PubMed Health⌕ Search

SEARCH · PubMed Health

Results for “Complement Activating Enzymes”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 145 records · Page 8Linked to original sources
PubMed1979

Regulation of the amplification C3 convertase of human complement by an inhibitory protein isolated from human erythrocyte membrane.

An activity that is inhibitory to the properdin-stabilized amplification C3 convertase (C3b,Bb,P) was solubilized from human erythrocyte (E(hu)) membranes by Nonidet P-40 and purified to homogeneity. The inhibitory membrane glycoprotein had an apparent M(r) of 1-1.2x10(6) on gel filtration in the presence of Nonidet P-40. On sodium dodecyl sulfate/polyacrylamide gel electrophoresis it presented a single stained band with an apparent M(r) of 205,000, with or without prior reduction of disulfides. The inhibitory protein of the E(hu) membrane produced a dose-related, first-order decay of C3b,Bb,P function on sheep erythrocytes (E(s)) and released (125)I-labeled Bb from these sites, indicating a mechanism of inhibition by decay-dissociation of the amplification C3 convertase. The 50% inhibitory dose of the E(hu) membrane protein was not altered by removal of sialic acid from the E(s) bearing C3b,Bb,P sites. E(hu) membrane protein also serves as a cofactor for C3b inactivator-induced cleavage of the alpha polypeptide chain of C3b. Thus, the inhibitory membrane protein can abrogate the activity of amplification convertase sites that have formed and also can prevent generation of such sites by augmenting irreversible inactivation of C3b.Discrimination between cells by the alternative complement pathway occurs after initial deposition of C3b and is related to the modulation by surface constituents of the capacity of bound C3b to function as a subunit of the amplification C3 convertase. The existence in the E(hu) membrane of a protein that can impair the functions of membrane-bound C3b and C3b,Bb,P could represent a molecular basis for preventing inappropriate self-recognition.

Binding Sites↗
PubMed1987

Regulation of the activity of platelet-bound C3 convertase of the alternative pathway of complement by platelet factor H.

The alternative pathway of complement is regulated on the surface of homologous blood cells at the C3 amplification step by the membrane protein decay-accelerating factor, as well as by the plasma protein factor H. We have reported elsewhere that platelets from patients with paroxysmal nocturnal hemoglobinuria regulate the activity of the C3 convertase C3bBb, even though they lack decay-accelerating factor. We now report that normal human platelets contain factor H, which was released from the platelet in response to complement deposition or thrombin stimulation. Factor H was localized to the platelet alpha granules by immunocytochemical techniques. As determined by a solid-phase radioimmunoassay, thrombin-stimulated platelets released approximately equal to 54 ng of factor H per 10(8) platelets. The release of factor H in response to complement or thrombin was inhibited by treating the platelets with metabolic inhibitors. Such inhibition resulted in a 3-fold increase in the activity of C3bBb. Platelets that released factor H bound only half as many molecules of radiolabeled factor B to platelet-bound C3b than platelets that could not release factor H. Treatment of platelets with anti-decay-accelerating factor antibody had no effect on the activity of C3bBb unless the release of factor H was blocked. Therefore, so far as we know, human platelets have a unique mechanism for the regulation of the alternative pathway of complement.

Complement Activating Enzymes↗
PubMed1981

Formation of the initial C3 convertase of the alternative complement pathway. Acquisition of C3b-like activities by spontaneous hydrolysis of the putative thioester in native C3.

Activation of the alternative pathway of complement commences with the formation of an initial fluid-phase C3 convertase. Treatment of C3 with the nucleophilic reagent methylamine has previously been shown to result in the cleavage of an intramolecular thioester bond and to induce C3b-like properties, including the ability to form a fluid-phase C3 convertase. This report examines the hypothesis that spontaneous hydrolysis of the thioester generates a derivative of C3 that is responsible for the formation of the initial C3 convertase of the alternative pathway. The rate of spontaneous decay of C3 hemolytic activity in buffer was found to be between 0.2 and 0.4%/h. In the presence of other alternative pathway proteins, the rate of inactivation was 1%/h. The rate of spontaneous inactivation was greatly accelerated by low concentrations of chaotrophic agents such as KSCN or guanidine. Liberation of a sulfhydryl group, not present in native C3, correlated with loss of hemolytic activity, indicating that exposure to chaotropic agents resulted in thioester hydrolysis. Unlike native C3, C3 bearing a single reactive sulfhydryl group was capable of generating fluid-phase C3 convertase with Factors B, D, and P and was cleaved by Factor I (C3b inactivator) in the presence of Factor H (beta 1H). The fragmentation patterns indicated that the C3a domain was covalently associated with the functionally C3b-like C3. Organomercurial agarose was employed for the rapid removal of sulfhydryl-bearing, hemolytically inactive forms of C3 and C3b from native hemolytically active C3.

