From the Department of Chemistry, Juntendo University
School of Medicine, Inba, Chiba 270-16, Japan and the ¶ Division
of Biochemical Analysis, Central Laboratory of Medical Sciences,
Juntendo University School of Medicine, Hongo, Bunkyo-ku,
Tokyo 113, Japan
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ABSTRACT |
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Peroxynitrite has recently been implicated in the
inactivation of many enzymes. However, little has been reported on the
structural basis of the inactivation reaction. This study proposes that
nitration of a specific tyrosine residue is responsible for
inactivation of recombinant human mitochondrial manganese-superoxide
dismutase (Mn-SOD) by peroxynitrite. Mass spectroscopic analysis of the peroxynitrite-inactivated Mn-SOD showed an increased molecular mass
because of a single nitro group substituted onto a tyrosine residue.
Single peptides that had different elution positions between samples
from the native and peroxynitrite-inactivated Mn-SOD on reverse-phase
high performance liquid chromatography were isolated after successive
digestion of the samples by staphylococcal serine protease and
lysylendopeptidase and subjected to amino acid sequence and molecular
mass analyses. We found that tyrosine 34 of the enzyme was exclusively
nitrated to 3-nitrotyrosine by peroxynitrite. This residue is located
near manganese and in a substrate O2 gateway
in Mn-SOD.
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INTRODUCTION |
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Peroxynitrite anion
(ONOO)1 is a
potent biological oxidant that has been implicated in diverse forms of
free radical-induced tissue injury (1). Peroxynitrite has been
demonstrated to readily oxidize or nitrate various enzymes such as
metalloproteinase-1 inhibitor (2), alcohol dehydrogenase (3), aconitase
(4), glutamine synthetase (5), and superoxide dismutase (6). However,
little is known about the molecular basis of the nitration or oxidation
of these enzymes. Peroxynitrite is produced by the reaction of
superoxide (O
2) and NO with a very rapid reaction rate (4 ~ 6.7 × 109 M
1
s
1) (7, 8). Therefore, various cell types, such as
macrophages (9), Kuppfer cells (10), and endothelial cells (11), that simultaneously produce and release O
2 and NO could produce
peroxynitrite.
Mammalian mitochondria are one of the most important targets of the
cytotoxicity of peroxynitrite. Recently, a mechanism for peroxynitrite-mediated dysfunction of mitochondria has been proposed as
follows. When mitochondria are exposed to NO, NO diffuses easily through the membranes and reversibly inhibits cytochrome oxidase. This
inhibition causes inhibition of the mitochondrial respiratory chain and
as a consequence increases mitochondrial O2
release, leading to peroxynitrite formation (12, 13). Peroxynitrite then irreversibly inhibits complexes I and II in the mitochondrial respiratory chain (13). In this mechanism, accumulation of
O
2 results in production of a large amount of
peroxynitrite. Mitochondrial Mn-SOD2 has a function to
eliminate O
2 from the mitochondrial matrix space.
However, O
2 can react with NO more than three
times faster than with mitochondrial Mn-SOD (14). Therefore, if
mitochondria are exposed to a relatively large amount of NO, a
sufficient amount of peroxynitrite could be formed even in the presence
of Mn-SOD. In this context, a clear understanding of the reactivity of
mitochondrial Mn-SOD with peroxynitrite is necessary to clarify the
mechanism of mitochondrial respiratory damage caused by NO.
Ischiropoulos et al. (6) and Smith et al. (15) reported that bovine Cu,Zn-SOD reacts with peroxynitrite to form nitrotyrosine at Tyr-108 without inactivation of enzymatic activity. In contrast, Mn- and Fe-SOD from Escherichia coli are inactivated by peroxynitrite (6). Nitration of tyrosine residues was also observed. MacMillan-Crow et al. (16) reported inactivation of Mn-SOD activity and concomitant increase of 3-nitrotyrosine in a tissue homogenate of transplanted allogripha during chronic rejection. They also reported that recombinant human Mn-SOD was inactivated by peroxynitrite with an IC50 of 10 µM and concomitant increase of nitration of tyrosine residues. However, the number of nitrated residues and an exact correlation between the nitration of tyrosine residue(s) and inactivation of human Mn-SOD has not been confirmed. Recently, we showed that nitration of one tyrosine residue of human Mn-SOD by peroxynitrite was linearly related to inactivation of human Mn-SOD by monitoring the absorption increase at 428 nm (17). In this study, we report that exclusive nitration of Tyr-34 is responsible for the inactivation of human Mn-SOD by peroxynitrite. This is the first example of peroxynitrite-mediated enzyme inactivation that is caused by nitration of a single specific amino acid.
