Multiplexed in-gel microfluidic immunoassays: characterizing protein target loss during reprobing of benzophenone-modified hydrogels

From whole tissues to single-cell lysate, heterogeneous immunoassays are widely utilized for analysis of protein targets in complex biospecimens. Recently, benzophenone-functionalized hydrogel scaffolds have been used to immobilize target protein for immunoassay detection with fluorescent antibody probes. In benzophenone-functionalized hydrogels, multiplex target detection occurs via serial rounds of chemical stripping (incubation with sodium-dodecyl-sulfate (SDS) and β-mercaptoethanol at 50-60 °C for ≥1 h), followed by reprobing (interrogation with additional antibody probes). Although benzophenone facilitates covalent immobilization of proteins to the hydrogel, we observe 50% immunoassay signal loss of immobilized protein targets during stripping rounds. Here, we identify and characterize signal loss mechanisms during stripping and reprobing. We posit that loss of immobilized target is responsible for ≥50% of immunoassay signal loss, and that target loss is attributable to disruption of protein immobilization by denaturing detergents (SDS) and incubation at elevated temperatures. Furthermore, our study suggests that protein losses under non-denaturing conditions are more sensitive to protein structure (i.e., hydrodynamic radius), than to molecular mass (size). We formulate design guidance for multiplexed in-gel immunoassays, including that low-abundance proteins be immunoprobed first, even when targets are covalently immobilized to the gel. We also recommend careful scrutiny of the order of proteins targets detected via multiple immunoprobing cycles, based on the protein immobilization buffer composition.

Mesenchymal stem cell-derived angiogenin promotes primodial follicle survival and angiogenesis in transplanted human ovarian tissue

BACKGROUND

We have recently reported that human bone marrow-derived mesenchymal stem cells (MSCs) facilitate angiogenesis and prevent follicle loss in xenografted human ovarian tissues. However, the mechanism underlying this effect remains to be elucidated. Thus, determining the paracrine profiles and identifying the key secreted factors in MSCs co-transplanted with ovarian grafts are essential for the future application of MSCs.

METHODS

In this study, we used cytokine microarrays to identify differentially expressed proteins associated with angiogenesis in frozen-thawed ovarian tissues co-transplanted with MSCs. The function of specific secreted factors in MSCs co-transplanted with human ovarian tissues was studied via targeted blockade with short-hairpin RNAi and the use of monoclonal neutralizing antibodies.

RESULTS

Our results showed that angiogenin (ANG) was one of the most robustly up-regulated proteins (among 42 protein we screened, 37 proteins were up-regulated). Notably, the targeted depletion of ANG with short-hairpin RNAi (shANG) or the addition of anti-ANG monoclonal neutralizing antibodies (ANG Ab) significantly reversed the MSC-stimulated angiogenesis, increased follicle numbers and protective effect on follicle apoptosis.

CONCLUSION

Our results indicate that ANG plays a critical role in regulating angiogenesis and follicle survival in xenografted human ovarian tissues. Our findings provide important insights into the molecular mechanism by which MSCs promote angiogenesis and follicle survival in transplanted ovarian tissues, thus providing a theoretical basis for their further application.

Bovine milk RNases modulate pro-inflammatory responses induced by nucleic acids in cultured immune and epithelial cells

Activation of innate immune receptors by exogenous substances is crucial for the detection of microbial pathogens and a subsequent inflammatory response. The inflammatory response to microbial lipopolysaccharide via Toll-like receptor 4 (TLR4) is facilitated by soluble accessory proteins, but the role of such proteins in the activation of other pathogen recognition receptors for microbial nucleic acid is not well understood. Here we demonstrate that RNase4 and RNase5 purified from bovine milk bind to Salmonella typhimurium DNA and stimulate pro-inflammatory responses induced by nucleic acid mimetics and S. typhimurium DNA in an established mouse macrophage cell culture model, RAW264.7, as well as in primary bovine mammary epithelial cells. RNase4 and 5 also modulated pro-inflammatory signalling in response to nucleic acids in bovine peripheral blood mononuclear cells, although producing a distinct response. These results support a role for RNase4 and RNase5 in mediating inflammatory signals in both immune and epithelial cells, involving mechanisms that are cell-type specific.

UNIQUE PEPTIDE IDENTIFICATION OF RNaseA SUPERFAMILY SEQUENCES BASED ON REINFORCED MERGING ALGORITHMS

Human ribonuclease A (RNaseA) superfamily consists of eight RNases with high similarity in which RNase2 and RNase3 share 76.7% identity. The evolutionary variation of RNases results in differential structures and functions of the enzymes. To distinguish the characteristics of each RNase, we developed reinforced merging algorithms (RMA) to rapidly identify the unique peptide motifs for each member of the highly conserved human RNaseA superfamily. Many motifs in RNase3 identified by RMA correlated well with the antigenic regions predicted by DNAStar. Two unique peptide motifs were experimentally confirmed to contain epitopes for monoclonal antibodies (mAbs) specifically against RNase3. Further analysis of homologous RNases in different species revealed that the unique peptide motifs were located at the correspondent positions, and one of these motifs indeed matched the epitope for a specific anti-bovine pancreatic RNaseA (bpRNaseA) antibody. Our method provides a useful tool for identification of unique peptide motifs for further experimental design. The RMA system is available and free for academic use at and .

A novel fully human antitumour immunoRNase targeting ErbB2-positive tumours

BACKGROUND

ErbB2 is an attractive target for immunotherapy, as it is a tyrosine kinase receptor overexpressed on tumour cells of different origin, with a key role in the development of malignancy. Trastuzumab, the only humanised anti-ErbB2 antibody currently used in breast cancer with success, can engender cardiotoxicity and a high fraction of patients is resistant to Trastuzumab treatment.

METHODS

A novel human immunoRNase, called anti-ErbB2 human compact antibody-RNase (Erb-hcAb-RNase), made up of the compact anti-ErbB2 antibody Erbicin-human-compact Antibody (Erb-hcAb) and human pancreatic RNase (HP-RNase), has been designed, expressed in mammalian cell cultures and purified. The immunoRNase was then characterised as an enzymatic protein, and tested for its biological actions in vitro and in vivo on ErbB2-positive tumour cells.

RESULTS

Erb-hcAb-RNase retains the enzymatic activity of HP-RNase and specifically binds to ErbB2-positive cells with an affinity comparable with that of the parental Erb-hcAb. Moreover, this novel immunoRNase is endowed with an effective and selective antiproliferative action for ErbB2-positive tumour cells both in vitro and in vivo. Its antitumour activity is more potent than that of the parental Erb-hcAb as the novel immunoconjugate has acquired RNase-based cytotoxicity in addition to the inhibitory growth effects, antibody-dependent and complement-dependent cytotoxicity of Erb-hcAb.

