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Journal of Bacteriology, August 2004, p. 5427-5431, Vol. 186, No. 16
0021-9193/04/$08.00+0 DOI: 10.1128/JB.186.16.5427-5431.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
Requirements for Nitric Oxide Generation from Isoniazid Activation In Vitro and Inhibition of Mycobacterial Respiration In Vivo
Graham S. Timmins,1 Sharon Master,2 Frank Rusnak,3,
and Vojo Deretic2*
College of Pharmacy, Toxicology Program,1
Department of Molecular Genetics and Microbiology, University of New Mexico Health Sciences Center, Albuquerque, New Mexico 87131,2
Department of Biochemistry and Molecular Biology and Section of Hematology Research, Mayo Clinic and Foundation, Rochester, Minnesota 559053
Received 16 February 2004/
Accepted 11 May 2004

ABSTRACT
Isoniazid (INH), a front-line antituberculosis agent, is activated
by mycobacterial catalase-peroxidase KatG, converting INH into
bactericidal reactive species. Here we investigated the requirements
and the pathway of nitric oxide (NO
·) generation during
oxidative activation of INH by
Mycobacterium tuberculosis KatG
in vitro. We also provide in vivo evidence that INH-derived
NO
· can inhibit key mycobacterial respiratory enzymes,
which may contribute to the overall antimycobacterial action
of INH.

INTRODUCTION
Mycobacterium tuberculosis infections are of serious concern;
they cause 2 million deaths every year and latently persist
in over a billion individuals worldwide (
41). Isoniazid (isonicotinic
acid hydrazide [INH]) remains a front-line antituberculosis
agent some 50 years after its development, and millions of doses
are prescribed worldwide.
M. tuberculosis is exceptionally sensitive
to INH (
12,
44), a prodrug, which is peroxidatively activated
intracellularly by the
M. tuberculosis catalase-peroxidase KatG
to produce a range of reactive radicals that act as damaging
species within the bacteria. Although the mechanism(s) of action
and cellular targets of KatG-activated INH continue to be uncovered
(
22,
27), its full range of effects on mycobacterial cells still
remains to be resolved (
20,
33).
The critical role of INH activation via KatG is clearly apparent based on the findings that the katG gene represents the main site for mutations causing INH resistance in M. tuberculosis (24, 45). Several INH-derived intermediates generated from isoniazid activation, such as isonicotinic acyl NADH (27), and mycobacterial targets including enzymes from the mycobacterial type II fatty acid synthase system (1, 22) have been identified. Other work on INH activation has centered upon INH-derived free radicals as important antimycobacterial intermediates (18, 31, 38). Despite these advances, the exact mechanism(s) of INH action that results in its exceptional and specific potency against M. tuberculosis are not yet fully delineated, as multiple targets and pathways have been considered (12, 20, 23, 32).
A range of reactive nitrogen species, such as nitric oxide (NO·) and peroxynitrite (ONOO) are known to have various levels of activity against M. tuberculosis (8, 21, 43). In addition to the action of exogenously added NO· immune-derived NO· from the action of inducible nitric oxide synthase is considered to contribute to defenses against mycobacterial infection (29). Previous reports of tyrosine nitration during oxidation of INH (34) and NO· formation from peroxidative activation of hydroxyurea (16, 17) led us to hypothesize that NO· might be generated as a result of INH activation by KatG. We further hypothesized that the exceptional sensitivity of M. tuberculosis to NO· could potentially account for at least some of the potency of INH (8, 21, 43). Here we used spin trapping techniques to document KatG generation of nitric oxide during activation of INH and to probe potential pathways for its production from INH. We also present in vivo analysis indicating that NO· production during INH activation acts against important respiratory enzymes and that this may potentially contribute to the antimycobacterial action of INH.

