Structural and biological properties of human recombinant myeloperoxidase produced by Chinese hamster ovary cell lines

Structure and function of recombinant cobra venom factor

Cobra venom factor (CVF) is the complement-activating protein from cobra venom. It is a structural and functional analog of complement component C3. CVF functionally resembles C3b, the activated form of C3. Like C3b, CVF binds factor B, which is subsequently cleaved by factor D to form the bimolecular complex CVF,Bb. CVF,Bb is a C3/C5 convertase that cleaves both complement components C3 and C5. CVF is a three-chain protein that structurally resembles the C3b degradation product C3c, which is unable to form a C3/C5 convertase. Both C3 and CVF are synthesized as single-chain prepro-proteins. This study reports the recombinant expression of pro-CVF in two insect cell expression systems (baculovirus-infected Sf9 Spodoptera frugiperda cells and stably transfected S2 Drosophila melanogaster cells). In both expression systems pro-CVF is synthesized initially as a single-chain pro-CVF molecule that is subsequently proteolytically processed into a two-chain form of pro-CVF that structurally resembles C3. The C3-like form of pro-CVF can be further proteolytically processed into another two-chain form of pro-CVF that structurally resembles C3b. Unexpectedly, all three forms of pro-CVF exhibit functional activity of mature, natural CVF. Recombinant pro-CVF supports the activation of factor B in the presence of factor D and Mg2+ and depletes serum complement activity like natural CVF. The bimolecular convertase pro-CVF,Bb exhibits both C3 cleaving and C5 cleaving activity. The activity of pro-CVF and the resulting C3/C5 convertase is indistinguishable from CVF and the CVF,Bb convertase. The ability to produce active forms of pro-CVF recombinantly ensures the continued availability of an important research reagent for complement depletion because cobra venom as the source for natural CVF will be increasingly difficult to obtain as the Indian cobra is on the list of endangered species. Experimental systems to express pro-CVF recombinantly will also be invaluable for studies to delineate the structure and function relationship of CVF and its differences from C3 as well as to generate human C3 derivatives with CVF-like function for therapeutic complement depletion ("humanized CVF").

Expression of myeloperoxidase (MPO) by neutrophils is necessary for their activation by anti-neutrophil cytoplasm autoantibodies (ANCA) against MPO

Anti-neutrophil cytoplasm autoantibodies (ANCA) directed against proteinase-3 and myeloperoxidase (MPO) activate tumor necrosis factor-alpha-primed neutrophils in vitro. We used neutrophils from one completely and one partially MPO-deficient donor to assess the requirement of MPO expression for neutrophil activation by anti-MPO antibodies. The MPO deficiencies were defined enzymatically, by immunocytochemistry and by immunoblotting. The mutations in the MPO genes of these donors were identified as a combination of a novel splice-site mutation at the 3' end of intron 11 (A-2-->C), a deletion of 14 nucleotides in exon 9 (A1555-C1568), and a novel C1907 --> T (636Thr-->Met) substitution in exon 11 in the completely MPO-deficient donor and as the same splice-site mutation and a novel C995 --> T (332Ala-->Val) substitution in exon 7 in the partially MPO-deficient donor. Monoclonal antibody 4.15 against MPO and MPO-ANCA-immunoglobulin G induced no superoxide anion production in these MPO-deficient neutrophils despite a normal production induced by other stimuli. Thus, the presence of MPO is a conditio sine qua non for neutrophil activation by anti-MPO antibodies. Moreover, we demonstrated that by means of these MPO-deficient cells, hydrogen peroxide may diffuse from neutrophils to surrounding cells, which may contribute to the damage induced by oxygen radicals in the pathology of systemic vasculitides.

Organic and inorganic substrates as probes for comparing native bovine lactoperoxidase and recombinant human myeloperoxidase

The interaction of native bovine lactoperoxidase (nbLPO) with four substrates has been studied and compared with that of recombinant human myeloperoxidase (rhMPO). Kinetic, spectroscopic and binding parameters extrapolated for each enzyme-substrate adduct have been interpreted in the light of the structural data available for myeloperoxidase (X-ray structure) and lactoperoxidase (3D-model), respectively. The differences in the reactivity and affinity of nbLPO and rhMPO towards SCN(-), catechol, dopamine and 3,4-dihydroxyphenylpropionic acid are here discussed and related to a different structure of the organic substrate access channel as well as to a different accessibility of the heme pocket in the two enzymes.

Alveolar macrophage activation by myeloperoxidase: a model for exacerbation of lung inflammation

Inflammation of the lung is characterized by the influx of increased numbers of various leukocytes including polymorphonuclear leukocyte (PMN) neutrophils. In addition to cells, numerous studies have pointed to the role of tumor necrosis factor-alpha in the inflammatory process. This study addresses a previously unrecognized interaction between neutrophil-derived myeloperoxidase (MPO) and resident alveolar macrophages (AMø). Rat AMø exposed to either enzymatically active recombinant MPO or enzymatically inactive MPO (iMPO) exhibited an increased respiratory burst (RB). When iMPO was employed, the enhancement of the RB was greater than that observed with MPO. Although the RB was greater with iMPO, macrophage (Mø)-mediated intracellular candidic activity was equivalent for both MPO and iMPO. It is known that pro- inflammatory cytokines contribute to the inflammatory process. When rat AMø were exposed to both forms of myeloperoxidase, iMPO demonstrated greater upregulation of cytokine genes as well as product. These data suggest that at the site of inflammation, neutrophil-derived MPO and iMPO stimulate AMø, resulting in an increased inflammatory and cytotoxic state, and thereby contributing to the general lung inflammatory response.

Asp-225 and glu-375 in autocatalytic attachment of the prosthetic heme group of lactoperoxidase

The heme in lactoperoxidase is attached to the protein by ester bonds between the heme 1- and 5-methyl groups and Glu-375 and Asp-275, respectively. To investigate the cross-linking process, we have examined the D225E, E375D, and D225E/E375D mutants of bovine lactoperoxidase. The heme in the E375D mutant is only partially covalently bound, but exposure to H(2)O(2) results in complete covalent binding and a fully active protein. Digestion of this mutant shows that the heme is primarily bound through its 5-methyl group. Excess H(2)O(2) increases the proportion of the doubly linked species without augmenting enzyme activity. The D225E mutant has little covalently bound heme and a much lower activity, neither of which are significantly increased by the addition of heme and H(2)O(2). The heme is linked to this protein through a single bond to the 1-methyl group. The D225E/E375D mutant has no covalently bound heme and no activity. A small amount of iron 1-hydroxymethylprotoporphyrin IX is obtained from the wild-type enzyme along with the predominant dihydroxylated derivative. The results establish that a single covalent link suffices to achieve maximum catalytic activity and suggest that the 5-hydroxymethyl bond may form before the 1-hydroxymethyl bond.