CONCLUSION

Erb-hcAb-RNase could be a promising candidate for the immunotherapy of ErbB2-positive tumours.

New anti-CD30 human pancreatic ribonuclease-based immunotoxin reveals strong and specific cytotoxicityin vivo

Although the therapy of Hodgkin lymphoma and anaplastic large cell lymphoma has been considerably improved during the last decades, high therapeutic toxicity, relapses, secondary tumors, and primary treatment failure(s) occur. Both malignancies are well suited for CD30-targeted immunotherapy because of their strong CD30 expression. We constructed an immunotoxin composed of a single chain variable fragment of a CD30 antibody fused to the human pancreatic ribonuclease, showing CD30-specific binding and ribonucleolytic activity resistant to the inhibitor RNasin. This immunotoxin revealed CD30-specific anti-tumor activity in BALB/c mice that were challenged with CD30-positive or CD30-negative syngeneic tumor cells.

High-level production of a humanized immunoRNase fusion protein from stably transfected myeloma cells

ImmunoRNases represent a highly attractive alternative to conventional immunotoxins for cancer therapy. Quantitative production of immunoRNases in appropriate expression systems, however, remains a major challenge for further clinical development of these novel compounds. Here we describe a method for high-level production and purification of a fully functional immunoRNase fusion protein from supernatants of stably transfected mammalian cells.

The RNase a superfamily: Generation of diversity and innate host defense

The Ribonuclease A superfamily includes an extensive network of distinct and divergent gene lineages. Although all ribonucleases of this superfamily share invariant structural and catalytic elements and some degree of enzymatic activity, the primary sequences have diverged significantly, ostensibly to promote novel function. We will review the literature on the evolution and biology of the RNase A ribonuclease lineages that have been characterized specifically as involved in host defense including: (1) RNases 2 and RNases 3, also known as the eosinophil ribonucleases, which are rapidly-evolving cationic proteins released from eosinophilic leukocytes, (2) RNase 7, an anti-pathogen ribonuclease identified in human skin, and (3) RNase 5, also known as angiogenin, another rapidly-evolving ribonuclease known to promote blood vessel growth with recently-discovered antibacterial activity. Interestingly, some of the characterized anti-pathogen activities do not depend on ribonuclease activity per se. We discuss the ways in which the anti-pathogen activities characterized in vitro might translate into experimental confirmation in vivo. We will also consider the possibility that other ribonucleases, such as the dimeric bovine seminal ribonuclease and the frog oocyte ribonucleases, may have host defense functions and therapeutic value that remain to be explored. (190 words).

Enhancing immunogenicity by limiting susceptibility to lysosomal proteolysis

T cells recognize protein antigens as short peptides processed and displayed by antigen-presenting cells. However, the mechanism of peptide selection is incompletely understood, and, consequently, the differences in the immunogenicity of protein antigens remain largely unpredictable and difficult to manipulate. In this paper we show that the susceptibility of protein antigens to lysosomal proteolysis plays an important role in determining immunogenicity in vivo. We compared the immunogenicity of proteins with the same sequence (same T cell epitopes) and structure (same B cell epitopes) but with different susceptibilities to lysosomal proteolysis. After immunizing mice with each of the proteins adsorbed onto aluminum hydroxide as adjuvant, we measured serum IgG responses as a physiological measure of the antigen's ability to be presented on major histocompatibility complex class II molecules and to prime CD4⁺ T cells in vivo. For two unrelated model antigens (RNase and horseradish peroxidase), we found that only the less digestible forms were immunogenic, inducing far more efficient T cell priming and antibody responses. These findings suggest that stability to lysosomal proteolysis may be an important factor in determining immunogenicity, with potential implications for vaccine design.

Inhibition of Pancreatic Ribonuclease A Aggregation by Antibodies Raised Against the Native Enzyme and Its N-Terminal Dodecapeptide

Pancreatic ribonuclease A (RNase A) has been shown to aggregate moderately and gradually at 65 degrees C. Antibodies raised against the dodecapeptide KETAAAKFERQG corresponding to the N-terminal 1-12 amino acid residues of RNase A (Npep) as well as native RNase A were effective in lowering RNase A aggregation at 65 degrees C. The antiRNase A antibodies were, however, more protective. The binding of antiNpep antibodies to the N-terminal region of RNase A may interfere with initiation of oligomerization of the enzyme and consequently its aggregation. The antiRNase A antibodies were presumably more effective in protecting RNase A against aggregation by binding to multiple epitopes of the enzyme including the N-terminal region and hence restricting the interaction of the monomers.

Investigation of conformational changes induced by binding of pancreatic RNase to anti-RNase IgG derived Fab monomer using optical procedures

The conformational changes induced in Fab fragments of polyclonal anti-RNase antibody molecules obtained by digestion with papain as a result of binding of pancreatic RNase have been studied. The RNase-Fab complex (RN-Fab), being soluble, could be subjected to thermodynamic investigations using optical strategies, also because of the absence of tryptophan in RNase. Internalization of the chromophores (tryptophans and tyrosines) of Fab occurs when it binds to RNase, suggesting an increase in the compactness of Fab due to the binding of RNase.

Mung bean sprout (Phaseolus aureus) nuclease and its biological and antitumor effects

Bovine seminal ribonuclease (BS RNase), a dimeric homolog of bovine pancreatic ribonuclease (RNase A), is known to display special biological activities namely cytotoxicity for human tumor cells. Because some plant ribonucleases have a similar mass weight and structure as the animal ribonuclease, effects of a commercial product of Mung bean (Phaseolus aureus) nuclease (PhA) were studied on proliferation of ML-2 human tumor cells, as well as it's aspermatogenic, embryotoxic, immunogenic, and immunosuppressive activity, and therapeutic efficiency in athymic mice bearing human melanoma tumor. Concerning the antiproliferative activity, PhA nuclease was almost non-effective in vitro on ML-2 cells and also immunosuppressive activity on human lymphocyte in mixed culture was very low compared to that of BS RNase. However, significant antitumor activity was detected on human melanoma tumor after intratumoral or intraperitoneal administration into the mice. Furthermore conjugate of PhA nuclease with polyethylene glycol (PEG) injected seven times at the dose of 10 microg intraperitoneally showed identical antitumor activity as that of bovine seminal ribonuclease (BS RNase) injected by the same way at ten times higher dose. Both PhA and BS RNases exerted strong aspermatogenic effect on the width of spermatogenic layers while RNase A administration at ten times higher concentration was ineffective. PhA nuclease when compared by means of antibody cross reaction with RNase A, BS RNase and wheat leaf neutral RNase (WLN-RNase) was found to be immunologically similar to RNase A and WLN-RNase, meanwhile BS RNase showed much higher antigenicity in comparison with them.