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EXPERIMENTAL PROCEDURES |
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Materials
Peroxynitrite, synthesized by the reaction of sodium nitrite and hydrogen peroxide, was purchased from Dojindo Laboratory Co., Japan. Staphylococcal serine protease (EC 3.4.21.19) and lysylendopeptidase (EC 3.4.21.50) were purchased from Pierce and Wako Chemical Co., Japan, respectively. Human Mn-SOD was generously provided by Bio-Technology General (Israel) Ltd. The SOD preparation showed a specific activity of 2,684 ± 296 units/mg/mol of manganese/mol of subunit and metal contents of 0.81 ± 0.028 and 0.036 ± 0.02 g atom/mol of subunit for manganese and iron, respectively. This preparation lacked two amino acids from the N terminus (Lys-His-) and therefore had a calculated molecular weight of 21,939 (18).
Methods
Inactivation of Mn-SOD by Peroxynitrite-- The reaction of peroxynitrite with SOD for structural studies was carried out as follows. 0.1-ml reaction mixtures containing 1 mg/ml of SODs in 150 mM potassium phosphate buffer, pH 7.4, with 0.1 mM diethylenetriamine pentaacetic acid and peroxynitrite at a concentration of 1 mM were incubated at 25 °C. For control experiments, peroxynitrite was added 1 min before the addition of SOD. The reaction mixtures were analyzed after a 2-min incubation. Superoxide dismutase activity was measured by the inhibition of cytochrome c reduction by xanthine oxidase (19) with minor modifications (20). Metal contents were determined by using atomic absorption spectrometry with a Hitachi Z-9000 atomic absorption spectrophotometer. Amino acid analyses were performed with a Hitachi L-8500 amino acid analyzer, and the samples were hydrolyzed with 6 M hydrochloric acid containing 2% thioglycolic acid for 10, 15, and 20 min at 165 °C.
Protease Digestion and Peptide Separation-- Native and peroxynitrite-inactivated Mn-SOD (5.8 nmol) were denatured in 150 µl of 67 mM ammonium acetate buffer, pH 4.0, at 95 °C for 10 min and then digested with staphylococcal serine protease (substrate:enzyme = 1:50, mol/mol) at 37 °C for 18 h. After freeze-drying the digested sample, 100 µl of 0.1% trifluoroacetic acid solution was added. The peptides were separated on a Jasco HPLC system with a ultraviolet/visible monitor (Tokyo, Japan). An octadecyl silica gel reverse-phase column, 4.6 × 250 mm (120T, TOHSO Co. Ltd., Tokyo, Japan), was used. Solvent A was 0.1% trifluoroacetic acid in ultra pure water (Milli-Q), and solvent B was 0.1% trifluoroacetic acid in 60% acetonitrile. The gradient program of the solvent was linear from 0-30% B in 7 min, 30-80% B in 60 min, and 80-100% B in 6 min with a flow rate of 1.0 ml/min. The ultraviolet-visible detector was used at 230 and 428 nm, respectively. The nine major peptides were collected and subjected to amino acid sequence analysis and molecular mass determination on a mass spectrometer. The amount of these peptides was estimated by the absorption at 210 nm (21). The single peptides from each sample that had different elution positions were further digested with lysylendopeptidase (S:E = 150:1, mol/mol) in 10 mM Tris-HCl buffer at pH 9.5 at 35 °C for 14 h. The peptides resulting from this digestion were separated by the same method as above except that the gradient program of the solvent was linear from 0 to 100% B in 60 min. The single peptides from each sample that had different elution positions were collected and subjected to amino acid sequence and molecular mass analysis.
