Amino-terminal sequence of human factor B of the alternative complement pathway and its cleavage fragments, Ba and Bb

C3 dysregulation due to factor H deficiency is mannan-binding lectin-associated serine proteases (MASP)-1 and MASP-3 independent in vivo

Uncontrolled activation of the complement alternative pathway is associated with complement-mediated renal disease. Factor B and factor D are essential components of this pathway, while factor H (FH) is its major regulator. In complete FH deficiency, uncontrolled C3 activation through the alternative pathway results in plasma C3 depletion and complement-mediated renal disease. These are dependent on factor B. Mannan-binding lectin-associated serine proteases 1 and 3 (MASP-1, MASP-3) have been shown recently to contribute to alternative pathway activation by cleaving pro-factor D to its active form, factor D. We studied the contribution of MASP-1 and MASP-3 to uncontrolled alternative pathway activation in experimental complete FH deficiency. Co-deficiency of FH and MASP-1/MASP-3 did not ameliorate either the plasma C3 activation or glomerular C3 accumulation in FH-deficient mice. Our data indicate that MASP-1 and MASP-3 are not essential for alternative pathway activation in complete FH deficiency.

Fragment Bb in amniotic fluid: evidence for complement activation by the alternative pathway in women with intra-amniotic infection/inflammation

OBJECTIVE

Fragment Bb is an activator of the alternative pathway of the complement system. Recently, increased first trimester maternal plasma concentrations of this fragment were reported in patients destined to have a spontaneous preterm delivery before 34 weeks of gestation. The aim of this study was to determine whether the amniotic fluid (AF) concentrations of fragment Bb change with gestational age, spontaneous labor (term and preterm) and in the presence of intra-amniotic infection/inflammation (IAI).

STUDY DESIGN

This cross-sectional study included patients in the following groups: (1) mid-trimester (n = 64); (2) term in spontaneous labor (n = 70); (3) term not in labor (n = 43); (4) spontaneous preterm labor (PTL) who delivered at term (n = 76); (5) PTL without IAI who delivered preterm (n = 73); (6) PTL with IAI (n = 76); (7) preterm prelabor rupture of membranes (PROM) without IAI (n = 71); and (8) preterm PROM with IAI (n = 71). Fragment Bb concentration in AF was determined by an enzyme-linked immunoassay. Non-parametric statistics were used for analyses.

RESULTS

(1) Fragment Bb was detected in all AF samples (n = 544); (2) The median AF concentration of fragment Bb in patients at term not in labor was significantly higher than that of those in the mid-trimester [2.42 microg/ml, interquartile range (IQR) 1.78-3.22 vs. 1.64 microg/ml, IQR 1.06-3.49; p < 0.001]; (3) Among patients with PTL, those with IAI had a higher median AF fragment Bb concentration than that of woman without IAI, who delivered preterm (4.82 microg/ml, IQR 3.32-6.08 vs. 3.67 microg/ml, IQR 2.35-4.57; p < 0.001) and than that of women with an episode of PTL, who delivered at term (3.21 microg/ml, IQR 2.39-4.16; p < 0.001); (4) Similarly, among patients with preterm PROM, the median AF fragment Bb concentration was higher in individuals with IAI than in those without IAI (4.24 microg/ml, IQR 2.58-5.79 vs. 2.79 microg/ml, IQR 2.09-3.89; p < 0.001). (5) Among patients at term, the median AF fragment Bb concentration did not differ between women with spontaneous labor and those without labor (term in labor: 2.47 microg/ml, IQR 1.86-3.22; p = 0.97).

CONCLUSIONS

(1) Fragment Bb, an activator of the alternative complement pathway, is a physiologic constituent of the AF, and its concentration increases with advancing gestational age; (2) AF concentrations of fragment Bb are higher in pregnancies complicated with IAI; and (3) labor at term is not associated with changes in the AF concentrations of fragment Bb. These findings suggest a role for fragment Bb in the host immune response against IAI.

