The Action of Ammonia on Complement. The Fourth Component

Plasma complement activation mechanisms differ in ornate (Terrapene ornata ornata) and eastern box turtles (Terrapene carolina carolina)

Eastern (Terrapene carolina carolina) and ornate (Terrapene ornata ornata) box turtles have robust plasma antibacterial activity, however, the mechanism behind this activity is unknown. We used sheep red blood cell (SRBC) hemolysis assays, mannan-affinity chromatography, sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), and matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) to explore the mechanisms of complement activity in box turtles. Plasma from both species demonstrated volume, time, and temperature-dependent SRBC hemolysis, with significantly greater hemolytic activity in ornate box turtle plasma. Hemolytic activity was highly attenuated following treatment with heat, EDTA, and salicylaldoxime in both species, but was unchanged after treatment with methylamine and ammonium hydroxide. Two abundant mannan-binding proteins (presumed C-type lectins) were identified in eastern box turtle plasma using SDS-PAGE and MALDI-TOF, but ornate box turtles did not express either protein. Eastern box turtles appear to rely on the lectin pathway of complement activation while ornate box turtles utilize the alternative pathway. This study provides further evidence that mechanisms underlying immune function are not always conserved between closely related species. This finding may have important implications for explaining species differences in susceptibility to emerging threats such as disease, toxicants, and climate change.

The covalent binding story of the complement proteins C3 and C4 (I) 1972-1981

Alex Law and Paul Levine recall their work to establish the covalent bond between C3 and target surfaces. It started with a naive experiment by analyzing the membrane polypeptides of sheep erythrocytes bound with ¹²⁵I-labelled C3. They found complexes with molecular weight higher than the individual C3 polypeptides. These complexes survived all conditions designed to disrupt non-covalent interactions. They then showed that the bond was an ester, with an active acyl group on C3 which reacted with a hydroxyl group on the acceptor molecule. With the discovery of an internal thioester by Jim Prahl, Jamila Janatova, Brian Tack and their colleagues, it became clear that the reaction was by an acyl transfer from the thioester of C3 to the target hydroxyl group. Later on they showed that C4 also bound covalently to target molecules. By establishing a fluid phase system to study the kinetics of the binding reactions of C3 and C4, Alex was able to continue the work in the MRC Immunochemistry Unit in Oxford from 1981, to eventually determine the chemical mechanism of the binding reaction. In order to give some sense of reality, this article is written as a narrative from Alex, who did the experiments. Both Alex and Paul are retired. Pauls lives on Martha's Vineyard where he writes occasional articles on science for one of the Island's newspapers. Alex lives in Hong Kong and tries to make some sense of the local politics.

Experimental confirmation of the C3 tickover hypothesis by studies with an Ab (S77) that inhibits tickover in whole serum

The complement component 3 (C3) tickover hypothesis was put forward in the early 1970s to account for the spontaneous activation of the alternative complement pathway that occurs after the genetic absence or in vitro depletion of Factor I, the enzyme that is essential for the breakdown of C3b. The hypothesis was widely accepted, but experimental demonstration of the tickover was elusive. A phage Ab against C3b that inhibited the alternative complement pathway, but not the classical pathway, was described in 2009. Studies using this Ab in a variety of assays have now demonstrated that it acts primarily by inhibiting tickover, thereby confirming that tickover really exists.-Lachmann, P. J., Lay, E., Seilly, D. J. Experimental confirmation of the C3 tickover hypothesis by studies with an Ab (S77) that inhibits tickover in whole serum.

