Apolipoprotein CII Amyloidosis Associated With p.Lys41Thr Mutation.

INTRODUCTION: Apolipoprotein CII amyloidosis (AApoCII) is a rare form of amyloidosis. Here, we report a novel mutation associated with AApoCII amyloidosis in 5 patients and describe their clinical, renal biopsy, and mass spectrometry findings. METHODS: Five patients with renal AApoCII p.Lys41Thr amyloidosis were identified from our amyloid mass spectrometry cohort. Clinical features, kidney biopsy, and mass spectrometry findings were analyzed in this rare type of amyloidosis. RESULTS: The patients were older adults (mean age of 71.6 years at diagnosis), presented with nephrotic-range proteinuria, and often had declining renal function. All renal biopsy specimens showed massive mesangial nodules composed of weakly eosinophilic, periodic acid-Schiff negative, Congo red-positive amyloid deposits. There were no interstitial, vascular, or medullary deposits. In all cases, immunofluorescence studies were negative for Igs and electron microscopy showed amyloid fibrils. Proteomic analysis of Congo red-positive amyloid deposits detected large amounts of apolipoprotein CII (APOC2) protein. We also detected APOC2 p.Lys41Thr mutant protein in amyloid deposits of all patients. DNA sequencing in 1 patient confirmed the presence of the mutation. Both mutant and wild-type forms of APOC2 were detected in amyloid deposits of all patients. Molecular dynamic simulations showed the variant mediating a collapse of the native structure of APOC2, thereby destabilizing the protein. CONCLUSION: We propose that AApoCII p.Lys41Thr amyloidosis is a new form of amyloidosis seen in elderly individuals, histologically exhibiting massive glomerular involvement, leading to nephrotic-range proteinuria and progressive chronic kidney disease.

C4 Glomerulopathy: A Disease Entity Associated With C4d Deposition.

Complement-mediated glomerulonephritis, which includes C3 glomerulopathy, is characterized by dominant staining of C3 with minimal or no immunoglobulin deposits on immunofluorescence studies. We describe a new entity of complement-mediated glomerulonephritis that is characterized by bright C4d staining but with no or minimal C3 or immunoglobulin deposits on immunofluorescence studies. We label this entity as C4 glomerulopathy. C4 glomerulopathy includes C4 dense deposit disease and C4 glomerulonephritis. C4 dense deposit disease is characterized by bright C4d staining and dense deposits along glomerular basement membranes. C4 glomerulonephritis is characterized by bright C4d staining and many mesangial electron-dense deposits, with or without rare intramembranous electron-dense deposits. We describe clinical features and kidney biopsy results in a short series of 3 patients to highlight these findings. All 3 patients presented with proteinuria, and 2 patients also had hematuria. Kidney function was preserved in 2 patients, whereas 1 patient presented with declining kidney function. Evaluation for autoimmune disease, infection, and paraprotein yielded negative results in all patients. Complement levels were normal, although 1 patient had borderline low C4 levels. Kidney biopsy showed mesangial proliferative or membranoproliferative glomerulonephritis with bright C4d staining and absent or minimal C1q, C3, and immunoglobulin. Laser microdissection and mass spectrometry of glomeruli in all 3 patients showed large to moderate numbers of spectra matching C4. Furthermore, analysis of amino acid sequences showed that they were localized to the C4d portion of C4, consistent with immunofluorescence findings. Further studies are required to determine the underlying cause. In summary, we describe a novel complement-mediated glomerulonephritis that is characterized by bright glomerular C4d staining with minimal or absent staining for C1q, C3, and immunoglobulin.

Proteomic detection of immunoglobulin light chain variable region peptides from amyloidosis patient biopsies.

Immunoglobulin light chain (LC) amyloidosis (AL) is caused by deposition of clonal LCs produced by an underlying plasma cell neoplasm. The clonotypic LC sequences are unique to each patient, and they cannot be reliably detected by either immunoassays or standard proteomic workflows that target the constant regions of LCs. We addressed this issue by developing a novel sequence template-based workflow to detect LC variable (LCV) region peptides directly from AL amyloid deposits. The workflow was implemented in a CAP/CLIA compliant clinical laboratory dedicated to proteomic subtyping of amyloid deposits extracted from either formalin-fixed paraffin-embedded tissues or subcutaneous fat aspirates. We evaluated the performance of the workflow on a validation cohort of 30 AL patients, whose amyloidogenic clone was identified using a novel proteogenomics method, and 30 controls. The recall and negative predictive values of the workflow, when identifying the gene family of the AL clone, were 93 and 98%, respectively. Application of the workflow on a clinical cohort of 500 AL amyloidosis samples highlighted a bias in the LCV gene families used by the AL clones. We also detected similarity between AL clones deposited in multiple organs of systemic AL patients. In summary, AL proteomic data sets are rich in LCV region peptides of potential clinical significance that are recoverable with advanced bioinformatics.

