Circulating proalbumin associated with a second case of antitrypsin Pittsburgh

We report here the conclusive identification of circulating proalbumin of normal N-terminal sequence (Arg Gly Val Phe Arg Arg Asp Ala) in a second child with the alpha 1-antitrypsin Pittsburgh 358 Met-->Arg mutation. As in the first case, the proalbumin made up 3-5% of the total serum albumin. The finding of proalbumin associated with a second de novo mutation at the inhibitory site bait of antitrypsin confirms our earlier hypothesis; that antitrypsin Pittsburgh was acting as a specific intracellular inhibitor of the hepatic proalbumin convertase and that antitrypsin Pittsburgh could be used as a probe to identify the proprotein convertase.

Inhibition of the alternative C3 convertase and classical C5 convertase of complement by group A streptococcal M protein

When Streptococcus pyogenes group A type 3 strain C203 (M+) and its M-protein-lacking derivative, strain C203S (M-), were treated with normal human serum in the presence of magnesium-EGTA [ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid], virulent M+ bacteria bound only 10 to 30% as much C3 and factors B and P as did avirulent M- bacteria. After treatment of M+ bacteria with trypsin, which inactivates M protein, their binding of these substances was similar to that of M- bacteria. Pretreatment of M+ bacteria with the Fab fragment of rabbit immunoglobulin G anti-M antibody also increased their binding of C3 in the absence of Ca2+. Therefore, M protein inhibits the alternative C3 convertase. In contrast, in the presence of Ca2+ and Mg2+, M+ bacteria bound 75% as much C3 as M- bacteria. This binding was mostly mediated by classical pathway activation, because M+ bacteria bound much smaller amounts of factors B and P than did M- bacteria but consumed amounts of C4 and C2 comparable to those consumed by M- bacteria. On the other hand, the amount of C5 bound to M+ bacteria was much less than that bound to M- bacteria, and the consumption of C5 and C8 by M+ bacteria was also much less than that by M- bacteria. Therefore, M protein does not inhibit the classical C3 convertase but does inhibit the classical C5 convertase. When M+ and M- streptococci were incubated with normal human serum containing radiolabeled C3 in the presence of Ca2+ and Mg2+, more than 85% of the C3 bound to either type of streptococcus was extractable by sodium dodecyl sulfate and alkali treatment. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of the C3 extracted from both strains showed that it was mostly C3b and iC3b. The proportions of C3b and iC3b, respectively, were 7.5 and 71.9% on M+ bacteria and 18.9 and 58.4% on M- bacteria. These results support and extend previous findings that the antiphagocytic activity of streptococcal M protein may be due to complement inhibition mediated by the binding of factor H.

Peptide inhibitors of C3 breakdown

We have investigated the development of substrate-based inhibitors of complement enzymes. Sequences around the scissile Arg77-Ser78 bond of C3 have been synthesized and tested as inhibitors of C3 convertase. The best inhibition was found with the tetrapeptide Ac-Arg-Ser-Asn-Leu-OH (H-576); extending this sequence in either direction reduced inhibitory activity. Preliminary experiments with peptides in which the scissile bond--CO--NH--was replaced with non-hydrolysable moieties such as--CO--CH2--(H-497) and--CH2--NH--(H-336) failed to show enhanced inhibition. One of the longer chain inhibitors H-416 containing DArg77-Ser78 was unexpectedly found to potentiate iC3 cleavage by factors I and H but did not inhibit the intact alternative pathway. The same peptide also bound to factor H. It is concluded that the binding requirements of the C3 convertase are more sophisticated than can be satisfactorily imitated simply by linear sequences around the scissile bond of C3.

[Effectiveness and tolerance of the C3 convertase inhibitor, N-acetyl-aspartyl-magnesium glutamate with anti-allergic action. Results of a double-blind study]

N-Acetyl-aspartyl magnesium glutamate (Rhinaaxia, NAAGA) is a topically active antiallergic dipeptide. The compound acts in two different ways. On the one hand NAAGA inhibits the mast cell-degranulation, on the other hand this compound blocks the activation of the C3-convertase, subsequently followed by a blocked cleavage of the fragments C3a and C5a, respectively. 20 patients suffering from pollinosis were treated for 2 weeks according to a randomized double-blind placebo-controlled study. Besides subjective complaints nasal obstruction was objectively documented via rhinomanometria. 9 out of 10 patients under placebo had to use the rescue drug tritoqualine, a histidine decarboxylase inhibitor, compared to none in the verum group (p less than 0.01). After 14 days of treatment with NAAGA the nasal peak flow rate increased by 21.5 l/min and 21.8 l/min in the tritoqualine/placebo group, respectively (not significant). Nasal obstruction improved statistically significantly after 7 and 14 days of treatment in both groups. Tolerance was reported to be good in either group.

