Reduction of toxic metabolite formation of acetaminophen
Biochemical and Biophysical Research Communications 291, 1089 –1094 (2002)
doi:10.1006/bbrc.2002.6541, available online at http://www.idealibrary.com on
Reduction of Toxic Metabolite Formationof Acetaminophen
Eszter Hazai, La´szlo´ Vereczkey, and Katalin Monostory1Chemical Research Center, Hungarian Academy of Sciences, P.O. Box 17, H-1525 Budapest, Hungary
diphosphoglucuronic acid and 3Ј-phosphoadenosine-5Ј-
Acetaminophen is a widely used over-the-counter drug that causes severe hepatic damage upon over-
water-soluble conjugates (4). About 3% of ingested
dose. Cytochrome P450-dependent oxidation of acet-
acetaminophen is bioactivated by cytochrome P450 en-
aminophen results in the formation of the toxic
zymes (CYP1A2, CYP2A6, CYP2E1 and CYP3A4 (4)). N-acetyl-p-benzoquinone-imine (NAPQI). Inhibition of
Cytochrome P450-dependent oxidation results in the
cytochrome P450 enzymes responsible for NAPQI for-
formation of the nontoxic 3-hydroxy-acetaminophen
mation might be useful— besides N-acetylcysteine
and the highly reactive N-acetyl-pbenzoquinone-imine
treatment—in managing acetaminophen overdose. In-
(NAPQI). NAPQI, which is formed only in a trace
vestigations were carried out using human liver mi-
amount upon intake of therapeutic doses, can be de-
crosomes to test whether selective inhibition of cyto-
toxified by conjugation with glutathione (Fig. 1). Upon
chrome P450s reduces NAPQI formation. Selective
intake of an overdose, the capacity of the organism—to
inhibition of CYP3A4 and CYP1A2 did not reduce,
cope with acetaminophen— overloads (the cofactors of
whereas the inhibition of CYP2A6 and CYP2E1 signif-
conjugation enzymes as well as glutathione are de-
icantly decreased NAPQI formation. Furthermore, se-
pleted) (5). The toxic effect of an overdose results in
lective CYP2E1 inhibitors that are used in human
covalent modification of proteins as well as impairment
therapy were tested for their inhibitory effect on
of cellular capacity to keep essential thiol groups in
NAPQI formation. 4-Methylpyrazole, disulfiram, and diethyl-dithiocarbamate were the most potent inhibi- tors with IC values of 50 M, 8 M, and 33 M, re-
Inhibition of cytochrome P450 enzyme(s) responsible
spectively. Although cimetidin is used in the therapy
for the formation of NAPQI might be one of the useful
of acetaminophen overdose as an inhibitor of cy-
tools besides N-acetylcysteine treatment in managing
tochrome P450, it is not able to reduce NAPQI
acetaminophen overdose. In this way the organism
may gain time to produce cofactors that are necessary
2002 Elsevier Science (USA) Key Words: acetaminophen; N-acetyl-p-benzoqui-
for the detoxifying pathways. Although in manage-
ment strategy of acetaminophen poisoning cimetidine
ylpyrazole; disulfiram; diethyl-dithiocarbamate; hu-
is often used as a cytochrome P450-inhibitor to reduce
man microsomes; cimetidine.
NAPQI formation, its efficacy is rather questionable (6,7). The aim of our investigation was i) to determine theenzymes responsible for NAPQI formation at lower andhigher doses; ii) to test whether cimetidine efficiently
Acetaminophen is one of the most widely used over-
inhibits NAPQI formation; iii) to inhibit the formation
the-counter drugs with analgesic and antipyretic prop-
of NAPQI by using selective cytochrome P450 enzyme
erties. Although regarded to be safe at therapeutic
doses, acetaminophen is known to cause acute hepaticcentrilobular necrosis in humans when consumed in
large doses (1, 2). Acetaminophen overdose results inmore calls to poison centers in the United States than
14C-acetaminophen labelled on the methyl group was
overdose with any other pharmacologic substance (3).
synthetized at Chemical Research Center of the Hungarian Academy
In biotransformation reactions in human about 90% of
of Sciences (Budapest, Hungary). The synthesis was carried out asfollows: 14C-acetic acid was reacted with N-hydroxy-succinimide. The
ingested acetaminophen is linked with uridine-5Ј-
product was reacted with p-aminophenol. After purification proce-dures, the specific activity of the endproduct was 1.95*1012 Bq ⅐
molϪ1. N-acetyl-p-benzoquinone-imine standard was purchased
ϩ36013257548. E-mail: [email protected]
from Sigma (Deisenhofen, Germany). D-Glucose-6-phosphate and
0006-291X/02 $35.00 2002 Elsevier Science (USA)All rights reserved.