Mass Spectrometry and Amino Acid Sequencing--
The molecular
weights of the native and peroxynitrite-inactivated Mn-SOD and
protease-digested peptides were determined with a TSQ 700 electrospray
ionization mass spectrometer (Thermo-Quest Finnigan Mat Co., Ltd., San
Jose, CA). The analytical conditions were as follows; spray voltage,
4.5 kV; electron multiplier, 1500 V; manifold vacuum, 7.0 × 106; manifold temperature, 70 °C; capillary
temperature, 150 °C; scan range, m/z
500-3000; scan time, 5 s. Mn-SOD and the peptides were dissolved
in a mixture of methanol and 0.5% acetic acid (1:1 v/v) to a final
concentration of 10 pmol/µl and infused into the ion source of the
TSQ 700 with a pump. Amino acid sequences of the peptides derived from
Mn-SOD were determined by using an Applied Biosystems 477A gas liquid
phase protein sequencer equipped with an on-line 120A PTH amino acid
analyzer. PTH-3-nitrotyrosine was obtained by application of
3-nitrotyrosine into the PTH analyzer.
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RESULTS |
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Inactivation and Nitration of Mn-SOD by Peroxynitrite-- Inactivation of recombinant human Mn-SOD by peroxynitrite shows different IC50 values dependent on the enzyme concentration, that is 0.43, 2.28, and 11.8 µM Mn-SOD exhibits IC50 values of 6, 30, and 170 µM peroxynitrite, respectively, at pH 7.4 (17). For this structural analysis study, we used 11.8 µM Mn-SOD and 1 mM peroxynitrite at pH 7.4. Under these conditions, more than 90% of the activity was lost after a 2-min incubation. The inactivated enzyme still contained over 75% of the original content of manganese in the native enzyme and showed two bands that migrated a little faster than those of the native enzyme in acrylamide gel electrophoresis (data not shown). Fig. 1 shows the results of mass spectrometry of the native and peroxynitrite-inactivated Mn-SOD. Deconvolution of the protein mass spectra (Fig. 1, A and B, insets) revealed that the molecular mass of the enzyme had increased by 46 Da (21,939 Da to 21,985 Da) after inactivation, consistent with the addition of a single nitro group into the inactivated enzyme. Deconvolution of the spectra also showed subpeaks corresponding to a molecular mass of 21,977 Da for the native enzyme and 22,023 Da for the inactivated enzyme. Each subpeak had an increase in molecular mass (38 Da) compared with the respective main peaks that corresponds to binding of potassium ion to each enzyme preparation. Amino acid analyses of the native and the inactivated enzymes showed a decrease of 0.81 ± 0.15 mol of tyrosine residue/mol of subunit in the inactivated Mn-SOD (average of three measurements). No significant differences in the composition of the other amino acids was observed between the two samples. From this evidence, we conclude that a single nitro group was introduced onto a tyrosine residue specifically by the reaction of peroxynitrite with Mn-SOD.
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Identification of the Position of the Nitrated Tyrosine in Mn-SOD-- After digestion of the native and peroxynitrite-inactivated Mn-SOD by staphylococcal serine protease, we separated nine major peptides by reverse-phase HPLC. Fig. 2 shows a chromatogram of the peptides from the native (A) and peroxynitrite-inactivated (B) Mn-SOD. Among the nine major peptides separated, peptides 5 and 5' had different elution positions between the two samples, which suggested that peptide 5' was nitrated by peroxynitrite. A minor peak that has the same elution position as peak 5 in Fig. 2A also appears in Fig. 2B. The ratio of the area between this peak and peak 5' in Fig. 2B is 17:83, which suggests that 83% of the intact peptide was converted to the modified peptide by the peroxynitrite treatment. Fig. 2C shows the elution pattern of the peptides from the peroxynitrite-inactivated enzyme monitoring the absorption at 428 nm, which arises from 3-nitrotyrosine. Although a few very small peaks were observed other than the major peak in Fig. 2C, the elution position of the major peak coincided with the position of peak 5' in Fig. 2B. Molecular masses of 4,681 and 4,727 Da were obtained for the peptides from the native and inactivated