A new apoptotic pathway for the complement factor B-derived fragment Bb

Apoptosis is involved in both the cellular and humoral immune system destroying tumors. An apoptosis-inducing factor from HL-60 myeloid leukemia cells was obtained, purified, and sequenced. The protein found has been identified as a human complement factor B-derived fragment Bb, although it is known that factor B is able to induce apoptosis in several leukemia cell lines. Monoclonal antibodies against fragment Ba and Bb inhibited the apoptotic activity of factor B. When the purified fragment Bb was used for apoptosis induction, only the anti-Bb antibody inhibited Bb-induced apoptosis, and not the anti-Ba antibody. The apoptosis-inducing activity was found to be enhanced under conditions facilitating the formation of Bb. Blocking TNF/TNFR or FasL/Fas interactions did not interfere with the factor B-induced apoptosis. CD11c (iC3bR) acts as the main subunit of a heterodimer binding to fragment Bb in the apoptosis pathway, and the factor B-derived fragment Bb was found to possess the previously unknown function of inducing apoptosis in leukemic cells through a suicide mechanism of myeloid lineage cells during the differentiation stage.

The Arthus reaction in rodents: species-specific requirement of complement

We induced reverse passive Arthus (RPA) reactions in the skin of rodents and found that the contribution of complement to immune complex-mediated inflammation is species specific. Complement was found to be necessary in rats and guinea pigs but not in C57BL/6J mice. In rats, within 4 h after initiation of an RPA reaction, serum alternative pathway hemolytic titers decreased significantly below basal levels, whereas classical pathway titers were unchanged. Thus the dermal reaction proceeds coincident with systemic activation of complement. The serine protease inhibitor BCX 1470, which blocks the esterolytic and hemolytic activities of the complement enzymes Cls and factor D in vitro, also blocked development of RPA-induced edema in the rat. These data support the proposal that complement-mediated processes are of major importance in the Arthus reaction in rats and guinea pigs, and suggest that BCX 1470 will be useful as an anti-inflammatory agent in diseases where complement activation is known to be detrimental.

Isolation from fetal bovine serum of a fragment b of complement factor B-like protein improving a long-term survival of human endothelial cells

It is known that serum is a most important factor supporting cell survival and growth. Particularly, the deprivation of serum would result in the death of human endothelial cell. Our previous paper reported an endothelial cell-viability maintaining factor (EC-VMFa) purified from fetal bovine serum and identified as an apolipoprotein. In the present further study, it is demonstrated that another potent serum factor (refer as EC-VMFb) is also possessed of the endothelial cell-viability maintaining activity, improving a long-term survival of human endothelial cells in serum-free medium. EC-VMFb has a molecular weight of 66,000 (reduced and nonreduced), pI of 4.5 and has been identified as fragment b of complement factor B (Bb)-like protein by amino-terminal amino acid sequence.

Crystal structure of a complement factor D mutant expressing enhanced catalytic activity

Complement factor D is a serine protease regulated by a novel mechanism that depends on conformational changes rather than cleavage of a zymogen for expression of proteolytic activity. The conformational changes are presumed to be induced by the single natural substrate, C3bB, and to result in reversible reorientation of the catalytic center and of the substrate binding site of factor D, both of which have atypical conformations. Here we report that replacement of Ser94, Thr214, and Ser215 of factor D (chymotrypsinogen numbering has been used for comparison purposes) with the corresponding residues of trypsin, Tyr, Ser, and Trp, is sufficient to induce substantially higher catalytic activity associated with a typical serine protease alignment of the catalytic triad residues His57, Asp102, and Ser195. These results provide a partial structural explanation for the low reactivity of "resting-state" factor D toward synthetic substrates.

Mutational analysis of the substrate binding site of human complement factor D

Complement factor D is a serine protease with a single natural substrate, C3b-complexed factor B, and very low catalytic activity against synthetic esters. The recently solved X-ray crystal structure of factor D has demonstrated certain key differences from other serine protease in the conformation of residues of the catalytic triad and the substrate-binding regions. To investigate possible contributions of unique amino acid substitutions to these distinct structural and functional features of factor D, we constructed a series of mutants by substituting trypsin substrate-binding residues for the corresponding factor D residues. Wild-type and seven mutant factor D cDNAs were expressed stably in Chinese hamster ovary cells, and the recombinant proteins were purified from culture supernatants and assayed by hemolytic, proteolytic, and esterolytic assays. The combined results indicate that residues Thr-198, Ser-199, Arg-202, and perhaps also Val-203 provide determinants for substrate binding and catalysis. The data also provide additional support for the hypothesis that the proteolytically active conformation of the active center of factor D is induced by its substrate, C3bB.