Disappearance of immunoglobulins from persistent renal amyloid deposits following stem cell transplantation for heavy-and light-chain amyloidosis.

BACKGROUND: Immunoglobulin (Ig)-related amyloidosis is the most common type of systemic amyloidosis in the developed countries and involves the kidney in most cases. Clinical remission can be achieved with chemotherapy and/or autologous stem cell transplantation (ASCT). Previous case reports have showed persistence of renal amyloid mass in light-chain amyloidosis (AL) even in the setting of hematologic and renal response. METHODS: We report a novel finding in two cases of heavy- and light-chain amyloidosis (AHL) in which monoclonal Ig but not serum amyloid P (SAP), apolipoprotein E (ApoE) or amyloid bulk in the kidney regressed after successful therapy. RESULTS: In the pre-treatment renal biopsies, the amyloid deposits stained for one heavy and one light chains (IgG + λ in one case and IgA + κ in one case). Laser microdissection followed by mass spectrometry (LMD/MS) in both cases showed abundant spectra for Ig heavy and light chains, SAP and ApoE. Both patients achieved a hematologic response with disappearance of the monoclonal protein from serum and urine and normalization of serum-free light chain ratio, but renal response occurred in only one patient. Repeat kidney biopsies showed persistence of fibrillar amyloid deposits, but regression of Ig from the amyloid deposits based on immunofluorescence. LMD/MS on the repeat biopsy performed in one case also showed disappearance of Ig but not SAP or ApoE. CONCLUSIONS: Our finding suggests that effective chemotherapy and/or ASCT in some patients with AHL not only eliminates the circulating pathogenic monoclonal Ig but also the Ig component of amyloid deposits, which may translate into renal response. This, however, may not lead to regression of amyloid deposits themselves. The latter may require more time or addition of therapeutic agents that target amyloid-associated proteins such as SAP, which are not commercially available.

IgD heavy-chain deposition disease: detection by laser microdissection and mass spectrometry.

Monoclonal Ig deposition disease (MIDD) is a rare complication of monoclonal gammopathy characterized by deposition of monoclonal Ig light chains and/or heavy chains along the glomerular and tubular basement membranes. Here, we describe a unique case of IgD deposition disease. IgD deposition is difficult to diagnose, because routine immunofluorescence does not detect IgD. A 77-year-old man presented with proteinuria and renal failure, and kidney biopsy analysis showed a nodular sclerosing GN with extensive focal global glomerulosclerosis, tubular atrophy, and interstitial fibrosis. Immunofluorescence was negative for Ig deposits, although electron microscopy showed deposits in the glomeruli and along tubular basement membranes. Laser microdissection of glomeruli and mass spectrometry of extracted peptides showed a large spectra number for IgD, and immunohistochemistry showed intense glomerular and tubular staining for IgD. Together, these findings are consistent with IgD deposition disease. Bone marrow biopsy analysis showed 5% plasma cells, which stained for IgD. The patient was treated with bortezomib and dexamethasone, which resulted in improvement of hematologic parameters but no improvement of renal function. The diagnosis of IgD deposition disease underscores the value of laser microdissection and mass spectrometry in further evaluating renal biopsies when routine assessment fails to reach an accurate diagnosis.

Transforming growth factor beta 1 induces CXCL16 and leukemia inhibitory factor expression in osteoclasts to modulate migration of osteoblast progenitors.

The processes of bone resorption and bone formation are tightly coupled in young adults, which is crucial to maintenance of bone integrity. We have documented that osteoclasts secrete chemotactic agents to recruit osteoblast lineage cells, contributing to coupling. Bone formation subsequent to bone resorption becomes uncoupled with aging, resulting in significant bone loss. During bone resorption, osteoclasts release and activate transforming growth factor beta 1 (TGF-ß1) from the bone matrix; thus, elevated bone resorption increases the level of active TGF-ß in the local environment during aging. In this study, we examined the influences of TGF-ß1 on the ability of osteoclasts to recruit osteoblasts. TGF-ß1 increased osteoclast expression of the chemokine CXCL16 to promote osteoblast migration. TGF-ß1 also directly stimulated osteoblast migration; however, this direct response was blocked by conditioned medium from TGF-ß1-treated osteoclasts due to the presence of leukemia inhibitory factor (LIF) in the medium. CXCL16 and LIF expression was dependent on TGF-ß1 activation of Smad2 and Smad3. These results establish that TGF-ß1 induces CXCL16 and LIF production in osteoclasts, which modulate recruitment of osteoblasts to restore the bone lost during the resorptive phase of bone turnover.