[Changes in the kallikrein-kinin and complement system in angiography using non-ionic contrast media]

To examine alterations of the kallikrein-kinin system and of the complement due to the bolus injection of newer non-ionic contrast agents, venous blood samples were taken before and 3 min after angiography. There were no adverse contrast reactions clinically evident. Prekallikrein, kallikrein inhibition, beta-factor XIIa inhibition, C1-esterase inhibitor, C1q, C3, ATIII, HMW-kininogen, fibrinogen and factor XII were determined. Bolus injection of the contrast medium caused an activation of the kallikrein-kinin system (p less than 0.05) with reduction of prekallikrein, kallikrein-inhibition, beta-factor XIIa inhibition and C1-esterase inhibitor. The levels of C1q and C3 were also decreased (p less than 0.05) indicating an activation of the complement. Our results demonstrate, that angiography causes a significant activation of the kallikrein-kinin as well as of the complement system in spite of the use of newer non-ionic contrast agents.

Rosmarinic acid: a new inhibitor of complement C3-convertase with anti-inflammatory activity

Rosmarinic acid (RA) is a naturally occurring compound, isolated from Rosmarinus officinalis or Melissa officinalis which inhibits the in vitro immunohaemolysis of antibody-coated sheep erythrocytes by guinea pig serum. In further experiments this reduced immunohaemolysis was found to be due to inhibition of the C3-convertase of the classical complement pathway. The threshold concentration for inhibition of C3-convertase was 10(-6) mol/l. RA with an optimal inhibitory concentration between 5 and 10 mumol/l., resulting in about 70% inhibition of haemolysis. However, higher concentrations of RA were less effective at inhibiting C3-convertase. The inhibition may not be specific for C3-convertase, since another serine protease, elastase, was also weakly inhibited by RA in vitro. RA also exhibited inhibitory activity in three in vivo models in which complement activation plays a role. Thus, RA (0.316-3.16 mg/kg i.m.) reduced paw oedema induced by cobra venom factor (CVF) in the rat, and at 1-100 mg/kg p.o. inhibited passive cutaneous anaphylaxis in the rat. In addition, at 10 mg/kg i.m. RA impaired in vivo activation by heat-killed Corynebacterium parvum (i.p.) of mouse macrophages, as measured by the decreased capacity of the activated macrophages to undergo the oxidative burst. RA (0.1-10 mg/kg i.m.) did not inhibit t-butyl hydroperoxide-induced paw oedema in the rat, indicating selectivity for complement-dependent processes.

The mechanism of action of decay-accelerating factor (DAF). DAF inhibits the assembly of C3 convertases by dissociating C2a and Bb

DAF is a 70,000-Mr membrane protein that inhibits the amplification of the complement cascade on the cell surface, and protects cells from damage by complement. The precise mechanism of action of DAF is not entirely clear. Purified DAF was incorporated into the membrane of EAC4b cells. EAC4b2 and EDAF AC4b2 cells were prepared with radiolabeled C2. The same amount of labeled C2 bound to both cells, showing that DAF does not prevent the binding of C2 zymogen to C4b. After adding Cl, the radioactivity of bound C2 dissociated more rapidly from EDAF AC4b cells than from EAC4b cells. In EAC4b cells, bound C2 was converted to C2a, which gradually dissociated into the supernatants. In the DAF-treated cells, on the other hand, a large amount of C2a rapidly appeared in the supernatants and only a small amount of C2a remained on the cells. In a similar experiment using EhuAC4b, DAF on human erythrocyte membrane also dissociated the C2a from the cells. These results were confirmed by hemolytic assay and the accelerated decay of C2a caused the rapid depletion of C2 from the fluid phase. In addition, we found that DAF functions on the alternative pathway C3 convertase, C3bBb in the same manner. Thus, DAF, which associates with C4b and C3b in the membrane, acts on C2a and Bb, but not on intact C2 and B, and dissociates them rapidly from the binding sites, thereby preventing the assembly of the classical and alternative pathways C3 convertases.

Prekallikrein activation, C1 esterase inhibitor, and factor XII as predictors of adverse reaction to contrast media. A prospective study

The susceptibility of 152 patients to idiosyncratic reactions resulting from the administration of radiographic contrast media was studied. The rate of activation of plasma prekallikrein was measured in samples taken from these patients before they received contrast agents. Kallikrein inhibitor and factor XII levels were also determined. The tests were of no value in selecting the ten patients who subsequently experienced mild reactions. However, the possibility remains that one or more of the tests may have predictive value for patients who experience moderate or severe reactions.