Human complement factor B: cDNA cloning, nucleotide sequencing, phenotypic conversion by site-directed mutagenesis and expression

A full-length cDNA clone, BHL4-1, encoding factor B was isolated from a human liver cDNA library and sequenced in its entirety. It consists of 2388 bp which include a 5'-untranslated region of 40 bp, a single open reading frame, 2292 bp in length, and a 3'-untranslated region of 56 bp followed by a poly-A tail. The deduced amino acid sequence comprises 25 residues of a putative leader peptide and 739 residues of the mature polypeptide chain of the F allele of factor B. We constructed an S allele-like Q7R mutant of BHL4-1 by site-directed mutagenesis. Both the wild-type and mutant factor B cDNA were expressed transiently in a eukaryotic system. The specific hemolytic activities of the two recombinant factor B alleles and of native B were not significantly different from each other.

Dye-ligand affinity purification of human complement factor B and beta 2 glycoprotein I

The rapid purification of human factor B using dye-ligand chromatography is described. The 50% ammonium sulphate supernatant of fresh human serum is equilibrated in pH 7.4, 25 mM Tris buffer containing 0.5 mM CaCl2 and 0.5 mM MgCl2, with 25 mM sodium caprylate and chromatographed on Cibacron Blue F3GA-agarose. Caprylic acid binds to the fatty acid binding site of albumin, preventing it from binding to the resin which thus retains a high capacity for binding factor B. Factor B together with the homologous protein beta 2I are eluted from the column by a linear gradient of KCl. Subsequent NaCl gradient FPLC on Hiload S-Sepharose, equilibrated in 10 mM potassium phosphate, 5 mM EDTA, pH 7.0, provides both factor B and beta 2I in homogeneous form.

The principal site of glycation of human complement factor B

Accumulating amino acid sequence data have made it increasingly evident that many essential complement proteins have potentially modifiable lysine residues in putative critical functional regions. Evidence is now presented that glucose is covalently attached to lysine-266 of purified human complement Factor B as a result of glycation. Purified B was treated with NaB3H4, which reduces such bound glucose to a mixture of radiolabelled hexitols. Amino acid analysis revealed the expected radiolabelled hexitol-lysine epimers. In addition, fluorography of dried gels resolving the major high-molecular-mass h.p.l.c.-fractionated CNBr-cleavage peptides of NaB3H4-reduced B indicated that this radioactivity was specifically associated with the 15 kDa fragment derived from the N-terminal region of fragment Bb. Amino acid sequence analysis suggested that the C-terminal lysine (residue 266 of B) of the N-terminal Lys-Lys doublet of this peptide is preferentially modified. If such glycation can subsequently be shown to occur in vivo, then perhaps this modification might also be found to affect the functional activity of B and offer a potential explanation for some of the immunopathological complications of diseases exposing key plasma proteins, such as this active-site-containing proteinase of the multimeric alternative-complement-pathway C3/C5 convertases, to long-term high concentrations of glucose, such as the decreased resistance to infection and impaired chemotaxis and phagocytosis characteristic of diabetes.

Proteolytic activity of the different fragments of factor B on the third component of complement (C3). Involvement of the N-terminal domain of Bb in magnesium binding

Kinetic experiments measuring the proteolytic activity of Bb and 33Kd fragment (the C-terminal domain of factor B) on C3 were performed in several conditions, in order to assess the role of factor B domains in the catalytic activity and magnesium binding. The experiments were carried out in fluid phase with 125I-C3 or C3(H2O) as substrates and in the presence of nonradioactive C3b as cofactor. The results indicate: (a) The C-terminal domain, 33Kd, possesses proteolytic activity on C3, which is Mg2(+)-independent, whereas proteolysis by Bb is enhanced in 5 mM Mg2+. (b) C3b behaves as cofactor of 33Kd proteolytic activity on C3 and factor H is able to inhibit this activity. (d) Kinetics of C3 proteolysis by 33Kd shows a lag phase which is also displayed by Bb in the absence but not in the presence of Mg2+. Taken together these data are consistent with the involvement of the N-terminal domain of Bb in Mg2+ binding, which results in an enhancement of the proteolytic activity on C3 of the adjacent C-terminal domain. A C3 convertase model accounting for these results is presented.