Complement in health and disease

The complement system consists of about 35-40 proteins and glycoproteins present in blood plasma or on cell surfaces. Its main biological function is to recognise "foreign" particles and macromolecules, and to promote their elimination either by opsonisation or lysis. Although historically complement has been studied as a system for immune defence against bacteria, it has an important homeostatic role in which it recognises damaged or altered "self" components. Thus complement has major roles in both immune defence against microorganisms, and in clearance of damaged or "used" host components. Since complement proteins opsonise or lyse cells, complement can damage healthy host cells and tissues. The system is regulated by many endogenous regulatory proteins. Regulation is sometimes imperfect and both too much and too little complement activation is associated with many diseases. Excessive or inappropriate activation can cause tissue damage in diseases such as rheumatoid arthritis, age-related macular degeneration (AMD), multiple sclerosis, ischemia-reperfusion injury (e.g. ischemic stroke). Insufficient complement activity is associated with susceptibility to infection (mainly bacterial) and development of autoimmune disease, like SLE (systemic lupus erythematosus).

The conglutination phenomenon: VII. A study of the interaction of complement components and conglutinin in the process of conglutination

Techniques for the fractionation of C′ 1 and C′ 2, and for the specific inactivation of C′ 4 of horse complement are described, and shown to be satisfactory. These three components of complement and also conglutinin are found to be essential to the process of conglutination.

The experiments reported do not exclude the possibility of additional or unidentified fractions of horse serum playing a part in the reaction. Whether or not C′ 3 is essential to the process of conglutination could not be determined from the evidence available.

It was found that by the procedures employed the C′ 4 fraction may be supplied either from the horse complement or from the heated bovine serum added as a source of conglutinin or from both.

When horse complement and bovine antibody against sheep cells are used it is found that conglutination will only result when the complement components are absorbed on to the immune complex in a particular sequence. The sequence is that C′ 1 is absorbed first on to the sensitized cells, and then C′ 2 and C′ 4 are absorbed together. Both these components must be presented together to the sensitized cells carrying C′ 1 if conglutination is to result. Finally, conglutinin acts on the sensitized cells which have absorbed the three complement components, and conglutination results.

The significance of these findings, together with matters of a more general nature, is discussed.

Complement before molecular biology

Complement activity was first described in the 1890s and the characterisation of this highly complex system has continued ever since. This review traces the history of complement research from its beginnings until it was transformed by the advent of molecular biology in the 1980s. It takes as a focus point the CIBA symposium on Complement held in London in May 1964 and reflects-and is slanted by-the views and research experience of the author.

Complement: A nostalgic journey

The scientific career and research contributions of Hans J. Müller-Eberhard to the field of complement research are presented in historical context, and interpreted with regard to the state of the field and the research technologies available when the contributions were made.

Identification of alternative pathway serum complement activity in the blood of the American alligator (Alligator mississippiensis)

Incubation of different dilutions of alligator serum with sheep red blood cells (SRBCs) that had not been sensitized with antibodies resulted in concentration-dependent hemolytic activity. This hemolytic activity was not affected by the presence of ammonium hydroxide and methylamine, known inactivators of the classical complement cascade. However, the hemolytic activities were inhibited by EDTA and salicylaldoxime, indicating that the alternate pathway is primarily responsible for these activities. Immunofixation of electrophoretically-resolved alligator serum proteins with antihuman C3 polyclonal antibodies resulted in detection of a protein antigenically similar to human C3 in alligator serum. SDS-PAGE, followed by Western blot analysis, revealed the presence of two alligator serum proteins with nearly identical molecular weights as human C3alpha and C3beta. SRBC hemolysis and antibacterial activity by alligator serum was significantly reduced in the presence of antihuman C3 antibodies. The hemolytic effect of alligator serum was shown to occur rapidly, with significant activity within 5 min and maximal activity occurring at 15 min. SRBC hemolysis was also temperature-dependent, with reduced activity below 15 degrees C and above 30 degrees C. These data suggest that the antibiotic properties of alligator serum are partially due to the presence of a complement-facilitated humoral immune response analogous to that described in mammalian systems.