TGF-ß induces Wnt10b in osteoclasts from female mice to enhance coupling to osteoblasts.

In young adults, bone lost through osteoclast-mediated resorption is precisely replaced in both location and amount. Understanding how these two processes are coupled is crucial to advancing treatments for osteoporosis, a disease that progresses when the processes become uncoupled. We documented that osteoclasts secrete the mammalian integration 1 gene that is the homolog of Drosophila Wngless (Wnt) 10b, bone morphogenetic protein 6 (BMP6), and the chemokine sphingosin 1 phosphate (S1P) to promote mesenchymal cell mineralization in vitro. During bone resorption, TGF-ß1 is released from the bone extracellular matrix and activated by osteoclasts. Thus, TGF-ß1 levels are elevated during the resorption phase of bone turnover. We therefore investigated the influences of TGF-ß1 on osteoclast-mediated support of mineralization. TGF-ß1 increased osteoclast production of Wnt10b, but not BMP6 or S1P. Blocking Wnt10b activity with the Wnt signaling inhibitor Dickkoph-related protein 1 suppressed the ability of TGF-ß-treated osteoclast-conditioned media to promote osteoblast mineralization. Examination of TGF-ß signaling in osteoclasts revealed that induction of Wnt10b expression was dependent on Smad2/3 activation and independent from TGF-ß1 stimulation of protein kinase B (AKT) or MAPK kinase. TGF-ß1-treated osteoclast-conditioned media from cells with blocked Smad signaling exhibited a reduced ability to support mineralization, demonstrating the importance of Smad signaling in this response. Parallel cultures with suppressed TGF-ß activation of AKT or MAPK kinase signaling retained their ability to elevate mineralization. These results demonstrate that TGF-ß1 stimulates Wnt10b production in osteoclasts, which may enhance restoration of the bone lost during the resorptive phase of bone turnover.

Clonally related follicular lymphomas and Langerhans cell neoplasms: expanding the spectrum of transdifferentiation.

The traditional model of hematopoiesis is based on unidirectional maturation of hematopoietic precursors into lineage-committed cells. However, recent studies indicate that mature B lymphocytes may demonstrate significant lineage plasticity. We and others have reported transdifferentiation of follicular lymphomas (FLs) into clonally related histiocytic/dendritic cell neoplasms. Here, we describe 2 patients with FL who developed clonally related Langerhans cell neoplasms. The first was a 52-year-old man diagnosed with FL, grade 1. He received immunochemotherapy and had stable disease for 8 years. He then developed increasing lymphadenopathy, and lymph node biopsy showed Langerhans cell sarcoma with no evidence of FL. The second patient was a 77-year-old woman who presented with lymphadenopathy, an abdominal mass, and pulmonary nodules. Lymph node biopsy showed both Langerhans cell histiocytosis and minimal involvement by FL, grade 1. In each case, a combination of immunoglobulin gene rearrangement and fluorescence in situ hybridization studies provided evidence to support a clonal relationship between the FL and Langerhans cell neoplasm. These cases provide striking examples of neoplastic transdifferentiation and expand the spectrum of lesions clonally identical to otherwise typical FL. Awareness of this phenomenon may aid in diagnosis when histologically dissimilar tumors arise synchronously or metachronously in patients with lymphoma.

Renal amyloidosis: origin and clinicopathologic correlations of 474 recent cases.

