Complexes in Human Plasma between α1-Antitrypsin and IgA, and α1-Antitrypsin and Fibrinogen

A Review of Alpha-1 Antitrypsin Binding Partners for Immune Regulation and Potential Therapeutic Application

Alpha-1 antitrypsin (AAT) is the canonical serine protease inhibitor of neutrophil-derived proteases and can modulate innate immune mechanisms through its anti-inflammatory activities mediated by a broad spectrum of protein, cytokine, and cell surface interactions. AAT contains a reactive methionine residue that is critical for its protease-specific binding capacity, whereby AAT entraps the protease on cleavage of its reactive centre loop, neutralises its activity by key changes in its tertiary structure, and permits removal of the AAT-protease complex from the circulation. Recently, however, the immunomodulatory role of AAT has come increasingly to the fore with several prominent studies focused on lipid or protein-protein interactions that are predominantly mediated through electrostatic, glycan, or hydrophobic potential binding sites. The aim of this review was to investigate the spectrum of AAT molecular interactions, with newer studies supporting a potential therapeutic paradigm for AAT augmentation therapy in disorders in which a chronic immune response is strongly linked.

Purification, Properties and Extended Solution Structure of the Complex Formed between Human Immunoglobulin A1 and Human Serum Albumin by Scattering and Ultracentrifugation

Immunoglobulin A (IgA) is unique amongst antibodies in being able to form polymeric structures that may possess important functions in the pathology of specific diseases. IgA also forms complexes with other plasma proteins, the IgA1-human serum albumin (HSA) complex (IgA1-HSA) being typical. We have purified this complex using a novel two-step purification based on thiophilic chromatography and gel filtration, and characterised this. HSA is linked covalently to the tailpiece of IgA1 by a disulphide bond between Cys471 in IgA1 and Cys34 in HSA. IgA1-HSA binds to IgA receptors on neutrophils and monocytes, and elicits a respiratory burst that is comparable in magnitude to that of monomeric IgA1. The solution arrangement of IgA1-HSA was identified by X-ray scattering and ultracentrifugation. The radius of gyration R(G) of 7.5(+/-0.3) nm showed that IgA1-HSA is more extended in solution than IgA1 (R(G) of 6.1-6.2 nm). Its distance distribution function P(r) showed two peaks that indicated a well-separated solution structure similar to that for IgA1, and a maximum dimension of 25 nm, which is greater than that of 21 nm for IgA1. Sedimentation equilibrium showed that the IgA1:HSA stoichiometry is 1:1. Sedimentation velocity resulted in a sedimentation coefficient of 6.4S and a frictional ratio of 1.87, which is greater than that of 1.56 for IgA1. The constrained modelling of the IgA1-HSA structure using known structures for IgA1 and HSA generated 2432 conformationally randomised models of which 52 gave good scattering fits. The HSA structure was located at the base of the Fc fragment in IgA1 in an extended arrangement. Such a structure accounts for the functional activity of IgA1-HSA, and supports our previous modelling analysis of the IgA1 solution structure. The IgA1-HSA complex may suggest the potential for creating a new class of targeted therapeutic reagents based on the coupling of IgA1 to carrier proteins.

Inhibition of neutrophil elastase by the alpha1-proteinase inhibitor-immunoglobulin A complex

Neutrophil elastase is thought to be involved in cartilage destruction occurring in rheumatoid arthritis despite the local presence of alpha1-proteinase inhibitor. Part of synovial fluid alpha1-proteinase inhibitor forms a mixed disulfide with immunoglobulin A, which has been postulated to lack inhibitory activity. We show here that the immunoglobulin-inhibitor complex tightly inhibits neutrophil elastase and cathepsin G, bovine pancreatic trypsin and chymotrypsin, and porcine pancreatic elastase. Although the rate constant of inhibition of neutrophil elastase by immunoglobulin A-bound alpha1-proteinase inhibitor (k(ass) = 9.2 X 10(5) M(-1) x s(-1)) is about 10-fold lower than that measured with the free inhibitor, it is high enough to enable efficient inhibition of elastase in vivo.

Role of oxidative mixed-disulfide formation in elastase-serine proteinase inhibitor (serpin) complex

To understand the role of thiol and oxidative mixed-disulfide exchange reaction in serpins, we analyzed the conformation of native and mixed-disulfide forms of alpha 1-proteinase inhibitor (alpha 1-PI), alpha 1-antichymotrypsin (alpha 1-ACT), alpha 2-antiplasmin (alpha 2-AP), angiotensinogen, and ovalbumin. The conformation of native and oxidized mixed-disulfide serpins was measured by transverse urea gradient (TUG) gels. The results suggest that the acute phase proteins alpha 1-PI and alpha 1-ACT undergo conformational changes following oxidative mixed-disulfide formation and that alpha 2-AP and angiotensinogen do not. The kinetics of disulfide formation was followed by measuring changes in absorbance at 412 nm resulting from Ellman's reaction of disulfide exchange. The rate of mixed-disulfide formation in albumin was 10-fold faster than in the serpin tested. The rate of disulfide exchange in alpha 1-PI was 2-fold faster than that of alpha 1-ACT. However, disulfide formation in alpha 1-PI and alpha 1-ACT was much slower than for any other serpin, e.g., alpha 2-AP and angiotensinogen. We present evidence that alpha 1-PI forms a dimer sensitive to thiol reduction, suggesting cysteinyl-mediated dimerization of alpha 1-PI. The alpha 1-PI also demonstrated two types of inter-protein disulfide linkages: one resulting in homodimer and other involving heterodimer formation. TUG-Western immunoblot methodology was developed to identify the conformational changes in serpins. We found that the conformational changes in serpins by mixed-disulfide formation are due to unfolding and not to decomposition or degradation in TUG gels. Using fluorescence measurements with isolated tryptic fragments of fluorescence-labelled elastase, we observed that the cysteinyl232 in alpha 1-PI interacted with the cysteinyl168 of elastase in the proteinase-inhibitor complex. Our data suggests that serpin thiols may play an important role in forming a stable serpin-proteinase complex.