Dancing with complement C4 and the RP-C4-CYP21-TNX (RCCX) modules of the major histocompatibility complex

The number of the complement component C4 genes varies from 2 to 8 in a diploid genome among different human individuals. Three quarters of the C4 genes in Caucasian populations have the endogenous retrovirus, HERV-K(C4), in the ninth intron. The remainder does not. The C4 serum proteins are highly polymorphic and their concentrations vary from 100 to approximately 1000 microg/ml. There are two distinct classes of C4 protein, C4A and C4B, which have diversified to fulfill (a) the opsonization/immunoclearance purposes and (b) the well-known complement function in the killing of microbes by lysis and neutralization, respectively. Many infectious and autoimmune diseases are associated with complete or partial deficiency of C4A and/or C4B. The adverse effects of high C4 gene dosages, however, are just emerging, as the concepts of human C4 genetics are revised and accurate techniques are applied to distinguish partial deficiencies from differential expression caused by unequal C4A and C4B gene dosages and gene sizes. This review attempts to dissect the sophisticated genetics of complement C4A and C4B. The emphases are on the qualitative and quantitative diversities of C4 genotypes and phenotypes. The many allotypic variants and the processed products of human and mouse C4 proteins are described. The modular variation of C4 genes together with the serine/threonine nuclear kinase gene RP, the steroid 21-hydroxylase CYP21, and extracellular matrix protein TNX (RCCX modules) are investigated for the effects on homogenization of C4 protein polymorphisms, and on the unequal genetic crossovers that knocked out the functions of CYP21 and/or TNX. Furthermore, the influence of the endogenous retrovirus HERV-K(C4) on C4 gene expression and the dispersal of HERV-K(C4) family members in the human genome are discussed.

The internal thioester and the covalent binding properties of the complement proteins C3 and C4

The covalent binding of complement components C3 and C4 is critical for their activities. This reaction is made possible by the presence of an internal thioester in the native protein. Upon activation, which involves a conformational change initiated by the cleavage of a single peptide bond, the thioester becomes available to react with molecules with nucleophilic groups. This description is probably sufficient to account for the binding of the C4A isotype of human C4 to amino nucleophiles. The binding of the C4B isotype, and most likely C3, to hydroxyl nucleophiles, however, involves a histidine residue, which attacks the thioester to form an intramolecular acyl-imidazole bond. The released thiolate anion then acts as a base to catalyze the binding of hydroxyl nucleophiles, including water, to the acyl function. This mechanism allows the complement proteins to bind to the hydroxyl groups of carbohydrates found on all biological surfaces, including the components of bacterial cell walls. In addition, the fast hydrolysis of the thioester provides a means to contain this very damaging reaction to the immediate proximity of the site of activation.

ISOLATION AND DESCRIPTION OF THE FOURTH COMPONENT OF HUMAN COMPLEMENT

Purification of the activity of the fourth component of human complement resulted in the isolation of a highly homogeneous serum protein. Since this protein has not been recorded previously it was called beta(1E)-globulin on the basis of its immunoelectrophoretic behavior. C'4 activity and beta(1E)-globulin were found to have highly similar, if not identical physicochemical characteristics. Moreover, beta(1E)-globulin was shown to exhibit the specific behavior of C'4 activity in that it is taken up only by cells which contain activated C'1. DFP-inactivated C'1 failed to catalyze uptake of the protein. Treatment with hydrazine which is known to destroy C'4 activity, led to changes in the physicochemical properties of beta(1E)-globulin and rendered the molecule incapable to combine with C'1-containing cells. The evidence indicates that beta(1E)-globulin represents the fourth component of human complement.

Limulus alpha 2-macroglobulin. First evidence in an invertebrate for a protein containing an internal thiol ester bond

Intra-chain thiol ester bonds are present in a limited number of proteins. The thiol ester class of proteins includes vertebrate alpha 2-macroglobulin and the complement proteins C3 and C4. We report here the first instance of a thiol ester protein from an invertebrate, the alpha 2-macroglobulin proteinase-inhibitor homologue present in the plasma of the American horseshoe crab Limulus polyphemus. Our evidence is of three kinds: (1) the proteinase-binding activity of Limulus alpha 2-macroglobulin is inactivated by the low-molecular-mass primary amine methylamine; (2) the native protein is subject to autolytic fragmentation during mild thermal denaturation, yielding fragments of approx. 125 kDa and 55 kDa, whereas the methylamine-treated protein is stable under these conditions of thermal treatment; (3) new thiol groups are generated rapidly during reaction of the protein with trypsin. The demonstration of the thiol ester bond in a protein from an ancient invertebrate provides evolutionary evidence for the importance of this bond in the function of plasma forms of the alpha 2-macroglobulin-like proteinase inhibitors.