Evidence that C4b-binding protein is an acute phase protein

C4b-binding protein is a regulatory factor for both complement and coagulation systems. We found that a human hepatoma cell line, Hep G2, was capable of synthesizing C4b-binding protein and that the secretion of C4b-binding protein was enhanced by interleukin-6 and tumor necrosis factor, which are known to be modulators of acute phase proteins. In addition, the plasma content of C4b-binding protein was found to increase in patients of acute pneumonia. These results suggest that C4b-binding protein is an acute phase protein.

Two forms of guinea pig factor B of the alternative complement pathway with different molecular weights

Two forms of guinea pig factor B (B) of the alternative complement pathway with different mol. wts (Mr) have been isolated from plasma and characterized. The Mr of the two B species, tentatively termed B1 and B2, were estimated to be about 100,000 and 96,000, respectively, as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Incubation of B with guinea pig C3 and human factor D (D) in the presence of Mg2+ generated two cleavage fragments of B, namely Ba and Bb. Although the Bb fragments showed the same migration corresponding to an Mr of 62,000, Ba fragments showed different mobilities corresponding to an Mr of 38,000 from B1 and 34,000 from B2. Digestion of B1-Ba, the Ba fragment derived from B1, and B2-Ba, the Ba fragment derived from B2, with endoglycosidase F resulted in a band at Mr 30,000 on an SDS-PAGE in both cases, indicating a difference in structure of the asparagine-linked oligosaccharide moiety in B1-Ba and B2-Ba. No difference in antigenicity was noted between B1 and B2 on immunodiffusion with anti-B sera. Immunoblotting analysis showed that all individual Hartley guinea pigs examined in this study possessed both B1 and B2 at similar levels, as determined by the intensity of staining of their sera. Furthermore, treatment of their serum with zymosan led to the generation of two Ba species corresponding to the Ba fragments from B1 and B2. The capacity to form C3/C5 convertase, as determined by hemolytic assay, was found to be similar between B1 and B2. Furthermore, kinetics of the decay of C3 convertase showed the same half-life of 3.0 min at 30 degrees C. The NH2-terminal amino acid sequences of B1 and B2 and their Bb fragments were determined and found to be identical.

Adipsin and complement factor D activity: an immune-related defect in obesity

Adipsin is a serine protease that is secreted by adipocytes into the bloodstream; it is deficient in several animal models of obesity, representing a striking example of defective gene expression in this disorder. Recombinant mouse adipsin was purified and its biochemical and enzymatic properties were studied in order to elucidate the function of this protein. Activated adipsin has little or no proteolytic activity toward most substrates but has the same activity as human complement factor D, cleaving complement factor B when it is complexed with activated complement component C3. Like authentic factor D, adipsin can activate the alternative pathway of complement, resulting in red blood cell lysis. Decreased (58 to 80 percent) complement factor D activity, relative to lean controls, was observed as a common feature of several experimental models of obesity, including the ob/ob, db/db, and monosodium glutamate (MSG)-injected mouse and the fa/fa rat. These results suggest that adipsin and the alternative pathway of complement may play an unexpected but important role in the regulation of systemic energy balance in vivo.

Analysis of complement factor B activation by PAGE-immunoblotting

Initiation of complement activation via the alternative pathway results in cleavage of Factor B into two fragments Ba and Bb. Employing PAGE and immunoblotting analysis we have demonstrated Ba and Bb activation fragments following in vitro and in vivo activation in a range of biological fluids including cerebrospinal fluid and synovial fluid. Quantitative linearity was obtained over the range 5-20 micrograms/ml of Factor B and its fragments in body fluids. Using Davis-PAGE a previously unrecognised triple component heterogeneity of the Ba fragment was observed. This study demonstrates that immunoblotting may be used as a sensitive and quantitative tool for the detection and characterisation of the individual cleavage products of Factor B in complex biological fluids without prior purification.

Synthetic peptide inhibitors of complement serine proteases--I. Identification of functionally equivalent protease inhibitor sequences in serpins and inhibition of C1s and D

Sequence homology comparisons between serum serine protease inhibitors led to the prediction that the C-terminal sequences are functionally equivalent and represent an essential protease binding domain. Inhibition of complement serine protease D cleavage of factor B and of C1s cleavage of C4 by synthetic peptides containing sequences from the C-termini of three serum serine protease inhibitors supports this prediction. These functionally equivalent peptides represent a new class of inhibitors of D and C1s as well as other serum serine proteases.

