The complement component C4 of mammals

Human complement component C4 is coded by tandem genes located in the HLA class III region. The products of the two genes, C4A and C4B, are different in their activity. This difference is due to a degree of 'substrate' specificity in the covalent binding reactions of the two isotypes. Mouse also has a duplicated locus, but only one gene produces active C4, while the other codes for the closely related sex-limited protein (Slp). In order to gain some insight into the evolutionary history of the duplicated C4 locus, we have purified C4 from a number of other mammalian species, and tested their binding specificities. Like man, chimpanzee and rhesus monkey appear to produce two C4 types with reactivities similar to C4A and C4B. Rat, guinea pig, whale, rabbit, dog and pig each expresses C4 with a single binding specificity, which is C4B-like. Sheep and cattle express two C4 types, one C4B-like, the other C4A-like, in their binding properties. These results suggest that more than one locus may be present in these species. If this is so, then the duplication of the C4 locus is either very ancient, having occurred before the divergence of the modern mammals, or there have been three separate duplication events in the lines leading to the primates, rodents and ungulates.

The leukocyte cell surface receptor(s) for the iC3b product of complement

CR3 is probably the major adhesion molecule on monocytes and neutrophils. Its function as a phagocytic receptor for iC3b-coated particles has been well characterized. CR3 also has binding affinity for other ligands, including those that compete with iC3b such as fibrinogen, factor X, and beta-glucan, and those that do not such as bacterial LPS. CR3 binding to endothelial cells probably plays an important role in the extravascular migration of monocytes and neutrophils, but the ligand that it recognizes on endothelial cells has not been identified. Structurally CR3 belongs to the integrin family, and it shares a common subunit with p150,95 and LFA-1. The expression of these three membrane antigens appear to be limited to leukocytes, and they are sometimes referred to collectively as the leukocyte integrins. All three antigens have a common binding affinity for bacterial LPS. p150,95 also has affinity for iC3b, but p150,95/iC3b-dependent cellular responses has not been demonstrated. Its status as a complement receptor therefore awaits further experimental support.

The low C5 convertase activity of the C4A6 allotype of human complement component C4

We have compared the C5-convertase-forming ability of different C4 allotypes, including the C4A6 allotype, which has low haemolytic activity and which has previously been shown to be defective in C5-convertase formation. Recent studies suggest that C4 plays two roles in the formation of the C5 convertase from the C3 convertase. Firstly, C4b acts as the binding site for C3 which, upon cleavage by C2, forms a covalent linkage with the C4b. Secondly, C4b with covalently attached C3b serves to form a high-affinity binding site for C5. Purified allotypes C4A3, C4B1 and C4A6 were used to compare these two activities of C4. Covalently linked C4b-C3b complexes were formed on sheep erythrocytes with similar efficiency by using C4A3 and C4B1, indicating that the two isotypes behave similarly as acceptors for covalent attachment of C3b. C4A6 showed normal efficiency in this function. However, cells bearing C4b-C3b complexes made from C4A6 contained only a small number of high-affinity binding sites for C5. Therefore a lack of binding of C5 to the C4b C3b complexes is the reason for the inefficient formation of C5 convertase by C4A6. The small number of high-affinity binding sites created, when C4A6 was used, were tested for inhibition by anti-C3 and anti-C4. Anti-C4 did not inhibit C5 binding, whereas anti-C3 did. This suggests that the sites created when C4A6 is used to make C3 convertase may be C3b-C3b dimers, and hence the low haemolytic activity of C4A6 results from the creation of low numbers of alternative-pathway C5-convertase sites.

Regional localization of CD18, the beta-subunit of the cell surface adhesion molecule LFA-1, on human chromosome 21 by in situ hybridization

The gene coding for the beta-subunit of the cell surface adhesion glycoprotein LFA-1 has been localized to the tip of the long arm of chromosome 21 at 21q22.1-qter.