Effects of inhibitors of C1q biosynthesis on macrophage Fc receptor subclass-mediated antibody-dependent cellular cytotoxicity and phagocytosis

We examined the effects of the inhibitors of C1q or collagen biosynthesis, 2,2'-dipyridyl (DP), and 3,4-dehydro-DL-proline (DHP) on murine macrophage (M phi) FcR subclass-mediated antibody-dependent cellular cytotoxicity (ADCC) and phagocytosis of sheep erythrocyte targets. Oil-elicited peritoneal M phi from C3HeB/FeJ mice which were cultured for 24 hr with DP (0.08 or 0.10 mM) or DHP (0.8 or 1.0 mM) showed a significant decrease in FcR subclass-mediated ADCC for murine monoclonal IgG2a (FcRI) and IgG2b/IgG1 (FcRII) as well as for heterologous polyclonal IgG. These collagen inhibitors also blocked phagocytosis mediated by both IgG2a- and IgG2b-opsonized erythrocytes. DP was more potent than DHP in blocking FcR effector functions in a reversible fashion and neither inhibitor affected M phi C3b receptor function. Pretreatment of M phi with collagenase resulted in significant reduction in FcR-mediated ADCC and phagocytosis. The inhibition of M phi FcR subclass-mediated ADCC and phagocytosis by collagen C1q synthetic inhibitors or by collagenase treatment further confirms a functional relationship between cell-associated C1q and FcR-dependent functions.

Hypocomplementemia with low C1s-C1 inhibitor complex in systemic lupus erythematosus

Ninety-three serum and plasma samples from 45 patients with systemic lupus erythematosus were analyzed for the complex formed by C1s and its inhibitor, as well as for C3, C4, C4a desarginine, and staphylococcal protein A-bound immune complexes. There were statistically significant correlations between C1s-C1 inhibitor complex and CH50, between C1s-C1 inhibitor complex and C4, and between C1s-C1 inhibitor complex and C4a desarginine. Serial studies were performed on 24 patients over a period of 6 months. Seven of 21 patients with hypocomplementemia had persistently normal levels of C1s-C1 inhibitor complex, 7 had transiently abnormal levels of C1s-C1 inhibitor complex, and 7 had sustained abnormal levels of C1s-C1 inhibitor complex. Two of 3 pregnant patients with normal levels of complement had abnormal levels of C1s-C1 inhibitor complex. Staphylococcal protein A-bound immune complexes demonstrated no correlation with any of the complement assays. Complement activation, as measured by C1s-C1 inhibitor complex, is often a transient phenomenon in systemic lupus erythematosus patients with persistent hypocomplementemia.

Glycoprotein C of herpes simplex virus 1 is an inhibitor of the complement cascade

Mammalian cells in culture express membrane receptors for C3b when infected with HSV-1. C3b binding is mediated by glycoprotein C (gC), a virus-specified membrane glycoprotein. In view of the inhibitory functions of other C3b-binding proteins, we studied the capacity of gC to modulate complement activation. Glycoprotein C was purified from HSV-1-infected cells by immunoaffinity chromatography. Glycoprotein C, but not a control viral glycoprotein, demonstrated dose-dependent acceleration of decay of C3bBb sites. In addition, gC produced a dose-dependent, time-independent depression of the overall hemolytic efficiency of C3bBb sites. Inhibition of C5b6-initiated reactive lysis of cells bearing C3b, but not cells bearing antibody alone, by gC suggests that the second effect represents interference with the C3b-C5/5b interaction. This hypothesis is supported by the failure of gC to inhibit reactive lysis when added after C5b67 insertion into target cells. Glycoprotein C does not accelerate C14b2a decay, nor does it impair classical pathway hemolytic efficiency when excess C5 is present. By limiting available C5/5b, some gC inhibition of C3b-C5/5b interactions can be unmasked in the classical pathway system. Glycoprotein C is devoid of factor I co-factor activity. HSV-1 gC is a modulator of complement activation, especially via the alternative pathway, and may represent a novel viral mechanism for evading host defense processes.

[Inhibition of the binding and activation of the first component of human complement. The effect of synthetic peptides, immunoglobulin fragments and various proteins]

A study has been carried out on the inhibition of the subcomponent Clq binding to sensitized sheep erythrocytes (EA) by the following synthetic peptides mimicking the structure of a putative complement binding site of immunoglobulin G: Boc-Trp-Tyr, Boc-Tyr-Trp, Trp-Tyr, Boc-Trp-Phe, Boc-D-Trp-D-Tyr, Boc-D-Tyr-D-Trp, Boc-Leu-Leu, Ac-Phe-Tyr, and commercial Thr-Lys-Pro-Arg (tuftsin). Boc-Trp-Tyr was found to be the most potent inhibitor of Clq binding to EA (Ki 2.86 X 10(-4) M), tuftsin ranking second with Ki 6 X 10(-4) M. The D,D-dipeptides failed to inhibit the Clq binding at the investigated concentrations. Insoluble Z-Trp-Tyr-OMe activated a classical pathway of complement system, as monitored by consumption of C4, C2 and C3 components. Synthetic octapeptide Boc-Glu-Val-Asp-Leu-Leu-Lys-Asp-Glu-OMe (corresponding to the sequence 36-43 of beta 2-microglobulin) inhibited the Clq binding with Ki 4.7 X 10(-4) M, which gave grounds for localizing the complement binding site in beta 2-microglobulin. The finding in the Clq structure of the peptide sequence homologous to than of the pepsin active site, as well as the close similarity in the specificity of these proteins towards hydrophobic amino acid residues justified the assumption on the same structural bases of their specificity. The results of the present study, along with the literature data, underlie the hypothesis on the involvement in the complement binding of the following IgG residues: Trp277, Tyr278, Lys320, Lys322, Glu318 and Lys290. The enlisted residues are closely located in the three-dimensional structure of the CH2 domain of IgG. Lysozyme and lactalbumin having the sequences homologous to Trp277-Tyr278 of IgG inhibited Clq binding to EA with Ki 3 and 1.5 microM respectively.