Glu375Gln and Asp225Val mutants: about the nature of the covalent linkages between heme group and apo-Protein in bovine lactoperoxidase

In analogy with studies previously reported for myeloperoxidase (Kooter, I. M.; Moguilevsky, N.; Bollen, A.; Van der Veen, L. A.; Otto, C.; Dekker, H. L.; Wever, R. J. Biol. Chem. 1999, 274, 26794), we examined for bovine lactoperoxidase the effect of mutation of Asp225 and Glu375, the residues thought to be responsible for the covalent binding of the heme group to the apoprotein. Starting from the plasmid encoding rbLPO (Watanabe, S.; Varsalona, F.; Yoo, Y.; Guillaume, J. P.; Bollen, A.; Shimazaki, K.; Moguilevsky, N. FEBS Letters 1998, 441, 476), which was engineered to carry mutations in correspondence of those residues, the mutants Asp225Val and Glu375Gln were expressed in CHO cells and their products purified and characterized. Unequivocal evidence about the existence of ester linkages as well as their relative contribution to the specific spectroscopic and catalytic properties of bLPO is here discussed.

A transient kinetic study on the reactivity of recombinant unprocessed monomeric myeloperoxidase

Spectral and kinetic features of the redox intermediates of human recombinant unprocessed monomeric myeloperoxidase (recMPO), purified from an engineered Chinese hamster ovary cell line, were studied by the multi-mixing stopped-flow technique. Both the ferric protein and compounds I and II showed essentially the same kinetic behavior as the mature dimeric protein (MPO) isolated from polymorphonuclear leukocytes. Firstly, hydrogen peroxide mediated both oxidation of ferric recMPO to compound I (1.9 x 10(7) M(-1) s(-1), pH 7 and 15 degrees C) and reduction of compound I to compound II (3.0 x 10(4) M(-1) s(-1), pH 7 and 15 degrees C). With chloride, bromide, iodide and thiocyanate compound I was reduced back to the ferric enzyme (3.6 x 10(4) M(-1) s(-1), 1.4 x 10(6) M(-1) s(-1), 1.4 x 10(7) M(-1) s(-1) and 1.4 x 10(7) M(-1) s(-1), respectively), whereas the endogenous one-electron donor ascorbate mediated transformation of compound I to compound II (2.3 x 10(5) M(-1) s(-1)) and of compound II back to the resting enzyme (5.0 x 10(3) M(-1) s(-1)). Comparing the data of this study with those known from the mature enzyme strongly suggests that the processing of the precursor enzyme (recMPO) into the mature form occurs without structural changes at the active site and that the subunits in the mature dimeric enzyme work independently.

Native and recombinant proteins to analyze auto-antibodies to myeloperoxidase in pauci-immune crescentic glomerulonephritis

The prevalence of Anti-Neutrophil Cytoplasmic Antibodies (ANCA) directed against myeloperoxidase (MPO) in pauci-immune necrotizing crescentic glomerulonephritis (NCGN) is dependent on the assay(s) used. We investigated the frequency of MPO-ANCA as detected by different assays for MPO-ANCA in a large cohort of patients with biopsy-proven pauci-immune NCGN. Sera from 121 consecutive untreated patients presenting with pauci-immune NCGN were tested for ANCA directed to proteinase-3 (PR3) at diagnosis. PR3-ANCA negative sera were tested by direct ELISA using recombinant or native MPO and by capture ELISA using two different specific monoclonal antibodies directed to MPO and three different antigenic sources. Sera from 80 relevant disease controls were tested to explore the specificity of the different assays. Thirty-eight out of 121 patients (31%) with pauci-immune NCGN did not have PR3-ANCA. Sufficient amounts of serum from 30 of these 38 PR3-ANCA negative patients were available for further testing. Recombinant and native MPO were recognized by similar numbers of sera in a direct ELISA (recombinant MPO: 93%, native MPO: 93%) and a capture ELISA (recombinant MPO: 77-87%, native MPO: 93%). Sera of patients with PR3-ANCA positive pauci-immune NCGN and disease controls were less frequently positive for MPO-ANCA in a capture ELISA (recombinant MPO: 3-7%, native MPO: 6-7%) than in a direct ELISA (recombinant MPO: 25%, native MPO: 13%). Both direct and capture ELISA assays using either native or recombinant MPO are sensitive techniques to detect MPO-ANCA in patients with pauci-immune NCGN. A capture ELISA performs better than a direct ELISA because it combines a higher specificity with a comparable sensitivity. Recombinant MPO is a good alternative for native MPO when used as antigen in a capture ELISA, but not when used in a direct ELISA because of lower specificity in this latter assay.

Inhibition of the myeloperoxidase chlorinating activity by non-steroidal anti-inflammatory drugs investigated with a human recombinant enzyme

Non-steroidal anti-inflammatory drugs (NSAIDs) were investigated for their ability to affect the chlorinating activity of human myeloperoxidase and to scavenge HOCl, the main myeloperoxidase system product. Fourteen drugs representative of various NSAIDs families were tested with the chlorination of taurine used as a detection system. All were unable to inhibit taurine chlorination in a system without myeloperoxidase. In contrast, most of them induced a dose-dependent inhibition of the taurine chlorination mediated by a myeloperoxidase/H2O2/Cl- system. This took place at variable drug concentrations and IC50 were calculated. The inhibitory effect was therefore due to a direct interaction with the enzyme rather than to HOCl scavenging. A spectroscopic method used to measure the myeloperoxidase compound II lifetime in presence of the different drugs showed that all the drugs, which inhibited chlorination activity were able to induce accumulation of compound II. The extent of chlorinating activity inhibition (IC50) was inversely related to the duration of the block of enzyme in compound II form. This further demonstrates that myeloperoxidase is an interesting target for anti-inflammatory therapy. The recombinant myeloperoxidase used for the first time in this kind of study was as convenient for pharmacological purposes as the purified one.

