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

Arylamine N-acetyltransferases: from drug metabolism and pharmacogenetics to drug discovery

Arylamine N-acetyltransferases (NATs) are polymorphic drug-metabolizing enzymes, acetylating arylamine carcinogens and drugs including hydralazine and sulphonamides. The slow NAT phenotype increases susceptibility to hydralazine and isoniazid toxicity and to occupational bladder cancer. The two polymorphic human NAT loci show linkage disequilibrium. All mammalian Nat genes have an intronless open reading frame and non-coding exons. The human gene products NAT1 and NAT2 have distinct substrate specificities: NAT2 acetylates hydralazine and human NAT1 acetylates p-aminosalicylate (p-AS) and the folate catabolite para-aminobenzoylglutamate (p-abaglu). Human NAT2 is mainly in liver and gut. Human NAT1 and its murine homologue are in many adult tissues and in early embryos. Human NAT1 is strongly expressed in oestrogen receptor-positive breast cancer and may contribute to folate and acetyl CoA homeostasis. NAT enzymes act through a catalytic triad of Cys, His and Asp with the architecture of the active site-modulating specificity. Polymorphisms may cause unfolded protein. The C-terminus helps bind acetyl CoA and differs among NATs including prokaryotic homologues. NAT in Salmonella typhimurium supports carcinogen activation and NAT in mycobacteria metabolizes isoniazid with polymorphism a minor factor in isoniazid resistance. Importantly, nat is in a gene cluster essential for Mycobacterium tuberculosis survival inside macrophages. NAT inhibitors are a starting point for novel anti-tuberculosis drugs. Human NAT1-specific inhibitors may act in biomarker detection in breast cancer and in cancer therapy. NAT inhibitors for co-administration with 5-aminosalicylate (5-AS) in inflammatory bowel disease has prompted ongoing investigations of azoreductases in gut bacteria which release 5-AS from prodrugs including balsalazide.

Chemical labelling of active serum thioester proteins for quantification

The complement serum proteins C3 and C4 and the protease inhibitor α-2 macroglobulin are all members of the C3/α-2M thioester protein family, an evolutionarily ancient and conserved family that contains an intrachain thioester bond. The chemistry of the thioester bond is a key to the function of the thioester proteins. All these proteins function by covalently linking to their target by acyl transfer of the protein via the thioester moiety. We show that the signature thioester bond can be targeted with nucleophiles linked to a bioreporter molecule, site-specifically modifying the whole, intact thioester protein. Conditions were optimised to label selectively and efficiently pull-down unprocessed thioester-containing proteins from serum. We demonstrated pull-down of full-length C3, α-2M and C4 from sera in high salt, using a biotinylated nucleophile and streptavidin-coated resin, confirmed by MALDI-TOF MS identification of the gel bands. The potential for the development of a quantitative method for measuring active C3 in serum was investigated in patient sera pre and post operation. Quantifying active C3 in clinical assays using current methods is difficult. Methods based on antibody detection (e.g. nephelometry) do not distinguish between active C3 and inactive breakdown products. C3-specific haemolytic assays can be used, but these require use of relatively unstable reagents. The current work represents a promising robust, enzyme- and antibody-free chemical method for detecting active thioester proteins in blood, plasma or serum.

Defective prevention of immune precipitation in autoimmune diseases is independent of C4A*Q0

Increased prevalence of C4 null alleles is a common feature of autoimmune diseases. We have shown previously that complement-dependent prevention of immune precipitation (PIP) is defective in patients with systemic lupus erythematosus (SLE), and correlated this defect with C4A*Q0 and low levels of the C4A isotype. To further clarify the role of C4A in the aetiology of SLE, we now extend our studies to other diseases which have been associated with C4A*Q0. The frequency of C4A*Q0 was increased in Icelandic patients with coeliac disease (0.50; P < 0.001), Grave's disease (0.30; P = 0.002) and insulin-dependent diabetes mellitus (0.23; P = 0.04) and in British patients with dermatitis herpetiformis (0.42; P = 0.002) and this was reflected in low levels of C4A. In spite of this, PIP was normal in these patients, and in marked contrast to our previous observations on connective tissue diseases, PIP measurements in these patient groups correlated more strongly with levels of C4B (r = 0.51, P = 0.0000004) than C4A. Patients with increased levels of anti-C1q antibodies had significantly lower PIP than patients without such antibodies (P < 0.01) and a negative association of PIP with anti-C1q antibodies was also reflected in an increased prevalence (P = 0.006) and levels (P = 0.006) of anti-C1q antibodies in patients with subnormal PIP, as well as a negative correlation between PIP and anti-C1q antibodies (r = - 0.25, P = 0.02). These results show that the PIP defect cannot be explained by low levels of C4A alone and suggest that measurements of anti-C1q antibodies may be useful in future studies on the molecular cause of the PIP defect in autoimmune connective tissue disease.