Structural basis of the binding specificity of the thioester-containing proteins, C4, C3 and alpha-2-macroglobulin

We have previously noted a large difference in the specificity of the covalent binding reaction of human C4-A and C4-B. Here we report data on three other thioester-containing proteins. Human C3 is unreactive with glycine but its reactivity with glycerol (k'/ko = 23.0 M-1) is similar to that of human C4-B (k'/ko = 15.5 M-1). Human alpha 2-macroglobulin reacts with glycine (k'/ko = 206 M-1) in a manner similar to C4-B (k'/ko = 119 M-1) but its reactivity with glycerol (k'/ko = 1.2 M-1) is C4-A like (k'/ko = 1.3 M-1). Mouse C4 is C4-B like in its reaction with both glycine (k'/ko = 136 M-1) and glycerol (k'/ko = 26.0 M-1). Of these proteins, only C4-A shows a very high rate of reaction with glycine (k'/ko = 13,400 M-1). The comparison of the primary structures of these proteins has allowed us to propose the Leu Asp:Ile His substitutions at positions 1105 and 1106 in the human pro-C4 molecule as the residues largely responsible for the binding specificities of these proteins. The Leu:Ile change would not markedly affect the reactivity of these proteins, but may be necessary for allosteric reasons. The Asp in C4-A and His in C4-B seem likely to be the major specificity-defining residues.

C3 receptors on macrophages

The complement receptors on macrophage are responsible for their binding and ingestion of opsonized targets. The two established receptors are CR1, which recognizes C3b, and CR3, which recognizes iC3b, the natural product of C3b from cleavage by the complement control protein factor I and its cofactors. CR1 belongs to a group of proteins that contain a structural element characterized by its size of 60-65 amino acids, and four conservatively positioned cysteines, which engage in a self-contained 1-3, 2-4 disulphide arrangement. This structural unit is called SCR (short consensus repeat) and is found in the complement proteins C1r, C1s, C2, factor B, factor H, C4BP, DAF, MCP and CR2, each of which interacts with some cleavage products of C3 and/or C4. CR1 has 30 SCR units accounting for its entire extracellular structure. It has a transmembrane segment and a small cytoplasmic domain. CR3 is a heterodimer containing an alpha and beta subunit held together by non-covalent forces. The beta subunit is also found in the two leukocyte antigens, LFA-1 and p150,95, which have alpha subunits distinct from that of CR3. The beta subunit contains 56 cysteine residues, 42 of which lie in a span of 256 residues immediately adjacent to the transmembrane segment. It shares extensive sequence homology with subunits of membrane protein complexes that bind fibronectin and vitronectin, implicating that they all belong to an extended set of surface adhesion molecules not restricted to the immune system. p150,95 is also expressed on macrophages and it has iC3b binding activity. It also shares some functional properties with CR3 as an adhesion surface molecule.

Serial sectioning techniques for a modified LKB Historesin

A glycol methacrylate-based plastic that is capable of producing serial sections has been introduced by LKB. This plastic, provided in the LKB 2218-500 Historesin Embedding Kit, has been tested in our laboratory for its ribbon forming capacity. Various block sizes, concentrations of the softening agent polyethylene glycol 400 (PEG), and tissue types have been examined to determine the optimal conditions for ribbon formation. Although unmodified LKB Historesin is capable of forming ribbons, these ribbons often break. The addition of PEG to the embedding solution enhances ribbon formation. When sectioning with glass knives the best results are achieved with the addition of 0.2 ml of PEG/5.0 ml of embedding medium. A conventional AO rotary microtome can be used to produce ribbons if, in addition to the added PEG (optimal concentration 0.25-0.30 per 5 ml of embedding medium) a thin layer of dental wax is added to the upper and lower surfaces of the block. Ribbons form more easily on microtomes, such as the LKB Historange, that have a retractable specimen arm. If serial sections are to be produced it is very important that the upper and lower faces of blocks be parallel.

The primary structure of the beta-subunit of the cell surface adhesion glycoproteins LFA-1, CR3 and p150,95 and its relationship to the fibronectin receptor

The lymphocyte-function-associated antigen-1 (LFA-1), the complement receptor type 3 (CR3) and the antigen p150,95 are cell-surface glycoproteins. They are heterodimeric complexes, each containing a unique alpha-subunit noncovalently associated with a common beta-subunit. We have purified the beta-subunit from human spleen and obtained limited peptide sequences. What appears to be the complete primary structure for the fully processed beta-subunit was obtained by cDNA sequencing of clones from a phorbol ester (PMA) stimulated U937 cDNA library. There are five possible glycosylation sites and a transmembrane segment. The sequence contains a high level of cysteine (7.6%), with 24 of the 57 cysteine residues being found in three repeating units each with eight residues. The entire primary structure has 47% identity to a subunit of a fibronectin binding protein from chicken fibroblasts. It seems that LFA-1, CR3 and p150,95 antigens may belong to an extended family of cell surface molecules including the fibronectin binding protein.

