Studies on the precursor form of the first component of complement--I. Isolation of the proenzyme forms of Clr and Cls

Inorganic supports coated with N-substituted polyacrylamides: application to biospecific chromatography of proteins

Wide porous glass (WPG) chemically coated with a poly-N-(2-hydroxyethyl)acrylamide layer is proposed as a carrier of biospecific ligands in affinity chromatography. The method of WPG chemical modification includes synthesis of the gamma-aminopropyl derivative followed by chemical adsorption of poly(p-nitrophenyl acrylate). Ester groups of the polyacrylate-coated WPG can be used for coupling the ligands bearing primary amino groups. Condensation of esters with ethanolamine yields a poly-N-(2-hydroxyethyl)acrylamide-coated support with non-specific adsorption properties resembling those of Sepharose 4B. Human IgG immobilized on the polyacrylate support was used for isolation of the first complement component from human serum and for its separation into subcomponents C1r, C1s and C1q by a one-step method. An unbound part of serum may be used as the R1 reagent for determining haemolytic C1 activity. The stepwise elution of C1r, C1s and C1q from the column reflects the course of C1 breakdown after its activation on immune complex formation.

Human complement component C1s. Partial sequence determination of the heavy chain and identification of the peptide bond cleaved during activation

Human C1s proenzyme (Mr 83 000) was isolated by a rapid two-stage method involving affinity chromatography of C1 on IgG-Sepharose and isolation of subcomponent C1s by ion-exchange chromatography on DEAE-Sephacel. Single-chain C1s proenzyme was activated to two-chain C1s with self-activated C1r. After reduction and S-carboxamidomethylation the heavy chain of C1s (Mr 57 000) was isolated by ion exchange chromatography on DEAE-Sephacel. Cleavage of C1s heavy chain with CNBr yielded five fragments whose N-terminal sequences were determined. The alignment of the fragments within the heavy chain was established by tryptic peptides containing methionine. C1s heavy chain comprises about 470 amino acid residues and 42% of its sequence was determined. An intrachain sequence homology and a homology to the alpha 2 chain of human haptoglobin were identified. The C-terminal CNBr fragment comprising 44 amino acid residues was completely sequenced. From BNPS-skatole cleavage of reduced and alkylated C1s proenzyme a fragment was isolated which overlaps the C1s heavy and light chain parts and which contains the peptide bond cleaved during activation. The results show that this is an Arg-Ile bond and that under standard conditions of activation no peptide material is liberated from this portion of the molecule. The sequence data and homology to two-chain serine proteases indicate a single interchain disulfide bond in C1s.

C1 dissociation in serum: estimation of free C1q by electroimmunoassay

A two-stage electroimmunoassay was developed for measuring macromolecular C1 (C1qrs) and free C1q. The method was based on Ca2+ dependent fixation of C1qrs to agarose, followed by immune precipitation of dissociated C1s in the presence of EDTA. Free C1q was estimated from the increase in C1qrs resulting from saturation of C1q in the samples with purified C1r-C1s. The assay system was studied under various experimental conditions. Combined analysis by electroimmunoassay and crossed immunoelectrophoresis indicated that part of the free C1q in undiluted normal serum could be attributed to physiological C1 activation. Owing to concentration dependent C1qrs dissociation the proportion of free C1q increased with the dilution of serum. Results obtained with serum and with purified C1qrs were consistent with the formation of an equimolar C1q:C1r-C1s complex. However, the capacity for C1r-C1s binding appeared to be higher in the purified system than in serum. Serum concentrations of free C1q were high in some of the patients with disease conditions characterized by increased C1 activation, such as systemic lupus erythematosus or primary biliary cirrhosis.

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.

A new simple method for determination of C1-esterase inhibitor activity in plasma

A convenient method for the determination of C1-esterase inhibitor activity in plasma samples is described. The method is based on addition of purified C1s to plasma and measuring excess C1s with a new chromogenic tripeptide-p-nitroanilide substrate with a recording spectrophotometer. Addition of C1s to C1-esterase inhibitor-depleted plasma did not result in any appreciable inactivation of the enzyme for three hours. The concentration of C1-esterase inhibitor in 19 healthy individuals was estimated as 1.63 +/- 0.27 (SD) mumol/l. The correlation with C1-esterase inhibitor antigen in these individuals and 19 patients with varying concentrations of C1-esterase inhibitor was excellent. The correlation with an antikallikrein assay was found to be poor.