Impact of missense mutations on biosynthesis of myeloperoxidase

We have examined the biosynthesis of normal and mutant forms of myeloperoxidase (MPO) in order to gain insights into the critical features of normal biogenesis of MPO. The expression of wild-type and mutant forms of MPO in a stably transfected cell line devoid of endogenous MPO as well as in established human promyelocytic cell lines has allowed understanding of several features of MPO biosynthesis. It is clear that heme insertion into apoproMPO is necessary for proper folding, egress from the endoplasmic reticulum (ER), and eventual entry into the maturation pathway. In addition, molecular chaperones calreticulin and calnexin interact with normal MPO precursors in a sequential and regulated fashion. Studies of naturally occurring mutants, specifically missense mutations underlying inherited MPO deficiency, and mutations in putatively important residues in MPO have highlighted special features of the ER quality control system in the context of MPO biosynthesis. With identification of additional genotypes of MPO deficiency and the recent solution of MPO crystal structure at 1.8 A, this approach provides a powerful technique to assess structure-function relationships in MPO that are likely applicable to other members of the family of animal peroxidases.

The endothelium and cytokine secretion: the role of peroxidases as immunoregulators

The endothelium is frequently exposed to many proinflammatory mediators. The present study was done to determine the effects of human recombinant myeloperoxidase (MPO) and porcine eosinophil peroxidase (EPO) on certain endothelial cell (HUVEC) functions. The following areas were evaluated: (1) production of reactive oxygen intermediates (ROI), (2) cytokine secretion, and (3) regulation of mRNA cytokine transcripts. Both MPO and EPO induced the production of ROI, but an enzymatically inactive form of MPO (iMPO) was the most effective. Enzymatically inactive MPO, but not MPO, induced the secretion of interleukins 6 and 8 and granulocyte-monocyte colony-stimulating factor. A ribonuclease protection assay indicated that both iMPO and MPO upregulated mRNA cytokine transcripts; however, the former was markedly more effective. The simultaneous addition of EPO and iMPO resulted in a decrease in cytokine-specific mRNA. These data indicate a major role for peroxidases in the regulation of inflammation.

Large-scale purification of myeloperoxidase from HL60 promyelocytic cells: characterization and comparison to human neutrophil myeloperoxidase

A large-scale purification procedure was developed for the isolation of myeloperoxidase from HL60 promyelocytic cells in culture. Initial studies showed the bulk of peroxidase-positive myeloperoxidase activity to be located in the cetyltrimethylammonium bromide solubilized particulate fraction of cell homogenates. The myeloperoxidase was then chromatographically purified using concanavalin A followed by gel filtration. SDS-PAGE analysis of the final preparation showed the presence of only two proteins with molecular masses of approximately 55 and 15 kDa, corresponding to the large and small subunits of myeloperoxidase. These data, along with Reinheit Zahl (RZ) values (A(430)/A(280)) of greater than or equal to 0.72, indicate that the myeloperoxidase prepared by this method is apparently homogeneous. Preparations routinely yielded 12-20 mg of pure myeloperoxidase per 10 ml of cell pellet. The HL60 myeloperoxidase was shown to be indistinguishable from purified human neutrophil myeloperoxidase by size exclusion chromatography, analytical ultracentrifugation, SDS-PAGE, Western blot, and NH(2)-terminal sequence analysis. The activities of the two myeloperoxidase samples, as measured using either the tetramethylbenzidine or the taurine chloramine assay, were indistinguishable. Finally, both enzymes responded identically to dapsone and aminobenzoic acid hydrazide, known inhibitors of myeloperoxidase. A protocol is presented here for the rapid, large-scale purification of myeloperoxidase from cultured HL60 cells, as well as evidence for the interchangeability of this myeloperoxidase and that purified from human neutrophils.

Determination of the carbohydrate composition and the disulfide bond linkages of bovine lactoperoxidase by mass spectrometry

The extent and distribution of N-glycosylation and the nature of most of the disulfide bond linkages were determined for bovine lactoperoxidase through proteolytic and glycolytic digestions combined with matrix-assisted laser desorption/ionization mass spectrometric analysis. In addition, 98% of the primary sequence of the protein was confirmed. All five of the asparagines present in sequons were found to be glycosylated, predominantly by high mannose and complex structures. Six disulfide bonds were assigned, including Cys 32-Cys 45, Cys 146-Cys 156, Cys 150-Cys 174, Cys 254-Cys 265, Cys 473-Cys 530 and Cys 571-Cys 596.

The sulfonium ion linkage in myeloperoxidase. Direct spectroscopic detection by isotopic labeling and effect of mutation

The heme group of myeloperoxidase is covalently linked via two ester bonds to the protein and a unique sulfonium ion linkage involving Met(243). Mutation of Met(243) into Thr, Gln, and Val, which are the corresponding residues of eosinophil peroxidase, lactoperoxidase, and thyroid peroxidase, respectively, and into Cys was performed. The Soret band in the optical absorbance spectrum in the oxidized mutants is now found at approximately 411 nm. Both the pyridine hemochrome spectra and resonance Raman spectra of the mutants are affected by the mutation. In the Met(243) mutants the affinity for chloride has decreased 100-fold. All mutants have lost their chlorination activity, except for the M243T mutant, which still has 15% activity left. By Fourier transform infared difference spectroscopy it was possible to specifically detect the (13)CD(3)-labeled methionyl sulfonium ion linkage. We conclude that the sulfonium ion linkage serves two roles. First, it serves as an electron-withdrawing substituent via its positive charge, and, second, together with its neighboring residue Glu(242), it appears to be responsible for the lower symmetry of the heme group and distortion from the planar conformation normally seen in heme-containing proteins.

Characterization of the Asp94 and Glu242 mutants in myeloperoxidase, the residues linking the heme group via ester bonds

The heme group of all mammalian peroxidases is covalently linked to the protein matrix via two esterbonds, as we have recently shown by Fourier transform infrared (FTIR) difference spectroscopy [Kooter, I. M., Pierik, A.J., Merkx, M., Averill, B.A., Moguilevsky, N., Bollen, A. & Wever, R. (1997) J. Am. Chem. Soc. 119, 11542-11543]. We have examined the effects of mutation of Asp94 and Glu242, responsible for those ester bonds in myeloperoxidase, on the spectroscopic properties and catalytic activity of this enzyme. Mutation of Asp94 in myeloperoxidase results in two species. The first species has spectroscopic characteristics similar to that of wild-type myeloperoxidase. The second species has spectroscopic characteristics similar to that of Met243-->Gln mutant, and it is therefore concluded that, besides loss of the ester bond involving Asp94, this species also has lost the sulfonium ion linkage that is also characteristic of myeloperoxidase. The Asp94-->Asn mutant still has about 30% residual peroxidase activity while for the Asp94-->Val mutant only a few percentage activity is left. When Glu242 is mutated the sulfonium ion linkage is not affected, but this residue together with its neighbouring residue Met243, according to resonance Raman spectra, is responsible for the low symmetry of the heme group. Mutation of either of these residues results in loss of the bowed distortion from the planar conformation, and in a heme group with higher symmetry. For the Glu242-->Gln mutant 8% residual peroxidase activity is found.