Differential lysosomal proteolysis in antigen-presenting cells determines antigen fate

Antigen-presenting cells (APCs) internalize antigens and present antigen-derived peptides to T cells. Although APCs have been thought to exhibit a well-developed capacity for lysosomal proteolysis, here we found that they can exhibit two distinct strategies upon antigen encounter. Whereas macrophages contained high levels of lysosomal proteases and rapidly degraded internalized proteins, dendritic cells (DCs) and B lymphocytes were protease-poor, resulting in a limited capacity for lysosomal degradation. Consistent with these findings, DCs in vivo degraded internalized antigens slowly and thus retained antigen in lymphoid organs for extended periods. Limited lysosomal proteolysis also favored antigen presentation. These results help explain why DCs are able to efficiently accumulate, process, and disseminate antigens and microbes systemically for purposes of tolerance and immunity.

Selective killing of B-cell hybridomas targeting proteinase 3, Wegener's autoantigen

Wegener's granulomatosis (WG) is a rare disease characterized by granulomatous lesions, small vessel vasculitis and the presence of anti-neutrophil cytoplasmic autoantibodies (C-ANCAs) in the sera of affected patients. Their main target antigen is proteinase 3 (PR3), a neutrophil and monocyte-derived neutral serine protease. Since the standard treatment of this severe autoimmune disease, with cyclophosphamide and corticosteroids, is associated with potential side-effects, the development of a more specific immunotherapeutic agent is warranted. The key role of ANCA in the pathogenesis of vasculitis and the effectiveness of anti-CD20 antibodies in patients with refractory WG points towards the importance of B cells in WG. We thus evaluated a new approach to selectively eliminate PR3-specific autoreactive B cells by targeting the B-cell receptor. For this purpose we used a bifunctional recombinant fusion protein consisting of the antigen PR3 and a toxin. The cytotoxic component of this novel fusion protein was the ribonuclease angiogenin, a human toxin with low immunogenicity. The toxin was stabilized by exchanging the catalytically relevant histidine in position 44 with glutamine to eliminate the autoproteolytic activity. PR3H44Q was fused either to the N terminus or to the C terminus of angiogenin. The recombinant proteins were expressed in 293T cells. Binding assays demonstrated the appropriate size and recognition by anti-PR3 antibodies. Using TUNEL technology, we demonstrated that these autoantigen toxins kill proteinase 3-specific B-cell hybridomas selectively by inducing apoptosis. The data indicate that autoantigen-toxins are promising tools in the treatment or co-treatment of autoimmune diseases in which the antigen is known.

The mannose receptor fails to enhance processing and presentation of a glycoprotein antigen in transfected fibroblasts

One function proposed for the mannose receptor found on dendritic cells as well as on macrophages and hepatic endothelial cells is in enhancing uptake and processing of glycoprotein antigens for presentation by major histocompatibility complex (MHC) class II molecules. In this study, a direct assessment of the possible role of the mannose receptor in this process was made in the absence of other endocytic receptors that can internalize glycoproteins. Presentation of RNase A and B peptides was compared in transfected fibroblasts coexpressing the mannose receptor and MHC class II molecules. RNase B bears a high-mannose oligosaccharide and is a ligand for the mannose receptor, whereas RNase A is not glycosylated and is taken up by pinocytosis. Incubation of RNase A or B with the transfected cells resulted in identical stimulation of ribonuclease-specific T cells, indicating that endocytosis of the glycosylated protein by the mannose receptor does not enhance presentation of this antigen. The postulated role of the mannose receptor in presentation of glycoprotein-derived antigen is reevaluated in light of these results.

Modeling formalin fixation and antigen retrieval with bovine pancreatic RNase A II. Interrelationship of cross-linking, immunoreactivity, and heat treatment

In this study, gel electrophoresis and capture enzyme-linked immunosorbent assay were used to assess the effect of formaldehyde treatment on the structural and immunological properties of bovine pancreatic ribonuclease A (RNase A). Prolonged incubation of RNase A in a 10% formalin solution leads to the formation of extensive intra- and intermolecular cross-links. However, these formaldehyde cross-links do not completely eliminate the recognition of RNase A by a polyclonal antibody. Comparative immunotitration of monomers, dimers, and oligomers greater than pentamers isolated from formalin-treated RNase A demonstrated that reduction of immunoreactivity due to intramolecular modifications prevails over the excluded volume effect of intermolecular cross-links. The latter only becomes important for intermolecular cross-links involving four or more molecules. The restoration of RNase A immunoreactivity during heating correlates with the reversal of formaldehyde cross-links if the incubation temperature does not exceed the denaturation temperature of the formalin-treated RNase A preparation. We conclude that formaldehyde cross-links stabilize antigens against the denaturing effects of high temperature, but the reversal of these cross-links is necessary for the restoration of immunoreactivity.

Modeling formalin fixation and antigen retrieval with bovine pancreatic ribonuclease A: I-structural and functional alterations

Understanding the chemistry of protein modification by formaldehyde is central to developing improved methods to recover proteins from formalin-fixed paraffin-embedded tissues for proteomic analysis and to improve protein immunoreactivity for immunohistochemical studies. We used biophysical techniques to investigate the effects of formaldehyde treatment on bovine pancreatic ribonuclease A (RNase A). Treatment of RNase A with formaldehyde was shown by gel electrophoresis to lead to the rapid formation of intra- and intermolecular protein cross-links. Thermal studies revealed that these protein cross-links significantly increased the thermal denaturation temperature of RNase A preparations. Analysis of formaldehyde-treated RNase A oligomers isolated by gel chromatography revealed that intramolecular protein cross-links are primarily responsible for the increase in protein thermostability. Formaldehyde treatment also lowered the isoelectric point of the enzyme from 9.45 to the 6.0-7.4 range. Optical spectroscopic studies demonstrated that the formaldehyde-induced modifications did not significantly alter the secondary or tertiary structure of RNase A. Heating formaldehyde-treated RNase A at 65 degrees C resulted in a significant reversal of the protein intra- and intermolecular cross-links and led to a partial restoration of enzymatic activity.