The molecular genetics of components of complement

Rapid progress has been made in establishing linkages and in chromosome allocation of the genes of some 9 complement components. In the MHC, C2, Factor B, and two C4 or C4 related genes have been placed in some detail in both man and mouse. The gene coding for the cytochrome P-450 21-hydroxylase has been shown to be duplicated and immediately 3' to the two C4 genes, though it appears to be functionally and structurally unrelated to the complement components. Thus six genes have been mapped to this region where particular haplotypes are associated with increased susceptibility to a number of diseases, some of which are autoimmune in character. The complete gene structure of Factor B has been solved in man and rapid progress is being made with the C2 and C4 genes. The structural basis of the polymorphisms of these genes is being established. In C4, the polymorphism is exceptionally complex with varying numbers of loci and probably more than 50 allotypes occurring in man. A structural basis has also been found for the big differences in the biological activity of some of the C4 allotypes in man. Apart from the genes in the MHC, linkage has been found between the genes coding for C4bp, CR1, and Factor H. Remarkably there are sequence homologies between these proteins and C2 and Factor B, probably related to the ability to bind to one or other of the structurally similar proteins C3b and C4b. The complete cDNA sequences of C3 and C4 in mouse and man have given much information on the many posttranslational modifications of these proteins. A partial structure has been obtained for the C3 gene and the homology shown between C3, C4, C5, alpha 2-macroglobulin, and pregnancy zone protein. Although the amount of detailed information in the molecular genetics of complement components is accumulating rapidly, there appears to be a reasonable prospect that linkages and homologies will classify the data into a comprehensible form.

Ammonia: its effects on biological systems, metabolic hormones, and reproduction

The physical, chemical properties of ammonia, its sources and detoxification, its effects in biological systems, its influence upon insulin action and glucose metabolism, and its possible effects on reproduction are discussed. Present chemical methods do not distinguish nonionic from ionic forms. At physiological pH, nonionic ammonia concentrations remain low but are primarily responsible for toxic effects. Thus, biologically significant changes of ammonia concentrations may not be revealed by determinations of ammonia in blood plasma. For these and other reasons the subacute toxicity of ammonia often is unrecognized, and its effects on intermediary metabolism and the hormonal milieu in normal and disease states remain poorly understood. Effects of ammonia may be stimulatory at low concentrations and inhibitory as concentrations rise or exposure is extended. Extensive experiments in eight ureotelic species, including man, show that urinary excretion of orotic acid becomes significantly elevated when the quantity of ammonia presented to the liver exceeds the capacity for normal detoxification. Present evidence with arginine and other intermediates of the urea cycle suggest that these substances influence glucose metabolism and insulin action. Recent studies of dairy cattle provide speculative evidence that high protein feeding or forms of protein that lead to elevated ammonia concentrations in tissue decrease conception rates and increase the calving to conception interval of dairy cows. Limited data concerning luteinizing hormone concentrations and steroid hormone metabolism are insufficient to establish whether differences in reproductive performance are due to changes of hormonal physiology, intrauterine environment, or metabolism.