BACKGROUND AND OBJECTIVES: The kidney is the organ most commonly involved in systemic amyloidosis. This study reports the largest clinicopathologic series of renal amyloidosis. DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS: This study provides characteristics of 474 renal amyloidosis cases evaluated at the Mayo Clinic Renal Pathology Laboratory from 2007 to 2011, including age, sex, serum creatinine, proteinuria, type of amyloid, and tissue distribution according to type. RESULTS: The type of amyloid was Ig amyloidosis in 407 patients (85.9%), AA amyloidosis in 33 (7.0%), leukocyte chemotactic factor 2 amyloidosis in 13 (2.7%), fibrinogen A α chain amyloidosis in 6 (1.3%), Apo AI, Apo AII, or Apo AIV amyloidosis in 3 (0.6%), combined AA amyloidosis/Ig heavy and light chain amyloidosis in 1 (0.2%), and unclassified in 11 (2.3%). Laser microdissection/mass spectrometry, performed in 147 cases, was needed to determine the origin of amyloid in 74 of the 474 cases (16%), whereas immunofluorescence failed to diagnose 28 of 384 light chain amyloidosis cases (7.3%). Leukocyte chemotactic factor 2 amyloidosis and Apo AI, Apo AII, or Apo AIV amyloidosis were characterized by diffuse interstitial deposition, whereas fibrinogen A α chain amyloidosis showed obliterative glomerular involvement. Compared with other types, Ig amyloidosis was associated with lower serum creatinine, higher degree of proteinuria, and amyloid spicules. CONCLUSIONS: In the authors' experience, the vast majority of renal amyloidosis cases are Ig derived. The newly identified leukocyte chemotactic factor 2 amyloidosis form was the most common of the rarer causes of renal amyloidosis. With the advent of laser microdissection/mass spectrometry for amyloid typing, the origin of renal amyloidosis can be determined in >97% of cases.

Exploring the amyloid proteome in immunoglobulin-derived lymph node amyloidosis using laser microdissection/tandem mass spectrometry.

Amyloidosis affecting lymph nodes (LN) may occur in the setting of systemic amyloidosis or as an entity localized to the site of production (peritumoral). Why some LN amyloid remains peritumoral is unknown. We speculated that the composition of amyloid in these two presentations differs. We analyzed the amyloid proteome in LN amyloid samples to identify differences between the systemic and peritumoral subtypes. In immunoglobulin-derived LN amyloidosis (N = 26), 70% had heavy chain amyloid (AH or mixed AH/AL). True localized LN amyloidosis was rare, with only 2 patients without a monoclonal protein component. Nineteen patients (73%) had typical amyloid syndromes (100% of AL vs 67% of AH/AL, P = 0.02). A trend to improved survival for the AH/AL group in comparison to AL (median 5-year survival 48 vs. 19 months, P = 0.06) was seen. Mass spectrometric amyloid analysis is a powerful tool for characterizing amyloid and may provide additional prognostic information.

Sclerostin is expressed in osteoclasts from aged mice and reduces osteoclast-mediated stimulation of mineralization.

Osteoclast-mediated bone resorption precedes osteoblast-mediated bone formation through early adulthood, but formation fails to keep pace with resorption during aging. We previously identified several factors produced by osteoclasts that promote bone formation. In this study, we determined if osteoclast-produced factors contribute to the impaired bone formation with aging. We previously found that mice between the ages of 18 and 22 months develop age-related bone loss. Bone marrow-derived pre-osteoclasts were isolated from 6-week, 12-month, and 18- to 24-month-old mice and differentiated into osteoclasts in vitro. Conditioned media were collected and compared for osteoblast mineralization support. Conditioned medium from osteoclasts from all ages was able to support mineralization of bone marrow stromal cells. Concentrating the conditioned medium from 6-week-old and 12-month-old mouse marrow cells-derived osteoclasts enhanced mineralization support whereas concentrated conditioned medium from 18- to 24-month-old mouse marrow-derived osteoclasts repressed mineralization compared to base medium. This observation suggests that an inhibitor of mineralization was secreted by aged murine osteoclasts. Gene and protein analysis revealed that the Wnt antagonist sclerostin was significantly elevated in the conditioned media from 24-month-old mouse cells compared to 6-week-old mouse cells. Antibodies directed to sclerostin neutralized the influences of the aged mouse cell concentrated conditioned media on mineralization. Sclerostin is primarily produced by osteocytes in young animals. This study demonstrates that osteoclasts from aged mice also produce sclerostin in quantities that may contribute to the age-related impairment in bone formation.

Laser microdissection and proteomic analysis of amyloidosis, cryoglobulinemic GN, fibrillary GN, and immunotactoid glomerulopathy.