Polymer-conjugated bovine pancreatic and seminal ribonucleases inhibit growth of human tumors in nude mice

The hydrophilic poly[N-(2-hydroxypropyl)methacrylamide] (PHPMA) was used for RNase A or BS-RNase modification to prevent their degradation in bloodstream or fast elimination. Two PHPMA chains (classic and star-like) were synthesized and their conjugates with both enzymes were tested on the CD-1 nude mice bearing various human tumors. These RNase conjugates injected intravenously or intraperitoneally into the mice bearing melanoma, neuroblastoma or ovarian tumor caused significant reduction of transplanted tumors following ten daily doses of 2.5 and/or 1 mg/kg, respectively, while free RNase A or BS-RNase injected in doses of 10 mg/kg exerted only negligible antitumor activity. Histological examination confirmed potent cytotoxic effect of RNase A conjugates in ovarian tumor. Despite the antitumor activity observed in vivo, the in vitro cytotoxic activity of RNase A conjugates was not pronounced and did not differ from that caused by the free RNase A. The in vitro experiments with 125I-labeled preparations demonstrated that polymer conjugates were internalized by tumor cells very poorly in contrast to the dose-dependent internalization of the wild enzyme preparation. Surprisingly, mice injected with EL-4 leukemic cells, which were preincubated for 4 h with BS-RNase conjugates, exerted significantly prolonged survival compared with the control non-treated mice. It may be supposed that both BS-RNase and RNase A conjugates with PHPMA act after administration in vivo by a mechanism different from that or those occurring under in vitro conditions because in vivo they exert an antitumor action, whereas in vitro, they are ineffective. The experiments proved that RNase A, when conjugated to PHPMA, produced identical aspermatogenic and antitumor effects as BS-RNase conjugated to this polymer and that this preparation may be regarded as a potential anticancer drug.

The crystal structure of human angiogenin in complex with an antitumor neutralizing antibody

The murine monoclonal antibody 26-2F neutralizes the angiogenic and ribonucleolytic activities of human angiogenin (ANG) and is highly effective in preventing the establishment and metastatic dissemination of human tumors in athymic mice. Here we report a 2.0 A resolution crystal structure for the complex of ANG with the Fab fragment of 26-2F that reveals the detailed interactions between ANG and the complementarity-determining regions (CDRs) of the antibody. Surprisingly, Fab binding induces a dramatic conformational change in the cell binding region of ANG at the opposite end of the molecule from the combining site; crosslinking experiments indicate that this rearrangement also occurs in solution. The ANG-Fab complex structure should be invaluable for designing maximally humanized versions of 26-2F for potential clinical use.

Angiogenins: a new class of microbicidal proteins involved in innate immunity

Although angiogenins have been implicated in tumor-associated angiogenesis, their normal physiologic function remains unclear. We show that a previously uncharacterized angiogenin, Ang4, is produced by mouse Paneth cells, is secreted into the gut lumen and has bactericidal activity against intestinal microbes. Ang4 expression is induced by Bacteroides thetaiotaomicron, a predominant member of the gut microflora, revealing a mechanism whereby intestinal commensal bacteria influence gut microbial ecology and shape innate immunity. Furthermore, mouse Ang1 and human angiogenin, circulating proteins induced during inflammation, exhibit microbicidal activity against systemic bacterial and fungal pathogens, suggesting that they contribute to systemic responses to infection. These results establish angiogenins as a family of endogenous antimicrobial proteins.

Inhibition of prostate carcinoma establishment and metastatic growth in mice by an antiangiogenin monoclonal antibody

A neutralizing monoclonal antibody (MAb) 26-2F to human angiogenin, a potent inducer of neovascularization, has been shown previously to prevent or delay the appearance of angiogenin-secreting human colon, fibrosarcoma and lung tumor cell xenografts implanted subcutaneously (s.c.) into athymic mice. In an analogous model system, we report here that the antibody also prevents the establishment of PC-3 androgen-independent human prostate cancer tumors in, on average, 40% of treated mice (p < 0.0001, survivor analysis). Intriguingly, combining MAb 26-2F together with cisplatin and suramin, 2 therapeutic agents that together showed little antitumor activity in the aforementioned model, resulted in an even greater degree of protection (71% protected, p = 0.009 compared to antibody treatment alone). This protective effect persisted several weeks after cessation of treatment. Additionally, prophylactic systemic administration of MAb 26-2F dramatically reduced by 50% the formation of spontaneous regional metastasis originating from primary growth in the prostate gland of PC-3M cells, highly metastatic variants of PC-3. Protection from metastasis was still significant when treatment with MAb 26-2F was delayed until after the primary tumor was well established. The antibody is not directly cytotoxic to either cell type, both of which secrete angiogenin in vitro and when growing as tumors in vivo, but changes the pattern of vascularity in primary tumors growing orthotopically. These findings, together with the observation that angiogenin protein and mRNA are apparently overexpressed in cancerous vs. normal human prostate tissues, demonstrate that angiogenin antagonism represents a promising new approach for preventing progression and metastasis of clinical prostate cancer.

Stabilization of pancreatic ribonuclease A by immobilization on Sepharose-linked antibodies that recognize the labile region of the enzyme

The stabilizing potential of the antibodies recognizing the labile region of pancreatic ribonuclease A (RNase) has been investigated. The dodecapeptide SRNLTKDRAKPV corresponding to the labile region 32--43 on RNase was synthesized by the solid-phase method. Antiserum raised against the dodecapeptide-bovine serum albumin conjugate showed good cross-reactivity with the peptide and native RNase. RNase immobilized on Sepharose support precoupled either with the antipeptide immunoglobulin (IgG) or anti-RNase IgG proved to be more resistant to thermal inactivation than the soluble enzyme. Besides, stability against inactivation by trypsin at 55 degrees C was markedly high when enzyme was immobilized on the antipeptide IgG support, as compared to the soluble and other immobilized preparations. These results suggest that matrices bearing antibodies recognizing specific labile regions of enzyme may be useful in selectively improving their stability against specific forms of inactivation.

Expression of vascular endothelial growth factor, basic fibroblast growth factor, and angiogenin immunoreactivity in asthmatic airways and its relationship to angiogenesis

BACKGROUND

Angiogenesis is a prerequisite for airway remodeling in bronchial asthma. Several growth factors may play important roles in inflammation and angiogenesis through effects on inflammatory cell infiltration or neovascularization.

OBJECTIVE

We sought to compare bronchial vascularity and expression of vascular endothelial growth factor (VEGF), basic fibroblast growth factor (bFGF), and angiogenin in bronchial biopsy specimens from asthmatic and healthy control subjects.