[Effect on human complement of blastolysin and the glycopeptide (MDP and GMDP) and carbohydrate fragments of peptidoglycans]

Along with complement activation by the classical pathway, blastolysin, an antitumor and adjuvant preparation of Lactobacillus bulgaricus peptidoglycans, effectively inhibits the transformation of C3 in to C5 convertase. Values of inhibition maximum and dissociation constants of the reversible C3b-acceptor complex for blastolysin and main immunological active structural moieties of peptidoglycans (GMDP, MDP) and their inactive carbohydrate components (N-acetylglucosaminyl-N-acetylmuramic acid, N-acetylglucosamine, and N-acetylmuramic acid) have been determined. Immunostimulator concentrations for blastolysin, GMDP, and MDP in inhibition of the C5 convertase formation (C3b binding) correlate with their doses in vivo (animal blood), displaying antitumor activity.

Evidence for arginine residues in the immunoglobulin-binding sites of human Clq

The immune complex binding activity of human Clq was lost following treatment of the protein with the arginine-selective reagents cyclohexane 1,2-dione and phenylglyoxal. Both inactivations followed pseudo-first-order kinetics. The affinity of Clq for immune complexes was reduced 7-fold following cyclohexane-1,2-dione treatment, and could be substantially restored by treatment of the modified protein with hydroxylamine. Heat-aggregated IgG protected Clq against inactivation by both reagents. Incorporation of 25 molecules of [7-14C]phenylglyoxal per Clq molecule completely inactivated the protein. These data are consistent with the presence of arginyl residues in the immunoglobulin recognition sites of human Clq.

[Binding of activated C3b component with complement effector]

Various nucleophilic agents (acceptors) react with thiolester group of nascent activated fragment (C3b) of the third complement component. The C3b-acceptors binding prevents transformation of C3 convertase to C5 convertase and results in inhibition of the cell-target lysis. A convenient method of monitoring the EAC142 to EAC1423 transformation was elaborated. Character of the inhibition suggests that the covalent binding follows a stage of the reversible C3b-acceptor complex formation. The method allows to determine the maximum of inhibition of the C5 convertase formation and the dissociation constant of the reversible C3b-acceptor complex, which reflects the C3b affinity to this acceptor.

A circulating inhibitor of fluid-phase amplification. C3 convertase formation in systemic lupus erythematosus

C3 nephritic factor (C3NeF) was used to assess the formation of the fluid-phase amplification convertase, C3b,Bb, in 37 serum specimens from 24 patients with systemic lupus erythematosus (SLE). C3b,Bb formation was measured by the concentration of Ba, released when C3b,B is activated. Incubation of normal human serum (NHS) with C3NeF accelerates C3b amplification loop turnover with the formation of large quantities of C3b,Bb. In contrast, sera from 22 of 24 patients with SLE formed little or no convertase when incubated with C3NeF. C3 conversion to C3b was commensurately reduced. The inhibition could not be attributed to depressed serum concentrations of C3, factor B, or classical pathway components. Inhibitor present in excess could be demonstrated in 23 of 34 specimens of SLE serum by mixing experiments. The spontaneous convertase formation that occurs when a portion of the serum H is inactivated with F(ab')2 anti-H was also shown to be inhibited in SLE serum. The inhibition was found, however, to be H dependent in that convertase formation was normal in SLE serum depleted of H. It is concluded that the C3b in most SLE sera is unusually susceptible to inactivation by H, but a functional abnormality was not demonstrable in either C3 or H isolated from SLE serum. The inhibition could be simulated in NHS by addition of heparin, 100 micrograms/ml. In vivo, inhibition of convertase formation could interfere with the solubilization and disposal of immune complexes by reducing the deposition of C3b on the immune complex lattice.

Deficiency of the complement regulatory protein, "decay-accelerating factor," on membranes of granulocytes, monocytes, and platelets in paroxysmal nocturnal hemoglobinuria

Erythrocytes from patients with paroxysmal nocturnal hemoglobinuria are deficient in decay-accelerating factor, a membrane protein that inhibits the complement C3 convertases. We studied the expression of this protein on leukocytes and platelets from four patients with paroxysmal nocturnal hemoglobinuria, using cytofluorographic analysis and antibody to decay-accelerating factor. The granulocytes and monocytes had a bimodal distribution of fluorescence, indicating antigen-deficient and antigen-positive subpopulations of cells. In contrast, granulocytes and monocytes from normal donors and patients with other diseases had no antigen-deficient cells. Platelets from the four patients with paroxysmal nocturnal hemoglobinuria had less fluorescence than normal platelets. Furthermore, surface-radiolabeled granulocytes and platelets from one of the four patients, which were maximally deficient in decay-accelerating factor, also lacked antigen that was immunoprecipitable by specific antibody to this protein. Thus, paroxysmal nocturnal hemoglobinuria is a clonal disorder characterized by deficient membrane expression of decay-accelerating factor on granulocytes, monocytes, and platelets, as well as on erythrocytes.

