Isolation and characterization of extracellular myeloperoxidase precursor in HL-60 cell cultures

Human myeloperoxidase (hMPO) is expressed in neurons in the substantia nigra in Parkinson's disease and in the hMPO-α-synuclein-A53T mouse model, correlating with increased nitration and aggregation of α-synuclein and exacerbation of motor impairment

α-Synuclein (αSyn) is central to the neuropathology of Parkinson's disease (PD) due to its propensity for misfolding and aggregation into neurotoxic oligomers. Nitration/oxidation of αSyn leads to dityrosine crosslinking and aggregation. Myeloperoxidase (MPO) is an oxidant-generating enzyme implicated in neurodegenerative diseases. In the present work we have examined the impact of MPO in PD through analysis of postmortem PD brain and in a novel animal model in which we crossed a transgenic mouse expressing the human MPO (hMPO) gene to a mouse expressing human αSyn-A53T mutant (A53T) (hMPO-A53T). Surprisingly, our results show that in PD substantia nigra, the hMPO gene is expressed in neurons containing aggregates of nitrated αSyn as well as MPO-generated HOCl-modified epitopes. In our hMPO-A53T mouse model, we also saw hMPO expression in neurons but not mouse MPO. In the mouse model, hMPO was expressed in neurons colocalizing with nitrated αSyn, carbamylated lysine, nitrotyrosine, as well as HOCl-modified epitopes/proteins. RNAscope in situ hybridization confirmed hMPO mRNA expression in neurons. Interestingly, the hMPO protein expressed in hMPO-A53T brain is primarily the precursor proMPO, which enters the secretory pathway potentially resulting in interneuronal transmission of MPO and oxidative species. Importantly, the hMPO-A53T mouse model, when compared to the A53T model, exhibited significant exacerbation of motor impairment on rotating rods, balance beams, and wire hang tests. Further, hMPO expression in the A53T model resulted in earlier onset of end stage paralysis. Interestingly, there was a high concentration of αSyn aggregates in the stratum lacunosum moleculare of hippocampal CA2 region, which has been associated in humans with accumulation of αSyn pathology and neural atrophy in dementia with Lewy bodies. This accumulation of αSyn aggregates in CA2 was associated with markers of endoplasmic reticulum (ER) stress and the unfolded protein response with expression of activating transcription factor 4 (ATF4), C/EBP homologous protein (CHOP), MPO, and cleaved caspase-3. Together these findings suggest that MPO plays an important role in nitrative and oxidative damage that contributes to αSyn pathology in synucleinopathies.

Neutrophil activation in response to monomeric myeloperoxidase

Myeloperoxidase (MPO) is an oxidant-producing enzyme that can also regulate cellular functions via its nonenzymatic effects. Mature active MPO isolated from normal human neutrophils is a 145 kDa homodimer, which consists of 2 identical protomers, connected by a single disulfide bond. By binding to CD11b/CD18 integrin, dimeric MPO induces neutrophil activation and adhesion augmenting leukocyte accumulation at sites of inflammation. This study was performed to compare the potency of dimeric and monomeric MPO to elicit selected neutrophil responses. Monomeric MPO (hemi-MPO) was obtained by treating the dimeric MPO by reductive alkylation. Analysis of the crucial signal transducer, intracellular Ca2+, showed that dimeric MPO induces Ca2+ mobilization from the intracellular calcium stores of neutrophils and influx of extracellular Ca2+ whereas the effect of monomeric MPO on Ca2+ increase in neutrophils was less. It was also shown that monomeric MPO was less efficient than dimeric MPO at inducing actin cytoskeleton reorganization, cell survival, and neutrophil degranulation. Furthermore, we have detected monomeric MPO in the blood plasma of patients with acute inflammation. Our data suggest that the decomposition of dimeric MPO into monomers can serve as a regulatory mechanism that controls MPO-dependent activation of neutrophils and reduces the proinflammatory effects of MPO.