Patients with systemic lupus erythematosus are deficient in complement-dependent prevention of immune precipitation

OBJECTIVE

A functional deficiency of complement has been implicated but not conclusively demonstrated in the pathogenesis of systemic lupus erythematosus (SLE). To test this, we studied several aspects of complement in 44 patients with SLE, 46 patients with rheumatoid arthritis and 102 blood donors.

METHODS

Prevention of immune precipitation (PIP) was measured by an enzyme immunoassay, levels of C1q, C4 and C3 by rocket immunoelectrophoresis, C4A, C4B and C3d by enzyme-linked immunosorbent assay (ELISA), complement haemolysis (CH50) by standard methods and C4 allotypes by high-voltage agarose electrophoresis and sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE).

RESULTS

PIP was significantly reduced in SLE (P<0.001); the defect was revealed by a sensitive assay measuring this function of complement but not by the other tests employed. The patients were clinically well at the time of study, and levels of C3d, which have been shown to correlate with disease activity, were normal. The defect was more common in patients with early disease (P = 0.009), supporting a role in aetiology or early pathophysiology. PIP was positively correlated with levels of C4 (P = 3 x 10(-5)) and in particular the C4A isotype (P = 9 x 10(-10)) whereas C4B was redundant.

CONCLUSIONS

Our results reveal a defect in prevention of immune precipitation in SLE that is apparent at an early stage in the disease and correlates with low levels of C4A. These results indicate that subtle deficiencies of complement may predispose to SLE.

Mechanisms of drug eruptions: Part I

The pathogenetic mechanisms underlying common, and less common but severe, adverse cutaneous drug reactions are reviewed. Pharmacogenetic variability may account for a susceptibility to serious drug reactions to sulphonamides and anticonvulsants, as well as to lupus erythematosus (LE)-like syndrome. Exanthematous drug reactions may have an immunological basis. Cell mediated cutaneous drug reactions, including lichenoid reactions, LE-like syndrome, fixed drug eruption, erythema multiforme, Stevens-Johnson syndrome and toxic epidermal necrolysis, will inevitably involve elements of the skin immune system. Graft-versus-host disease provides a useful model for aspects of these drug-induced disorders. Urticaria, angioedema, anaphylaxis and anaphylactoid reactions may involve Type I immunoglobulin (Ig)-mediated or Type III hypersensitivity, or may be caused by pharmacological, non-allergic means. Drug-induced vasculitis, serum sickness and the Arthus phenomenon are manifestations of the immune complex disease. Drug-induced pemphigus may involve immune dysregulation, but several thiol-containing drugs are able to cause antibody-independent acantholysis directly.

Effect of thiol compounds on human complement component C4

Thiol compounds have been investigated as inhibitors of the covalent binding reaction of human complement protein C4 using Sepharose-C1s as a combined activating and binding surface. o- and p-substituted aminothiophenols are equally effective inhibitors, whereas the m-substituted compound is a less potent inhibitor. The anti-hypertensive drug captopril is also shown to inhibit the covalent binding reaction. A comparison of the effects of these compounds on the covalent binding reaction of isolated C4A and C4B has been made. Results suggest that a Pro-to-Leu substitution in C4B is likely to account for the differences in inhibitory potency of C4B compared with C4A observed with the aromatic inhibitors.