[Changes in the ultrastructure of subcomponent C1q of human complement during spontaneous inactivation in diluted solutions]

Dilution of human serum or solutions of highly purified subcomponent C1q of human complement results in the drop of C1q activity. Electron microscopy of highly purified subcomponent C1q revealed that a certain part of molecules has a changed ultrastructure and C1q subunits are dissociated. As the preparations for electron microscopy have been obtained from dilute solutions, the changes in the ultrastructure and C1q inactivation should be interrelated phenomena. The conformational liability of the C1q structure is supposed to have a functional role.

Inhibition of complement activation on the surface of cells after incorporation of decay-accelerating factor (DAF) into their membranes

Decay-accelerating factor (DAF), extracted from the stroma of human erythrocytes, was purified to homogeneity and incorporated into the membrane of sheep red cell complement intermediates, where its functional properties were analyzed. Incorporation of DAF into the cell membranes was temperature dependent, took place on pronase- or trypsin-treated erythrocytes, and did not depend on prior deposition of antibody, C1 or C4. Serum lipoproteins (high and low density) effectively inhibited DAF incorporation, but had no effect on the activity of DAF after its association with the cell membrane. The incorporated DAF could not be removed from the red cell surface by repeated washings in the presence of high salt concentration but was solubilized when the stroma were extracted with 0.1% Nonidet P-40. The presence of DAF in the membrane of EA did not affect the deposition of C1 and C4, but as few as 10(2) DAF molecules per cell profoundly inhibited the assembly of C3 and C5 convertases of both the classical and alternative pathways. The DAF inhibitory effect on EAC14 or EAC43 was not overcome by supplying an excess of C2 or factor B, but the alternative pathway C3 convertase could be assembled in the presence of Ni++, or nonphysiological concentrations of Mg++, which enhances the binding affinity of factor B for C3b. The DAF effect on EAC14 or EAC143 was entirely reversed by treating the cells with specific anti-DAF antibodies, showing that DAF did not alter the structure of C4b or C3b. Taken together, the experimental evidence suggests that DAF interacts directly with membrane-bound C3b or C4b and prevents subsequent uptake of C2 and factor B. DAF can function only within the cell membrane. Indeed, the decay dissociation of the C4b2a enzyme on DAF-containing sheep intermediates was not changed by varying the cell concentration. DAF-treated EA had no influence on the decay of nontreated EAC142 present in the same mixture. Moreover, the inhibitory activity of intact human erythrocytes on C4b2a was not blocked by antibodies to DAF, but was abolished by antibodies to the C3b/C4b receptor (CR1). When incorporated into the membrane of rabbit erythrocytes, human DAF inhibited their lysis by human complement. In conclusion, on the basis of these and previous results, it appears that DAF plays a central role in preventing the amplification of the complement cascade on host cell surfaces.(ABSTRACT TRUNCATED AT 400 WORDS)

Detection of intermediary Clr with complete active site, using a synthetic proteinase inhibitor

The synthetic proteinase inhibitor, FUT-175 (6-amidino-2-naphthyl-4-guanidinobenzoate), strongly suppressed activation of Clr at 37 degrees C, causing 50% inhibition at 0.03 mM. To clarify whether the inhibitor was incorporated into the active site of intermediary Clr formed during the incubation, determination of the active site was tried using this inhibitor. Consequently, release of amidinonaphthol equimolar with the amount of Clr used was observed in the early period of incubation, in which the activation to Clr- was about 5%. These results indicate that intermediary Clr already has a complete active site.

The effect of ethylenedinitrilotetraacetic acid (EDTA) on the reaction between the guinea-pig C5 convertase and guinea-pig C5

Guinea-pig C5 was reacted with EAC1423 in the washed-cell intermediate assay in the presence of glucose gelatin veronal buffer (GGVB), Zn2+-GGVB (0.025 mM), GGVB2+ containing Ca2+ and/or Mg2+ or EDTA (0.013 M)-GGVB. The EDTA inhibited the formation of competent SAC14235, while Ca2+ and/or Mg2+ had a slight enhancing effect compared to GGVB alone and Zn2+ gave a four-fold increase. Similar results were obtained by using human C5 with guinea-pig C5 convertase and functionally pure guinea-pig C6, C7, C8 and C9. When guinea-pig C6 was incorporated into these various reaction mixtures with guinea-pig C5, its addition markedly reduced the inhibition by EDTA, while Zn2+ still showed an enhancing effect. These results demonstrate that EDTA inhibited formation of competent SAC14235 by preventing activation of C5. The association of C6 with C5 can partially overcome the inhibition of C5 conversion by EDTA and may account for C5 activity in reaction mixtures containing C-EDTA.