Autolytic fragmentation of complement components C3 and C4 and its relationship to covalent binding activity

The autolytic cleavage reaction of C3 and C4 and the covalent binding reaction of these proteins, are both aspects of the reactivity of an activated thiolester within these proteins. Autolytic cleavage occurs by internal nucleophilic attack on one face of the planar thiolester, while the covalent binding reaction of the activated proteins follows exposure of the opposite face of the thiolester to attack by external nucleophiles. Although the autolytic cleavage reaction does not occur under physiological conditions, the study of this phenomenon has provided valuable evidence in support of the mechanisms postulated for the physiological covalent binding reactions. The ease with which autolysis can be induced and observed in C3, C4, and alpha 2 M has provided a valuable method for detecting the active forms of these proteins in circumstances where other assays are impracticable, as, for example, in the examination of the uptake of active C3 by lymphocytes. Autolytic cleavage has also been used by Karp and colleagues to produce fragments used in characterizing genetic and biosynthetic variants of mouse C4 and the mouse protein Slp, which is structurally similar to C4. Gross structural comparisons made among C3, C4, and alpha 2 M on the basis of alignment of the autolytic cleavage sites and the protease-activation sites in these proteins were useful in predicting how the alpha-, beta-, and gamma-chains of C4, or the alpha- and beta-chains of C3, were aligned in the single polypeptide chain pro-forms of these proteins. The beta-alpha-gamma alignment deduced for C4 was also found by Goldberger and Colten. Similar alignments of cleavage sites have been used as a basis for evolutionary comparisons of complement proteins and alpha 2 M from species other than man. Although autolytic cleavage has been described only for C3, C4, alpha 2 M, and Slp, it is likely that other proteins will be found that exhibit this phenomenon. A possible candidate is pregnancy-associated plasma protein A (PAPP-A) which resembles alpha 2 M in many respects. The autolytic cleavage reaction will serve as a useful indicator in the detection of other proteins that undergo covalent binding by the mechanism discussed above.

Bactericidal and bacteriolytic activity of serum against gram-negative bacteria

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Evolution of alpha 2-macroglobulin. The purification and characterization of a protein homologous with human alpha 2-macroglobulin from plaice (Pleuronectes platessa L.) plasma

A papain-binding protein (PB-protein) was purified to homogeneity from the plasma of plaice (Pleuronectes platessa L.). PB-protein inhibited the activity of trypsin and pancreatic elastase (serine proteinases), thermolysin (a metalloproteinase) and papain (a cysteine proteinase). Presaturation of PB-protein with trypsin prevented the subsequent inhibition of thermolysin, and vice versa. Only catalytically active endopeptidases were bound by PB-protein. The catalytic activity of trypsin bound by PB-protein was inhibited by 95% against an insoluble protein substrate, but only by 38% against a low-molecular-weight synthetic substrate. The remaining activity of the bound trypsin was partially protected against further inhibition by soya-bean trypsin inhibitor. Trypsin bound by PB-protein showed a decrease of 67% in its reactivity with antibodies. The inhibitory activity of PB-protein was inactivated at pH 8.0 by methylamine (0.2M) or dithiothreitol (1 mM). The inhibition of proteinases by plaice PB-protein shows the distinctive characteristics of inhibition by human alpha 2-macroglobulin, and it is concluded that the plaice protein is a homologue of the human macroglobulin.

Sequence determination of the thiolester site of the fourth component of human complement

The fourth component of complement (C4) is inactivated by treatment with methylamine. This property is shared wit the third component (C3) and with alpha 2-macroglobulin. In each instance, the reaction with methylamine is stoichiometric, covalent, and accompanied by the appearance of a thiol group. These data are consistent with the presence of an internal thiolester bond. Incubation of C4 with [14C] methylamine in the presence of activated thiol-Sepharose resulted in immobilization of the protein via its active-site thiol. Analysis of bound C4 indicated incorporation of 1.12 mol of [14C]methylamine per mol of protein. Digestion of the immobilized protein with porcine elastase resulted in the release of C4 beta- and gamma-chains and lower molecular weight fragments. The 14C label, however, was retained on the Sepharose beads. Subsequent release of bound material with L-cysteine indicated that the radiolabel was associated with two polypeptides of Mr 25,000 [C4d(ela25)]. The released material was dialyzed and the active-site thiol was radioalkylated with iodo[2-3H]acetic acid. C4d(ela25) was further purified by chromatography on Sephadex G-100 and, after reduction and alkylation, on Sepharose CL-6B in 0.2% NaDodSO4. The C4d(ela25) pool, containing 0.83 mol of [14C]methylamine per mol of iodo[2-3H]acetic acid, was subjected to automated sequence analysis. S-carboxy-[3H]methylcysteine was released at step 21 and gamma-glutamyl-[14C]methylamide was released at step 24. The recovery of radiolabel at positions 21 and 24 confirmed the originally calculated 14C/3H incorporation ratio and further indicated that the radiolabels were present at single sites in the C4 molecule. Comparison of the derived primary structure for the thiolester site in C4 with those for the corresponding regions in C3 and alpha 2-macroglobulin has shown sequence identity. Further comparisons among these three proteins have indicated additional homologies on both the NH2- and COOH-terminal sides of the thiolester site.