The purification and properties of some less common allotypes of the fourth component of human complement

Human complement component C4 is coded by two genes situated between HLA-D and HLA-B. Both genes are highly polymorphic; C4-A gene products normally carry the blood group antigen Rodgers and C4-B proteins usually carry the Chido antigen. Using a monoclonal antibody which binds Rodgers-positive and Chido-positive proteins with different affinities, we have purified a number of less common C4 allotypes and compared their properties. All C4-B allotypes tested have similar specific hemolytic activities and binding efficiencies to small molecules. All C4-A proteins tested had similar binding to small molecules and hemolytic activities except for the C4-A6 proteins from two individuals with different extended haplotypes, both of which had identical hemolytic activities and much lower ones than other C4-A allotypes. Two allotypes, C4-A1, Rodgers-negative but Chido-positive, and C4-B5, Chido-negative but probably Rodgers-positive, were found to behave as typical C4-A and C4-B proteins, respectively, apart from the switch in their antigenic properties.

The primary structure of the fourth component of human complement (C4)-C-terminal peptides

C-terminal CNBr peptides of the three polypeptide chains of C4 were obtained and sequenced. These results supplement previously obtained data, notably the protein sequence derived from cDNA sequencing of pro-C4 (Belt KT, Carroll MC & Porter RR (1984) Cell 36, 907-914) and the N-terminal sequences of the three polypeptides (Gigli I, von Zabern I & Porter RR (1977) Biochem. J. 165, 439-446), to define the complete primary structure of the plasma form of C4. The beta (656 residues), alpha (748 residues), and gamma (291 residues) chains are found in positions 1-656, 661-1408, and 1435-1725 in the pro-C4 molecule.

The origin of the very variable haemolytic activities of the common human complement component C4 allotypes including C4-A6

The human complement component C4 occurs in many different forms which show big differences in their haemolytic activities. This phenomenon seems likely to be of considerable importance both physiologically and pathologically. C4 is coded by duplicated genes between HLA-D and HLA-B loci in the major histocompatibility complex in man. Several fold differences in haemolytic activity between products of the two loci C4-A and C4-B have been correlated with changes of six amino acid residues in this large protein of 1722 residues and with differences of several fold in the covalent binding of C4 to antibody-antigen aggregates. Some allotypes of one locus also differ markedly, notably C4-A6 which has 1/10th the haemolytic activity of other C4-A allotypes. A monoclonal antibody affinity column has been prepared which is able to separate C4-A from C4-B proteins and, using serum from an individual expressing only the C4-A6 allele at the C4-A locus, C4-A6 protein has been prepared. Investigation has shown C4-A6 to have the same reactivity as other C4-A allotypes except in the formation of the complex protease, C5 convertase. This protease is formed from C4, C2 and C3 and if C4-A6 is used it has approximately 1/5th the catalytic activity compared with other C4-A allotype. Allelic differences in sequence identified in C4 proteins so far are few and it is probable that the big difference in catalytic activity of C5 convertase is caused by very small changes in structure.

Hydralazine binds covalently to complement component C4. Different reactivity of C4A and C4B gene products

Long-term treatment with hydralazine is sometimes associated with deposition of immune complexes and development of systemic lupus erythematosus (SLE) as an adverse side-effect. Hydralazine inhibits the covalent binding reaction of the complement protein C4. We show that when hydralazine inhibits C4, it becomes covalently bound to the polypeptide chain containing the active site thiol ester. C4 is encoded at 2 adjacent polymorphic loci, C4A and C4B, within the major histocompatibility complex. We show that hydralazine binds more efficiently to the C4A than to the C4B gene product and suggest that C4 type may predispose patients to hydralazine-induced SLE.