MATERIALS AND METHODS
In vitro spin trapping of INH-derived NO·.
An NO
·-specific electron paramagnetic resonance (EPR)
spin trapping technique was used (
19). Ten millimolar Fe(II)
(
N-methyl-
D-glucamine dithiocarbamate)
2 complex was prepared
by anoxic mixing of FeSO
4 and
N-methyl-
D-glucamine dithiocarbamate
and was incubated with 0.471 mg of purified
M. tuberculosis H37Rv KatG ml
1 (
37) with 10 mM INH and 10 mM H
2O
2 in
10 mM phosphate buffer, pH 7, at 37°C for 5 min. These reaction
conditions were similar to those previously described (
38),
with one modification: the overwhelming catalase activity of
KatG necessitated higher levels of H
2O
2 than the previously
used catalase-insensitive
tert-butyl hydroperoxide (
38).
N-Methyl-
D-glucamine
dithiocarbamate was synthesized by the method of Shinobu et
al. (
30), EPR spectrometry of incubated samples was then performed
using a Bruker Elexsys series spectrometer operating at X-band
frequencies at 25°C with samples held in 20-µl capillaries.
Authentic NO was from a commercial source (Sigma-Aldrich). To
produce an authentic NO spin adduct, NO gas was bubbled through
an aqueous solution (10 mM) of the spin trap Fe(II) (
N-methyl-
D-glucamine
dithiocarbamate)
2 complex.
In vitro spin trapping of oxygen and carbon-centered INH-derived free radicals.
EPR spectra were recorded after incubation of 0.471 mg of KatG ml1 with 10 mM INH, 5,5-dimethyl-1-pyrolline-N-oxide (DMPO; Sigma, St. Louis, Mo.) and 0.4 mM tert-butylhydroperoxide. DMPO was purified by charcoal treatment (6) and was used at a final concentration of 0.058 M. Anaerobic samples were prepared by deoxygenation of reagents with O2-free argon and sample loading under argon. Adduct assignments were made upon the hyperfine couplings (7), with peroxyl radical-derived adducts being dependent upon the presence of O2.
Aconitase and cytochrome c oxidase activities upon exposure to INH.
Exponentially growing cultures of Mycobacterium bovis BCG were treated overnight with various concentrations of INH. After treatment, cell extracts were obtained by beadbeating with 0.1-mm zirconia beads (two 30-s cycles) in a minibeadbeater (Biospec Products Inc., Bartsville, Okla.). Cell extracts were assayed for protein with a Pierce bicinchoninic acid (BCA) kit (Pierce, Rockford, Ill.). For aconitase activity, exponentially growing cultures of M. bovis BCG (as above) were treated overnight with 73 µM INH at 37°C, and aconitase was assayed in cell extracts with a Oxis Bioxytech Aconitase-340 kit (Oxis International Inc., Portland, Oreg.). For cytochrome c oxidase activity, exponentially growing cultures of M. bovis BCG were treated overnight with 5.5 µM INH at 37°C. Cytochrome c oxidase (in cell extracts, as previously described) was assayed using a Sigma cytochrome c oxidase kit (Sigma).
Isocitrate dehydrogenase assay.
Exponentially growing cultures of M. bovis BCG (as above) were treated overnight with 73 µM INH at 37°C, and isocitrate dehydrogenase was assayed in cell extracts by using components of the Oxis Bioxytech Aconitase-340 kit (Oxis International Inc.), except for the substitution of citrate with 230 µM isocitrate (Sigma).