Inhibition of classical C5 convertase in the complement system by factor H

This paper described the influence of factor H on the haemolytic activity of the classical C5 convertase. Factor H showed little effect on the interaction of C5 with EAC1,4b,2a,3b cells bearing low numbers of C3b sites, but displayed the inhibitory effect on the interaction of C5 with the intermediate cells bearing high numbers of C3b sites. The higher the number of C3b sites on the cells, the greater the degree of the inhibition by factor H. The inhibition by factor H was accompanied by the inhibition of consumption of C5 from the fluid phase, indicating that factor H inhibits the activity of C5 convertase, not the binding of activated C5 to the cells.

Heparin prevents formation of the human C3 amplification convertase by inhibiting the binding site for B on C3b

Fluid-phase heparin prevents generation of the C3 amplification convertase of human complement, C3b, Bb most likely by inhibiting the formation of the bimolecular complex between cell-bound C3b and B. The effect of heparin on the binding of B to C3b was examined using 125I-labelled B and C3b-bearing sheep erythrocytes (EsC3b). In the absence of heparin, B bound to EsC3b with an affinity of 0.5-1 X 10(6) M-1 in the presence of 5 mM Mg2+. Incremental amounts of heparin (100-700 micrograms/10(7) EsC3b) inhibited the binding of 125I-B to C3b in a dose-dependent manner. Scatchard analysis of the binding data in the presence of four inhibitory concns of heparin revealed that heparin did not affect the binding affinity of B for C3b but decreased the number of C3b sites recognized by B on the cells. No inhibition of binding occurred in the presence of totally (N- and O-) desulfated heparin which has no anticomplementary activity. These results demonstrate that heparin prevents generation of the C3 amplification convertase by binding to cell-bound C3b and masking the binding site for B on C3b.

Structural and circular-dichroism studies on the interaction between human C1-esterase inhibitor and C1s

The reaction between complement factor C1s and C1-esterase inhibitor has been investigated by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis, N-terminal amino acid analysis and c.d. studies. It is confirmed that a very stable stoichiometric 1:1 complex with a molecular weight of about 180000 is formed, involving the light chain of C1s. On the sodium dodecyl sulphate/polyacrylamide gels a small peptide with a molecular weight of about 5000 can be seen, which may be released from the C-terminal portion of the inhibitor moiety in a manner analogous to that occurring in other similar proteinase-inhibitor reactions. By N-terminal amino acid analysis, a newly formed threonine residue is found in the complex, suggesting that the inhibitor peptide chain is cleaved in the complex between C1s and C1-esterase inhibitor. The stabilizing bond may therefore be an ester bond. C.d. studies of the native C1-esterase inhibitor indicated the presence of about 38% alpha-helix, about 24% beta-structure and about 38% unordered structure. By gradual cleavage of the disulphide bridges under non-denaturating conditions, gradual changes in the c.d. spectra occurred, suggesting loss of ordered secondary structures. The c.d. spectra of the complex between C1s and C1-esterase inhibitor indicate that tryptophan residues are affected by the complex-formation.

Regulation of the C3 nephritic factor stabilized C3/C5 convertase of complement by purified human erythrocyte C3b receptor

Activation of complement may result in the generation of the amplification convertase C3bBb. This convertase can be stabilized by properdin (P) or C3NeF. C3bBbP is susceptible to inactivation by beta 1H, while C3bBbNeF is relatively resistant. Since it has been shown that the human erythrocyte C3b receptor (CR1) is able to inactivate C3bBbP, the inactivating action of CR1 on C3bBbNeF was investigated CR1 is at least five times more efficient than beta 1H in inactivating C3bBbNeF. Kinetic studies revealed that CR1 induces an enhanced biphasic kinetics of decay of C3bBbNeF; further purification of this C3NeF preparation by cation exchange chromatography showed that this phenomenon is dependent on the population of C3NeF. Finally CR1 is also able to inactivate fluid phase C3bBbNeF.

Inhibition of alternative pathway factor D by factor B-related synthetic hexapeptides

Hexapeptides mimicking the partial amino acid sequence of factor B surrounding the bond that is cleaved by factor D have been synthesized. These peptides have been assessed for their ability to inhibit factor D enzymatic activity and for their susceptibility to serine proteases. The synthetic peptides were cleaved by bovine trypsin and C1s but not by alpha-thrombin and factor D. The peptides inhibited factor B cleavage and fluid-phase or cell-bound alternative pathway C3 convertase activation by factor D. Altogether, these results suggest that peptides analogous to factor B specifically inhibit factor D enzymatic activity. Thus, they constitute an interesting tool for study of alternative pathway activation and can be of use when attempting to manipulate this pathway, since factor D is an essential component for alternative pathway initiation and amplification.