Anti-neutrophil cytoplasmic autoantibodies (ANCA) to bactericidal/permeability-increasing (BPI) protein recognize the carboxyl terminal domain

OBJECTIVES

to identify the region of bactericidal/permeability-increasing protein (BPI) recognized by anti-BPI ANCA.

METHODS

sera from 140 patients with a variety of clinical diagnoses (20 systemic vasculitis, 12 cystic fibrosis, 22 bronchiectasis/chronic obstructive airways disease, three diabetes mellitus, 13 chronic renal failure, 12 primary sclerosing cholangitis, eight ulcerative colitis, three Crohn's disease, seven cancer, and 40 other or unknown diagnoses) known to be reactive against native (nBPI), were screened by solid phase enzyme linked immunosorbent assay (ELISA) against a panel of recombinant fusion proteins; holo BPI (rBPI), recombinant lipopolysaccharide binding protein (rLBP), an N-terminal fragment of rBPI (rBPI21 ) and 'fusion' proteins containing the C- or N-terminal ends of BPI spliced with N-or C-ends of LBP, respectively.

RESULTS

a strong correlation was seen between the degree of reactivity to rBPI and the BPI C-terminal fusion protein, r=0.69, P < 0.001, as well as between nBPI and rBPI protein, r=0.55, P < 0.001, but not between nBPI and the N-terminal region of BPI (rBPI21), or proteins containing only the N-terminal fragment. Binding to proteins containing the BPI C-terminus was confirmed to be specific by fluid phase inhibition ELISA and Western blot analyses.

CONCLUSIONS

together these data suggest that circulating autoantibodies to BPI from patients with different diseases recognize the C-terminal region of BPI.

Neutrophilic myeloperoxidase-macrophage interactions perpetuate chronic inflammation associated with experimental arthritis

Rheumatoid arthritis is a systemic disease of unknown etiology. The purpose of this study was to elucidate an unrecognized interaction between neutrophilic myeloperoxidase (MPO) and macrophages (Mphi) which could perpetuate the inflammatory response associated with arthritis. A monoarticular arthritis was induced by intra-articular injection of group A streptococcus cell wall fragments (PG-APS) into the ankle joint of female Lewis rats. After swelling/erythema subsided, joints were reinjected with either recombinant MPO or enzymatically inactive MPO (iMPO). Joint measurements were made daily and arthritis was confirmed by histology. Neither iMPO nor MPO could initiate "clinical" arthritis; however, either form of the enzyme injected after PG-APS induced a dose-dependent increase in erythema and swelling. Mannans, which block the binding of MPO to Mo, ablated clinical symptoms. Also, the presence of tumor necrosis factor alpha was observed only in diseased joints using immunocytochemistry.

The inhibition of myeloperoxidase by ceruloplasmin can be reversed by anti-myeloperoxidase antibodies

BACKGROUND

The purpose of this study was to characterize the recently reported inhibition of myeloperoxidase (MPO) by ceruloplasmin and to determine whether this may be disturbed in the presence of anti-MPO antibodies.

METHODS

Specificity of the binding between ceruloplasmin and MPO was confirmed by Western blotting and enzyme-linked immunosorbent assay (ELISA), and the enzymatic activity of MPO was measured in the presence of ceruloplasmin, affinity-purified anti-MPO antibodies, or both. The affinity of the binding between MPO and ceruloplasmin and MPO and the anti-MPO antibodies was measured using a biosensor, with the results confirmed by chaotrope ELISA.

RESULTS

Affinity-purified anti-MPO antibodies from patients with microscopic polyangiitis and florid renal vasculitis inhibited the binding between ceruloplasmin and MPO to a maximum of 72.9 +/- 12.8%, whereas those from patients with Wegener's granulomatosis and only minimal renal involvement inhibited the binding to a maximum of only 36.8 +/- 10.9% (P < 0. 001), with comparable reversal of the ceruloplasmin-mediated inhibition of MPO activity. Measurement of the affinity of the interactions demonstrated that binding between MPO and the anti-MPO antibodies is stronger than that between MPO and ceruloplasmin (1.61 x 107 to 1.33 x 108 vs. 7.46 x 106 m-1), indicating that binding to the autoantibody would be favored in vivo.

CONCLUSIONS

This study confirms a role for ceruloplasmin as a physiological inhibitor of MPO, and demonstrates how the inhibition may be disrupted in the presence of anti-MPO antibodies. Because a majority (16 of 21) of the antibodies did not themselves inhibit MPO activity, their interference with the inhibition mediated by ceruloplasmin may be brought about by steric hindrance consequent upon the binding of the antibody to a dominant epitope at or near the active site.

Recombinant bovine lactoperoxidase as a tool to study the heme environment in mammalian peroxidases

The cDNA encoding bovine lactoperoxidase (LPO) has been expressed in CHO cells. The recombinant LPO was secreted as an enzymatically active single chain molecule presenting two immunoreactive forms of 88 kDa and 82 kDa, differing by their glycosylation. rLPO exhibited the characteristic absorbance spectrum with a Soret peak at 413 nm. Engineering of rLPO into a myeloperoxidase (MPO)-like molecule was attempted by substituting Gln-376 by Met, a residue known to achieve covalent binding with the heme in MPO. However, the resulting bovine LPO mutant failed to acquire the peculiar absorbance spectrum and the chlorinating activity of MPO, underlining the complex nature of interactions in the heme vicinity.

Sulphoxidation reaction catalysed by myeloperoxidase from human leucocytes

The oxidation of alkyl aryl sulphides by myeloperoxidase (MPO) at the expense of hydrogen peroxide was investigated under steady-state conditions. The sulphide concentration effect was studied under saturating H2O2 concentrations at pH 5.0 and 20 degreesC. The kinetic constants, kcat and Km, of the different substrates were determined and the values were in the 1-10 s-1 range and around 43+/-26 microM respectively, whatever the sulphide considered. In the case of p-substituted thioanisoles, the oxidation rate was dependent upon the substituent effect. The correlation of log(kcat) with the substituent constants (sigma+ values) (Hammett equation) could be explained by a reaction mechanism involving the enzyme compound II and a sulphenium radical cation. This conclusion was also supported by spectrophotometric analysis of catalytic intermediates of the enzyme, showing the accumulation of compound II. Moreover, chiral HPLC analyses showed that MPO oxidation of alkyl aryl sulphides produced the corresponding (R)-sulphoxides with a low enantioselectivity (4-8%). Chloride ion effects on the MPO-catalysed oxygenation of sulphides were also studied. Chloride acted as a substrate for MPO and as an activator in MPO-catalysed sulphoxidation. Inhibition occurred at chloride concentrations above 120 mM, whereas below 120 mM, chloride increased the reaction rate when using p-tolyl methyl sulphide as the substrate. In the presence of 100 mM chloride the catalytic efficiency (kcat/Km) of MPO increased 3-4-fold, whatever the sulphide considered, but racemic products were obtained. These data have been interpreted in the light of known structural information on the accessibility of the distal haem cavity.