A myeloperoxidase precursor, pro-myeloperoxidase, is present in human plasma and elevated in cardiovascular disease patients

Myeloperoxidase (MPO)-derived oxidants have emerged as a key contributor to tissue damage in inflammatory conditions such as cardiovascular disease. Pro-myeloperoxidase (pro-MPO), an enzymatically active precursor of myeloperoxidase (MPO), is known to be secreted from cultured bone marrow and promyelocytic leukemia cells, but evidence for the presence of pro-MPO in circulation is lacking. In the present study, we used a LC-MS/MS in addition to immunoblot analyses to show that pro-MPO is present in human blood plasma. Furthermore, we found that pro-MPO was more frequently detected in plasma from patients with myocardial infarction compared to plasma from control donors. Our study suggests that in addition to mature MPO, circulating pro-MPO may cause oxidative modifications of proteins thereby contributing to cardiovascular disease.

Biosynthesis of human myeloperoxidase

Members of Chordata peroxidase subfamily [1] expressed in mammals, including myeloperoxidase (MPO), eosinophil peroxidase (EPO), lactoperoxidase (LPO), and thyroid peroxidase (TPO), express conserved motifs around the heme prosthetic group essential for their activity, a calcium-binding site, and at least two covalent bonds linking the heme group to the protein backbone. Although most studies of the biosynthesis of these peroxidases have focused on MPO, many of the features described occur during biosynthesis of other members of the protein subfamily. Whereas MPO biosynthesis includes events typical for proteins generated in the secretory pathway, the importance and consequences of heme insertion are events uniquely associated with peroxidases. This Review summarizes decades of work elucidating specific steps in the biosynthetic pathway of human MPO. Discussion includes cotranslational glycosylation and subsequent modifications of the N-linked carbohydrate sidechains, contributions by molecular chaperones in the endoplasmic reticulum, cleavage of the propeptide from proMPO, and proteolytic processing of protomers and dimerization to yield mature MPO. Parallels between the biosynthesis of MPO and TPO as well as the impact of inherited mutations in the MPO gene on normal biosynthesis will be summarized. Lastly, specific gaps in our knowledge revealed by this review of our current understanding will be highlighted.

Structure of human promyeloperoxidase (proMPO) and the role of the propeptide in processing and maturation

Myeloperoxidase (MPO) is synthesized by neutrophil and monocyte precursor cells and contributes to host defense by mediating microbial killing. Although several steps in MPO biosynthesis and processing have been elucidated, many questions remained, such as the structure-function relationship of monomeric unprocessed proMPO versus the mature dimeric MPO and the functional role of the propeptide. Here we have presented the first and high resolution (at 1.25 Å) crystal structure of proMPO and its solution structure obtained by small-angle X-ray scattering. Promyeloperoxidase hosts five occupied glycosylation sites and six intrachain cystine bridges with Cys-158 of the very flexible N-terminal propeptide being covalently linked to Cys-319 and thereby hindering homodimerization. Furthermore, the structure revealed (i) the binding site of proMPO-processing proconvertase, (ii) the structural motif for subsequent cleavage to the heavy and light chains of mature MPO protomers, and (iii) three covalent bonds between heme and the protein. Studies of the mutants C158A, C319A, and C158A/C319A demonstrated significant differences from the wild-type protein, including diminished enzymatic activity and prevention of export to the Golgi due to prolonged association with the chaperone calnexin. These structural and functional findings provide novel insights into MPO biosynthesis and processing.

Proconvertase proteolytic processing of an enzymatically active myeloperoxidase precursor