RESULTS AND DISCUSSION
Generation of NO· from INH depends on KatG and H2O2.
To examine the requirements for NO
· production during
INH activation, we used an in vitro system consisting of purified
components. NO
· production was detected during KatG-mediated
oxidation of INH in a reaction mixture comprised of the following:
(i) purified and previously characterized
M. tuberculosis H37Rv
KatG (
38), (ii) INH, and (iii) H
2O
2 (
39) (Fig.
1, spectrum v).
This mixture, consisting of purified components, was identical
to the previously published INH activation system (
38), with
the single modification of substituting the previously used
catalase-insensitive
tert-butyl hydroperoxide (
38) with H
2O
2,
at concentrations bypassing the intrinsic catalase activity
of KatG. Identification of NO
· was based on the
14N-hyperfine
coupling (1.25 mT), the
g value (2.04), and by the identity
of the spectra with an authentic NO
· standard (Fig.
1,
spectrum i). The generation of NO
· was reproducible and
had absolute requirements (Fig.
1, spectra ii to v) for enzyme
(KatG), substrate (INH), and oxidant (H
2O
2).
Initial investigations of the potential pathway of NO· production from INH.
We next addressed the pathway of NO
· production from
INH. From the analogy with hydroxyurea, the NO
· must
derive from oxidation occurring at the nitrogen atoms of the
hydrazide group. Prior work also indicated the importance of
oxidation at the inner nitrogen atom of the hydrazide group,
since alkyl substitution here destroys activity in vivo, whereas
that at the terminal nitrogen does not (
14). Furthermore, since
INH is only effective against aerobic mycobacteria (
36), we
hypothesized that addition of molecular oxygen, O
2, to one of
the initial radicals might be important. To follow up on this
lead, we examined the formation of other free radical species
formed by KatG oxidation of INH, by using EPR spin trapping
with DMPO, a spin trap able to form stable adducts with a wide
range of radical species. DMPO is superior to the