BIOCHEMICAL AND BIOPHYSICAL RESEARCH COMMUNICATIONS
acetaminophen. The incubation mixture contained 5 mg human livermicrosomal protein, selected concentrations of 14C-acetaminophen,10 mM glutathione in 0.1 M Tris-HCl buffer (pH 7.4). The reactionwas initiated by the addition of NADPH-generating system (1 mMNADPH, 5 mM MgCl2, 10 mM glucose-6-phosphate and 2 units/ml ofglucose-6-phosphate dehydrogenase). The final incubation volumewas 500 l. The assay was conducted at 37°C for 40 min, and wasterminated by the addition of 50 l 33% sulfosalicilic acid. NAPQIformation was linear up to 50 min. The mixture was centrifuged thenthe pH of the supernatant was adjusted to 5.0 (by the addition of 2NNaOH) and the sample was analyzed for the formation of NAPQI. Since NAPQI is a highly reactive molecule, it also binds to proteinspresent in the incubation mixture, which makes the detection ofNAPQI impossible. Therefore, NAPQI was detected as its glutathi-one conjugate, 3-(glutathione-S-yl)acetaminophen (GS-acetamino-phen). GS-acetaminophen is immediately formed in the presence ofreduced glutathione and NAPQI (14).
The incubation process was the same as de-
scribed above. Acetaminophen was used at 0.25 mM concentration. The following specific cytochrome P450 enzyme inhibitors were used:␣-naphtoflavone (100 M, for CYP1A2), 8-methoxypsoralen (10 M,for CYP2A6), diethyl-dithiocarbamate (100 M, for CYP2E1), trole-andomycin (200 M, for CYP3A4). The following published cyto-chrome P450 inhibitors at 100 M concentration were tested forNAPQI production: cimetidine, chlorpromazine, diclofenac, fluphen-azine, thioridazine, isoniazid, diallyl sulfide, 4-methyl pyrazole, di-sulfiram.
A new chromatographic method has been de-
veloped in our laboratory for the detection of GS-acetaminophen. Compounds were separated on a Merck Lichrospher RP-18 column 5m (250 mm*4 mm) column (Darmstadt, Germany), protected by acartridge guard column (30*4 mm) packed with identical solid phase. The mobile phase consisted of 40 mM NH4-acetate (pH ϭ 4.8):methanol ϭ 87:13 (v/v) at a flow rate of 1 mL ⅐ minϪ1. The compoundswere detected both by their UV absorption at 250 nm and accordingto 14C activity.
conjugate and selective cytochrome P450 enzyme activities (phenac-etin O-deethylation of CYP1A2, coumarin 7-hydroxylation of
Cytochrome P450-dependent metabolism of acetamino-
CYP2A6, chlorzoxazone 6-hydroxylation of CYP2E1, nifedipine oxi-
dation of CYP3A4) for each preparation of liver microsomes werecompared by simple linear regression analysis for each pair of data.
D-glucose-6-phosphate dehydrogenase (E.C. 126.96.36.199.) were obtainedfrom Calbiochem (La Jolla, CA, USA). 5-Sulfosalicylic acid, reduced
glutathione and methanol of HPLC grade were obtained from Merck(Darmstadt, Germany). All other chemicals were from Reanal(Budapest, Hungary). Correlation of NAPQI formation and specific enzymePreparation of human liver microsomes.
kidney transplant donors were from Transplantation and Surgery
nors were characterized with substrates selective for
Clinic of Semmelweis University (Budapest, Hungary). Permission
different cytochrome P450s that are responsible for the
of Local Research Ethics Committee was obtained for the use of
oxidative metabolism of majority of drugs. Results of
human liver tissues. Samples of human liver microsomes were pre-
pared as described in (8). Microsomal protein concentration was
The formation of NAPQI (assayed as its glutathione
determined by the method of Lowry (9), with bovine serum albuminas standard.