Analysis of the interaction between properdin and factor B, components of the alternative-pathway C3 convertase of complement

The interactions between Factor B (B), its activation products Ba and Bb, properdin (P) and C3i or C3b, components that together form the alternative-pathway C3 convertase enzyme of human complement, have been analysed. Fluid-phase complexes of the purified components C3i, B and P were probed with the homobifunctional cross-linking reagent disuccinimidyl tartarate, and efficient cross-linking of B to P was observed. The 140 kDa B-P conjugate formed was cleaved by Factor D to yield a single product of 85 kDa. This is consistent with a Ba-P heterodimer, and suggests that the initial interaction of B and P includes an interaction of P with the Ba domain of intact B. (The Ba fragment is not retained in the active P-stabilized complex, C3bBbP). By contrast, no cross-linking of P to the Bb domain of B could be demonstrated. Binding studies on cellular intermediates also provided evidence for a site of interaction between B and P, with high concentrations of B inhibiting P binding to EAC3b (sheep erythrocytes coated with antibody and C3b). Neither isolated Ba nor Bb had any effect on the P-EAC3b interaction. High concentrations of B also accelerated the decay of the functional EAC3bBbP complex. These data indicate that the positive co-operativity of binding to C3i or to C3b between B and P is mediated, at least in part, through a direct interaction between B and P.

Purification of proteins from polyacrylamide gels, free of detergent or dye

A two step procedure recovers proteins from sodium dodecyl sulfate polyacrylamide gels. The proteins are eluted by electrophoretic dialysis. The eluent is then passed through an Amberlite CG-400 anion-exchange resin. The recovery of protein is nearly total. The recovered proteins have no detectable sodium dodecyl sulfate contamination. With gels that have been stained with Coomassie Brilliant Blue R, the procedure recovers the proteins free of the dye. We have used this procedure successfully during the purification of epidermal glycoproteins.

Radioassays for quantitation of intact complement proteins C2 and B in human serum

Availability of polyclonal and monoclonal antibodies recognizing determinants on the major cleavage fragments of complement proteins C2 and B enabled development of sensitive radioassays which can be used to quantitate the intact proteins in human sera. Changes in C2 and B concentrations indicative of classical or alternative pathway activation, or both, were seen in normal serum after incubation with complement activators. We determined the normal range (mean +/- 2 SD) of C2 concentration to be 11-35 micrograms/ml in 32 healthy individuals, and that of protein B to be 74-286 micrograms/ml. Sera from patients with systemic lupus erythematosus (SLE), septic shock, infections, and following orthopedic surgery were then assayed. Mean protein B concentration was significantly higher in SLE sera (P = 0.002) and in the infected and post-operative (acute-phase) sera (P less than 0.001), and the mean C2 concentration in the septic shock group (P less than 0.001) was significantly lower than the mean of healthy individuals. Intact C2 was not detected in known C2-deficient individuals. These assays allow parallel quantitation of the structurally and functionally homologous proteins of the classical (C2) and alternative (B) pathways, which is of interest in patients with genetic and acquired hypocomplementemia.

Renal filtration and catabolism of complement protein D

Complement protein D, a serine protease participating in the formation of the C3 convertase of the alternative complement pathway, has the lowest molecular weight (23,750) and serum concentration of all complement proteins. In normal serum, D is the rate-limiting protease of the alternative pathway of complement activation. We report that the serum concentrations of D in 20 patients with chronic renal failure (mean +/- S.D., 0.42 +/- 0.28 mg per deciliter) and in 16 patients on long-term dialysis (1.53 +/- 0.39 mg per deciliter) were significantly higher (P less than 0.001) than in 22 healthy adults (0.18 +/- 0.04 mg per deciliter). In chronic renal failure the serum concentration of D correlated with that of creatinine (r = 0.75, P less than 0.001). The serum concentrations of D found in patients with renal failure reached and in some cases exceeded those at which the protease is no longer rate-limiting. Thus, enhanced activity of the alternative pathway of complement should be expected in patients with advanced renal failure. Urinary D was undetectable (less than 0.2 micrograms per deciliter) in 17 normal adults and either undetectable or below the concentration expected from the degree of proteinuria in 10 patients with nephrotic syndrome. However, in a patient with Fanconi's syndrome the urinary concentration of D (1.3 mg per deciliter) was an order of magnitude higher than the serum concentration, representing 0.5 per cent of the total protein. The urinary D in this patient had normal hemolytic activity, antigenicity, and size. These results indicate that D is filtered through the glomerular membrane and is probably catabolized in the proximal renal tubules.