Optimal and efficient killing of ingested microbes by human neutrophils is mediated in large part by the action of hypochlorous acid produced by the myeloperoxidase-H(2)O(2)-chloride system in phagosomes. Myeloperoxidase gene transcription is limited to early myeloid precursors in the bone marrow, when myeloperoxidase is synthesized and stored in azurophilic granules for subsequent release from stimulated neutrophils. Promyeloperoxidase, the 90 kDa myeloperoxidase precursor synthesized in the endoplasmic reticulum (ER), contains a 125-amino acid pro-region whose function and fate during myeloperoxidase biosynthesis are unknown. Promyeloperoxidase has two fates during myeloperoxidase biosynthesis; the majority undergoes proteolytic processing to generate mature myeloperoxidase, while the remainder is constitutively secreted from the cells in bone marrow. We used a promyelocytic cell line that produces endogenous myeloperoxidase as well as human embryonic kidney cells stably expressing normal and mutant forms of myeloperoxidase to examine proteolytic processing of promyeloperoxidase. We demonstrated that CMK-RVKR, an inhibitor of subtilisin-like proteinases, blocked cleavage of the pro-peptide of promyeloperoxidase in a post-ER compartment. Mutants with alanine substitution of basic residues in the predicted proteinase cleavage site failed to undergo maturation to normal myeloperoxidase subunits and were arrested at the promyeloperoxidase stage. Whereas specific mutants varied as to their stability, secreted promyeloperoxidase from the mutants retained the capacity to generate hypochlorous acid. Taken together, these studies demonstrate proconvertase-dependent cleavage of promyeloperoxidase as an essential step in normal proteolytic processing and granule targeting of myeloperoxidase. Furthermore, although mutations in the proteinase cleavage site reduced intracellular stability of the mutants, the integrity of the heme group was not compromised, as chlorinating activity was retained in the secreted promyeloperoxidase.

Vascular peroxidase-1 is rapidly secreted, circulates in plasma, and supports dityrosine cross-linking reactions

Members of the peroxidase-cyclooxygenase superfamily catalyze biochemical reactions essential to a broad spectrum of biological processes, including host defense, thyroid hormone biosynthesis, and modification of extracellular matrix, as well as contributing to the pathogenesis of chronic inflammatory diseases. We recently identified a novel member of this family, vascular peroxidase-1 (VPO1), that is highly expressed in the human cardiovascular system. Its biosynthesis and enzymatic properties are largely unknown. Here, we report that VPO1 was rapidly and efficiently secreted into the extracellular space when the gene was stably expressed in human embryonic kidney (HEK) cells. Secreted VPO1 is a monomer with complex N-linked oligosaccharides and exhibits peroxidase activity. Biosynthesis of endogenous VPO1 by cultured human umbilical vein endothelial cells (HUVECs) shares features exhibited by heterologous expression of recombinant VPO1 (rVPO1) in HEK cells. The proinflammatory agents lipopolysaccharide and tumor necrosis factor-α induce expression of VPO1 mRNA and protein in HUVECs. Furthermore, murine and bovine sera and human plasma contain enzymatically active VPO1. rVPO1 exhibits spectral and enzymatic properties characteristic of the peroxidase-cyclooxygenase family, except with regard to its heat stability. rVPO1 catalyzes tyrosyl radical formation and promotes dityrosine cross-linking. Taken together, these data demonstrate that VPO1 is a glycosylated heme peroxidase that is actively secreted into circulating plasma by vascular endothelial cells and shares several features with other members of the peroxidase-cyclooxygenase family, including the catalysis of dityrosine formation.

Conformational and thermal stability of mature dimeric human myeloperoxidase and a recombinant monomeric form from CHO cells

Myeloperoxidase (MPO) is a lysosomal heme enzyme present in the azurophilic granules of human neutrophils and monocytes. It is a critical element of the human innate immune system by exerting antimicrobial effects. It is a disulfide bridged dimer with each monomer containing a light and a heavy polypeptide and its biosynthesis and intracellular transport includes several posttranslational processing steps. By contrast, MPO recombinantly produced in Chinese hamster ovary cell lines is monomeric, partially unprocessed and contains a N-terminal propeptide (proMPO). It mirrors a second form of MPO constitutively secreted from normal bone marrow myeloid precursors. In order to clarify the impact of posttranslational modifications on the structural integrity and enzymology of these two forms of human myeloperoxidase, we have undertaken an investigation on the conformational and thermal stability of leukocyte MPO and recombinant proMPO by using complementary biophysical techniques including UV-Vis, circular dichroism and fluorescence spectroscopy as well as differential scanning calorimetry. Mature leucocyte MPO exhibits a peculiar high chemical and thermal stability under oxidizing conditions but is significantly destabilized by addition of dithiothreitol. Unfolding of secondary and tertiary structure occurs concomitantly with denaturation of the heme cavity, reflecting the role of three MPO-typical heme to protein linkages and of six intra-chain disulfides for structural integrity by bridging N- and C-terminal regions of the protein. Recombinant monomeric proMPO follows a similar unfolding pattern but has a lower conformational and thermal stability. Spectroscopic and thermodynamic data of unfolding are discussed with respect to the known three-dimensional structure of leukocyte MPO as well as to known physiological roles.