METHODS

Bronchial biopsy specimens were obtained from 16 asthmatic subjects and 9 normal control subjects. The number of vessel profiles and the vascular area per unit area on a histologic section were estimated by using computerized image analysis after staining for type IV collagen in vessel walls. Numbers of VEGF+, bFGF+, and angiogenin+ cells were determined by means of immunoreactivity.

RESULTS

The airways of asthmatic subjects had significantly more vessels (P < .05) and greater vascular area (P < .001) than that observed in control subjects. Asthmatic subjects exhibited higher VEGF and bFGF and angiogenin immunoreactivity in the submucosa than did control subjects (P < .001, respectively). Significant correlations were detected between the vascular area and the numbers of angiogenic factor-positive cells (VEGF: rs = 0.93, P < .001; bFGF: rs = 0.83, P < .001; angiogenin: rs = 0.88, P < .001) within the asthmatic airways. Furthermore, the degree of vascularity was inversely correlated with airway caliber and airway responsiveness. Colocalization analysis revealed that the angiogenic factor-positive cells were CD34+ cells, eosinophils, and macrophages.

CONCLUSION

Our results suggest that increased vascularity of the bronchial mucosa in asthmatic subjects is closely related to the expression of angiogenic factors, which may then contribute to the pathogenesis of asthma.

Detection of pET-vector encoded, recombinant S-tagged proteins using the monoclonal antibody ATOM-2

The 15-meric S-tag is a truncated form of the S-peptide, which builds together with the 103 amino acid large S-protein the whole ribonuclease S-protein. Its small size and excessive solubility have made the S-tag an excellent fusion partner in the production of recombinant proteins, and a large variety of applications have been reported using the S-tag as a carrier. While S-tagged proteins were mostly detected and analyzed so far by use of their affinity to S-proteins, monoclonal antibodies (MAbs) for this tag have been not available. The generation of antibodies specific for S-tagged proteins is expected to broaden the range of applications of such S-tag fused recombinant proteins, and in this context, a novel MAb termed ATOM-2 was generated that specifically binds S-tagged proteins, which have been expressed using pET-vectors. Antigen specificity of ATOM-2 was confirmed in Western blot and enzyme-linked immunoadsorbent assay analysis, and using a series of amino acid deletion mutants, the binding epitope of ATOM-2 was precisely mapped. This showed that ATOM-2 recognizes the C-terminal part of the 15-meric S-tag in context with a few residues of vector encoded sequences. The core sequence for ATOM-2 binding epitope is "His-Met-Asp-Ser-Pro-Asp-Leu-Gly-Thr," which is present in all pET-expression vectors encoding S-tag fusion proteins. Because the ATOM-2 binding region does not overlap with the S-protein binding sequence, a convenient tool is provided for the simultaneous or alternative detection, purification, and analysis of recombinant S-tagged proteins to conventional S-proteins.

Formaldehyde-mediated aggregation of protein antigens: comparison of untreated and formalinized model antigens

A formaldehyde-mediated aggregation pathway (FMAP) is suggested as being primarily responsible for the aggregation of lyophilized tetanus toxoid (TT; a formalinized antigen) in the presence of moisture. The general occurrence of the FMAP was examined by using bovine serum albumin (BSA) and ribonuclease A (RNase) as model antigens; both protein antigens were formalinized according to a method commonly used to detoxify bacterial toxins. To clearly delineate the FMAP from other aggregation mechanisms, the aggregation kinetics and mechanism of both unmodified antigens (BSA and RNase) and formalinized antigens (f-BSA and f-RNase) were evaluated. We report that formaldehyde treatment introduces more rapid and extensive aggregation in antigens under conditions that favor the FMAP (i.e., 80% relative humidity and 37 degrees C). Consistent with formaldehyde-mediated crosslinking, f-antigen aggregates were covalent and non-disulfide-bonded, whereas BSA aggregates were disulfide-linked and RNase even did not aggregate under the same conditions. Coincorporation of amino acids (histidine and lysine), which strongly interact with formaldehyde, as well as prior antigen reduction with cyanoborohydride, significantly inhibited f-BSA aggregation, but showed no selective effect on BSA aggregation. Mechanistic analysis of f-BSA aggregates, inhibition studies, and similar reactivity of f-BSA with TT all confirmed the existence of the FMAP at moisture levels intermediate between the dry and solution state. This study demonstrates the potential for covalent reactions between formalinized protein antigens and neighboring chemical or biochemical species even after formalinization, and provides a general approach to inhibit the FMAP.

Small molecule-based chemical affinity system for the purification of proteins

A new chemical affinity system is described for the purification of proteins. The Linx Affinity Purification System enables researchers to quickly and easily bind a protein ligand to a chromatographic matrix and use the resulting affinity resin to purify a second protein from crude mixtures. The entire process takes approximately 2 h.

Human pancreatic ribonuclease 1: expression and distribution in pancreatic adenocarcinoma

BACKGROUND

Human pancreatic ribonuclease (RNase 1) is a pancreatic enzyme that is present at high levels in the serum of most patients with pancreatic adenocarcinoma. For this reason, the authors studied its patterns of expression at the single-cell level in pancreatic adenocarcinoma tissues by immunohistochemical analysis and in situ hybridization (ISH).

METHODS

Immunohistochemical analysis with polyclonal antibodies against RNase 1 and by ISH with digoxigenin-labeled RNase 1 probe were used to detect RNase 1 in the neoplastic cells of ductal type pancreatic adenocarcinomas.

RESULTS

Fifteen of 18 carcinoma samples were positive for RNase 1, demonstrating that the expression of ribonuclease that the authors observed previously in human pancreatic adenocarcinoma cell lines was not an artifact of cell culture. The authors also found RNase 1 in some of the metaplastic ducts and atrophic islets in 4 of 6 chronic pancreatitis samples, and they observed RNase 1 immunostaining in hyperplastic ducts adjacent to one of the well-differentiated adenocarcinomas.

CONCLUSIONS

The expression levels of RNase 1 by tumor cells from pancreatic adenocarcinomas are consistent with the high RNase 1 levels found in the serum of most patients with pancreatic adenocarcinoma. This expression of RNase 1, which is an acinar protein, demonstrates that the patterns of gene expression in pancreatic adenocarcinoma are distinct from those of normal pancreatic duct cells. Conversely, RNase 1 expression levels in altered ductal cells from some chronic pancreatitis tissues and hyperplastic ducts from carcinoma tissues suggest that abnormal expression levels may be an early event in pancreatic tumorigenesis.