Complement Activating Enzymes↗
PubMed1988

Human monoclonal IgG isotypes differ in complement activating function at the level of C4 as well as C1q.

Humanized antibodies are likely to have a major role in therapy and it is important to define their interaction with physiological effectors. By comparing a matched series of chimeric human mAbs we found that igG1 was most efficient in complement lysis, although IgG3 bound more C1q. To resolve this paradox we compared the ability of human IgG1, IgG2, IgG3, IgG4, and IgE and rat IgG2b to cause C1q binding, C1 binding and activation, C4 activation, C4b binding, and C3b binding. Rat IgG2b was included because this isotype has already successfully been used for therapy. Human IgG1 was less efficient than IgG3 and fixing C1q and C1 on the cell surface, but the number of C4 molecules bound per C1 was 10-fold greater for IgG1 than for IgG3. This difference, amplified through later stages of the complement cascade, can account for the superiority of IgG1 for cell lysis. The efficiency of IgG1 in fixing C4 was not due to a favored binding site on the antibody molecule, since virtually all of the bound C4b was attached to the cells. Rather, it appeared that the activation of C4 by C1s was greatly favored by IgG1 compared with IgG3. It should be possible to combine the optimal properties of IgG1 and IgG3 antibodies to produce an improved therapeutic reagent.

Antibodies, Monoclonal↗
PubMed1985

Purification and characterization of the C3 convertase of the classical pathway of human complement system by size exclusion high-performance liquid chromatography.

The C3 convertase of the classical pathway of the complement system is a liable complex, C4b,2a, and is activated by limited proteolysis of two components, C4 and C2, by C1s. By utilizing iodine-treated C2 and size exclusion high-performance liquid chromatography (HPLC), we have succeeded in isolating for the first time the classical pathway C3 convertase. Size exclusion HPLC demonstrated that the apparent molecular mass of the C3 convertase was 280K daltons. The C3 convertase decay-dissociates spontaneously into C4b and C2a. The decay-dissociation is a temperature-dependent reaction and the half-lives of the C3 convertase at 24, 30, and 37 degrees C were estimated to be 400, 180, and 60 min, respectively. The decay-dissociation was also dependent on pH and was accelerated by increasing pH. In addition, the decay-dissociation of the C3 convertase was accelerated by C2b. This result suggests that C2b acts as a feedback inhibitor on the activation of the classical pathway of complement system.

Chemical Phenomena↗
PubMed1982

Effect of nonviable tissue and abscesses on complement depletion and the development of bacteremia.

Complement concentrations and blood cultures were compared in 58 patients within 24 hours of injury and weekly thereafter. Extensive amounts of nonviable tissue (n = 40) were associated with a mean depletion of C4, C3, and C5 by 56%, and minor injuries (n = 18) by 22% of normal concentration within 14 hours after injury. The C4, C3, and C5 concentrations returned to normal or above within a week after minor injuries, but not following major injuries. However, C4, c3, and C5 levels remained depressed after major injuries unless necrotic tissue was removed or abscesses were drained. If complement concentration was below 50% of normal for more than a week all patients developed bacteremia. Following debridement or drainage complement returned to normal in 11 patients and blood cultures became negative in seven. Possible consequences by activation and altered availability of complement for chemotaxis, opsonization, and lysis of bacteria have been analyzed and related to the development of bacteremia.

Abscess↗
PubMed1980

The Croonian Lecture, 1980. The complex proteases of the complement system.

The assembly and activation of the early components of complement, after their interaction with antibody-antigen complexes, are described in terms of the structures of the different proteins taking part. C1q, a molecule of unique half collagen--half globular structure, binds to the second constant domain of the antibody molecules through its six globular heads. A tetrameric complex of C1r2-C1s2 binds to the collagenous tails and leads to formation of the serine-type proteases C1r and C1s. C1s activates C4, which forms a covalent bond between its alpha' chain and the Fab section of the antibody. C2 is also activated by C1s and associates with the bound C4 molecule to form C42, a labile protease that activates C3, but which loses activity as the C2 peptide chains dissociate from C4. C2, by analogy with factor B, the equivalent component of the alternative pathway of activation, appears to be a novel type of serine protease with a similar catalytic site but different activation mechanism to the serine proteases that have been described previously.

Amino Acid Sequence↗
PubMed1984

Activation of C1.