Impact of Two Novel Mutations on the Structure and Function of Human Myeloperoxidase

The heme protein myeloperoxidase (MPO) contributes critically to O(2)-dependent neutrophil antimicrobial activity. Two Japanese adults were identified with inherited MPO deficiency because of mutations at Arg-499 or Gly-501, conserved residues near the proximal histidine in the heme pocket. Because of the proximity of these residues to a critical histidine in the heme pocket, we examined the biosynthesis, function, and spectral properties of the peroxidase stably expressed in human embryonic kidney cells. Biosynthesis of normal MPO by human embryonic kidney cells faithfully mirrored events previously identified in cells expressing endogenous MPO. Mutant apopro-MPO was 90 kDa and interacted normally with the molecular chaperones ERp57, calreticulin, and calnexin in the endoplasmic reticulum. However, mutant precursors were not proteolytically processed into subunits of MPO, although secretion of the unprocessed precursors occurred normally. Although delta-[(14)C]aminolevulinic acid incorporation demonstrated formation of pro-MPO in both mutants, neither protein was enzymatically active. The Soret band for each mutant was shifted from the normal 430 to approximately 412 nm, confirming that heme was incorporated but suggesting that the number of covalent bonds or other structural aspects of the heme pocket were disrupted by the mutations. These studies demonstrate that despite heme incorporation, mutations in the heme environs compromised the oxidizing potential of MPO.

Biosynthesis, processing, and sorting of human myeloperoxidase

Exclusively synthesized by normal neutrophil and monocyte precursor cells, myeloperoxidase (MPO) functions not only in host defense by mediating efficient microbial killing but also can contribute to progressive tissue damage in chronic inflammatory states such as atherosclerosis. The biosynthetic precursor, apoproMPO, is processed slowly in the ER, undergoing cotranslational N-glycosylation, transient interactions with the molecular chaperones calreticulin and calnexin, and heme incorporation to generate enzymatically active proMPO that is competent for export into the Golgi. After exiting the Golgi the propeptide is removed prior to final proteolytic processing in azurophil granules, resulting in formation of a symmetric MPO homodimer linked by a disulfide bond. Some proMPO escapes granule targeting and becomes constitutively secreted to the extracellular environment. Although the precise mechanism is unknown, the pro-segment is required for normal processing and targeting, as propeptide-deleted MPO precursor is either degraded or constitutively secreted. Characterizing the molecular consequences of naturally occurring mutations that cause inherited MPO deficiency provides unique insight into the structural determinants of MPO involved in biosynthesis, processing and targeting.

Perfluoroalkylphosphocholines are poor protein-solubilizing surfactants, as tested with neutrophil plasma membranes