Resistance to phagocytosis by group A streptococci: failure of deposited complement opsonins to interact with cellular receptors

The previous finding that phagocytosis-resistant M+ group A streptococci bear quantities of C3 which are sufficient for phagocytosis of their M- derivatives was investigated at two levels. It was first established that the C3 associated with M+ streptococci was not able to promote adherence to cells bearing the complement receptors CR1 and CR3 under conditions in which M- streptococci readily attached. The molecular form of C3 bound to M+ and M- streptococci was then defined by adding 125I-C3 to serum used for opsonization. C3 eluted from the bacteria by chaotropic and hydrolytic agents was analyzed by SDS-PAGE, and revealed that both cell types bound the opsonic forms of C3, C3b, and iC3b. Furthermore, approximately 80% of the C3b and iC3b associated with both cell types was covalently bound to a surface component, although most of the C3 bound to M+ streptococci was detergent-extractable, whereas greater than 50% of that bound to M- streptococci was not. These findings demonstrate that the M+ surface is interfering with the receptor binding of deposited C3b and iC3b, and that this contributes to resistance to phagocytosis by these organisms.

Resistance to phagocytosis by group A streptococci: failure of deposited complement opsonins to interact with cellular receptors

The previous finding that phagocytosis-resistant M+ group A streptococci bear quantities of C3 which are sufficient for phagocytosis of their M- derivatives was investigated at two levels. It was first established that the C3 associated with M+ streptococci was not able to promote adherence to cells bearing the complement receptors CR1 and CR3 under conditions in which M- streptococci readily attached. The molecular form of C3 bound to M+ and M- streptococci was then defined by adding 125I-C3 to serum used for opsonization. C3 eluted from the bacteria by chaotropic and hydrolytic agents was analyzed by SDS-PAGE, and revealed that both cell types bound the opsonic forms of C3, C3b, and iC3b. Furthermore, approximately 80% of the C3b and iC3b associated with both cell types was covalently bound to a surface component, although most of the C3 bound to M+ streptococci was detergent-extractable, whereas greater than 50% of that bound to M- streptococci was not. These findings demonstrate that the M+ surface is interfering with the receptor binding of deposited C3b and iC3b, and that this contributes to resistance to phagocytosis by these organisms.

A comparison of the properties of two classes, C4A and C4B, of the human complement component C4

A remarkable difference has been observed between the reactivity of the two forms of human complement component C4. C4B binds twice as effectively as C4A to antibody-coated red cells, but the reverse occurs with protein-antigen complexes. C4B reacts much more effectively with hydroxyl groups than C4A and this is reversed for reaction with amino groups in spite of the very small difference in amino acid sequence between the two forms of C4. No other differences in stability, activation or inactivation were observed. These findings emphasise the biological advantage of the duplication of the C4 gene in its reaction with a wide range of antigenic structures. The correlation of the presence of different forms of C4 with susceptibility to autoimmune diseases may be explicable by these big differences in binding reactivity.

Covalent binding efficiency of the third and fourth complement proteins in relation to pH, nucleophilicity, and availability of hydroxyl groups

The binding of [3H]glycerol and [3H]putrescine to C3 was studied in a fluid-phase system using trypsin as the C3 convertase. The binding of glycerol showed little variation in the pH range between 6.0 and 10.0. The binding of putrescine (pKa = 9.0) is rather ineffective below pH 7.5 but becomes more efficient as the pH of the reaction mixture increases. These results agree with the contention that the final step of the binding reaction is the transfer of the acyl group of the exposed thio ester of C3 to a nucleophile since the nucleophilicity of hydroxyl groups is rather independent of pH whereas only the unprotonated form of amino groups is nucleophilic. The inefficient reaction of amino groups with the exposed thio ester of C3 is also supported by the study of the inhibitory activity of serine and its two derivatives, N-acetylserine and O-methylserine, to the binding of [3H]glycerol to C3. N-Acetylserine showed an inhibitory activity equivalent to that of serine, whereas O-methylated serine showed only minimal activity. It can be concluded, therefore, that serine reacts with the thio ester of C3 by its hydroxyl group but not by its alpha-amino group. The ability of the alcohol group of various alkanes to inhibit the binding of [3H]glycerol to C3 was also studied. The primary alcohols inhibit the binding reaction with an efficiency that is similar to glycerol, and there are no significant differences in the binding efficiencies of methanol, ethanol, 1-propanol, and 1-butanol.(ABSTRACT TRUNCATED AT 250 WORDS)