Role for specific complement phenotypes and deficiencies in the clinical expression of IgA nephropathy

IgA nephropathy, the most commonly occurring type of chronic glomerulonephritis in individuals of European and Asian descent, exhibits marked heterogeneity of clinical signs and ultimate prognosis. Based upon their studies of regional clustering of the ancestors of related patients in eastern Kentucky, the authors have postulated the existence of an inherited disease susceptibility for IgA nephropathy. They examined serum concentrations of individual complement proteins and phenotypes for C3, C4A, C4B, and factor B (Bf) for related and unrelated patients with IgA nephropathy from Kentucky and for patients from the Mid-South region of Tennessee, Mississippi, and Alabama. In these populations, they have described partial complement deficiencies or specific phenotypes which may be associated with the disease. Their findings include the following: (1) partial deficiencies for C2, beta 1H (H), properdin (P), or C4 binding protein (C4BP) in four patients with end-stage renal disease, (2) an association between the C3*F allele with IgA nephropathy in the combined group of unrelated patients from Kentucky and the Mid-South, (3) the occurrence of C4B deficiency in two siblings with IgA nephropathy, and (4) an association between C4A deficiency and poor outcome in patients with IgA nephropathy diagnosed as adults. In addition, the related patients differ from the unrelated patients from Kentucky with respect to frequencies of Bf*F and the BfF (FF + FS + F1F + F1S) phenotype, suggestive of immunogenetic difference between these groups. Important functional differences exist between C4A and C4B isotypes and functional differences are also possible based upon C3 or Bf phenotype.(ABSTRACT TRUNCATED AT 250 WORDS)

Drug metabolism by leukocytes and its role in drug-induced lupus and other idiosyncratic drug reactions

This review presents a unifying hypothesis that provides a connection between several types of hypersensitivity reactions associated with several types of drugs and explains some of the therapeutic effects (antiinflammatory activity and antithyroid effects) of these same drugs. This hypothesis centers on the oxidation of these drugs to chemically reactive metabolites by peroxidases. The drugs of interest have functional groups that are easily oxidized. The major peroxidase involved in this hypothesis is MPO because of its critical location in leukocytes which play a key role in the function of the immune system. However, thyroid peroxidase can probably also oxidize many of the same drugs to reactive metabolites, and this may be responsible for the thyroid autoimmunity observed in connection with some hypersensitivity reactions. Peroxidases have also been described in the skin and in platelets, and their presence may be responsible for the high incidence of skin reactions in the hypersensitivity response and the occurrence of immune-mediated thrombocytopenia, respectively. Involvement of other peroxidases, such as prostaglandin peroxidase, may also be important for antiinflammatory effects of drugs. In addition, leukocytes contain prostaglandin synthetase, and the activation of leukocytes leads to the release of arachidonic acid and the production of prostaglandins. This process may also lead to the metabolism of drugs to reactive metabolites. In studies of the metabolism of procainamide and dapsone, aspirin and indomethacin did not inhibit the formation of the hydroxylamine by neutrophils and mononuclear leukocytes. This is evidence against the involvement of prostaglandin synthetase in these oxidation; however, preliminary studies with other drugs suggest that prostaglandin synthetase may contribute to the metabolism of some drugs by leukocytes. Furthermore, the metabolism of phenylbutazone, phenytoin, and tenoxicam, as well as our preliminary work with other drugs such as carbamazepine, suggests that the range of drugs that are metabolized to reactive metabolites by peroxidases may be broader than initially suspected. There are several other drugs that do not fit into the functional group classes covered in this review but have similar properties. A good example is alpha-methyldopa, which is associated with drug-induced lupus, immune-mediated hemolytic anemia, and other hypersensitivity reactions. Such drugs may also be metabolized to reactive metabolites by peroxidases. Another aspect of the hypothesis is that an infection, or other inflammatory condition, may be an important risk factor for a hypersensitivity reaction because such a stimulus leads to activation of leukocytes which can lead to formation of reactive metabolites from certain drugs.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibition of the covalent binding reaction of complement component C4 by penicillamine, an anti-rheumatic agent