Myeloperoxidase autoantibodies distinguish vasculitis mediated by anti-neutrophil cytoplasm antibodies from immune complex disease in MRL/Mp-lpr/lpr mice: a spontaneous model for human microscopic angiitis

Anti-neutrophil cytoplasm antibodies (ANCA) with specificity for myeloperoxidase (MPO) occur in the sera of patients with microscopic angiitis, an autoimmune disease characterized by necrotizing vasculitis and crescentic glomerulonephritis. These autoantibodies have been shown to stimulate neutrophil degranulation and are believed to participate in pathogenesis. A neutrophilic vasculitis has been reported in MRL-lpr mice which has histological appearances similar to microscopic angiitis. In the present study we show that 22% of female MRL-lpr mice develop MPO autoantibodies. These animals develop a clinical syndrome of vasculitis and glomerulonephritis that is distinct from immune complex disease. Anti-MPO monoclonal antibodies derived from these mice are polyreactive and react with double-stranded DNA. They bind a conformational epitope on human MPO which is also expressed by activated human neutrophils. The results suggest that a subset of MRL-lpr mice develop ANCA-related vasculitis rather than systemic lupus erythematosus and may be used as a model for human microscopic angiitis.

Augmentation of human macrophage candidacidal capacity by recombinant human myeloperoxidase and granulocyte-macrophage colony-stimulating factor

Phagocyte myeloperoxidase (MPO) is believed to be particularly important in defense against candida infection. We reported earlier that monocytes, rich in MPO, killed Candida albicans at a significantly higher rate and extent than did monocyte-derived macrophages, known to lack MPO, and that C. albicans is less resistant to MPO-dependent oxidants than less pathogenic Candida species. We hypothesized, therefore, that the capacity of macrophages to kill C. albicans might be improved in the presence of MPO. In this study, we evaluated the ability of recombinant human MPO (rhMPO) to augment the killing of C. albicans by resident macrophages and macrophages activated by recombinant human granulocyte-macrophage colony-stimulating factor. Addition of rhMPO (concentration range, 0.8 to 6.4 U/ml) to suspensions of resident and activated macrophages and opsonized C. albicans resulted in concentration-dependent and significant increases in candida killing. This enhancement was particularly pronounced with activated macrophages, whether C. albicans was opsonized or unopsonized and ingested through the macrophage mannose receptor. rhMPO did not affect the killing of C. albicans by monocytes, nor did it affect phagocytosis of opsonized or unopsonized C. albicans. These results indicate that exogenous rhMPO can augment the candidacidal capacity of both resident and activated macrophages, with a more profound effect on activated cells. We suggest that rhMPO may be effective in the treatment of invasive candidiasis.

The role of the propeptide for processing and sorting of human myeloperoxidase

Myeloperoxidase (MPO), stored in azurophil granules of neutrophils, is critical for an optimal oxygen-dependent microbicidal activity of these cells. Pro-MPO goes through a stepwise proteolytic trimming with elimination of an amino-terminal propeptide to yield one heavy and one light polypeptide chain. The propeptide of MPO may have a role in retention and folding of the nascent protein into its tertiary structure or in targeting of pro-MPO for processing and storage in granules. A propeptide-deleted pro-MPO mutant (MPODeltapro) was constructed to determine if deletion of the propeptide interferes with processing and targeting after transfection to the myeloid 32D cell line. Transfection of full-length cDNA for human MPO results in normal processing and targeting of MPO to cytoplasmic dense organelles. Although the efficiency of incorporation was lower for MPODeltapro, both pro-MPO and MPODeltapro showed heme incorporation indicating that the propeptide is not critical for this process. Deletion of the propeptide results in synthesis of a protein that lacks processing into mature two-chain forms but rather is degraded intracellularly or secreted. The finding of continued degradation of MPODeltapro in the presence of lysosomotrophic agents or brefeldin A rules out that the observed degradation takes place after transfer to granules. Intracellular pro-MPO has high mannose oligosaccharide side chains, whereas stored mature MPO was found to have both high mannose and complex oligosaccharide side chains as judged by only partial sensitivity to endoglycosidase H. The propeptide may normally interfere with the generation of certain complex oligosaccharide chain(s) supported by the finding of high mannose side chains in secreted pro-MPO and lack of them in MPODeltapro that contained complex oligosaccharide side chains only. In conclusion, elimination of the propeptide of pro-MPO blocks the maturation process and abolishes accumulation of the final product in granules suggesting a critical role of the propeptide for late processing of pro-MPO and targeting for storage in granules.

Use of native and recombinant bactericidal/permeability-increasing proteins (BPI) as antigens for detection of BPI-ANCA

Anti-neutrophil cytoplasmic antibodies (ANCA) against native bactericidal/permeability-increasing protein (nBPI) have gained increasing diagnostic significance in inflammatory bowel disease and cystic fibrosis. However, routine detection of BPI-ANCA requires pure antigen in large quantities. As nBPI is difficult to isolate and is very susceptible to proteolytic cleavage with subsequent epitope loss, it was the aim of this study to determine whether recombinant BPI (rBPI) can be used as an alternative to nBPI as target antigen for ANCA in diagnostic procedures. Therefore, 93 BPI-ELISA-positive sera and controls were compared in different ELISAs using nBPI, rBPI, unglycosylated rBPI and a 21-kDa amino-terminal fragment of rBPI. ELISA results were confirmed by immunoblotting and all sera were tested in indirect immunofluorescence (IFT). There was an 88% (82/93) agreement in recognition of nBPI and rBPI by ANCA in both ELISA systems, yet the quantitation of BPI-ANCA in relative units showed a less optimal result and correlated only by 45% (p < 0.01). Most sera recognized nBPI, rBPI and unglycosylated rBPI equally suggesting that glycosylation has no influence on antigen recognition. Only two sera were positive for the 21-kDa nBPI indicating that the binding sites for ANCA are either conformational epitopes and/or are located mainly on the carboxy-terminal part of the BPI molecule. Most BPI-ELISA-positive sera were negative in IFT (43%), but a perinuclear (pANCA, 30%), a cytoplasmic (cANCA,10%) or an atypical ANCA (aANCA, 2%) staining pattern, as well as a cytoplasmic pattern only on formaldehyde-fixed granulocytes (13%) were also observed. Overall, no characteristic pattern was seen for BPI-ELISA-positive sera in IFT. Taken together, these data suggest that rBPI offers an excellent alternative to nBPI for broad-based BPI-ANCA ELISA and will be of great value in further investigations of BPI-ANCA interactions.