BACKGROUND AND OBJECTIVES: Organized deposits are present in amyloidosis, fibrillary GN, and immunotactoid glomerulopathy. However, the constituents of the deposits are not known. DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS: Laser microdissection of glomeruli followed by mass spectrometry was performed to determine the composition of the deposits. The results were compared with cryoglobulinemic GN. RESULTS: The results are divided into four major groups: amyloidogenic proteins, structural/other proteins, complement proteins, and Igs. With regards to amyloidogenic proteins, large spectra numbers of apolipoprotein E are noted in amyloidosis (41.8±20.9) compared with fibrillary (15.6±12.5) and immunotactoid (12.3±12) glomerulopathy. Apolipoprotein E was absent in cryoglobulinemic GN. Serum amyloid P component is present in large spectra numbers in amyloidosis (14.1±6.7) and small spectra numbers in immunotactoid glomerulopathy, but it is absent in fibrillary and cryoglobulinemic GN. However, large spectra numbers of Ig γ-1 chain C region are present in immunotactoid glomerulopathy (47.3±34.6) compared with fibrillary (16.25±19.7) and cryoglobulinemic (13.3±4.9) GN. All cases of Ig light chain-associated amyloidosis showed spectra for the respective Ig light-chain C region (mean=10±1.7). CONCLUSIONS: Based on the spectra numbers, the study shows that the relative amount of apolipoprotein E to Ig light-chain C region/amyloidogenic proteins or Ig γ-1 chain C region is associated with the organization of the deposits in amyloidosis, fibrillary GN, and immunotactoid glomerulopathy. However, the absence of apolipoprotein E correlates with the lack of fibrillar deposits in cryoglobulinemic GN.

Sphingosine 1-phosphate (S1P) receptors 1 and 2 coordinately induce mesenchymal cell migration through S1P activation of complementary kinase pathways.

Normal bone turnover requires tight coupling of bone resorption and bone formation to preserve bone quantity and structure. With aging and during several pathological conditions, this coupling breaks down, leading to either net bone loss or excess bone formation. To preserve or restore normal bone metabolism, it is crucial to determine the mechanisms by which osteoclasts and osteoblast precursors interact and contribute to coupling. We showed that osteoclasts produce the chemokine sphingosine 1-phosphate (S1P), which stimulates osteoblast migration. Thus, osteoclast-derived S1P may recruit osteoblasts to sites of bone resorption as an initial step in replacing lost bone. In this study we investigated the mechanisms by which S1P stimulates mesenchymal (skeletal) cell chemotaxis. S1P treatment of mesenchymal (skeletal) cells activated RhoA GTPase, but this small G protein did not contribute to migration. Rather, two S1P receptors, S1PR1 and S1PR2, coordinately promoted migration through activation of the JAK/STAT3 and FAK/PI3K/AKT signaling pathways, respectively. These data demonstrate that the chemokine S1P couples bone formation to bone resorption through activation of kinase signaling pathways.

Renal amyloidosis associated with a novel sequence variant of gelsolin.

We present a case of a 75-year-old woman who presented with progressive kidney failure. Kidney biopsy performed to determine the cause of kidney failure showed amyloidosis of undetermined type. Laser microdissection of the Congo Red-positive glomeruli followed by mass spectrometry studies showed a large number of spectra matching apolipoprotein E, serum amyloid P component, and gelsolin, consistent with a diagnosis of gelsolin-associated renal amyloidosis. Sequencing of the gelsolin gene revealed a previously undescribed sequence variant, a guanine to adenine substitution at nucleotide 580 of the coding sequence, corresponding to a predicted glycine to arginine mutation at amino acid 194. Gelsolin amyloidosis typically involves the nerves and skin, with only rare reported involvement of the kidney. An atypical finding on electron microscopy was that of a swirling pattern of the amyloid fibrils. The novel gelsolin variant may be responsible for the unusual clinical and pathologic presentation. The report also highlights the usefulness of laser microdissection and mass spectrometry in the typing of difficult cases of amyloidosis.

Role of a glutamate bridge spanning the dimeric interface of human manganese superoxide dismutase.

The function in the structure, stability, and catalysis of the interfaces between subunits in manganese superoxide dismutase (MnSOD) is currently under scrutiny. Glu162 in homotetrameric human MnSOD spans a dimeric interface and forms a hydrogen bond with His163 of an adjacent subunit which is a direct ligand of the manganese. We have examined the properties of two site-specific mutants of human MnSOD in which Glu162 is replaced with Asp (E162D) and Ala (E162A). The X-ray crystal structures of E162D and E162A MnSOD reveal no significant structural changes compared with the wild type other than the removal of the hydrogen bond interaction with His163 in E162A MnSOD. In the case of E162D MnSOD, an intervening solvent molecule fills the void created by the mutation to conserve the hydrogen bond interaction between His163 and residue 162. These mutants retain their tetrameric structure and their specificity for manganese over iron. Each has catalytic activity in the disproportionation of superoxide that is typically 5-25% of that of the wild-type enzyme and a level of product inhibition greater by approximately 2-fold. Differential scanning calorimetry indicates that the hydrogen bond between Glu162 and His163 contributes to the stability of MnSOD, with the major unfolding transition occurring at 81 degrees C for E162A compared to 90 degrees C for wild-type MnSOD. These results suggest that Glu162 at the tetrameric interface in human MnSOD supports stability and efficient catalysis and has a significant role in regulating product inhibition.

