Vol.2, No.3, 202-207 (201 1)
doi:10.4236/jbpc.2011.23024
C
opyright © 2011 SciRes. Openly accessible at http://www.scirp.org/journal/JBPC/
Journal of Biop hysical Chemistry
A predictive kinetic model for inhibitory effect of
nitrite on myeloperoxidase catalytic activity
towards oxidation of chloride
Yahya R. Tahboub1,2*, Mohammad M. Fares1
1Department of Applied Chemistry, Faculty of Science & Arts, Jordan University of Science and Technology, Irbid, Jordan;
2Department of Applied Chemistry, Faculty of Applied Sciences, Taibah University, Almadinah Almonawwarah, Saudi Arabia;
*Corresponding Author:
Received 13 April 2011; revised 19 May 2011; accepted 2 June 2011.
ABSTRACT
Myeloperoxidase (MPO) is a neutrophil enzyme
that employs hydrogen peroxide (H2O2) to cata-
lyze the oxidation of chloride (Cl–) to hy-
pochlorous acid (HOCl). Accepted mechanism
is based on rapid reaction of native MPO with
H2O2 to produce Compound I (MPO-I) which
oxidizes Cl– through a 2e– transition generating
MPO and HOCl. MPO-I also reacts with H2O2 to
generate Compound II (MPO-II) which is inac-
tive in 2e oxidation of Cl–. Nitrite (2 NO ) inhibits
the 2e oxidation of Cl– by reaction with MPO-I
through 1e transition generating MPO-II and
nitrite radical. H2O2 consumption during stead y-
state catalysis w as monitored amperometrically
by a carbon fiber based H2O2-biosensor a t 25˚C.
Results demonstrated that in absence of 2 NO


reactions were monophasic and rapid (complete
H2O2 consumption occurs in <10 s). As con-
centration of 2 increases, reactions change
to biphasic (rapid step followed by a slow step)
and both steps have been inhibited by 2
NO
NO

. A
predictive kinetic model describing the inhibit-
tory effect of 2 was developed and applied
to experimental results. The model is based on
the assumption that MPO-I cannot be detected
during steady-state catalysis. Calculated rate
constants are in agreement w ith those obtained
from pre-steady state kinetic methods.
NO
Keywords: MPO-Hydrogen Peroxide-Chloride
System; Nitrite Inhibitor; 4
3
k
1. INTRODUCTION
Myeloperoxidase (MPO) is a human peroxidase en-
zyme and a lysosomal protein stored in azurophilic
granules of the neutrophil. Its deficiency can severely
cause quantitative or functional genetic disorder [1]. The
major role of MPO is to aid in microbial killing. It
oxidizes tyrosine to tyrosyl radical using hydrogen
peroxide as oxidizing agent [2]. Furthermore, the MPO
catalyzed production of HOCl from hydrogen peroxide
(H2O2) and chloride ion (Cl), together with tyrosyl
radical, are aimed at killing bacteria and other pathogens.
The MPO-hydrogen peroxide-chloride system has been
considered an important pathophysiologic factor in
kidney disease [3], to enhance lipid oxidation in LDL in
presence of SCN catalyst [4], to lead to oxidative dam-
age of apolipoprotein A-I [5], to oxidizes free α-amino
acids to aldehydes [6], leading to advanced glycation
products present in human lesion material [7].
Vast range of inflammatory diseases was found to be
correlated with products of MPO nitration of tyrosine
residues. Myeloperoxidase can oxidize nitrite ions to an
intermediate capable of nitrating tyrosine and tyrosyl
residues in proteins [8-10].
The simplified mechanism that governs the catalytic
activity of MPO can be represented by the classic per-
oxidases catalytic cycle as follows:
Chloride oxidation starts by rapid reaction of ground
state MPO with H2O2 to form Compound I (MPO-I).
Compound I is capable of oxidizing chloride (Cl)
through a 2e transition generating the ground state
MPO and hypochlorous acid (HOCl). During turnover,
some MPO-I is converted to Compound (II) (MPO-II)
by reaction with 1e donors such as nitrite (2
NO

) or
(H2O2). As could be seen in equations 1 to 4 [11,12,17].
k
, 4
2
k
k
, and 2
3
k
k
Ratios;
Theoretical Kinetic Model 1
22 2
MPOH OMPO-IHO
k
 (1)