The first component of complement, C1, is a calcium-dependent complex of two loosely interacting subunits: C1q, responsible for the binding of activators to C1; C1r2-C1s2, which supports the autoactivation potential of C1, together with the proteolytic activity of activated C1- on its two substrates, C4 and C2. Isolated dimeric C1r2 is able to autoactivate through an intradimer cross-proteolysis; this capacity is lost when C1r2 is associated with two molecules of C1s inside the calcium-dependent C1r2-C1s2 subunit; this capacity is again observed in reconstituted C1. A model for reconstituted soluble C1 is proposed, based on electron microscopy, neutron diffraction, ultra-centrifugation, various biochemical findings, as well as functional properties of C1 or of its subcomponents. The flexible rod-like structure of C1r2-C1s2 is folded around two arms of C1q, with the catalytic domains of C1r and C1s inserted inside the cone defined by the C1q stalks. Activation of C1 which, in vivo, is controlled by C1 inhibitor, can be achieved by various activators, such as immune complexes; it appears to result from the suppression of a negative control and resides in a positive modulation of the intrinsic autocatalytic potential of C1r inside C1.

Amino Acid Sequence↗
PubMed1985

The killer molecule of complement.

Cell injury by complement occurs as a consequence of activation of either the classical or the alternative pathway on the surface of a cell. It is accomplished by the membrane attack complex (MAC). Its precursor proteins, C5, C6, C7, C8, and C9, are hydrophilic glycoproteins with Mr ranging from 70,000-180,000. When C5 is cleaved by the serine protease C5 convertase which covalently attaches to target cells, nascent C5b is produced and forms together with C6 a soluble and stable bimolecular complex (C5b,6). Upon binding of C5b,6 to C7 a trimolecular complex (C5b-7) is formed which expresses a metastable membrane-binding site. Membrane-bound C5b-7 constitutes the receptor for C8 and the tetramolecular C5b-8 complex binds and polymerizes C9. During the assembly process the proteins undergo hydrophilic-amphiphilic transition and the end product consists of C5b-8 (Mr approximately 550,000) and of tubular poly C9 (Mr approximately 1,100,000). The functional channel size varies but its maximal diameter is approximately 100 A. C9 polymerization appears to involve initial reversible association of several C9 molecules which is followed by temperature-dependent, constrained unfolding. Unfolded C9 monomers then associate laterally with each other and polymerization terminates with closure of the circular structure which consists of 12-18 C9 monomers. Amino acid composition and sequence indicate that the N-terminal half of the single chain C9 molecule is hydrophilic and the C-terminal half rather hydrophobic. Phospholipid-binding and insertion into membranes are functions of the C-terminal portion of the molecule. Control of the MAC is exerted by the S-protein (Mr 80,000) which binds to the forming complex and prevents its attachment to the cell membrane. Control is also exerted by certain species-specific membrane proteins which interfere with C5 convertase and C9 function.

Binding Sites↗
PubMed1982

Antibody-independent binding of Clq and activation of serum complement by human skin in vitro.

By exposing frozen sections of human skin to fresh normal human serum, binding and activation of complement was observed. Attached complement components Clq, C4, and C3 were detected by immunofluorescence microscopy using anticomplement conjugates. Isolated Clq bound to the same structures as serum Clq, C4, and C3 and binding of C4 and C3 was dependent on binding of Clq. The complement components bound to capillary endothelial cells and to fibrillar structures in the dermis and reacted with the epidermis. Complement binding dermal fibrillar structures in adult skin were scant but abundant in fetal skin. They had similar distribution as "microfibrils" demonstrable by human autoantibodies. Antibody independence of the Clq binding was shown using isolated Clq and by the observed constant and reproducible binding of complement of normal sera lacking antibodies to the described target structures. The observations suggest that antibody independent binding of complement should be considered as a possible mechanism leading to dermal complement deposition in vivo.

Adult↗
PubMed1982

In vitro complement activation by intercellular antibodies.

By in vitro complement immunofluorescence, 6 sera from pemphigus with intercellular antibodies were tested for their capability to fix C1q, C4, C3, and properdin. All 6 serum samples yielded positive reaction for C3 staining. Three serum samples gave positive staining for C1q, 5 serum samples for C4, and 3 serum samples for properdin, respectively. Substitution of C2 deficient serum as a complement source inhibited C3 and properdin staining but not positive C1q and C4 staining. These results are best explained by the concept that complement activation in vitro by intercellular antibodies occurs via the classical pathway followed by assembly of the C3 amplification mechanism.