Myeloperoxidase mediates cell adhesion via the alpha M beta 2 integrin (Mac-1, CD11b/CD18)

Myeloperoxidase is a leukocyte component able to generate potent microbicidal substances. A homologous invertebrate blood cell protein, peroxinectin, is not only a peroxidase but also a cell adhesion ligand. We demonstrate in this study that human myeloperoxidase also mediates cell adhesion. Both the human myeloid cell line HL-60, when differentiated by treatment with 12-O-tetradecanoyl-phorbol-13-acetate (TPA) or retinoic acid, and human blood leukocytes, adhered to myeloperoxidase; however, undifferentiated HL-60 cells showed only minimal adhesion. No cells adhered to horseradish peroxidase, and cell adhesion to myeloperoxidase was not decreased by catalase, thus showing that peroxidase activity, per se, was neither sufficient nor necessary for the adhesion activity. Mannan, which has been reported to inhibit the binding of peroxidases to cells, did not affect adhesion to myeloperoxidase. However, adhesion to myeloperoxidase was inhibited by monoclonal antibodies to alpha M (CD11b) or to beta2 (CD18) integrin subunits, but not by antibodies to alpha L (CD11a), alpha M (CD11c), or to other integrins. Native myeloperoxidase mediated dose-dependent cell adhesion down to relatively low concentrations, and denaturation abolished the adhesion activity. It is evident that myeloperoxidase supports cell adhesion, a function which may be of considerable importance for leukocyte migration and infiltration in inflammatory reactions, that alpha M beta2 integrin (Mac-1 or CD11b/CD18) mediates this adhesion, and that the alphaM beta2 integrin-mediated adhesion to myeloperoxidase is distinct from the previously reported ability of this integrin to bind to certain denatured proteins at high concentrations.

Uptake of recombinant myeloperoxidase, free or fused to Fc gamma, by macrophages enhances killing activity toward micro-organisms

A chimeric antibody-like molecule consisting of the human myeloperoxidase (rMPO) fused to the second and third constant-sequence (CH2 and CH3) Fc domains of human immunoglobulin G-1 has been constructed and expressed in Chinese hamster ovary (CHO) cells. This fusion molecule was designed to combine the binding specificity of Fc with the antimicrobial properties of rMPO. The rMPO-Fc fusion dimerized through the Fc fragment, while retaining the enzymatic activity of rMPO. The chimeric molecule was glycosylated and most of the propeptide was eliminated, indicating a better processing of the polypeptide than for rMPO alone. Both rMPO and rMPO-Fc bound to and were internalized by macrophage-like U937 promonocytic cells. Unexpectedly, the chimera failed to bind to the Fc receptor but interacted with a higher affinity than rMPO with the same binding sites. The presence of the Fc fragment in the chimera, in addition, did not extend the plasma half-life of the fusion protein. In vitro, rMPO-Fc exhibited a stronger killing effect than rMPO toward Candida albicans in the presence of either H202 alone or human macrophages. In vivo, rMPO-Fc similarly conferred a better protection than rMPO in a lethal model of murine cowdriosis. These properties could be related to the Fc-induced dimerization of the fusion protein in CHO cells.

Effect of the R569W missense mutation on the biosynthesis of myeloperoxidase

Human neutrophil microbicidal activity is largely mediated by reactive species generated by the oxygen-dependent myeloperoxidase (MPO) system. Peroxidase-negative neutrophils from many patients with hereditary MPO deficiency possess a 90-kDa MPO-related protein. We recently identified a missense mutation, R569W, in the MPO gene of many subjects with MPO deficiency. In these studies we examined the consequences of R569W on MPO biosynthesis and processing, using stably transfected K562 cells expressing normal MPO or the R569W mutation. K562 cells expressing normal MPO mimicked faithfully many features of MPO biosynthesis in myeloid cells. 1) apopro-MPO was synthesized; 2) a functional heme group was inserted into apopro-MPO, and enzymatically active pro-MPO was thereby generated; 3) pro-MPO underwent proteolytic processing to mature MPO; and 4) hemin augmented the processing of pro-MPO. pREP-R569W cells synthesized apopro-MPO, but heme was not inserted. Neither enzymatically active pro-MPO nor mature MPO was synthesized by transfectants expressing mutated cDNA, confirming our hypothesis that the R569W mutation results in a form of apopro-MPO which does not undergo post-translational processing to enzymatically active MPO species. In addition, these data support previous suggestions that heme insertion into apopro-MPO is necessary for its subsequent proteolytic processing into mature MPO subunits.

Neutrophil and recombinant myeloperoxidase as antigens in ANCA positive systemic vasculitis

Myeloperoxidase (MPO) is one of the major autoantigens recognized by anti-neutrophil cytoplasm antibodies. The association of this antigen with specific disease entities requires that there is a source of pure antigen present in large quantities. Further delineation of the molecular mechanisms involved in the antigen-antibody interaction requires the ability to manipulate the molecule. The expression of recombinant MPO in Chinese hamster ovary cells has produced a source of pure protein, suitable for molecular studies. We have shown that this protein is an antigen recognized by 95% of anti-MPO antibodies from patients with systemic vasculitis. This recombinant molecule will be of use in providing an additional specific solid-phase assay for these antibodies and further forms of this protein which mirror the antigenicity of native MPO more exactly may replace chemically purified antigen. It will also be of great value in studies examining the epitopes recognized by anti-MPO antibodies and in studies of immunoregulation and T cell activation.