Y. R. Tahboub et al. / Journal of Biophysical Chemistry 2 (2011) 202-207
Copyright © 2011 SciRes. Openly accessible at http://www.scirp.org/journal/JBPC/
203203
2
2
MPO-I ClHMPOHOCl
k 
 (2)
3
2
MPO-I NOMPO-II NO
k 
 (3)
4
22 22
MPO-IH OMPO-IIH O
k
 (4)
MPO-II is believed to be inactive in chloride oxida-
tion. The decay of MPO-II to ground state is considered
as the rate-limiting step during steady-state catalysis
[11,12].
Pre-steady-state and steady-state studies based on
stopped-flow mixing and optical detection were em-
ployed for studies of MPO-hydrogen peroxide-chloride
system in presence and absence of nitrite [13,14]. In
such studies, larger than physiological plasma concen-
trations of MPO and/or 2 were employed to moni-
tor measurable changes in absorbance at selected wave-
lengths. Steady-state methods with amperometric moni-
toring have advantages over optical methods. In such
methods, the oxidation or reduction of a targeted reac-
tant or product is directly monitored at the surface of a
selective electrochemical biosensor. However, lack of
biosensors with enough sensitivity, selectivity and short
response time limited their role to initial rate measure-
ments [13-17].
NO
Recently, combination H2O2-biosensors with adequate
sensitivity (2 pA/nM) and a relatively short response
time (<2 s) were developed [18-20].
In a previous study [20], we employed a carbon fiber
based H2O2-biosensor to study the effect of 2
NO

on
catalytic activity of MPO towards oxidation of chloride
under respective physiological concentrations. Our re-
sults confirmed the inhibitory nature of 2 NO

. In this
study, we utilized experimental data to develop a kinetic
model capable of explaining the monophasic and bi-
phasic phenomena. Additionally, 4
3
k
k, 4
2
k
k, and 2
3
k
k
ratios were estimated and the dependence of k1 on [Cl–]
and [] was determined.
2
NO
2. EXPERIMENTAL
2.1. Reagents
Chemicals used for preparation of buffer, stock and
standard solutions were of analytical grade reagents and
purchased from Sigma Chemical Co. (St. Louis, MO,
USA). Phosphate buffer, 100 mM and pH 7.00, was
prepared by mixing appropriate volumes of 0.10 M
NaH2PO4 and 0.10 M Na2HPO4 to achieve pH 7.00. A
3.00 mM H2O2 solution was freshly prepared from stock
solutions prepared by sequential dilutions from 30%
H2O2 solution. Standard solutions of chlorides and 2
NO

were prepared by sequential dilutions from their respec-
tive sodium salts. All solutions were bubbled with high
purity N2 gas before use. MPO was purified from human
leukocytes [21-23]. A 30 μM MPO solution was freshly
prepared by diluting measured amounts with buffer.
2.2. Electrochemical Measurements
The Amperometric system consisted from an Apollo
4000 free radical analyzer, ISO-HPO-100 H2O2-biosen-
sor and a thermostated measurement chamber (WPI,
Sarasota, FL, USA). All experiments were performed at
room temperature.
For each experiment, 3.00 mL of 100 mM phosphate
buffer solution containing 30 μM EDTA were placed in
the measurement chamber. For effect of nitrite meas-
urements, a 100 mM Cl– and varied concentrations of
2
NO

(0 - 100) μM were pre-incubated with buffer solu-
tion in the chamber. For effect of chloride measurements,
a 100 μM 2
NO

and varied concentrations of Cl– (0 -
100) mM were pre-incubated with buffer solution in the
chamber. The electrode was immersed and magnetic
stirrer was turned on at fixed moderate speed. Continu-
ous amperometric monitoring started after addition of 30
μL H2O2 (10 μM). Reactions started with addition of 5.0
μL MPO solution (50 nM) and allowed to proceed until
complete decay of initial current signal. H2O2 concentra-
tions (μM) versus time (s) plots were obtained by setting
the initial current signal to 10 μM H2O2 [19-20].
3. RESULTS AND DIS CUS SION
Due to its much higher concentration (100 - 140 mM)
relative to other halides, chloride is assumed to be the
physiological substrate for MPO. Time course H2O2-
decay plots for MPO-catalyzed oxidation of Cl–, at a
selected normal plasma level (100 mM), in presence of
increasing 2
NO

concentrations were studied by con-
tinuous amperometric monitoring of H2O2 consumption
(Figure 1) [20]. A monophasic plot prevails in absence
of 2 NO