Amino acid sequence around the thiol and reactive acyl groups of human complement component C4

Activation of the fourth component of complement (C4) by C1s results in the generation of a reactive acyl group, able to react with putrescine, and in the release of a free thiol group that cannot be detected in the native haemolytically active molecule. Both the reactive acyl group and the free thiol group have been shown to reside in C4d, a fragment of the alpha'-chain of C4b derived from digestion of the molecule with the control proteins C3b inactivator and C4-binding protein. Peptides derived from CNBr digestion of [1,4-14C]putrescine-labelled and iodo(2-14C]acetic acid-labelled C4d have been obtained and used to establish a continuous sequence of 88 residues from the N-terminus of the molecule. The thiol and reactive acyl groups are contained in an octapeptide that shows near identity with the equivalent sequences reported for alpha 2-macroglobulin and C3. Other adjacent short sections also show homology of sequence between the three proteins, and it is highly likely that they contribute to the overall structure that gives a unique reactivity to the thiol ester bond postulated to exist in the native forms of the three proteins.

Studies on the possible involvement of complement component C3 in the initiation of acid hydrolase secretion by macrophages. I. Correlation between enzyme-releasing and complement-activating capacities of several secretagogues

A possible relationship between activation of the alternative pathway of complement and acid hydrolase secretion by macrophages has been investigated in vitro by examining the dose--response characteristics of several immunological and non-immunological stimuli of these two processes. Zymosan particles, insoluble immune complexes, methylamine and several other primary aliphatic monoamines were all found to elicit the selective release of lysosomal enzymes from macrophages by a process that correlated well with the ability of these agents to bring about consumption of haemolytically-active components of the alternative complement pathway. By contrast, substances which failed to activate the alternative complement pathway, i.e. soluble aggregated immunoglobulin and several primary aliphatic diamines, were found to be likewise incapable of inducing the selective release of lysosomal glycosidases from macrophages. These observations are interpreted as further evidence for imputing a role for complement C3 in the initiation of lysosomal enzyme release from macrophages.

Treatment of human complement components C4 and C3 with amines or chaotropic ions. Evidence of a functional and structural change that provides uncleaved C4 and C3 with properties of their soluble activated froms, C4b and C3b

Treatment of human components C4 and C3 with amines like hydrazine, ammonium hydroxide, and neutral ammonium salts or with chaotropic salts like KSCN and NaBr leads to complete loss of haemolytic activity. The pretreated components are, however, still active in formation of soluble C3 convertases. This activity pattern is reminiscent of the activities of C4 and C3 that have been activated by cleavage in the fluid phase. Indeed, the antigenic properties of pretreated C4 and C3 are similar to soluble C4b and C3b. The polypeptide chain structure of pretreated C4 and C3, is, however, identical to that of the untreated components when investigated by SDS gel electrophoresis. Pretreatment even reduces greatly the susceptibility of C4 to cleavage by C1s and of C3 to cleavage by classical and alternative pathway C3 convertases. Pretreated components have lost the ability to combine with EAC1 and EAC142, respectively; this fact explains their failure to exhibit haemolytic activity. In serum, pretreated C4 and C3 are cleaved in a manner similar to C4b and C3b. Amines and chaotropic ions cause the same functional and structural alterations, which are best explained by assumption of a conformational change. A similar transformation can also occur in C4 and C3 during preparation or storage.