Molecular basis for recognition of an arthritic peptide and a foreign epitope on distinct MHC molecules by a single TCR

KRN TCR transgenic T cells recognize two self-MHC molecules: a foreign peptide, bovine RNase 42-56, on I-Ak and an autoantigen, glucose-6-phosphate isomerase 282-294, on I-Ag7. Because the latter recognition event initiates a disease closely resembling human rheumatoid arthritis, we investigated the structural basis of this pathogenic TCR's dual specificity. While peptide recognition is altered to a minor degree between the MHC molecules, we show that the receptor's cross-reactivity critically depends upon a TCR contact residue completely conserved in the foreign and self peptides. Further, the altered recognition of peptide derives from discrete differences on the MHC recognition surfaces and not the disparate binding grooves. This work provides a detailed structural comparison of an autoreactive TCR's interactions with naturally occurring peptides on distinct MHC molecules. The capacity to interact with multiple self-MHCs in this manner increases the number of potentially pathogenic self-interactions available to a T cell.

Is nitrotyrosine generated in human erythrocytes in circulation?

Nitrotyrosine is considered a stable biomarker of reactive nitrogen species, including nitrogen dioxide (NO2) and peroxynitrous acid (ONOOH) in biomaterials. There are inconsistent observations on the detection of free and protein-associated nitrotyrosine in normal human plasma. Human erythrocytes, differentiated from erythrocyte precursor cells in the bone marrow, circulating in the body for an average of 120 d, and finally removed by spleen macrophages, may be exposed to reactive nitrogen species. In the present study, membrane proteins and hemoglobin from the senescent erythrocyte population were compared with those from young erythrocytes separated from the same individuals in their nitrotyrosine presence using newly prepared rabbit polyclonal anti-nitrotyrosine-ribonuclease A and anti-nitro(N-butoxycarbonyl)tyrosine-bovine serum albumin antibodies. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the membranes and hemoglobin, and subsequent Western blot analysis, showed that these antibodies only slightly bind to the bands of the proteins from both young and senescent erythrocytes, whereas these antibodies definitely bind to the protein bands of membranes and hemoglobin nitrated by NO2 or ONOOH in vitro. This result indicates that nitrotyrosine is not detected in the membrane proteins and hemoglobin in human normal erythrocytes in circulation. However, this does not conclude that erythrocytes are not exposed to reactive nitrogen species in the circulation.

A single-domain antibody fragment in complex with RNase A: non-canonical loop structures and nanomolar affinity using two CDR loops

BACKGROUND

Camelid serum contains a large fraction of functional heavy-chain antibodies - homodimers of heavy chains without light chains. The variable domains of these heavy-chain antibodies (VHH) have a long complementarity determining region 3 (CDR3) loop that compensates for the absence of the antigen-binding loops of the variable light chains (VL). In the case of the VHH fragment cAb-Lys3, part of the 24 amino acid long CDR3 loop protrudes from the antigen-binding surface and inserts into the active-site cleft of its antigen, rendering cAb-Lys3 a competitive enzyme inhibitor.

RESULTS

A dromedary VHH with specificity for bovine RNase A, cAb-RN05, has a short CDR3 loop of 12 amino acids and is not a competitive enzyme inhibitor. The structure of the cAb-RN05-RNase A complex has been solved at 2.8 A. The VHH scaffold architecture is close to that of a human VH (variable heavy chain). The structure of the antigen-binding hypervariable 1 loop (H1) of both cAb-RN05 and cAb-Lys3 differ from the known canonical structures; in addition these H1 loops resemble each other. The CDR3 provides an antigen-binding surface and shields the face of the domain that interacts with VL in conventional antibodies.

CONCLUSIONS

VHHs adopt the common immunoglobulin fold of variable domains, but the antigen-binding loops deviate from the predicted canonical structure. We define a new canonical structure for the H1 loop of immunoglobulins, with cAb-RN05 and cAb-Lys3 as reference structures. This new loop structure might also occur in human or mouse VH domains. Surprisingly, only two loops are involved in antigen recognition; the CDR2 does not participate. Nevertheless, the antigen binding occurs with nanomolar affinities because of a preferential usage of mainchain atoms for antigen interaction.

T cell cross-reactivity to heavy metals: identical cryptic peptides may be presented from protein exposed to different metals

The mechanisms by which metals induce activation of T cells and thus produce allergic and/ or autoimmune reactions are still obscure, and the same is true for the mechanisms that underly T cell cross-reactivity to different heavy metal ions. In the present study, we investigated induction by metals of T cell reactions to cryptic peptides of bovine RNase A. Murine CD4+ T cell hybridomas specific for cryptic RNase peptides presented from Au(III)-treated RNase were used as detection probes. We showed that in vitro treatment of RNase with Pd(II), Pd(IV), Ni(IV), and partially Pt(IV), but not Au(I), Ni(II), or Pt(II), induced presentation of the same cryptic peptides as those presented from Au(III)-treated RNase. That the former heavy metal ions, but not the latter, were able to alter the antigenicity of RNase was reflected by their ability to induce conformational changes of RNase, as detected by circular dichroism spectroscopy. Furthermore, upon immunization against RNase pretreated with these metals, CD4+ T cell hybridomas specific for unidentified cryptic peptides were obtained. In conclusion, "metal-specific" T cell reactions may be directed against cryptic peptides, and metal cross-reactivity in allergic individuals might be due to metal-induced presentation of overlapping, but not identical, panels of cryptic peptides.

Organogenesis and angiogenin

Our search for an angiogenesis-inducing factor in culture medium conditioned by human colon adenocarcinoma cells (HT-29) was inspired by the 'organizer' hypothesis originally postulated by Spemann. It led us to the isolation of angiogenin, a 14 kD protein homologous to pancreatic ribonuclease and one of the most potent stimulators of blood vessel formation known. This review summarizes the properties of angiogenin, its enzymatic and three-dimensional relationship to ribonuclease A (RNase A), those aspects of its structure that are critical for its biological function, and the therapeutic potential of angiogenin inhibition. Despite having the same arrangement of catalytic residues as RNase A, angiogenin has very low enzymatic activity. It lacks one of the four disulphide loops of RNase A; instead, the corresponding residues form part of a cell binding region. Both the catalytic activity and cell binding site are essential for angiogenesis. Angiogenin binds to cell-surface actin in confluent endothelial cells and to an as yet uncharacterized receptor on proliferating cells. Internalization and translocation to the nucleolus are also required for activity. Inhibitors of angiogenin can block angiogenesis in vitro and prevent tumour growth in vivo. Thus, a noncytotoxic neutralizing monoclonal antibody prevents the establishment of HT-29 human tumour xenografts in up to 65% of treated athymic mice. In those tumours that develop, the number of vascular elements is reduced. Actin also prevents the establishment of tumours while exhibiting no toxic effects at daily doses > 50 times the molar amount of circulating mouse angiogenin. These antagonists also inhibit the appearance of tumours derived from two other human tumour cell lines. Inhibition of the action of angiogenin may prove to be an effective therapeutic approach for the treatment of malignant disease.