conjugate, 3-(glutathione-S-yl)acetaminophen, GS-acet-aminophen) was investigated i) at low (0.05 mM) and
termine specific enzyme activities of CYP1A2 (10), CYP2A6 (11),
ii) at high (0.25 mM) acetaminophen concentrations
CYP2E1 (12) and CYP3A4 (13). The incubation mixtures contained
(Table 1). The structure of GS-acetaminophen was
human liver microsomal protein, a NADPH-generating system (1
also confirmed by LC/MS study with authentic stan-
mM NADPH, 10 mM glucose 6-phosphate, 5 mM MgCl2 and 2
dard (Hazai et al., 2001 submitted for publication in
units/ml glucose 6-phosphate dehydrogenase); and various selective
substrates for cytochrome P450 forms (0.2 mM phenacetin forCYP1A2, 0.2 mM coumarin for CYP2A6, 0.5 mM chlorzoxazone for
At lower acetaminophen concentration no correla-
CYP2E1, 0.2 mM nifedipine for CYP3A4).
tion was found between NAPQI formation and the spe-
Biotransformation of acetaminophen.
cific cytochrome P450 activities (CYP1A2, CYP2A6,
NAPQI were identified at 0.05 mM and 0.25 mM concentrations of
CYP2E1 and CYP3A4 activities) (Table 1). At 0.25 mM
BIOCHEMICAL AND BIOPHYSICAL RESEARCH COMMUNICATIONS
Characterization of Human Liver Microsomes (HHM) for Selective Substrates of Cytochrome P450 Enzymes (Phenacetin
O-Deethylation of CYP1A2, Coumarin 7-Hydroxylation of CYP2A6, Chlorzoxazone 6-Hydroxylation of CYP2E1, NifedipineOxidation of CYP3A4), and for the Formation of GS-Acetaminophen Conjugate at Different Acetaminophen Concentrations
Note. The values are expressed as pmol product/(mg microsomal protein*min). The correlation of the production of GS-acetaminophen
conjugate and selective cytochrome P450 enzyme activities are presented. Each pair of data was compared by simple linear regression. R2values of the linear regression analysis are presented for each specific enzyme activities at two acetaminophen concentrations.
concentration there was a marked correlation (with an
further evidence for the participation of cytochrome
R2 value of 0.9381) between the formation of NAPQI
P450 forms in the formation of NAPQI. Selective inhi-
and the specific enzyme activity of CYP2E1 (Table 1,
bition of CYP3A4 and CYP1A2 did not reduce the
Fig. 2). However, no correlation was found between the
amount of NAPQI, whereas the inhibition of CYP2A6
formation of NAPQI and other specific cytochrome
and CYP2E1 resulted in a marked decrease of NAPQI
P450 enzyme activities (CYP1A2, CYP2A6, and
Furthermore, it has been studied if known CYP2E1
and CYP2A6 inhibitors, which can be administered to
bition studies were also carried out in order to provide
humans, are able to reduce the formation of NAPQI at0.25 mM acetaminophen concentration. The followinginhibitors of CYP2E1 and/or CYP2A6 were subject toour investigation: chlorpromazine, diclofenac, fluphen-azine, thioridazine, isoniazid, diallyl sulfide, 4-methylpyrazole, disulfiram and its metabolite, diethyl-dithiocarbamate.
carbamate (at 100 M concentration) reduced NAPQIformation to 7 Ϯ 1% and 23 Ϯ 13%, respectively (Fig. 4). 4-Methyl pyrazole (at 100 M concentration) de-creased NAPQI formation to 35 Ϯ 1%. Isoniazid anddiallyl sulfide showed a modest inhibitory effect on theformation of NAPQI (to 74 Ϯ 5% and 45 Ϯ 2%, respec-tively) (Fig. 4). The use of 100 M cimetidine, chlor-promazine, diclofenac, fluphenazine and thioridazinedid not reduce the formation of NAPQI (Fig. 4). How-ever, these substances listed above were considered tobe CYP2E1 inhibitors (15).
The inhibitory effect at different inhibitor concentra-
tions was studied with the most potent inhibitors in
Formation of NAPQI as the function of CYP2E1 activity
at 0.25 mM acetaminophen concentration. HHM means samples of
human microsomes. Specific CYP2E1 enzyme activity was measured
M for disulfiram, 33 M for diethyl-dithio-
with chlorzoxazone 6-hydroxylase reaction.
carbamate and 50 M for 4-methyl pyrazole (Fig. 5).
