured forms of a protein

The effect of heat denaturation on the physicochemical and immunological properties of a model protein, ovalbumin, and its formaldehyde/lysine-treated form was investigated. Polyacrylamide gel electrophoresis and gel filtration showed that heat denaturation converted ovalbumin to high Mr polymers, whereas formaldehyde/lysine-treated ovalbumin remained monomeric with only a small proportion forming oligomers. NMR analysis demonstrated that non-denatured structures could easily be differentiated from the denatured structures. Intraperitoneal immunization of rabbits and mice showed that both native and denatured forms of ovalbumin induced an immune response, but denatured forms of ovalbumin were found to be less immunogenic and to have a lower epitope density than native ovalbumin. Analysis of the antisera in crossed immunoelectrophoresis showed that they were specific for either native or denatured forms of ovalbumin. These findings were further investigated by ELISA and immunoaffinity chromatography, and the high specificity and low cross-reactivity was confirmed. We conclude that the immunogenic epitopes on denatured ovalbumin are different from those on ovalbumin, and that these epitopes reflect a continuum of denatured conformations.

In situ detection of diamine oxidase activity using enhanced chemiluminescence

In need of a simple and sensitive method for detection of diamine oxidase (EC 1.4.3.6) activity in connection with diamine oxidase purification from human placenta, we have developed an enhanced chemiluminescence method using putrescine as substrate and horseradish peroxidase and luminol for the detection of the H2O2 produced by diamine oxidase. The method allows direct detection of small amounts of diamine oxidase in serum samples after agarose gel electrophoresis and allows visualization of diamine oxidase activity in tissue sections. Employing this method we have detected diamine oxidase in sera from cow, horse, monkey, rabbit, and pregnant women. On tissue sections from term human placenta diamine oxidase activity was exclusively localized to the maternal side and was concentrated in vessels and fibrinoid areas.