Natural release of covalently bound C3b from cell surfaces and the study of this phenomenon in the fluid-phase system

Covalently bound C3b is released from cell surfaces (EAC1423 and zymosan-C3b) on incubation under physiologic conditions. The release of C3b from cell surfaces occurs by the cleavage of the covalent bond. Sodium dodecyl sulfate (SDS) abolishes the release, thereby indicating the requirement of the native structure of C3b in this process. The phenomenon of release of C3b from cell surfaces has also been observed in the fluid-phase system by using C3b-[3H]glycerol. The kinetics of the release of [3H]glycerol from C3b-[3H]glycerol were studied at 37 degrees C in 0.15 M phosphate buffer, pH 7.4. The first-order rate constant was found to be 0.028 +/- 0.003 hr-1. The release does not take place in either 8 M urea or 6 M guanidine hydrochloride, at pH 7.4. Under alkaline conditions, the rate of release is unaffected in the presence of SDS, indicating that the release in this pH range is not dependent on the native structure of the protein. From the Arrhenius plot in the temperature range 18 to 37 degrees C, an apparent activation energy for the hydrolysis reaction of 21.2 kcal/mol was calculated. The release phenomenon is exclusive for ester-linked complexes, as inferred by the absence of release of [3H]threonine from C3b-[3H]threonine, wherein the linkage is of the amide type. The presence or absence of the C3a portion of the molecule has no effect on the rate of release. The modification of the -SH group of C3i-/C3b-[3H]glycerol alters the rate of hydrolysis of the ester bond between C3i/C3b and [3H]glycerol. Protease inhibitors (PMSF, benzamidine HCl, and DFP) do not alter the rate of release, indicating that the hydrolysis reaction is not due to trace amounts of contaminating proteases. Thus, it appears that some chemical group(s) of C3i/C3b is (are) involved in the intramolecular hydrolysis of the ester bond between C3i/C3b and small molecules. This phenomenon may play an important role in the release of C3b from receptive surfaces once the biologic functions that require covalently bound C3b have been mediated.

Natural release of covalently bound C3b from cell surfaces and the study of this phenomenon in the fluid-phase system

Covalently bound C3b is released from cell surfaces (EAC1423 and zymosan-C3b) on incubation under physiologic conditions. The release of C3b from cell surfaces occurs by the cleavage of the covalent bond. Sodium dodecyl sulfate (SDS) abolishes the release, thereby indicating the requirement of the native structure of C3b in this process. The phenomenon of release of C3b from cell surfaces has also been observed in the fluid-phase system by using C3b-[3H]glycerol. The kinetics of the release of [3H]glycerol from C3b-[3H]glycerol were studied at 37 degrees C in 0.15 M phosphate buffer, pH 7.4. The first-order rate constant was found to be 0.028 +/- 0.003 hr-1. The release does not take place in either 8 M urea or 6 M guanidine hydrochloride, at pH 7.4. Under alkaline conditions, the rate of release is unaffected in the presence of SDS, indicating that the release in this pH range is not dependent on the native structure of the protein. From the Arrhenius plot in the temperature range 18 to 37 degrees C, an apparent activation energy for the hydrolysis reaction of 21.2 kcal/mol was calculated. The release phenomenon is exclusive for ester-linked complexes, as inferred by the absence of release of [3H]threonine from C3b-[3H]threonine, wherein the linkage is of the amide type. The presence or absence of the C3a portion of the molecule has no effect on the rate of release. The modification of the -SH group of C3i-/C3b-[3H]glycerol alters the rate of hydrolysis of the ester bond between C3i/C3b and [3H]glycerol. Protease inhibitors (PMSF, benzamidine HCl, and DFP) do not alter the rate of release, indicating that the hydrolysis reaction is not due to trace amounts of contaminating proteases. Thus, it appears that some chemical group(s) of C3i/C3b is (are) involved in the intramolecular hydrolysis of the ester bond between C3i/C3b and small molecules. This phenomenon may play an important role in the release of C3b from receptive surfaces once the biologic functions that require covalently bound C3b have been mediated.