Fluorescence polarization study of a salt bridge between a single-chain Fv and its antigen ribonuclease A

The interaction between a single-chain Fv (sFv) of the monoclonal antibody 3A21 and its antigen, bovine pancreatic ribonuclease A (RNase A), was studied by site-directed mutagenesis of the hypervariable regions and fluorescence polarization analysis. The affinity constants of wild-type sFv and a mutant sFv D31A (Asp31 of heavy chain was replaced by Ala) for RNase A were found to be 2.7 x 10(7) and 4.7 x 10(6) M-1 in PBS at pH 7.2 and 37 degrees C, respectively. While the affinity constant of D31A is not affected by NaCl concentration, that of wild-type sFv is almost the same as that of D31A in the presence of more than 1 M NaCl. These results demonstrate that Asp31 of the heavy chain interacts electrostatically with a positively charged amino acid residue of RNase A.

Cloning and cytotoxicity of a human pancreatic RNase immunofusion

BACKGROUND

Immunotoxins based on plant and bacterial proteins are usually very immunogenic. Human ribonucleases could provide an alternative basis for the construction of less immunogenic reagents. Two members of the human RNase family, angiogenin and eosinophil-derived neurotoxin (EDN), have been fused to a single chain antibody against the transferrin receptor, which is known to be internalised by endocytosis. The fusion proteins proved to be very efficient inhibitors of protein synthesis using various cell lines. It is not yet known whether the side effects of angiogenin and EDN will compromise their potential use as immunotoxins.

OBJECTIVES

The goal of this work was to construct a human immunotoxin with no harmful side effects. Bovine pancreatic ribonuclease has been shown to be as potent as ricin at abolishing protein synthesis on injection into oocytes. We therefore decided to clone its human analogue, which is fairly ubiquitous and per se non-toxic. An immunofusion of human pancreatic RNase with a single chain antibody against the transferrin receptor was tested for its ability to inhibit protein synthesis in three different human tumor cell lines.

STUDY DESIGN

DNA coding for the human pancreatic RNase was cloned partially from a human fetal brain cDNA library and then completed by PCR using a human placental cDNA library as a template. The RNase gene was then fused with a DNA coding for an single chain antibody against the transferrin receptor (CD71). After expressing the fusion protein in E. coli, the gene product was isolated from inclusion bodies and tested for cytotoxicity.

RESULTS

This fusion protein inhibited the protein synthesis of three human tumor cell lines derived from a melanoma, a renal carcinoma and a breast carcinoma, with IC50s of 8, 5 and 10 nM, respectively. These values were comparable with those using a similar fusion protein constructed with eosinophil derived neurotoxin (EDN) as the toxic moiety (IC50s of 8, 1.2 and 3 nM, respectively). The slightly lower activities of the human pancreatic RNase-scFv (pancRNase-scFv) with two of the cell lines suggests that fewer molecules are reaching the cytoplasmic compartment, since it was twice as active as EDN-scFv in inhibiting the protein synthesis of a rabbit reticulocyte lysate.

CONCLUSION

These results demonstrate that the human pancreatic RNase, which is expected to have a very low immunogenic potential in humans with no inherent toxicity, may be a potent cytotoxin for tumor cells after antibody targeting.

Different endocytic compartments are involved in the tight association of class II molecules with processed hen egg lysozyme and ribonuclease A in B cells

The processing of exogenous antigens and the association of peptides with class II molecules both occur within the endocytic pathway. 2A4 B lymphoma cells of the H-2k haplotype were grown in the presence or the absence of two different exogenous antigens (hen egg lysozyme and ribonuclease A) internalized by fluid-phase endocytosis. Using subcellular fractionation techniques, we demonstrate that, in the presence of hen egg lysozyme, newly synthesized SDS-stable class II molecules are detected in a dense endocytic compartment which does not have the characteristics of neither early and late endosomes nor lysosomes. In contrast, no SDS-stable class II molecules are observed between ribonuclease A and newly synthesized class II molecules. Interestingly, when class II molecules are analyzed at steady state, SDS-stable class II molecules induced by ribonuclease A are found in a compartment cosedimenting with late endosomes. These results suggest that the tight associations between ribonuclease A or hen egg lysozyme with class II molecules occur in distinct endocytic compartments and that these associations may depend on the sensitivity of antigens to proteolysis.

Antibody assisted protein refolding

In this study, we provide an initial demonstration of the use of monoclonal antibodies (MAbs) to enhance the yield of native protein during protein refolding. The presence of an anti-native MAb was found to enhance the refolding of reduced S-Protein (a fragment of Ribonuclease A) by as much as 360 percent over controls. This increase in recovered enzymatic activity was directly related to the MAb concentration and was saturable with excess antibody, suggesting that the antibodies are assisting through direct interaction at specific epitopes.

Antigen presentation of hen egg-white lysozyme but not of ribonuclease A is augmented by the major histocompatibility complex class II-associated invariant chain

The influence of the class II-associated invariant chain (Ii) on the presentation of the protein antigens hen egg-white lysozyme (HEL) and ribonuclease A (RNase) was investigated. For this purpose the Ii- rat-2 fibroblasts were transfected with I-Ak genes with or without Ii. Transfectants expressing Ii were superior in the presentation of the complete HEL protein to a panel of I-Ak-restricted T hybridomas characterized by distinct specificities for different HEL peptides and by different sensitivities to antigen concentration. There appeared to be a correlation between the antigen-presenting capacity and the amount of Ii, in that transfectants expressing large amounts of Ii were the best antigen presentors. The presentation of synthetic HEL peptides was not influenced by Ii. In contrast to the findings with HEL, the presentation of RNase by the same set of transfectants was clearly independent of Ii. Both antigens, HEL and RNase, required processing in the chloroquine-sensitive compartment. However, only the presentation of HEL but not of RNase could be efficiently blocked by brefeldin A. These data confirm that presentation of HEL depends on de novo synthesized class II molecules, whereas the presentation of RNase seems to be predominantly mediated by a pool of pre-existing class II molecules whose interaction with endocytosed antigen does not depend on Ii. These results suggest different mechanisms for the presentation of HEL and RNase and they raise the possibility that different antigens intersect the class II pathway at distinct intracellular locations.