(Figure 1(a)) which is demonstrated by a rapid
consumption of H2O2. As 2 concentration increases,
plots became biphasic and the rapid step is followed by a
slower second step which is observed as an exponential
decay of H2O2 signal (Figures 1(b)-(d)). The second
step dominates at larger concentration (Figure
1(d)).
NO
2
NO
Further prevailing of second phase was observed,
when Cl– was decreased in presence of 100 μM 2 NO

(Figure 2). Both steps are observed in Figure 2(e)
which is actually Figure 1(d). As Cl– concentration con-
tinues to decrease, the second step is further extended
and first step is disappeared. (Figures 2 (a)-(c)). Exten-
sion of second step is accompanied by increase in reac-
tion time.

Y. R. Tahboub et al. / Journal of Biophysical Chemistry 2 (2011) 202-207
Copyright © 2011 SciRes. Openly accessible at http://www.scirp.org/journal/JBPC/
204
Figure 1. Effect of on MPO-catalytic activity towards
oxidation of Cl– (Courtesy of portugaliae electrochimica acta
with permission). H2O2 consumption plots as a function of
[]. Reactions were started by the addition of 50 nM MPO
to 10 μM H2O2 in 100 mM phosphate buffer, pH 7.0, contain-
ing 30 μM EDTA pre-incubated with 100 mM Cl– (a) and 25
(b), 50 (c), 100 μM (d). Reactions were carried at 25˚C.
Plots are average of four replicates.
2
NO
2
NO
2
NO
Figure 2. Effect of Cl– on MPO-catalytic activity towards oxi-
dation of Cl– in presence of . H2O2 consumption plots as a
function of [Cl–]. Reactions were started by the addition of 50
nM MPO to 10 μM H2O2 in 100 mM phosphate buffer, pH 7.0,
containing 30 μM EDTA, pre-incubated with 100 μM
2
NO
2
NO

and 5 (a), 10 (b), 25 (c), 50 (d) and100 mM Cl– (e) Reactions
were carried at 25˚C. Plots are average of four replicates.
3.1. Proposed Kinetic Model
Referring to mechanism in introduction, reaction 1 is
known to be very fast, k1 is in the order of 107 [24],
whereas reaction 2 is slower with k2 ~ 104 [25]. Interest-
ingly, reaction 3 is found to be extremely fast, k3 in the
order of 107 [13] and k4 (102 - 104) consequently much
slower than k1 [17,26-28].
Let rate of consumption of H2O2 be written as:


 
22
1224 2
dHO MPOH OMPO-IHO
dkk t
 

2
(5)
On Parallel, the rate of consumptions of MPO, Cl,
and 2
NO

ions together with rate of formation of HOCl
can be written as:


2
dCl d HOClMPO-I Cl
dd
k
tt



 
 

(6)


 
1222
dMPO MPOH OMPO-ICl
dkk t



 

 (7)

2
3
dNO MPO-INO
dk t



 
 
2


(8)
Furthermore the rate of formation of MPO-II is:




32
42
d MPO-IIMPO-I NO
d
MPO-IHO
k
t
k





2
(9)
All Eqs.5-9 contain an unstable MPO-I intermediate.
Thus the rate of consumption of MPO-I intermediate can
be obtained from the use of steady-state method as fol-
lows:


 
 
1222
32422
dMPO-I MPOHOMPO-ICl
d
MPO-INOMPO-IHO0
kk
t
kk




 




 
(10)
Then






122
23242
MPOH O
MPO-I ClNOH O
k
kk k

 

 2
(11)
Pre-steady states studies reported that k4 is much
smaller than k2 and k3 [17] and thus reaction 4 is very
slow with respect to reactions 1, 2 and 3 respectively,
then k4[H2O2] could be neglected and omitted from de-
nominator and thus, Eq.11 becomes:





12
23
MPOH O
MPO-I Cl NO
k
kk

2
2






(12)
The reaction could be studied by monitoring the rate
of consumption of H2O2 and/or the rate of formation of
MPO-II. In our case we were able to experimentally