Fluid-phase interaction of C1 inhibitor (C1 Inh) and the subcomponents C1r and C1s of the first component of complement, C1

Interactions between proenzymic or activated complement subcomponents of C1 and C1 Inh (C1 inhibitor) were analysed by sucrose-density-gradient ultracentrifugation and sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. The interaction of C1 Inh with dimeric C1r in the presence of EDTA resulted into two bimolecular complexes accounting for a disruption of C1r. The interaction of C1 Inh with the Ca2+-dependent C1r2-C1s2 complex (8.8 S) led to an 8.5 S inhibited C1r-C1s-C1 Inh complex (1:1:2), indicating a disruption of C1r2 and of C1s2 on C1 Inh binding. The 8.5 S inhibited complex was stable in the presence of EDTA; it was also formed from a mixture of C1r, C1s and C1 Inh in the presence of EDTA or from bimolecular complexes of C1r-C1 Inh and C1s-C1 Inh. C1r II, a modified C1r molecule, deprived of a Ca2+-binding site after autoproteolysis, did not lead to an inhibited tetrameric complex on incubation with C1s and C1 Inh. These findings suggest that, when C1 Inh binds to C1r2-C1s2 complex, the intermonomer links inside C1r2 or C1s2 are weakened, whereas the non-covalent Ca2+-independent interaction between C1r2 and C1s2 is strengthened. The nature of the proteinase-C1 Inh link was investigated. Hydroxylamine (1M) was able to dissociate the complexes partially (pH 7.5) or totally (pH 9.0) when the incubation was performed in denaturing conditions. An ester link between a serine residue at the active site of C1r or C1s and C1 Inh is postulated.

New synthetic inhibitors of C1r, C1 esterase, thrombin, plasmin, kallikrein and trypsin

p-Guanidinobenzoate derivates were prepared and their inhibitory effects on trypsin, plasmin, pancreatic kallikrein, plasma kallikrein, thrombin, C1r and C1 esterase were examined. Among the various inhibitors tested, 6'-amidino-2-naphthyl-4-guanidinobenzoate dihydrochloride, 4-(beta-amidinoethenyl)phenyl-4-guanidinobenzoate dimethanesulfonate and 4-amidino-2-benzoylphenyl-4-guanidinobenzoate dimethanesulfonate were the most effective inhibitors of trypsin, plasmin, pancreatic kallikrein. plasma kallikrein and thrombin and they strongly inhibited the esterolytic activities of C1r and C1 esterase, and then strongly inhibited complement-mediated hemolysis.

Purification and characterization of two functionally distinct forms of C1 inhibitor from a patient with angioedema

A minority of patients with hereditary angioedema (HAE) have normal concentrations of a dysfunctional C1 inhibitor protein (C1INH) in their plasmas. We purified C1INH from the plasmas of one such patient before and during treatment with the anabolic steroid stanozolol. Both the pretreatment plasma and plasma obtained during stanozolol treatment contained varying amounts of two extremely similar C1INH proteins that were functionally distinct. The pretreatment plasma contained primarily (94%) dysfunctional C1INH that did not inactivate or complex with either purified C1s, activated Hageman factor, or kallikrein and small amounts (6%) of functionally normal C1INH. Stanozolol treatment increased the plasma concentrations of both of these proteins as well as the proportion (23%) of functional C1INH in the plasma. The purified dysfunctional and functional C1INHs had identical or nearly identical molecular sizes, charges, amino acid compositions, and amino sugar contents, and could not be distinguished physicochemically from each other or from normal C1INH. From these studies of purified C1INH proteins we concluded that HAE associated with dysfunctional C1INH is due to a defect at the structural locus for one C1INH gene and that both the dysfunctional C1INH gene and the normal C1INH gene products are present in the plasma of the affected subject. Treatment with stanozolol comparably increased the synthesis of both C1INH proteins. The disproportionate rise in the level of the normal C1INH protein is consistent with the view that it is more rapidly catabolized as a consequence of its interaction with the proteases it inactivates.

The C1q inhibitor in serum is a chondroitin 4-sulfate proteoglycan

An inhibitor of human C1q has been purified from serum and identified as a chondroitin 4-sulfate proteoglycan. A typical preparation contained 22% uronic acid, 20% hexosamine, 12% sulfate, and 9% protein. When chromatographed on Sepharose CL-2B, the proteoglycan was eluted as a broad peak with a mean Kav of 0.6, which indicates that it is polydisperse and has an average Mr = approximately 175,000 (range, 45,000-750,000). Unlike the major species of cartilage proteoglycans, the serum proteoglycan did not form a complex with hyaluronic acid. Additional evidence for the noncartilaginous origin of C1q inhibitor is that its glycosaminoglycan chains totally lack chondroitin 6-sulfate isomers. Furthermore, the glycosaminoglycan component of C1q inhibitor was eluted from Sepharose CL-6B with a Kav of 0.52, indicating that these polysaccharide chains are considerably larger than those of human articular cartilage proteoglycan. The interaction between the proteoglycan and C1q was clearly evident in 0.15 M NaCl, as demonstrated by a radial immunodiffusion technique. The interaction decreased with increasing ionic strength but was not entirely abolished even at 0.3 M NaCl. These findings suggest that the interaction between C1q and the C1q inhibitor may occur under physiological conditions and may be of importance in modulating C1q activity in vivo.