Human eosinophil peroxidase enhances tumor necrosis factor and hydrogen peroxide release by human monocyte-derived macrophages

Inhibition of growth or eradication of experimentally induced tumors has been shown to be accompanied by infiltration of eosinophils and macrophages into the tumor mass. Since macrophages are important mediators of host antitumor activity, the possibility arises that a collaboration may exist between these two cell types in the control of tumor growth. In this study, we report the effect of eosinophil peroxidase (EPO), a basic protein contained in eosinophils that binds to several cell types including macrophages, on tumor necrosis factor (TNF) production and hydrogen peroxide release by human monocyte-derived macrophages. After incubation with EPO, the macrophages produced large amounts of TNF and displayed an enhanced phorbol 12-myristate 13-acetate-triggered hydrogen peroxide release. These effects were accompanied by an increased cell protein content and by morphologic changes leading the large, round macrophages of the control cultures to become elongated, pear-like or spindle shaped cells after treatment with EPO. The stimulatory effect of EPO on hydrogen peroxide release was insensitive to addition of exogenous catalase, a H2O2-degrading enzyme, suggesting that an extracellular catalytic activity of EPO was not involved. In addition, myeloperoxidase, the homologous peroxidase of neutrophils with a catalytic activity similar to that of EPO, was ineffective. The EPO-induced effects differed in several aspects from the effects of lipopolysaccaride and interferon-gamma, two well-known macrophage activators. These findings provide supportive evidence for a functional interrelationship between eosinophils and macrophages that may be physiologically relevant in the tumoricidal activity of macrophages.

Peroxidase-catalysed oxidation of different dibenzazepine derivatives

According to a recent hypothesis suggesting the potential role of free radical formation in the clozapine-induced agranulocytosis, we have evaluated the susceptibility to the peroxidase-mediated oxidation of different dibenzazepine analogues. On the one hand, compounds with an arylamine group such as clozapine or isoclozapine present a high reactivity in the horseradish peroxidase or myeloperoxidase systems and, on the other hand, fluperlapine, though known to induce agranulocytosis, and other dibenzothiazepine and dibenzoxazepine derivatives appear insensitive to oxidation. Consequently, among tricyclic derivatives, the way of diaryloxa- and diarylthiazepine compounds could be an alternative for the development of safer drugs such as antipsychotics.

Stable expression of human H1-histamine-receptor cDNA in Chinese hamster ovary cells. Pharmacological characterisation of the protein, tissue distribution of messenger RNA and chromosomal localisation of the gene

A cDNA clone for the histamine H1 receptor was isolated from a human lung cDNA library; it encoded a protein of 487 amino acids which showed characteristic features of G-protein-coupled receptors. The percentages of identity of the deduced amino acid sequence with bovine, rat and guinea pig H1 histamine receptors were 82.6%, 79.4% and 73.3%, respectively, whereas these percentages decreased to 74.6%, 66% and 56.7% for the amino acid sequence of the third intracellular loop. The human H1-receptor cDNA was transfected into Chinese hamster ovary cells (CHO) via an eukaryotic expression vector; the receptor protein present on cell membranes specifically bound [3H]mepyramine with a Kd of 3.7 nM. The binding was displaced by H1-histamine-receptor antagonists and histamine. Northern blot analysis indicated the presence of two histamine H1 receptor mRNAs of 3.5 kb and 4.1 kb in various human tissues and an additional mRNA of 4.8 kb restricted to the human brain. Finally, by means of somatic cell hybrids segregating either human or rat chromosomes, the gene for histamine H1 receptor was found to reside on human chromosome 3 and rat chromosome 4.

Virucidal effect of myeloperoxidase on human immunodeficiency virus type 1-infected T cells

Myeloperoxidase is virucidal to human immunodeficiency virus type 1 (HIV-1) in the persistently infected CEM human T-cell line or in acutely infected human peripheral blood mononuclear cells, as judged by viral infectivity and P24 radioimmunoassay. HIV-1 was specifically inactivated by low doses of the human myeloperoxidase (1.4 to 14.3 mU/ml) and the cells were spared. A higher enzyme concentration (143 mU/m) was cytotoxic, but uninfected CEM cells and normal lymphocytes were resistant to > or = 143 mU of myeloperoxidase per ml. The enzyme was virucidal with the Cl- present in medium and did not require exogenous H2O2. Catalase, an antioxidant enzyme, partially inhibited the virucidal effect of myeloperoxidase. Hence, the H2O2 probably came from the HIV-infected cells themselves. These in vitro findings indicate that the myeloperoxidase system is capable of inactivating HIV-1 of infected cells.

Regulation of macrophage function by human recombinant myeloperoxidase

Myeloperoxidase is an enzyme which is found in the azurophilic granules of neutrophils and is associated with bactericidal, fungicidal, and tumoricidal activity. The present studies show that human recombinant myeloperoxidase (rec-MyPo) can regulate a number of macrophage (M phi) capacities and functions. Macrophages from mice exposed to rec-MyPo in vitro released reactive oxygen intermediates, tumor necrosis factor alpha (TNF alpha), and interferon alpha/beta (IFN alpha/beta). Enhanced target cell killing was also demonstrated with TNF alpha sensitive but not TNF alpha insensitive cells. Intravenous injection of rec-MyPo induced high titers of systemic TNF alpha and IFN alpha/beta. These results indicate that MyPo can function as an immunomodulator both in vitro and in vivo. Because of these actions, it is apparent that MyPo represents a previously unrecognized endogenous immunomodulator.

Candidicidal activity of the human recombinant myeloperoxidase system

The effect of the human recombinant myeloperoxidase (recMPO) system on the viability of Candida albicans blastospores was determined. Cells were incubated in the presence of the enzyme, H2O2, and various substrates. In optimal conditions, 100% mortality was reached with only 41 mU of recMPO per ml, confirming its powerful candidicidal activity.

Expression of recombinant myeloperoxidase using a baculovirus expression system

Myeloperoxidase (MPO) is a glycosylated heme-containing enzyme present in the azurophilic granules of normal human polymorphonuclear neutrophils. This enzyme plays a major role in the microbicidal activity of the host defense system by catalyzing the formation of the potent oxidant, hypochlorous acid. Although the amino acid sequence of MPO has been deduced from the cDNA, the structural basis for the observed heterogeneity of this enzyme is not known. Furthermore, the nature of the prosthetic group and its mode of linkage to the apoprotein has not been determined. To address questions regarding the structural features of MPO, which arise during the complex posttranslational processing of this enzyme, we utilized a baculovirus system to express MPO in Sf9 insect cells. Two glycosylated, single-chain precursor species of MPO were observed: an 84 kDa species that was secreted and a 74 kDa species that was cell-associated. This is the first report of an expression system in which a cell-associated MPO precursor undergoes posttranslational proteolytic processing.