Antibodies↗
PubMed1980

A newly described control mechanism of complement activation in patients with mixed cryoglobulinemia (cryoglobulins and complement).

Levels in serum of components of complement were studied in a group of 10 patients with mixed cryoglobulinemia. The profiles found in most patients showed decreased levels of the early complement components C1, C4, and C2, with normal levels of C3. Experiments performed to define the mechanism(s) responsible for this unusual complement profile showed that activation of the early complement components in serum was due to the activation of the classical pathway by mixed cryoglobulins. They also showed that the characteristic lack of effect on C3 was due to the action of a previously unrecognized regulatory mechanism upon C3 convertase of the classical pathway mediated by 2 normal serum proteins, namely, the C4 binding protein (C4-bp) and the C3b inactivator (C3bINA).

Complement Activating Enzymes↗
PubMed1983

Antibody-independent binding and activation of complement by Schistosoma mansoni adult worms.

The in vitro binding serum complement (C) components to Schistosoma mansoni worms was studied. By exposing frozen sections of adult male and female parasites to normal human serum, binding of both classical and alternative pathway C components was observed. By immunofluorescence (IFL) microscopy selective binding of Clq, C4 and C3 to the schistosomal tegument, internal structures, the intestinal tract and eggs was seen. The C4 and C3 binding was completely abolished in the presence of 10 mM EDTA. EGTA also completely inhibited binding of C4 and most of the C3 binding. These results suggest that C binding to S. mansoni adult worms occurs mostly by the classical activation pathway. However, the partial Ca2+ independence of C3 binding and the demonstrated binding of the regulatory protein beta 1H suggests that worms are capable also of C3 binding by the alternative pathway. No C binding occurred to intact worms. Although some in vivo bound immunoglobulin appeared to be present occasionally at the parasite surface and in the gut, this material did not account for the demonstrated extensive C deposition upon incubation of frozen sections with normal human serum in vitro. Antibody independence of the observed classical pathway C binding was further indicated by binding of isolated Clq to the same structures capable of binding C components from serum.

Animals↗
PubMed1987

Opsonic and physicochemical characteristics of intravenous immunoglobulin preparations.

The composition and opsonizing activity of five commercially available immunoglobulin preparations for intravenous use (Venoglobulin I, Venilon, Gammagard, Polyglobin, and Sandoglobulin) were studied. The composition of these preparations does not differ very much as far as total protein, immunoglobulin class and IgG subclass concentrations are concerned. The only exceptions were that Veniglobulin I, Gammagard and Sandoglobulin contain IgA, which might cause side effects in patients with anti-IgA antibodies, Gammagard contains very little IgG4, and Venilon and Polyglobin contain no and almost no IgG3, respectively, which might explain their very low opsonic activity. It was found that Venilon and Gammagard activate complement in the ready-for-infusion state. The opsonic activity of Venoglobulin I, Sandoglobulin and Gammagard is about equal to that of inactivated serum: Staphylococcus aureus, Escherichia coli with K antigen, Streptococcus pyogenes and Streptococcus group B are well opsonized and E. coli without K antigen and Streptococcus pneumoniae are poorly opsonized.

Blood Bactericidal Activity↗
PubMed1986

Activation of complement by myelin: identification of C1-binding proteins of human myelin from central nervous tissue.

Myelin isolated from central nervous tissue activates the classic pathway of complement by directly activating C1. Activation of C1 can proceed to form membrane attack complex, C5b-9, in the myelin. Such an interaction between myelin and complement may be important in diseases involving myelin damage, in view of the role of complement in membrane attack and inflammation. To identify the C1-activating protein, myelin was subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and Western blot. The blots were incubated with C1 or with whole serum complement, followed by immunostaining for C1 or C3, respectively. A duplicate strip was stained with amido black or anti-myelin antibody to visualize the myelin proteins. The results showed that two major protein bands were capable of activating C1. An approximately 56-58-kilodalton band comigrated with the W2 protein and an approximately 45-47-kilodalton band migrated along with, but slightly behind, the W1 Wolfgram doublet.

Complement Activating Enzymes↗
PubMedDate not supplied

The complement system in pregnancy.

Maternal recognition of fetal antigens is well-documented. Antibodies directed against fetal antigens are the rule rather than the exception. Maternal antibodies seem to bind fetal antigens at the placental level and apparently activate the complement system even in normal pregnancy, yet multiple studies confirm an increase in C1q, C4, C3 and CH50 levels in pregnancy and an absence of complement activation. More sensitive assays and a broadened concept of the timing of crucial immunologic events may lead to a greater understanding of the importance of the complement system in pregnancy.

Cell Membrane↗