D(-)-Penicillamine [D(-)-beta beta-dimethylcysteine] is an anti-arthritic drug, but its use is limited by adverse side effects, which include problems in immune-complex clearance. Complement is important as a source of inflammatory mediators in rheumatoid arthritis and is also involved in immune-complex clearance. Thus inhibition of the complement cascade would be likely to contribute to both the therapeutic and the toxic effects of penicillamine. It is shown that penicillamine and cysteine are potent inhibitors of the covalent binding of activated complement component C4 to immune complexes. [35S]Cysteine itself becomes covalently bound to C4b through the thioester site. Penicillamine and cysteine are more reactive with the C4A isotype than with the C4B isotype of the HLA class III protein C4. The limited amino acid sequence differences between C4A and C4B include a cysteine/serine interchange, and it is suggested that the cysteine residue in C4A contributes to the increased rate of reaction of C4A with the alpha-amino-beta-thiol compounds.

Immunogenetics of SLE and primary Sjögren's syndrome

SLE is a syndrome defined by clinical criteria and by the presence of autoantibodies reactive with nucleic acids and proteins concerned with transcription and translation. Breeding experiments in mice have illustrated the enormous genetic heterogeneity of this syndrome, of which the final common pathway is a widespread immune complex disease. The causes of SLE in humans are likely to be equally multifactorial. Family studies have demonstrated that genetic factors exist, but each factor appears to be a relatively weak disease-susceptibility gene. The major exceptions to this are the very rare complete deficiencies of classical pathway complement components, which are almost invariably accompanied by the development of SLE. Observations of these patients have led to the formulation of hypotheses relating complement and its receptor, CR1, to the defective removal of immune complexes from the circulation.

Molecular mechanisms of autoimmunity

Autoimmune diseases result from a combination of genetic susceptibility factors and exogenous influences such as infection or chemical (including drug) exposure. Germline DNA variations in genetic type as well as defects in antigen recognition acquired during thymic education of developing T-lymphocytes both contribute to impaired self: nonself discrimination and set the stage for later development of such diseases as myasthenia gravis, polymyositis, or systemic lupus erythematosus. In addition, drugs such as D-penicillamine, hydralazine, procainamide, or quinidine induce T-cell or B-cell changes which precipitate auto-reactivity and cause drug-induced disease. Intervention in autoimmune diseases with prednisone, alkylating agents or the future use of more selective monoclonal antibody reagents may be life-saving in some of these disorders.

Metabolites of procainamide and practolol inhibit complement components C3 and C4

Drug-induced systemic lupus erythematosus arises from toxic side-effects of administration of hydralazine, isoniazid, procainamide and practolol. Hydralazine and isoniazid are nucleophilic drugs and inhibit the covalent binding reaction of complement components, C3 and C4, an effect likely to lead to deposition of immune complexes (a feature of systemic lupus erythematosus). Procainamide and practolol do not themselves inhibit C3 and C4. A range of metabolites and putative metabolites of procainamide and practolol were synthesized, and tested for their ability to inhibit the covalent binding reactions of C3 and C4. The highly nucleophilic hydroxylamine metabolite of procainamide was strongly inhibitory in both tests, as was a putative hydroxylamine metabolite of practolol. These studies indicate a potential role for the hydroxylamine metabolites in mediating the toxic side-effects of procainamide and practolol, and emphasize the need for adequate measurements of hydroxylamine metabolites in human tissue.

Phenotyping of human complement component C4, a class-III HLA antigen

The plasma complement protein C4 is encoded at two highly polymorphic loci, A and B, within the class-III region of the major histocompatibility complex. At least 34 different polymorphic variants of human C4 have been identified, including non-expressed or 'null' alleles. The main method of identification of C4 polymorphic allotypes is separation on the basis of charge by agarose-gel electrophoresis of plasma. On staining by immunofixation with anti-C4 antibodies, each C4 type gives three major bands, but, since individuals can have up to five allotypes, the overlapping banding pattern is difficult to interpret. We show that digestion of plasma samples with carboxypeptidase B, which removes C-terminal basic amino acids, before electrophoresis, produces a single, sharp, distinct band for each allotype and allows identification of the biochemical basis of the multiple banding pattern previously observed in C4 phenotype determination.