-phenylbutylnitrone
previously used in such studies (
38) as it possesses a much
wider dynamic range of hyperfine coupling constants that can
allow much better identification of the radical adducts (
7).
To allow for anoxic incubations, we used
tert-butylhydroperoxide
as the oxidant, as previously described (
38), to avoid the O
2 formation that occurs from the catalase domain acting upon H
2O
2.
In the absence of oxygen, we could observe species assigned
(on the basis of their hyperfine couplings) as carbon-centered
(R
·) (A
N = 1.6 mT, A
H = 2.3 mT) and alkoxyl (RO
·)
(A
N = 1.5 mT, A
H = 1.6 mT) radical adducts (Fig.
2a and reactions
1 and 2).
 | (1) |
 | (2) |
As
predicted, in the presence of oxygen, an additional species
assigned as deriving from peroxyl radical (ROO
·) (A
N = 1.46 mT, A
H = 1.1 mT) was formed (Fig.
2b and reaction 3).
It can be seen that the ratio of intensity of the carbon-centered
to alkoxyl radical adducts is not greatly changed by addition
of O
2. This indicates that either (i) both alkoxyl and carbon-centered
species react with O
2 to form peroxyl radicals at similar rates
(which is unlikely, as oxygen addition to alkoxyl radicals is
not at all favorable) or (ii) one of the species is a precursor
to the other, so that lowered levels of one from its scavenging
by O
2 equally lowers levels of both. The latter is much more
probable.
 | (3) |
Thus, we confirmed that
O
2 reacted with an initial INH-derived radical, formed by KatG
oxidation of INH, to form a peroxyl radical. The requirement
for O
2 for optimal activity of INH (
42) would suggest that this
peroxyl species may be an important intermediate in INH activation.
In the absence of INH, the only product observed was simply
the oxidation product of DMPO, DMPOX (
13,
26) (Fig.
2c), demonstrating
that the adducts detected in Fig.
2a and b are INH derived.
No EPR signals at all were observed in the absence of KatG.
Biological activity of NO· generated during INH-derived NO· activation.
NO
· exerts its antimicrobial activities primarily through
two different mechanisms. Firstly, NO
· reacts with superoxide
(O
2·) at diffusion-controlled rates to form peroxynitrite
(ONOO
; reaction 4), a reactive species capable of oxidizing
and nitrating biomolecules (
28). Secondly, NO
· acts directly
upon metalloproteins such as aconitases (
15), cytochrome
c oxidases
(
10), and a range of other targets (
11), thereby inhibiting
respiratory activity and iron homeostasis.
 | (4) |
However,
there are many reports that
M. tuberculosis has potent detoxifying
systems against peroxynitrite, including the peroxynitritase
activity of KatG (
4,
5,
21,
37,
43). Furthermore, the importance
of inhibition of
M. tuberculosis respiration by NO
· was
recently demonstrated, even under conditions when the inhibition
of respiration was modest (
35). We therefore focused our attention
upon NO
·-sensitive respiratory metalloproteins.
Aconitase is an important respiratory enzyme and also has additional roles in iron homeostasis (40). Aconitase contains an 4S-4Fe iron-sulfur center and is known to be sensitive to NO·-induced damage (15). A significant decrease in the aconitase activity was detected in M. bovis BCG treated overnight with 73 µM INH (Fig. 3a). This finding is consistent with the predicted effects of INH-derived NO· upon metalloproteins, although this concentration of INH is approximately 10 times the MIC.
Next, we tested another prototypical NO
· target, the
respiratory enzyme cytochrome
c oxidase, which is exquisitely
sensitive to NO
· (
10). Although whole-cell oxygen consumption
measurements can prove useful (
35), there are many potential
sources of oxygen consumption other than cytochrome
c oxidase,
not least of which is oxidative detoxification of NO
· by the truncated hemoglobins trHbN and trHbO (
25). Thus, we
used a specific assay for cytochrome
c oxidase, based upon cytochrome
c oxidation, to study its inhibition by INH. Overnight treatment
with 5.5 µM INH, a value close to the MIC for
M. tuberculosis (3.7 to 7.3 µM in our hands) caused a 64% inhibition of
cytochrome
c oxidase activity (Fig.
3b). The much greater sensitivity
of cytochrome
c oxidase compared to that of aconitase is in
accord with its known greater sensitivity to NO
· over
FeS proteins (
2,
3). As a control, isocitrate dehydrogenase
was assayed in the same extracts, and no differences upon INH
treatment were observed (Fig.
3c).
Conclusions.
In this work we have demonstrated that NO· is formed by oxidative activation of INH by KatG, in a reaction that requires KatG and H2O2. We have also provided initial evidence for a pathway of KatG oxidation of INH. Our enzymological assays suggest that the NO· generated in vivo during oxidation of INH can have appreciable activity against respiratory enzymes. It is evident from the experimental data, however, that INH-derived NO· did not fully inhibit either aconitase or cytochrome c oxidase. This is consistent with the importance of other known antimycobacterial products of INH, such as isonicotinic acyl NADH (27). Nevertheless, the generation of NO· from INH and its detectable effects on the bacteria suggest the prospects of enhancing this property of INH as a potential strategy for generating new antituberculosis drugs. The addition of NO·-releasing groups to another antibiotic, ciprofloxacin, greatly increases its activity against M. tuberculosis (9), supporting the hypothesis that NO· release can synergize with other antimycobacterial activities. Since even modest inhibition (
50%) of respiration by NO· has profound effects on the physiology of M. tuberculosis (35), the levels of inhibition observed in this study may prove similarly important.

ACKNOWLEDGMENTS
We thank E. J. H. Bechara, K. J. Liu, and T. Wilson for discussions.
EPR facilities were provided by the UNMHSC Center of Biomedical
Research Excellence, NCRR P20-R15636.
This work was supported by NIH grant AI42999.

FOOTNOTES
* Corresponding author. Mailing address: Department of Molecular Genetics and Microbiology, University of New Mexico Health Sciences Center, 915 Camino de Salud NE, Albuquerque, NM 87131. Phone: (505) 272-0291. Fax: (505) 272-5309. E-mail:
vderetic{at}salud.unm.edu.

Deceased. 

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Journal of Bacteriology, August 2004, p. 5427-5431, Vol. 186, No. 16
0021-9193/04/$08.00+0 DOI: 10.1128/JB.186.16.5427-5431.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
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