Non-enzymic activation of the covalent binding reaction of the complement protein C3

The covalent binding of [3H]glycerol to C3 by the transfer of the acyl group of the internal thioester of C3 to the hydroxy group of glycerol can be activated either proteolytically by trypsin or by various chaotropes and denaturants. The activation of binding by trypsin or KBr showed similar dependence on the concentration of glycerol, indicating a similar activation mechanism. It is therefore concluded that the conformational change of the protein is the critical step in the binding reaction, and that the conversion of C3 into C3b under physiological conditions is only a means to induce the conformational change. Guanidinium chloride induces the binding of glycerol to C3 at concentrations of about 1 M. On increasing the concentration of guanidinium chloride the extent of binding declines and is accompanied by an increase in the autolytic cleavage reaction [Sim & Sim (1981) Biochem. J. 193, 129-141]. The autolytic cleavage reaction is therefore not independently activated with respect to the binding reaction. Its occurrence, however, is structurally restricted under physiological or limited denaturing conditions and is permissible only when C3 is brought to a higher denaturation state.

Ligand binding specificity of a rabbit alveolar macrophage receptor for C3b

We have recently reported the isolation from rabbit alveolar macrophages of a receptor which retained its ligand-binding activity for the third component of complement (C3) and for its major proteolytic derived activation fragment (C3b). The isolated receptor demonstrated a greater ability to bind C3b than an equimolar amount of C3. C3b differs from C3 in at least two ways: it is a proteolytic cleavage product of C3 and it lacks the internal thiolester bond of C3. We have analyzed the binding ability to isolated receptor to various C3 and C3b analogs and we demonstrate that the specificity of the C3b-C3b receptor interaction depends upon the lysis of the C3 thiolester bond and accompanying conformational change rather than upon proteolytic cleavage of the C3 molecule. Minimal, if any, binding of C3 with an intact thiolester bond to the isolated receptor was demonstrable.

Binding reaction between the third human complement protein and small molecules

The covalent binding reaction of the third complement protein (C3) to receptive surfaces is thought to proceed by the following mechanism. An internal thioester [Tack, B. F., Harrison, R. A., Janatova, J., Thomas, M. L., & Prahl, J. W. (1980) Proc. Natl. Acad. Sci. U.S.A. 77, 5764-5768; Law, S. K., Lichtenberg, N. A., & Levine, R. P. (1980b) Proc. Natl. Acad. Sci. U.S.A. 77, 7194-7198], which is usually hidden within the C3 molecule, is exposed upon proteolytic activation of C3 to C3b* (the hypothetical conformation of C3b which has the capacity to bind to receptive surfaces and small molecules). The exposed thioester is accessible to attack by hydroxyl groups on receptive surfaces. An acyl transfer reaction takes place, leading to the binding of C3b to the receptive surfaces via an ester linkage [Law, S. K., Lichtenberg, N. A., & Levine, R. P. (1979) J. Immunol. 123, 1388-1394]. We have used a fluid-phase system to demonstrate the specific binding of different small molecules to the labile binding site of C3. The small molecules include glycerol, different hexose monomers, sucrose, raffinose, and four amino acids. These molecules bind to C3b with different efficiencies, indicating that there is an order of preference of C3b* for these molecules. In certain cases, the small molecules bind to C3b via ester linkages (e.g., glucose); in others, the bond is an amide linkage (e.g., lysine). We have also studied the concentration dependence of the binding of small molecules to C3b. The binding is consistent with the following reaction scheme: (Formula: see text).

Isolation of a biologically active macrophage receptor for the third component of complement

C3b, the major cleavage fragment of the third component of complement (C3), has been demonstrated to bind to a specific receptor on various mammalian cells. Tissue-bound C3b receptors have also been demonstrated, most conclusively in renal glomeruli. On interaction with C3b, C3b receptors are thought to initiate cellular functions such as phagocytosis and to determine the fate of complement-fixing soluble and particulate immune complexes. We report here the isolation by affinity chromatography of a macrophage glycoprotein with an apparent molecular weight (MW) of approximately 64,000 that has properties expected of the C3b receptor. It is a cell-surface macromolecule (labelled with 125I and lactoperoxidase) which, in its isolated state, retains the ability to bind both C3 and C3b.