The human complement system: assembly of the classical pathway C3 convertase

The assembly of the classical pathway C3 convertase in the fluid phase has been studied. The enzyme is assembled from C2 and C4 on cleavage of these proteins by C1s. Once assembled, the enzyme activity decays rapidly. Kinetic evidence has been obtained that this decay is even more rapid than previously suggested (kdecay is 2.0 min-1 at 37 degrees C). As a result, optimal C3 convertase activity is only observed with high C1s levels, which result in rapid rates of cleavage of C2 and increased rates of formation of the C3 convertase. Using high concentrations of C1s at lower temperatures (22 degrees C) in the presence of excess substrate we have demonstrated kinetically that the enzyme comprises an equimolar complex of C4b and cleaved C2. We have obtained direct evidence from gel-filtration experiments for the role of C2a as the catalytic subunit of the enzyme. C2b appears to mediate the interaction between C4 (or C4b) and C2 at pH 8.5 and at low ionic strength where the interactions can easily be detected. It may therefore be important in the assembly of the enzyme, though it is not involved in the catalytic activity. The decay of the C3 convertase reflects the release of C2a from the C4b x (C2b) x C2a complex, and the stabilizing effect of iodine on the C3 convertase is therefore apparently one of stabilizing the C4b-C2z interaction, which is otherwise weak. C1s is not a part of the C3 convertase enzyme.

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.

Inhibition of C5 convertase by epsilon amino caproic acid (EACA): a limiting factor in the generation of C5a anaphylatoxin

The effect of EACA on spontaneous C5 consumption and on the mechanisms of C5 activation through the alternative pathway of complement were analyzed. In contrast to the effect upon C3, ADDITION OF 1 M EACA to normal human serum (NHS) did not enhance spontaneous consumption of C5, but augmented its haemolytic efficiency. In the presence of 1 M EACA, activation of serum with zymosan (Z), resulted in inhibition of the formation of C5 convertase, and thus of C5 cleavage and C5a anaphylatoxin release. The generation of a C5 convertase on activated zymosan (Z) was inhibited when Z was formed at 17 degrees with NHS containing 1 M EACA. In contrast, if Z was formed in the absence of EACA the C5 convertase could be regenerated with fresh serum containing 1M EACA. Nearly maximal C5 consumption was obtained if 5 mM EGTA and 10 mM MgCl2 were added to the mixture. These conditions are considered optimal for generation and maximum recovery of fully active anaphylatoxin.

Modulation of the formation of the amplification convertase of complement, C3b, Bb, by native and commercial heparin

Native rat mast cell macromolecular heparin proteoglycan and commercial hog heparin glycosaminoglycan chains inhibit generation of the amplification convertase, C3b, Bb. The inhibitory action of heparin is not due to chelation of magnesium. Heparin is most active in inhibiting convertase formation on cellular intermediates formed with the lowest C3b input and developed with the highest B concentration, thereby suggesting the receptor site for B on C3b as the point of heparin action. This interpretation is consistent with the demonstration that heparin prevents B utilization during the fluid phase interaction of C3b, B, and D. Inhibition is observed also when C3b,Bb generation takes place on cellular intermediates in the presence of P or C3NeF, which yield stabilized forms of the convertase. 50 times the concentration of heparin required to inhibit convertase generation does not accelerate the decay of the unstabilized or the C3NeF-stabilized convertases and has only a modest effect on the P-stabilized convertase. An additional effect of heparin is to impair beta1H-mediated decay-dissociation of C3b,Bb. The concentration of native or commercial heparin which prevents convertase formation is in the same range as that required for the demonstration of its anti-coagulant and anti-thrombin III cofactor activities. The additional finding that this inhibitory action of heparin can be expressed by the isolated mast cell granule suggests that native heparin may contribute to the modulation of the amplification pathway of complement.

Synthetic inhibitors of trypsin, plasmin, kallikrein, thrombin, C1r-, and C1 esterase

p-Carbethoxyphenyl episol-guanidinocaproate and p-(p'-guanidinobenzoyloxy)-phenyl derivatives were prepared, and their inhibitory effects on trypsin, plasmin, plasma kallikrein, thrombin, C1r- and C1 esterase were examined. Among the various inhibitors tested, p-nitrophenyl p'-guanidinobenzoate, N,N-dimethylamino p-(p'-guanidinobenzoyloxy)-benzoyl glycolate and N,N-dimethylamino p-(p'-guanidinobenzoyloxy)-benzilcarbonyloxy glycolate were the most effective inhibitors of trypsin, plasmin, plasma kallikrien and thrombin, and they strongly inhibited the esterolytic activities of C1r- and C1 esterase.