Structural mobility in human manganese superoxide dismutase.

Human manganese superoxide dismutase (MnSOD) is a homotetramer of 22 kDa subunits, a dimer of dimers containing dimeric and tetrameric interfaces. We have investigated conformational mobility at these interfaces by measuring amide hydrogen/deuterium (H/D) exchange kinetics and 19F NMR spectra, both being excellent methods for analyzing local environments. Human MnSOD was prepared in which all nine tyrosine residues in each subunit are replaced with 3-fluorotyrosine. The 19F NMR spectrum of this enzyme showed five sharp resonances that have been assigned by site-specific mutagenesis by replacing each 3-fluorotyrosine with phenylalanine; four 19F resonances not observed are near the paramagnetic manganese and extensively broadened. The temperature dependence of the line widths and chemical shifts of the 19F resonances were used to estimate conformational mobility. 3-Fluorotyrosine 169 at the dimeric interface showed little conformational mobility and 3-fluorotyrosine 45 at the tetrameric interface showed much greater mobility by these measures. In complementary studies, H/D exchange mass spectrometry was used to measure backbone dynamics in human MnSOD. Using this approach, amide hydrogen exchange kinetics were measured for regions comprising 78% of the MnSOD backbone. Peptides containing Tyr45 at the tetrameric interface displayed rapid exchange of hydrogen with deuterium while peptides containing Tyr169 in the dimeric interface only displayed moderate exchange. Taken together, these studies show that residues at the dimeric interface, such as Tyr169, have significantly less conformational freedom or mobility than do residues at the tetrameric interface, such as Tyr45. This is discussed in terms of the role in catalysis of residues at the dimeric interface.

Crystal structure of nitrated human manganese superoxide dismutase: mechanism of inactivation.

A cellular consequence of the reaction of superoxide and nitric oxide is enhanced peroxynitrite levels. Reaction of peroxynitrite with manganese superoxide dismutase (MnSOD) causes nitration of the active-site residue Tyr34 and nearly complete inhibition of catalysis. We report the crystal structures at 2.4 A resolution of human MnSOD nitrated by peroxynitrite and the unmodified MnSOD. A comparison of these structures showed no significant conformational changes of active-site residues or solvent displacement. The side chain of 3-nitrotyrosine 34 had a single conformation that extended toward the manganese with O1 of the nitro group within hydrogen-bonding distance (3.1 A) of Nepsilon2 of the second-shell ligand Gln143. Also, nitration of Tyr34 caused a weakening, as evidenced by the lengthening, of a hydrogen bond between its phenolic OH and Gln143, part of an extensive hydrogen-bond network in the active site. Inhibition of catalysis can be attributed to a steric effect of 3-nitrotyrosine 34 that impedes substrate access and binding, and alteration of the hydrogen-bond network that supports proton transfer in catalysis. It is also possible that an electrostatic effect of the nitro group has altered the finely tuned redox potential necessary for efficient catalysis, although the redox potential of nitrated MnSOD has not been measured.

Exchange of (18)O in the reaction of peroxynitrite with CO(2).

We have observed an exchange of (18)O in the reactions of CO(2) with peroxynitrite using membrane-inlet mass spectrometry and HPLC negative electrospray ionization mass spectrometry. The exchange appeared on addition of peroxynitrite to a solution containing (18)O-labeled CO(2) in equilibrium with bicarbonate. It was observed as a temporarily enhanced rate of depletion of (18)O from CO(2), a rate that was greater than the rate of (18)O depletion caused by the hydration/dehydration cycle of CO(2). In addition, we detected the appearance of mass peaks attributed to (18)O in product NO(3)(-). As a further measure of the (18)O exchange, there was a redistribution of (18)O such that the ratio of doubly to singly labeled CO(2) could not be described by the binomial expansion. This is not due to the hydration/dehydration cycle of CO(2) but most likely to recycling of CO(2) in the reaction with peroxynitrite. This (18)O exchange associated with the reactions of CO(2) and peroxynitrite may open a new methodology for studying this significant process.