We have tested the membrane-protein solubilizing properties of two perfluoroalkylphosphocholines. These compounds belong to a series of fluorinated amphiphiles which are being investigated as potential stabilizing agents for a variety of fluorocarbon-based systems. We are particularly interested in cytochrome b558 from phagocytes, the redox component of NADPH oxidase. Its heavy subunit is believed to carry binding sites for NADPH and FAD. Nevertheless, when the cytochrome is purified in the presence of classical detergents, it carries no FAD. This could be due to a delipidating, denaturing effect of these detergents (octyl glucoside, Triton, etc). The first perfluoroalkyphosphocholine, C8F17(CH2)2O-P(O2-)-O(CH2)2N+(CH3)3(F8C2PC), extracted about as much protein from neutrophil plasma membranes into a 100,000 g supernatant as octyl glucoside. The second compound, C8F17(CH2)11O-P(O2-)-O(CH2)2N+(CH3)3(F8C11PC), was less efficient. We found that flavin was still protein-bound in the crude F8C2PC extract at a FAD to heme ratio of about 1, and a good NADPH oxidase activity was obtained without addition of exogenous FAD, even after dialysis or gel filtration, whereas dialysis eliminated most of the FAD from the octyl glucoside extracts. These experiments appeared to make F8C2PC an interesting membrane-solubilizing agent. Nevertheless, no protein in the F8C2PC extract could be adsorbed on the chromatographic supports normally used for purification. After dilution of the extract and addition of 15 mM octyl glucoside, some of the proteins, such as myeloperoxidase, could be adsorbed (and eluted), but not cytochrome b558. Freeze-fracture electron microscopy showed that the F8C2PC extracts contained numerous vesicles and aggregates of small shapeless particles. Higher centrifugal forces sedimented most proteins of the 100,000 g supernatant. As a check, the effect of F8C2PC was tested on sarcoplasmic reticulum vesicles, the behavior of which with respect to the usual non-denaturating detergents has been well studied. There was little, if any, solubilization. We conclude that, although supernatants of F8C2PC extracts of neutrophil membranes are optically clear, proteins are not really solubilized. This result is in keeping with the absence of lytic effects of F8C2PC on erythrocyte membranes.

Abundant mRNAs in the squid light organ encode proteins with a high similarity to mammalian peroxidases

A library derived from mRNA in the bacterial light organ of the squid, Euprymna scolopes, contained an unexpectedly high proportion of cDNAs that encode proteins with approximately 30% similarity to a family of mammalian peroxidases (PO) including myelo-PO, eosinophil PO, and thyroid PO (donor:hydrogen-peroxide oxidoreductase; EC 1.11.1.7). Two nearly full-length cDNAs were determined to encode putative PO of nearly 93 kDa each that are 97% identical in amino acid sequence to each other. Each contains four potential glycosylation sites, and His416, believed to be within the active site of the human PO, is conserved in the putative PO from the squid light organ. The mRNAs for the putative squid PO were approximately 250 times more abundant in the tissue housing the bacterial symbiont than in the ocular lens or mantle and were undetectable in the light organ lens. By analogy with the bacteriocidal function of PO in mammalian neutrophils, the putative squid PO may be important for modulating or limiting the population of bacteria within the light organ. The possibility that the squid light organ contains a high concentration of PO raises the possibility that the light organ lens is under oxidative stress, providing a possible rationale for the recruitment of its aldehyde dehydrogenase-like crystallin.

Tissue distribution of constitutive and induced soluble peroxidase in rat. Purification and characterization from lacrimal gland