Purification and characterization of human C1-esterase inhibitor

A new purification method for C1-esterase inhibitor is described, which is essentially a three-step procedure: precipitation with poly(ethylene glycol), chromatography on DEAE-cellulose and hydrophobic interaction chromatography on hexyl-Sepharose. The final product is a single-chain glycoprotein with a molecular weight of about 100 000 and NH2-terminal asparagine. The molecule is fully active as judged by complex formation with C1s. Two of its three disulphide bridges can be easily reduced and S-carboxymethylated under non-denaturing conditions without loss of activity. However, at high dithioerythritol concentration the third disulphide bridge is also cleaved and accompanied by loss of the activity, indicating that this disulphide bridge is involved in maintaining the conformation around the reactive site in the inhibitor.

A study on the structure and interactions of the C1 sub-components C1r and C1s in the fluid phase

1. Both proenzyme and activated C1r, which are dimers at pH 7.4, dissociated into monomers at pH 5.0 (C1r) and 4.0 (C1r), as shown by the decrease of apparent molecular weight and of sedimentation coefficient, which was shifted from 7.1 S (dimer) to 5.0 S (monomer). 125I-labelling of C1r in the presence of lactoperoxidase occurred, for the dimer, 16-20% in the A chain and 80-84% in the B chain, whereas the distribution was 67.5% and 32.5%, respectively, for the monomer. It appears likely that the two monomers of C1r interact through their A chain and that the A and B chains are relatively independent from each other. 2. 125I-labelling of C1s in the presence of lactoperoxidase confirmed the calcium-dependent dimerization of this subcomponent. In the monomer, the B chain appears to be embedded in the A chain, as shown by the 125I- distribution in these chains, which was 5% and 95%, respectively. This changed after dimerization to 25% and 75%, respectively, which suggests that interactions occur through the A chain of each monomer and lead to an unfolding of the B chain. 3. C1r dimer and C1s monomer were found to interact in the absence of calcium to form a C1r2-C1s complex (7.7 S), whereas in the presence of calcium the two sub-components were associated into a C1r2-C1s2 complex (8.7S). It appears likely that the formation of this tetrameric complex involves both calcium-dependent, and calcium-independent binding forces, and that C1r and C1s interact through their respective A chain which, in the case of C1s, is hidden upon association.

Inhibition of immune complex-mediated activation of complement. Effects of agents modulating activation of, and the activated C1 complex

Several known chemical compounds were shown to selectively inhibit the interaction between immune aggregates and C1q, the activation of C1r-C1s complex by immune aggregate-bound C1q, and the esterolytic activity of the activated C1s, C1s. These reactions are relevant to the functions of the first complement component, C1, and its activation induced by immune complexes. The effects of these inhibitors on tissue injury mediated by immune complex-induced complement activation, such as immune hemolysis, passive cutaneous anaphylaxis, and experimental glomerulonephritis were examined. The results suggest an approximate correlation between the activity shown on the molecular level and that obtained in vivo. One such compound, suramin, was shown to be an effective inhibitor of PCA and the proteinuria manifestation of EGN while not affecting antibody fixation to tissue or histamine-mediated skin reaction. These results suggest that effective suppression of the initial steps of complement activation may be of value of controlling immune complex-mediated tissue injuries in disease.

Purification of the first component of guinea-pig complement by gel chromatography

A method has been described for the purification of the first component (C1) of complement from guinea-pig serum. The procedure consists in euglobulin precipitation followed by gel filtration on agarose columns. The final product has low protein content and high specific activity. The protein obtained by this procedure has a molecular weight of about one million and has been further characterized using immunochemical and polyacrylamide gel electrophoresis techniques. The protein reacts with anti-C1 antiserum and forms the EAC1, 4-intermediate. The experiments indicated the existence of electrophoretic variants of C1. The dissociation of the C1 molecule by increasing the ionic strength is confirmed. The described procedure appears to be a useful method of obtaining functionally purified C1.