The structure and evolution of vertebrate fibrinogen: a comparison of the lamprey and mammalian proteins

The blood plasmas of all vertebrate animals contain a six-chained fibrinogen molecule that is polymerized into fibrin upon the thrombin-catalyzed removal of fibrinopeptides. In all cases, also, the polymerization reaction is inhibited by Gly-Pro-Arg-ending peptides. The complete amino acid sequences of human, rat and lamprey fibrinogens are known, permitting an assessment of just which sequence features are essential for polymerization. To an extent, the same approach can also be applied to the associated phenomena of fibrin cross-linking by factor XIII, plasminogen and plasminogen activator binding, and vessel wall-fibrinogen interactions.

Aggregation of plasma Z type alpha 1-antitrypsin suggests basic defect for the deficiency

The abnormal type of alpha 1-antitrypsin, PI (protease inhibitor) type Z, is associated with inclusion bodies in the liver, which contain non-secreted alpha 1-antitrypsin. Our studies show that Z protein has an inherent tendency to aggregate, even in plasma. Depending upon conditions, from 15 to 70% of the Z protein in plasma was in a high-Mr form, compared with 1.5% of M type alpha 1-antitrypsin. The high-Mr complex in plasma cannot be disaggregated using Triton X detergent or reducing conditions. This increased tendency to aggregate can be explained by the mutation affecting, tertiary structure and salt bridge formation in Z protein. We have observed this same tendency to aggregate for Mmalton alpha 1-antitrypsin, a rarer variant also associated with a plasma deficiency.

Isolation and characterization of an inter-alpha-trypsin inhibitor-IgG complex from human serum

Inter-alpha-trypsin inhibitor is a human serum protease inhibitor of Mr 180 000 which may release physiological derivatives. A complex between IgG and an inter-alpha-trypsin inhibitor derivative of Mr 30 000 has been recently detected in human serum and was found to be inactive against trypsin, in contrast with the known inhibitory activity of the free 30-kDa derivative. The present study deals with detailed characterization of an inter-alpha-trypsin inhibitor-IgG complex following its purification by affinity chromatography techniques (anti-inter-alpha-trypsin inhibitor immunoadsorbent and Protein A-Sepharose) in mild conditions. The resulting product reacted simultaneously with anti-IgG and anti-inter-alpha-trypsin inhibitor antibodies. This complex contained Mr 180 000 inhibitor at least to some extent. It migrated in the beta-gamma zone in agarose; its molecular weight was estimated to be 1 500 000 or more; part of it displayed covalent bonding between inter-alpha-trypsin inhibitor and IgG; it had a trypsin inhibitor activity. Immunoelectrophoresis allowed one to demonstrate the native complex in serum owing to the use of anti-inter-alpha-trypsin inhibitor and anti-gamma radioactively labelled antibodies. The double immunoreactivity thus evidenced proved to be heterogeneous with respect to its level and location in the native as well as in the purified complex.

Crossed immunoelectrophoresis: qualitative and quantitative considerations

Invented 20 years age, crossed immunoelectrophoresis (X-IEP) today is a technique of unusual power and myriad application. It combines very high resolution with exquisite specificity by alloying 2-dimensional electrophoresis with immunoprecipitation for symbiotic new potentialities. The consequent matchless quantitative/qualitative capabilities of X-IEP for analyzing antigens in complex mixtures, particularly by their idiomatic internal comparison, are still not widely recognized. Because of this and the supposed complications of its use and interpretation, X-IEP is more rarely used than it should be. This essay discusses contemporary X-IEP with the particular aims of demonstrating that it is not difficult to use and of explaining with selected examples why it is peculiarly powerful for analyzing antigen mixtures like the body fluids, tissue and cell extracts, and microbial homogenates.

Identification and incidence of a urinary fragment of IgG

Urine from some patients, when concentrated approximately three hundred fold and immunoelectrophoresed against anti-IgG, shows an unexpected additional precipitin line in the alpha-globulin region. This reaction has been shown to be due to the presence of a low molecular weight (approximately 20 000) fragment of the heavy chain of IgG. A retrospective examination of immunoelectrophoretic plates run over a period of five years has revealed that this fragment was present in 30 out of 110 patients.

D-penicillamine treatment in rheumatoid arthritis monitored by plasma alfa-1-antitrypsin-IgA complexes and plasma and urinary sulphur containing aminoacids

D-Penicillamine treatment in rheumatoid arthritis resulted in a fall in plasma alfa-1-antitrypsin-IgA complexes which was significantly more pronounced among responders than among non-responders. Plasma free cystine levels also fell during D-penicillamine treatment, the fall being dose dependent up to a daily dose of 500 mg. Higher doses did not result in further lowering, and no difference was detected between responders and non responders. The dominating sulphur containing aminoacid excreted in the urine was penicillamine-cysteine disulphide, the concentration of which did not correlate to either toxicity or clinical effectiveness, but showed much individual variation. The slow kinetics of change in the complex concentration taken together with in vitro experiments, suggest that the effect of D-penicillamine is more likely to be on the de novo formation of the complexes, rather than on their reduction.