Structure and activation of complement components C2 and factor B

The activation of complement is initiated by two independent pathways. Each leads to the formation of a complex protease, C3 convertase, with equivalent specificity and function but different composition. The convertase derived from the classical pathway is composed of complement components C4 and C2 while that from the alternative pathway consists of components C3 and Factor B. C2 and Factor B contain the catalytic site of each convertase respectively. The amino acid sequence of Factor B has been determined. Limited sequence of CNBr-peptides isolated from C2 has also been obtained. The two enzymes are shown to be homologous and to represent a novel type of serine proteinase, characterized by their unusual structure and mechanism of activation, when compared to known serine proteinases.

MHC Class III products: an electron microscopic study of the C3 convertases of human complement

We have reported a transmission electron microscopic study of the two C3 convertases of human complement and their precursors. The corresponding proteins and complexes of the classical and alternative pathway appear very similar. Cofactors C3b and C4b are nearly indistinguishable and display a characteristic but highly irregular substructure. C2 and Factor B are globular with diameters of 85 +/- 8 A and 80 +/- 8 A and both consist of three discrete globular domains each approximately 40 A in diameter. Bb and C2a each contain two domains connected by a short linker segment. Both domains of Bb and one domain of C2a are 42 A in diameter (28 kd), while the second domain of C2 is 47 A in diameter (39 kd). Attachment of the enzymatic subunits to cofactors occurs through one domain only.

Amino acid sequence of the Bb fragment from human complement Factor B. Alignment of the cyanogen bromide-cleavage peptides

The alignment of all the CNBr-cleavage peptides of fragment Bb from human Factor B (a component of the alternative pathway of complement) was determined. This was derived from cleavage of the fragment Bb at arginine residues by using trypsin and clostripain. Details of the isolation and amino acid sequences of these peptides are given. Together with previously published N-terminal sequences of the CNBr-cleavage peptides [Christie & Gagnon (1982) Biochem. J. 201, 555-567], this provides the amino acid sequence of the N-terminal half of fragment Bb.

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.

Isolation, characterization and N-terminal sequences of the CNBr-cleavage peptides from human complement Factor B. Localization of a free thiol group and a sequence defining the site cleaved by factor D

Nine CNBr-cleavage peptides from Factor B (a component of the alternative pathway of complement) were isolated. Each was characterized by amino acid analysis and automated Edman degradation. One peptide contained a methionyl bond resistant to cleavage by CNBr. The number of CNBr-cleavage peptides is in agreement with the results of amino acid analysis of Factor B and the fragments Ba and Bb. A total of 358 unique residues were identified from the N-terminal sequences of the CNBr-cleavage peptides. These represent approx. 50% and 60% of the total residues of Factor B and fragment Bb respectively. Alignment of two CNBr-cleavage peptides (CB-VIc and CB-IV) provided a continuous segment of 140 residues. This sequence contained the site cleaved by Factor D to generate the Ba and Bb fragments during the activation of complement. Peptide CB-IV contained a free thiol group at a position corresponding to residue 33 of fragment Bb. Amino sugar analyses of Factor B and of fragments Bb and Ba indicated that all the carbohydrate structures of factor B are N-linked to asparagine through N-acetylglucosamine. The two carbohydrate-attachment sites of the Bb fragment were identified.

Characterization of the CNBr peptides generated from the factor B cleavage fragments, Ba and Bb, by molecular exclusion high performance liquid chromatography

Highly purified human factor B of the alternative complement pathway was treated with factor D in the presence of cobra venom factor to generate its Ba and Bb cleavage fragments. These cleavage fragments were isolated by preparative polyacrylamide gradient gel electrophoresis followed by electrodialysis elution and treatment with CNBr. The resultant CNBr cleavage peptides were isolated by molecular exclusion high performance liquid chromatography and characterized by SDS polyacrylamide gel electrophoresis. Results of these experiments indicate that the Ba fragment essentially consisted of a 28,000 CNBr peptide, whereas 34,700 (28,000 + 3,500 when characterized under reducing polyacrylamide gel electrophoresis conditions); 14,500 (=20,000); and 8,300 CNBr peptides were derived from the Bb fragment.