A thorough search for a soluble peroxidase in 31 different tissues of rat indicated the presence of a constitutive activity only in lacrimal, preputial and submaxillary gland. An induced soluble peroxidase activity was also detected in the lactating mammary gland and in the estrogen-induced uterine secretory fluid. The lacrimal gland was the richest source of the enzyme. No peroxidase activity was detected in the lactating mammary gland of mouse and hamster nor in the preputial gland of mouse and uterine fluid of hamster. The three constitutive and two induced soluble peroxidases of rat had a native molecular mass of 73 kDa by gel filtration and they showed a similar mobility in native PAGE. Lactoperoxidase of cow's milk and solubilized rat membrane-bound peroxidases of uterus, intestine and bone marrow showed in native PAGE a mobility which was distinctly different from that of rat soluble peroxidases. As the lacrimal gland of rat was the richest source of soluble peroxidase, the enzyme was purified from this gland to apparent homogeneity; SDS/PAGE then showed a single band of molecular mass 75 kDa which was similar to that obtained by gel filtration. Peroxidase also purified from preputial and submaxillary gland, as well as commercial lactoperoxidase, had a similar molecular mass on SDS/PAGE to purified lacrimal peroxidase. The visible spectrum of lacrimal peroxidase was similar to that of lactoperoxidase but different from membrane-bound peroxidase of rat neutrophils. On isoelectric focussing, purified lacrimal peroxidase resolved into about 14 multiple forms spanning a pI range of 6.5-3.5 while lactoperoxidase focussed at the cathode. Evidence presented suggests that the multiple forms are possibly due to differences in glycosylation. Immunodiffusion, immunoprecipitation and Western blot using antilacrimal peroxidase serum showed a similar interacting species for all five soluble peroxidases of rat while membrane-bound peroxidases showed no interaction. Although in immunodiffusion, the antiserum failed to cross-react with lactoperoxidase it did interact with lactoperoxidase on Western blot. The results indicate that the various constitutive and induced soluble peroxidases of rat tissues are similar to lacrimal peroxidase but are distinctly different from the known membrane-bound peroxidases of rat. However the lacrimal peroxidase shows both similarities as well as dissimilarities with bovine lactoperoxidase. This soluble peroxidase system of rat could be useful to study tissue-specific regulation of gene expression at the molecular level.

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.

Preferential localisation of elastase in cytosol of human myeloid leukemia HL-60 cells

The subcellular distribution of the elastase in human myeloid leukemia HL-60 cells was studied in comparison with that in normal leukocytes. On differential centrifugation, most of the elastase activity of HL-60 cell lysates was recovered in the 105,000 x g supernatant, while that of human peripheral blood leukocyte lysates was recovered in the 500 x g precipitate (azurophil granule-rich fraction). Moreover, on Percoll density gradient centrifugation, the elastase activity in HL-60 cell extracts was recovered in the lightest fraction with none in the azurophil granule-rich fractions, whereas most of the activity in leukocyte extracts was recovered in the azurophil granule-rich fractions. This subcellular localization of elastase did not change when HL-60 cells differentiated into monocytes and granulocytes by induction with 12-O-tetradecanoyl phorbol-13-acetate and retinoic acid, respectively. Furthermore, on Sephadex G-75 gel filtration, the elastase activity in HL-60 cell extracts was eluted earlier than that in leukocyte extracts. The size estimation indicated that the elastase of HL-60 cells was 36-30 kDa, corresponding to the size of an elastase precursor reported. The relevance of a large form of the elastase in HL-60 cells to its subcellular localization is discussed.

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

The cDNA encoding human myeloperoxidase carries three ATG codons in frame; 144, 111 and 66 bp upstream from the proprotein DNA sequence. In order to determine the most efficient signal sequence, three cDNA modules starting at each of the ATG were cloned into an eucaryotic expression vector and stably expressed in Chinese hamster ovary cell lines. In all three cases, recombinant human myeloperoxidase (recMPO) was secreted into the culture medium of transfected cells, indicating that each of the signal peptides functions efficiently. One of the recombinant cell lines, which was amplified using methotrexate, overexpresses enzymatically active recMPO up to 6 micrograms.ml-1.day-1. The recombinant product was purified by a combination of ion-exchange and metal-chelate chromatography, and characterized in terms of molecular mass, amino-terminal amino acid analysis, glycosylation, physicochemical properties and biological activity. The data show that recMPO is secreted essentially as a monomeric, heme-containing, single-chain precursor of 84 kDa which exhibits peroxidase activity. Amino-terminal analysis indicated that cleavage of the signal peptide occurs between amino acids 48 and 49. In addition, recMPO appeared to be glycosylated up to the last stage of sialylation, to an extent similar to that of the natural enzyme. Specific activity measurements as well as stability data, in various pH, temperature, ionic strength and reducing conditions, indicated that the recombinant single-chain enzyme behaves essentially in the same way as the natural two-chain molecule. Finally, recMPO was shown to exert potent cytotoxicity towards Escherichia coli when provided with its physiological substrates, i.e. hydrogen peroxide and chloride ions.