Y. R. Tahboub et al. / Journal of Biophysical Chemistry 2 (2011) 202-207
Copyright © 2011 SciRes. Openly accessible at http://www.scirp.org/journal/JBPC/
205205
monitor the rate of consumption of H2O2. Thus, substitu-
tion of Eq.12 in Eq.5 yields the net rate of consumption
of H2O2 through the entire reaction:

 
2
2241 22
122
23
First part
Second part
dHOMPO HO
MPOH O
dCl NO
kk
k
tkk

 
 

 


2



(13)
Eq.13 illustrates the rate change of consumption of
H2O2 through time scale. At the very early time of reac-
tion (first 10 s) fast drop of H2O2 signal was observed
(first phase), indicating domination of first part which is
attributed to no significant formation of MPO-II (en-
zyme is swinging between MPO-1 and ground state
MPO). Afterwards, and with presence of increasing
concentrations of 2, part of MPO-I is reduced to
MPO-II causing the enzyme to work under partial activ-
ity (second phase) indicating domination of second part
of Eq.13.
NO
Consequently, at the early time of the reaction, the
rate of consumption of H2O2 depends only on the first
part which is a second order reaction that depends en-
tirely on initial concentrations of MPO and H2O2 and
conclusively, the rate constant (k1) could be estimated
from slope of curve.



22
12
dHO MPOH O
dk t


2
(14)
After the passage of few seconds (10 s), second phase
is dominated and H2O2 consumption will follow the
second part of Eq.13 which is observed as exponential
decay of H2O2 signal (Figures 1(b)-(d)). The second
part of Eq.13 introduces new variables such as [Cl–],
[2] together with [MPO], [H2O2] and rate constant
values k1, k2, k3 and k4 , that affect the rate of consump-
tion of H2O2. Collectively, Eq.13 expresses a two-step
sequential decay of H2O2 signal.
NO
By applying extreme values for 2 concentration
in Eq.13 assuming that [2
NO
NO

] = ∞, then [MPO] ap-
proaches [MPO-II] and the rate of consumption of H2O2
will be zero indicating complete inhibition of reaction 2.
Additionally if [2] = 0 then k3[2] = 0, and Eq.13
could still be used even in the absence of 2
NONO
NO

ions.
Biphasic plots were observed for [Cl–] < 20 mM (data
not shown). This confirms the solidarity and consistency
of Eq.13 to present the rate of consumption of H2O2 in
the presence of MPO, Cl and presence or absence of
.
2
Furthermore Eq.13 is a separable differential equa-
tion, which can re-write as:
NO



2
22
22 22
dHO HO HO
dAB t


(15)
where A and B are constants, and
1[MPO]Ak
41
23
[MPO]
[Cl ][NO ]
kk
Bkk
2


. This differential equation can
be solved by having different


22
dHO
dt
 and [H2O2]
values at increasing time and then by using ordinary
differential computer program namely Mathematica, A
and B values could be determined. The negative B value
explains the decrease in the overall rate of consumption
of H2O2 at prolonged time. This finding strengthen the
discussion of the solidarity of Eq.13 to present the situa-
tion, because it simply states that the second part of
Eq.13 is the parameter responsible for the altering H2O2
consumption from a rapid step to a slower step. Finally,
calculated B values were used to estimate the ratio of
rate constants 2
3
k
k, 4
3
k
k and 4
2
k
k values. Rate constant
ratios are important, because they show the competi-
tiveness of Cl– and 2 NO

towards reaction with MPO-I,
and further explain the slowness of reaction 4 relative to
reactions 2 and 3.
Ta bl e 1 and Figure 3 show the change of k1 values,
deduced from Eq.14 with respect to changes in Cl
concentrations.
Furthermore, Ta bl e 2 and Figure 4 show the change
of k1 value, deduced from Eq.14 with respect to changes
in 2 NO

concentrations.
After the passage of 10 s, H2O2 consumption pattern
changes entirely, where the second part of Eq .1 3 became
the major factor. This factor introduces new parameters
that affect the rate of consumption of H2O2 such as Cl,
2
NO