enzymes, respectively. The difference in molecular mass (45 Da) corresponds to the molecular mass of a nitro group substituted onto a tyrosine residue. The same sequence of 23 amino acids from the N terminus was obtained for both peptides as Ser-Leu-Pro-Ala-Leu-Pro-Tyr-Asp-Tyr-Gly-Ala-Leu-Glu-Pro-His-Ile-Asn-Ala-Gln-Ile-Met-Gln-Leu-, which are the same as the reported sequence from Ser-3 to Leu-25. These results suggest that the fifth peptide in both HPLC elutants corresponds to a peptide of 41 amino acids at Ser-3 to Glu-43 (S3LPALPYD10YGALEPHINA20QIMQLHHSKH30HAAYVNN L NV40TEE). The calculated molecular mass of the peptide from the native Mn-SOD (4,681 Da) was exactly the same as that obtained from the mass spectrometry of the peptide. Because each peak in Fig. 2B had the same molecular mass as corresponding peaks in Fig. 2A (except for peaks 5 and 5') and the whole amino acid sequence of the enzyme was covered by the peptides of the peak 1 (Lys-44-Glu-47, 567 Da), peak 6 (Ala-48-Glu-95, 4,969 Da), peak 7 (Ala-96-Glu-131, 3,963 Da), peak 9 (dimer of Arg-132-Glu-191, 13,897 Da), and peak 8 (heterodimer between Arg-132-Glu-191 and Arg-192-Lys-198, 7,847 Da), we conclude that no other amino acid was modified by peroxynitrite. The peak 3 in Fig. 2 (A and B) corresponded to the peptide Ala-96-Gln-109, which might be caused by the conversion of glutamine109 to glutamic acid during the course of the protease digestion or purification of the enzyme.
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DISCUSSION |
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Peroxynitrite is known to attack many amino acids including
cysteine (22), methionine (23), tryptophan (24), phenylalanine (25,
26), and tyrosine (25, 26). Although human Mn-SOD contains one
methionine residue, two cysteine residues, six tryptophan residues, six
phenylalanine residues, and nine tyrosine residues (18), only Tyr-34
was nitrated by peroxynitrite to cause inactivation of the enzyme. This
tyrosine has a specific position in the three-dimensional structure of
human Mn-SOD. Structural studies on human Mn-SOD (18, 27), along with
studies on other bacterial Fe-SODs (28-31), Mn-SODs (29, 32, 33), and
cambialistic SOD (34), show that Tyr-34 is conserved in all these SODs
and that the phenolic oxygen atom of Tyr-34 is located 5.4 Å from
manganese (18, 27) at the vertex of the substrate funnels (18, 29).
These funnels are proposed to lead the substrate,
O2, from the bulk solvent toward the metal ions by
an electrostatic guidance mechanism (35). Peroxynitrite may be
introduced to manganese through the position of Tyr-34 by the same
mechanism as O
2. Although we cannot clarify whether the active
site manganese participates in the nitration reaction of Tyr-34 or not,
we are in favor of the mechanism of a manganese-catalyzed nitration for
the following reason: the proportion of nitrated tyrosine residue,
which was estimated by the decrease of tyrosine contents in the amino
acid analysis of the peroxynitrite-inactivated Mn-SOD (
0.80 mol/mol
of subunit) and by the proportion of the new peak in the first HPLC
(83%, Fig. 2B), closely coincides with the manganese
content of the native Mn-SOD (0.81 g atom/mol of subunit). Therefore,
peroxynitrite may react with manganese to form an active nitrating
species, probably nitronium ion (NO2+), similar to the case
of Cu,Zn-SOD (6, 26). This cation then reacts with the adjacent
tyrosine residue directly. We cannot rule out the possibility that the
nitration of tyrosine 34 could take place without metal catalysis,
because tyrosine 34 is located in the gateway of
O
2 and may have a chance to react directly with
peroxynitrite. Ramezanian et al. (25) reported that
peroxynitrite could nitrate tyrosine by the reactions catalyzed by
metals or without metals. Recently Gow et al. (36), Denicola
et al. (37), and Lemercier et al. (38) reported
that peroxynitrite nitrates tyrosine and phenol by a mechanism that is
catalyzed by carbon dioxide at physiological concentrations (36, 37) or
at low concentrations such as an amount achieved by equilibrating with air in an aqueous reaction solution for nitration (38). Further study
is required to elucidate the mechanism of the nitration of Tyr-34 in
human Mn-SOD.