Interaction between the labile binding site of human C4 and methylamine

Human complement component C4 was irreversibly inactivated by low concentrations of methylamine at slightly alkaline pH. The inactivated C4 molecules lost the ability to bind to EAC1 cells but retained th capacity to participate in the formation of classical pathway C3 convertase in the fluid phase. 14C-methylamine was incorporated into the alpha-chain at a ratio of 1 mol methylamine per mol C4.

Autolytic fragmentation of complement components C3 and C4 under denaturing conditions, a property shared with alpha 2-macroglobulin

The alpha polypeptide chain of the complement protein C3 splits into two fragments of 74 000 and 46 000 apparent mol.wt. under certain conditions used to prepare the protein for SDS (sodium dodecyl sulphate)/polyacrylamide-gel electrophoresis. The cleavage reaction occurs over a wide range of temperatures and from pH 4.6 to 10.6 in the presence of denaturants such as urea, SDS and guanidine hydrochloride. It is also induced by heat-denaturation of C3 in the absence of chemical denaturants. The reaction occurs only with haemolytically active C3, and is not observed with hydroxylamine-inactivated C3 or with C3b. A similar cleavage of the alpha-chain of complement component C4 occurs under the same conditions, forming fragments of 53 000 and 41 000 apparent mol.wt. This reaction is again specific for haemolytically active C4, and does not occur with C4b or hydroxylamine-inactivated C4. The complement component C5, although structurally similar to C3 and C4, does not undergo a reaction of this type. The characteristics of the denaturation-induced cleavage of C3 and C4 match those described for the 'heat-induced' cleavage of alpha 2-macroglobulin [Harpel, Hayes & Hugli (1979) J. Biol. Chem. 254, 8669-8678]. Cleavage of alpha 2-macroglobulin is also specific for the active form of the protein, and does not occur with chemically inactivated or proteinase-cleaved forms. The unusual conditions and specificity of the peptide-bond cleavage in all three proteins suggest that it is an autolytic process rather than being the result of trace proteinase contamination. The active forms of C3, C4 and alpha 2-macroglobulin have the transient ability to form covalent bonds after activation. The autolytic cleavage reaction is likely to be related to the covalent-bond-forming reactions of these proteins.

The binding of human complement component C4 to antibody-antigen aggregates

The binding of human complement component C4 to antibody-antigen aggregates and the nature of the interaction have been investigated. When antibody-antigen aggregates with optimal C1 bound are incubated with C4, the C4 is rapidly cleaved to C4b, but only a small fraction (1-2%) is bound to the aggregates, the rest remaining in the fluid phase as inactive C4b. It has been found that C4b and th antibody form a very stable complex, due probably to the formation of a covalent bond. On reduction of the C4b-immunoglobulin G (IgG) complex, the beta and gamma chains, but not the alpha' chain, of C4b are released together with all the light chain, but only about half of the heavy chain of IgG. The reduced aggregates contain two main higher-molecular-weight complexes, one shown by the use of radioactive components to contain both IgG and C4b and probably therefore the alpha' chain of C4b and the heavy chain of IgG, and the other only C4b and probably an alpha' chain dimer. The aggregates with bound C1 and C4b show maximal C3 convertase activity, in the presence of excess C2, when the alpha'-H chain component is in relatively highest amounts. When C4 is incubated with C1s in the absence of aggregates, up to 15% of a C4b dimer is formed, which on reduction gives an alpha' chain complex, probably a dimer. The apparent covalent interaction between C4b and IgG and between C4b and other C4b molecules cannot be inhibited by iodoacetamide and hence cannot be catalysed by transglutaminase (factor XIII). The reaction is, however, inhibited by cadaverine and putrescine and 14C-labelled putrescine is incorporated into C4, again by a strong, probably covalent, bond. It is suggested that a reactive group, possibly an acyl group, is generated when C4 is activated by C1 and that this reactive group can react with IgG, with another C4 molecule, or with water.