and reaction 4.
Mathematical salvation of Eq.15 at variant consump-
tion rates of H2O2 versus H2O2 concentrations under
variant chloride and nitrite concentrations (i.e. Tables 1
and 2) yielded A and B constant values, and conse-
quently the determined B constant values were employed
to estimate 4
3
k
k, 4
2
k
k and 2
3
k
k ratios (Table 3).
Rate constant ratios tell explicitly which reaction is
faster or slower in the mechanism, and hence several
conclusive remarks could be concluded from Table 3.
Firstly, is that k3 is much larger than k2, which empha-
sizes that the nitrite inhibition reaction 3 producing
MPO-II, is faster than the catalyzing reaction 2 producing
HOCl, (i.e.
3
2
k
k= 1.15 × 103), and that confirms the
inhibition nature of 2
NO

in the mechanism.
Secondly, the previously assumed that 34 has
been mathematically justified and it was also proven that
k4 is very much smaller than k3 or k2. Finally, reaction 4
kk

Y. R. Tahboub et al. / Journal of Biophysical Chemistry 2 (2011) 202-207
Copyright © 2011 SciRes. Openly accessible at http://www.scirp.org/journal/JBPC/
206
Table 1. Change of rate constant (k1) versus chloride ion con-
centration [Cl].
k1 (M–1·s–1) [Cl], mM
2.33 × 105 5
2.67 × 105 10
3.02 × 105 25
3.96 × 105 50
5.23 × 105 100
[MPO] = 50 nM, [H2O2] = 10 M, [] = 100 M, in 100 mM phos-
phate buffer, pH = 7.0, containing 30 M EDTA at 25˚C.
2
NO
Table 2. Change of rate constant (k1) versus nitrite ion concen-
tration [].
2
NO
k1 (M–1·s–1) [], M
2
NO 
1.5 × 106 0
1.4 × 106 25
7.13 × 105 50
5.75 × 105 100
[MPO] = 50 nM, [H2O2] = 10 M, [Cl] = 100 mM, in 100 mM phosphate
buffer, pH = 7.0, containing 30 M EDTA at 25˚C.
Table 3. Rate constant ratios in the presence and absence of
() inhibitor.
2
NO
Rate constant ratios 2
3
k
k 4
3
k
k 4
2
k
k
Presence of ()
inhibitor
2
NO 
1.15 × 10–3 7.0 × 10–8 6.09 × 10–5
Absence of ()
inhibitor
2
NO 
- - 4.4 × 10–3
Figure 3. Change of rate constant (k1) versus chloride ion

concentration [Cl].
y = –1.0 × 10
4
x + 1 × 10
6
R
2
= 0.8344
Figure 4. Change of rate constant (k1) versus nitrite ion con-
centration [2 NO

].
becomes significant in absence of
. CONCLUSIONS
alytic activity towards oxida-
tio
steady-state model was able to ex-
pl
1
2
NO.
4
Assessment of MPO-cat
n of chloride and other halides is a complex and
multifunctional process [24,29]. MPO catalytic activity
is dependent on initial concentrations of MPO, H2O2, Cl–,
pH, H2O2 to MPO concentration ratio and order of
mixing. Thus, development of a comprehensive kinetic
model is a complex task. We acknowledge that our
proposed kinetic model is limited to describing our
experimental data.
Proposed kinetic
ain the monophasic and biphasic phenomena in ab-
sence and presence of nitrite. Additionally, the model
was able to estimate values for k and
3
4
k,
k2
4
k
k and
3
2
k
k rate constant ratios. Interestingly, this model supports
previous findings that MPO has two binding sites that
5. ACKNOWLEDGEMENTS
Obstetrics and Gy-
ne
. and O’Gorman, M.R. (2007) Chronic
have distinct impact on the heme iron microenvironment
[29]. Chloride occupied one site as substrate and nitrite
occupied the other site as inhibitor.
Experimental work was done at Department of
cology, Wayne State University, MI, USA. Principal author would
like to thank Prof. Husam Abu-Soud for supplying facilities to conduct
this research. Also thanks to Jordan University for Science and Tech-
nology for financing a research sabbatical to principal and correspond-
ing author.
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Y. R. Tahboub et al. / Journal of Biophysical Chemistry 2 (2011) 202-207
Copyright © 2011 SciRes. Openly accessible at http://www.scirp.org/journal/JBPC/
207207
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