X-ray crystal structure of canine myeloperoxidase at 3 A resolution

The three-dimensional structure of the enzyme myeloperoxidase has been determined by X-ray crystallography to 3 A resolution. Two heavy atom derivatives were used to phase an initial multiple isomorphous replacement map that was subsequently improved by solvent flattening and non-crystallographic symmetry averaging. Crystallographic refinement gave a final model with an R-factor of 0.257. The root-mean-square deviations from ideality for bond lengths and angles were 0.011 A and 3.8 degrees. Two, apparently identical, halves of the molecule are related by local dyad and covalently linked by a single disulfide bridge. Each half-molecule consists of two polypeptide chains of 108 and 466 amino acid residues, a heme prosthetic group, a bound calcium ion and at least three sites of asparagine-linked glycosylation. There are six additional intra-chain disulfide bonds, five in the large polypeptide and one in the small. A central core region that includes the heme binding site is composed of five alpha-helices. Regions of the larger polypeptide surrounding this core are organized into locally folded domains in which the secondary structure is predominantly alpha-helical with very little organized beta-sheet. A proximal ligand to the heme iron atom has been identified as histidine 336, which is in turn hydrogen-bonded to asparagine 421. On the distal side of the heme, histidine 95 and arginine 239 are likely to participate directly in the catalytic mechanism, in a manner analogous to the distal histidine and arginine of the non-homologous enzyme cytochrome c peroxidase. The site of the covalent linkage to the heme has been tentatively identified as glutamate 242, although the chemical nature of the link remains uncertain. The calcium binding site has been located in a loop comprising residues 168 to 174 together with aspartate 96. Myeloperoxidase is a member of a family of homologous mammalian peroxidases that includes thyroid peroxidase, eosinophil peroxidase and lactoperoxidase. The heme environment, defined by our model for myeloperoxidase, appears to be highly conserved in these four mammalian peroxidases. Furthermore, the conservation of all 12 cysteine residues involved in the six intra-chain disulfide bonds and the calcium binding loop suggests that the three-dimensional structures of members of this gene family are likely to be quite similar.

Lethal oxidative damage to human immunodeficiency virus by human recombinant myeloperoxidase

Human recombinant myeloperoxidase was evaluated in a cell-free system for its inactivation properties on the replication of human immunodeficiency virus, HTLV-IIIB. In the presence of a hydrogen peroxide generating system (glucose and glucose oxidase) and sodium thiocyanate, the recombinant enzyme inhibited virus-induced syncytium formation and viral replication without causing any cytopathic effects on SupT1 reporter cells. In addition, U937 monocytoid cells, chronically infected with HIV1, were exposed to recombinant myeloperoxidase (10 U/ml) and monitored during 48 h for the accumulation of intracellular p24 viral antigen. Under these conditions, the recombinant enzyme significantly reduced intracellular viral replication without affecting cell viability.

Site-directed mutants of human myeloperoxidase. A topological approach to the heme-binding site

Two site-directed mutants of human promyeloperoxidase, MPO(His416----Ala) and MPO(His502----Ala), have been expressed in Chinese hamster ovary cells and purified. Overall purification yields and apparent molecular masses of the mutant proteins were similar to those of the wild-type enzyme. Both mutant species were analyzed spectroscopically to check the presence of the hemic iron in the proteins and were assayed for peroxidase activity. The data show that substitution of His502 leads to the loss, or to an inappropriate configuration, of the heme together with the loss of activity, suggesting that this residue could be the proximal His involved in the binding to the iron centers. On the other hand, substitution of His416 by alanine had no effect on either of the studied parameters.

Human hemi-myeloperoxidase. Initial chlorinating activity at neutral pH, compound II and III formation, and stability towards hypochlorous acid and high temperature

Human neutrophilic myeloperoxidase (MPO) is involved in the defence mechanism of the body against micro-organisms. The enzyme catalyses the generation of the strong oxidant hypochlorous acid (HOCl) from hydrogen peroxide and chloride ions. In normal neutrophils MPO is present in the dimeric form (140 kDa). The disulphide-linked protomers each consist of a heavy subunit and a light one. Reductive alkylation converts the dimeric enzyme into two promoters, 'hemi-myeloperoxidase'. We studied the initial activities of human dimeric MPO and hemi-MPO at the physiological pH of 7.2 and found no significant differences in chlorinating activity. These results indicate that, at least at neutral pH, the protomers of MPO function independently. The absorption spectra of MPO compounds II and III, both inactive forms concerning HOCl generation, and the rate constants of their formation were the same for dimeric MPO and hemi-MPO, but hemi-MPO required a slightly larger excess of H2O2 for complete conversion. Hemi-MPO was less stable at a high temperature (80 degrees C) as compared to the dimeric enzyme. Furthermore, the resistance of the chlorinating activity of hemi-MPO against its oxidative product hypochlorous acid was somewhat lower (IC50 = 32 microM HOCl) compared to dimeric MPO (IC50 = 50 microM HOCl). The higher stability of dimeric MPO in the presence of its oxidative product compared to that of monomeric MPO might be the reason for the occurrence of MPO as a dimer.

Spectral and enzymatic properties of human recombinant myeloperoxidase: comparison with the mature enzyme

Human recombinant myeloperoxidase (recMPO), purified from an engineered Chinese hamster ovary (CHO) cell line, has been characterized and compared to the mature enzyme isolated from polymorphonuclear leukocytes. Both molecules appear essentially similar in physicochemical enzymatic terms according to the following observations. 1. The unprocessed recombinant protein displays the characteristic light absorption spectra of ferric mature MPO and exhibits its typical spectral changes in the presence of dithionite or hydrogen peroxide. 2. The addition of 14C-labeled 5-aminolevulinic acid, a heme precursor, to the culture medium of recombinant CHO cells yields labeled recMPO, indicating the presence of a heme-like structure in the molecule. 3. Like mature MPO, recMPO has a peroxidatic activity and catalyzes the oxidation of chloride ions in the presence of hydrogen peroxide, producing hypochlorous acid as measured by the monochlorodimedon assay. For both enzymes, the chlorinating activity optimally occurs around pH 5.0 at about 100 microM of hydrogen peroxide and is strongly inhibited by methimazole. 4. Diethylpyrocarbonate significantly reduces the enzymatic activity of both molecules, suggesting that histidine residues may be of prime importance in the active site of the enzymes. 5. According to infrared spectroscopy data, both enzymes present a very similar secondary structure organization. In conclusion, the data suggest that the processing of the precursor enzyme (recMPO) into the mature form occurs without major structural and functional consequences.