Covalent binding and hemolytic activity of complement proteins

We report the inactivation of the third component of complement (C3) by hydroxylamine. C3 hemolytic and covalent binding activities decline with identical kinetics, demonstrating a direct correlation between the two activities. We conclude that covalent, surface-bound C3b is hemolytically active. The inactivation of C3 is first order with respect to hydroxylamine. We also studied C3 inactivation with [14C]methylamine. The inactivation corresponds quantitatively with the labeling of C3 in the C3d domain. The data obtained support the following hypothesis: there is an internal thioester within C3 which becomes highly reactive on activation to C3b, and C3b binds to receptive surfaces by transfer of the acyl function of the thioester to a hydroxyl group on the receptive surface. This proposed model for the reaction of C3 with receptive surfaces also applies to C4, which binds to membrane surfaces covalently and is able to be inactivated by hydroxylamine and methylamine. C5, on the other hand, is not inactivated by treatment with the amines.

Interaction between the labile binding sites of the fourth (C4) and fifth (C5) human complement proteins and erythrocyte cell membranes

We have shown that the labile binding site of C3b interacts covalently with receptive surfaces. We report here an analogous study of the interaction between the labile binding sites of the closely related complement proteins, C4 and C5, with sheep erythrocyte membranes. We find that i) C4b binds covalently to cell surface components; ii) the bond between C4b and receptive molecules is hydroxylamine sensitive; iii) the alpha-polypeptide of C4b binds to receptive molecules; and iv) C5b does not interact covalently with cell surfaces.

Interaction between the labile binding sites of the fourth (C4) and fifth (C5) human complement proteins and erythrocyte cell membranes

We have shown that the labile binding site of C3b interacts covalently with receptive surfaces. We report here an analogous study of the interaction between the labile binding sites of the closely related complement proteins, C4 and C5, with sheep erythrocyte membranes. We find that i) C4b binds covalently to cell surface components; ii) the bond between C4b and receptive molecules is hydroxylamine sensitive; iii) the alpha-polypeptide of C4b binds to receptive molecules; and iv) C5b does not interact covalently with cell surfaces.

Action of the C3b-inactivator on the cell-bound C3b

The action of C3bINA and beta 1H on cell-bound C3b is described in this paper. The alpha-polypeptide of C3b that binds covalently to cell surfaces is cleaved by the C3bINA and beta 1H into two fragments: one of 60,000 (C3b alpha-60) and another of 40,000 (C3b alpha-40) daltons. The beta-chain of C3b is unaffected by the C3bINA and beta 1H. The three polypeptides, C3b alpha-60, C3b alpha-40, and C3 beta, are held together as a single unit by disulfide bonds. This unit, referred to as C3b' is covalently bound to cell surfaces via the C3b alpha-60 polypeptide. The conversion of C3b to C3b' by C3bINA and beta 1H abolishes the ability of the C3b-bearing cells to adhere to human erythrocytes as well as the ability to form, on the cell surface, the B, D, and properdin-dependent amplification C3-convertase. However, the agglutinability of the cells with either anti-C3c or anti-C3d is not affected. Treatment of the C3b'-bearing cells with trypsin releases fragments of C3b' into solution, leaving a polypeptide of 32,000 daltons covalently linked to the membrane. Since the trypsinized cells are agglutinable by anti-C3d but not by anti-C3c, the 32,000 dalton polypeptide appears to correspond antigenically to C3d.

Urban-rural differences in student mental health: the Hong Kong scene

Student mental health of Hong Kong urban and rural fifth formers and sixth-formers was assessed by examining their scores obtained from the General Health Questionnaire twice administered. The first administration was done soon after commencement of a school term and served as a baseline measure, and the second was done six weeks before the fifth-formers took an important public examination, a significantly stressful event. The results indicated that all groups of fifth-formers showed an increase in mean scores, reaching statistical significance only in urban boys and rural girls. The latter finding was used to explain urban-rural differences in mean scores. The significance of severe examination distress and its possible late psychiatric sequelae remained unanswered.

Interaction between the third complement protein and cell surface macromolecules

The activated form of the third complement protein, C3b, forms a stable complex with components of plasma membranes and particulate entities such as zymosan. The complex resists the action of detergents and protein denaturants as well as extremes of temperature, salt concentration, and pH. It can, however, be broken by exposure to hydroxylamine or by ammonolysis followed by incubation with sodium dodecyl sulfate. Thus, the complex appears to result from a hydrophobic interaction as well as a bond susceptible to nucleophilic attack.