The purification and characterization of bovine C4, the fourth component of complement

The fourth component of complement, C4, was isolated from bovine plasma in high yield, by using simple purification techniques. The protein, like human component C4, is a beta-globulin with a mol.wt. of about 200 000 and consists of three polypeptide chains, alpha, beta and gamma, with apparent mol. wts. of 98 000, 82 000 and 32 000 respectively. The chains of C4 have been separated by methods previously used for human C4. Their amino acid compositions are very similar to those of the human component, but differences in carbohydrate distribution have been observed. The haemolytic activity of bovine C4 is totally destroyed by incubation with bovine C1s, the activated subcomponent of the first component of complement. Component C4, treated in this way, was shown to be cleaved in the alpha chain, which was decreased in mol.wt. by about 9000, corresponding to the removal of subcomponent C4a.

Complement-mediated immune mechanisms in renal infection

The belief that the inactivation of complement by renal ammonia enhances the susceptibility of renal tissue to infection has been held for some years. This thesis has been investigated in the present experiments using cultures of renal tissue maintained in vitro under physiological conditions. The experiments have confirmed that exposure of normal serum to renal issue in culture does result in the rapid loss of complement activity, but that the inactivation was not due to renal ammonia. Furthermore, in quantitative experiments, the liver was found to have even greater anti-complementary activity than renal tissue. In experiments where the biological significance of this phenomenon was examined, it was shown that the bactericidal capacity of serum was maintained even after exposure to renal tissue. The results of these investigations suggest that the biological significance of the inactivation of complement by renal tissue in vitro has been over-emphasized and requires further studies in vivo.

Studies on the murine Ss protein: demonstration that the Ss protein is functionally the fourth component of complement

This report describes a simple and reproducible assay for the fourth component of murine complement (C4). Utilizing this assay, experiments are described which conclusively demonstrate that the murine Ss protein (serum substance) is functional C4 and that ascitic fluid may be used as a source for isolating Ss. The evidence that Ss is functional C4 is: (i) pretreatment of mouse plasma with F(ab')2 anti-Ss completely abrogated the C4 hemolytic titer; and (ii) in the isolation of the Ss protein from ascitic fluid, Ss immunochemical activity and C4 functional activity coeluted in each step of a four-step purification procedure.

[Demonstration of total complement in normal cerebrospinal fluid]

Até o presente momento, todos os autores afirmam que o líquido cefalorraqueano (LCR) normal do homem não apresenta atividade hemolítica do complemento total. Alguns pesquisadores verificaram, entretanto, a presença de todos os quatro componentes em muitas amostras de LCR normal sem conseguir, porém, demonstrar a atividade hemolítica do complemento total. Este fato parecia indicar que o LCR normal era desprovido da atividade hemolítica do complemento total provavelmente por motivo da concentração insuficiente de suas proteínas. Para investigar esta hipótese foi feita uma prova preliminar, concentrando 10 amostras de LCR normal, dividindo-se o concentrado em duas partes, uma das quais foi inativada à temperatura de 56°C, durante 20 minutos. Esta prova revelou hemólise das hemácias sensibilizadas somente em relação com o LCR concentrado ativo. Em prosseguimento foram feitas pesquisas em 108 amostras de LCR normal obtidas por punção cisternal, selecionadas de pacientes sem doença de ordem geral, porém com perturbações neurológicas ou psíquicas que, em geral, não costumam determinar alterações do LCR. As amostras de LCR foram concentradas vinte vezes o seu volume, pelo método da diálise sob pressão negativa em camisa de colódio, segundo o processo de Mies. Esta pesquisa proporcionou resultados que permitiram concluir que o líquido cefalorraqueano normal concentrado apresenta atividade hemolítica do complemento total em 98% dos casos. O título do complemento total variou de 0 a 23 unidades de hemólise 50% em 1 ml de líquido concentrado.