Because the nitrated enzyme gave only a slightly faster protein band compared with that of the native enzyme in polyacrylamide gel electrophoresis (data not shown) and retained over 75% of the original manganese contents, the nitrated enzyme may still have the same gross structure as the native enzyme. Therefore, a change in Tyr-34 function may more likely cause the inactivation of Mn-SOD than a change in the gross protein structure. Recent mutational studies on Tyr-34 of E. coli Mn-SOD (39) and Fe-SOD (40, 41) proposed three major functions of Tyr-34 as follows. 1) Tyr-34 may not be essential for catalysis and may function to maintain the appropriate protonation state of the proton cluster, which is required to supply a proton in the second cycle of the dismutation reaction of the enzyme, consisting of Tyr-34, Gln-143, and coordinated solvent (Fig. 4) (39-41). 2) Tyr-34 may be required to make a proper active site environment by forming a hydrogen bond network through Tyr-34, Gln-143, and the coordinated solvent (Fig. 4) (41). 3) Tyr-34 may control the accessibility of both substrate and inhibitor molecule to the active site metal. Nitration of the Tyr-34 residue may result in a change of the pKa of the phenol group of the residue from around 10 to 7.5, which may cause deprotonation of the phenol group of the tyrosine residue and break the hydrogen bonding network of coordinated solvent Gln-143-Tyr-34 at physiological pH (Fig. 4). Because the substrate is believed to bind the active site metal through a position between His-30 and Tyr-34, the nitro group of the modified tyrosine could have a steric hindrance effect for access of the substrate to the active site. A combination of these effects originating from nitration of Tyr-34 may result in the loss of the enzymatic activity of human Mn-SOD by peroxynitrite.
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Finally, we can suggest a pathophysiological significance of the
nitration of Tyr-34 of human Mn-SOD. Although physiological concentrations (<1 µM) of NO reversibly inhibit
cytochrome oxidase in an oxygen concentration-dependent
manner and therefore NO was proposed as a regulator of mitochondrial
respiration under these conditions (12, 42), a higher concentration of
NO (5-200 µM) is proposed to cause mitochondrial
dysfunction (13). In the latter case, NO will compete with Mn-SOD for
mitochondrially produced O2, production of which
may be enhanced by the inhibition of cytochrome oxidase with NO, to
form peroxynitrite (13). Nitration and inactivation of Mn-SOD by
peroxynitrite could be a turning point in this mitochondrial
dysfunction, because inactivation of Mn-SOD leads to accumulation of
O
2 and as a consequence more accumulation of
peroxynitrite. Both accumulated O
2 and
peroxynitrite may cause oxidative damage of mitochondrial components.
Nitration and inactivation of Mn-SOD was observed in a tissue
homogenate of transplanted allogripha during chronic rejection (16).
Therefore, the presence of Tyr-34-nitrated Mn-SOD could be a marker for
NO- and/or peroxynitrite-induced mitochondrial dysfunction under some pathological conditions such as inflammation. To evaluate this possibility, a study of the exact correlation between the NO
concentration-dependent nitration of Tyr-34 of Mn-SOD and
dysfunction of mitochondrial respiration by using isolated mitochondria
is progressing in our laboratory.
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ACKNOWLEDGEMENTS |
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We thank Dr. Moshe Werber of Bio-Technology General (Israel) Ltd. for kindly supplying human recombinant Mn-SOD. We thank Prof. Takashi Matsumoto for measurement of the metal contents by atomic spectrometry. We are pleased to thank Prof. Joseph S. Beckman for helpful discussions. We are also grateful to Dr. Joseph A. Gardner for critical manuscript review.
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FOOTNOTES |
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* The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
§ To whom correspondence should be addressed: Dept. of Chemistry, Juntendo University School of Medicine, 1-1 Hiraga-gakuendai, Inba, Chiba 270-16, Japan. Tel.: 81-476-98-1001; Fax: 81-476-98-1011; E-mail: yamakura{at}sakura.juntendo.ac.jp.
1
The term peroxynitrite is used to refer to both
peroxynitrite anion (ONOO) and peroxynitrous acid
(ONOOH). IUPAC recommended names are oxoperoxonitrate(1
) and hydrogen
oxoperoxonitrate, respectively.
2 The abbreviations used are; SOD, superoxide dismutase; Mn-SOD, manganese-SOD; Cu,Zn-SOD, copper,zinc-SOD; Fe-SOD, iron-SOD; PTH, phenylthiohydantion; HPLC, high performance liquid chromatography.
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REFERENCES |
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