Effects of monomethoxypoly(ethylene glycol) modification of ribonuclease on antibody recognition, substrate accessibility and conformational stability

The effects of modification of bovine pancreatic ribonuclease A by monomethoxypoly(ethylene glycol) (MPEG) were examined for changes in recognition by antiRNase antibodies, enzymatic activity against low and high molecular weight substrates and conformational stability to temperature elevation. Modified forms of RNase were prepared containing an average of 4, 9, and 11 mol of MPEG/mol protein, by amino group modification. These were analysed by binding to RNase antibodies crosslinked to solid phase-immobilized protein A. The affinity column was incorporated into a high performance liquid chromatograph and the RNase species were studied by both zonal and frontal analytical affinity chromatography. An antibody dissociation constant of 7.6 x 10(-8) M was found for unmodified RNase, as compared to values of 1.3 x 10(-7) and 1.2 x 10(-6) M for RNase with 4 and 9 covalently bound MPEG chains, respectively. Modification also led to progressive loss of enzymatic activity against RNA, down to 3% for the most highly modified enzyme. In contrast, enzymatic activity against cytidine-2',3'-cyclic monophosphate was suppressed to a maximum of only 33% at the highest modification level, and the stability to temperature, as followed by circular dichroism, was reduced only partially, from 67 degrees C for native protein to 57 degrees C for RNase with 11 mol equivalents MPEG incorporated. The above differential effects on enzymatic activity, antibody binding and temperature effects are consistent with the view that MPEG modification has relatively small effects on conformational stability and small molecule accessibility, but more dramatic effects on large molecule (substrate as well as antibody) accessibility.(ABSTRACT TRUNCATED AT 250 WORDS)

Production and characterization of antibodies to advanced glycation products on proteins

Antibodies directed against advanced glycation products formed during Maillard reaction have been generated and characterized. These antibodies reacted specifically with advanced glycation products in common among proteins incubated with glucose, but not early-stage compounds such as a Schiff base adduct and Amadori rearrangement products. Incubation of bovine serum albumin with glucose caused a time-related increase in immunoreactivity and a concomitant increase in fluorescence intensity. These antibodies may serve as a useful tool to elucidate pathophysiological roles of advanced Maillard reaction in diabetic complications and aging processes.

Antigenic sites on ribonuclease A. Synthetic peptides corresponding to the sites 37-42 and 83-88 on ribonuclease A show immunogenicity

Two hexapeptides NH2-Lys-Asp-Arg-Cys-Lys-Pro-COOH and NH2-Asp-Cys-Arg-Glu-Thr-Gly-COOH corresponding to the strong hydrophilic regions 37-42 and 83-88, respectively, on ribonuclease A were synthesized by solid-phase method. These synthetic peptides showed antigenic characteristics and provided an experimental validity to the prediction made earlier, supporting the view that highly hydrophilic regions on the protein have a good correlation with their being potentially antigenic.

T cell recognition of bovine ribonuclease. Self/non-self discrimination at the level of binding to the I-Ak molecule

Bovine RNase A specific T-cell hybridomas were generated to study the recognition of foreign Ag by T lymphocytes. One hybrid, TS12, was shown to recognize RNase in association with I-Ak. This hybridoma required bovine RNase to be processed before recognition. The immunogenic determinant on the RNase molecule recognized by TS12 was localized to the tryptic fragment RNase(40-61). All of the stimulatory ability of this determinant was shown to be contained within the synthetic 14mer RNase(43-56). When this segment of bovine RNase was compared with the self murine sequence, only one amino acid difference was found, a substitution of a proline residue at position 50 for a serine residue. This substitution completely abolishes binding to the I-Ak molecule, as shown by both functional and direct binding assays. This finding shows that self/non-self discrimination not only occurs at the level of the T cell, but also can be caused by an inability of the self peptide to associate with a class II molecule.

Prediction of strong antigenic determinant of seminalplasmin and ribonuclease from the amino acid sequence

It has been reported earlier [Hopp, T.P. & Woods, K.R. (1981) Proc. Natl. Acad. Sci. US 78, 3824-3828] that the antigenic determinants of a protein can be delineated by examining the average local hydrophilicity values along the peptide chain. I have used this method to predict the strong antigenic determinants of two proteins, seminalplasmin and ribonuclease, of known sequence. In the former case, the N-terminal segment 1-14, and in the latter case the segments 27-38 and 80-86, are predicted to possess the antigenic determinants of the two proteins. Experimental verification already exists for the former case.

Influence of an extrinsic cross-link on the folding pathway of ribonuclease A. Conformational and thermodynamic analysis of cross-linked (lysine7-lysine41)-ribonuclease a

A cross-linked ribonuclease A derivative, Lys7-dinitrophenylene-Lys41-ribonuclease A, has been prepared and characterized for ultimate use in protein-folding experiments. Immunochemical assays and spectroscopic measurements demonstrated that the introduction of this artificial cross-link does not perturb the native conformation of ribonuclease A. The cross-linked protein exhibited a reversible thermal transition with Tm = 53 degrees C at pH 2.0, which is 25 degrees C higher than that of unmodified ribonuclease A under the same conditions. The denatured form of the cross-linked ribonuclease A has a conformational chemical potential that is 4.9 kcal/mol higher than that of the denatured form of unmodified ribonuclease A at 40 degrees C and pH 2.0, assuming that the cross-linked and the unmodified proteins have the same conformational chemical potential in the native conformation. This is in good agreement with a theoretical value of 5.2 kcal/mol, calculated from the reduction of chain entropy of the denatured form upon introduction of the extrinsic cross-link. Thus, it is concluded that the extrinsic cross-link between Lys7 and Lys41, formed by the dinitrophenylene group, does not affect the native conformation of ribonuclease A but destabilizes the denatured conformation, probably by decreasing its chain entropy.

Sialic acid residues contribute to the heterogeneity of human serum ribonuclease: demonstration by isoelectric focusing and neuraminidase treatment of serum

Ribonuclease (RNase) activity from human serum appears as multiple zones of activity following isoelectric focusing in thin layer polyacrylamide gel. At least one but not all of these zones is cross reactive with rabbit antibovine pancreatic RNase A antiserum. Treatment of serum or partially purified serum RNase with neuraminidase reduces the complexity of the serum RNase banding pattern to a major band which focuses at a pH of 9.5 or greater and a minor zone of activity which focuses at about pH 6.0-6.2. Trypsin does not affect the pattern. Thus, sialic acid residues account for a large portion of the heterogeneity of human serum RNase. Neuraminidase treatment is requisite for evaluating RNase from serum and certain other sources.