BIOCHEMICAL AND BIOPHYSICAL RESEARCH COMMUNICATIONS
Formation of NAPQI in the presence of specific cytochrome P450 inhibitors. Control means the microsomal activity in the absence
of inhibitors. The following specific inhibitors were used: troleandomycin (200 M, CYP3A4); ␣-naphtoflavone (100 M, CYP1A2);8-methoxypsoralen (10 M, CYP2A6) diethyl-dithiocarbamate (100 M, CYP2E1).
There are controversial reports on the role of differ-
Several forms of cytochrome P450 in humans, in-
CYP2E1, CYP3A4) in NAPQI formation at higher dose.
cluding 1A2, 2A6, 2E1, 3A4 have been shown to cata-
All of the cytochrome P450 forms listed above have
lyze the oxidation of acetaminophen to NAPQI (5).
been reported to contribute significantly to acetamino-
CYP3A4 was suggested to play an appreciable role at
phen toxicity (5, 17–19). However, studies using
lower doses (16). Our study demonstrated that at low
CYP1A2 null mice did not confirm the role of CYP1A2
concentration there is no enzyme primarily responsible
in acetaminophen toxicity (20). Our correlation study
confirmed the role of CYP2E1 in NAPQI formation at
The effect of cytochrome P450 inhibitors on the formation of NAPQI. All inhibitors were used at 100 M concentration.
BIOCHEMICAL AND BIOPHYSICAL RESEARCH COMMUNICATIONS
patients, methanol and ethylene-glycol poisoning (Fo-mepiazole), can be administered intravenously as well(26, 27). The other potent inhibitor, disulfiram (Antetil,Antabuse) is used in the therapy of alcoholic patientsas alcohol-dehydrogenase inhibitor. This compound isalso a known specific CYP2E1 inhibitor (28, 29). Thefirst step in the biotransformation of disulfiram is thethiol-dependent reduction to two molecules of diethyl-dithiocarbamate (30). Diethyl-dithiocarbamate is aknown inhibitor of both enzymes (CYP2E1, CYP2A6)(21) that are responsible for the formation of NAPQI. According to our present knowledge it does not inhibitany other known cytochrome P450 enzymes. This sub-stance is, however, not yet used in human therapy.
Isoniazid also showed inhibitory effect toward
NAPQI formation. However, it is not only an inhibitorbut also acts as an inducer for CYP2E1. The mecha-nism of induction is the stabilization of the enzymeprotein (31, 32). There has been a case report, where
Formation of NAPQI in the presence of various concen-
the administration of isoniazid potentiated the toxic
trations of 4-methyl pyrazole, diethyl-dithiocarbamate, and disul-
effect of acetaminophen (33). The toxicity might be due
to the induction of CYP2E1 as a consequence of isoni-azid administration. Therefore, the use of isoniazid
higher concentration. 0.25 mM acetaminophen concen-
seems to be disadvantageous in the case of acetamin-
tration is consistent with a toxic dose of acetaminophen
ophen poisoning. Diallyl-sulfide, a component of garlic
as can be concluded from the Rumack-Mathew nomo-
has also shown a modest inhibitory effect. However,
gram (6). On the other hand, as the inhibition studies
this substance is not used in the clinical practice.
revealed, CYP2A6 also participated in NAPQI forma-
The dose dependence of the inhibitory effect was also
tion. Considering the results of correlation and specific
studied in the case of three potent inhibitors (4-
inhibition studies and the fact that CYP2E1 is 6.9 Ϯ
methylpyrazole, disulfiram, diethyl-dithiocarbamate)
2.9% and CYP2A6 is 4.0 Ϯ 3.2% of total cytochrome
(Fig. 5). It can be concluded that 100 M concentration
P450 in human liver (21), it can be concluded that
of the inhibitor is required in order to achieve the
these two enzymes are mainly responsible for the for-
maximal inhibitory effect. The further, fivefold in-
mation of NAPQI at higher concentration. Further-
crease of the amount of inhibitor is unnecessary, since
more, our results suggest that CYP1A2 and CYP3A4
it does not improve its inhibitory effect.
do not contribute significantly to NAPQI formation at
Finally, it should be concluded that at lower dose
higher acetaminophen concentration. This finding is
several enzymes participate in the formation of
consistent with the studies of bacolovirus-expressed
NAPQI. In overdose conditions CYP2E1 and CYP2A6
and purified human liver microsomes, where CYP2A6
are mainly responsible for the toxic effect of acetamin-
and CYP2E1 were suggested to play a major role in
ophen. Cimetidine does not play a role in inhibiting the
formation of NAPQI as both in vitro and in vivo human
Cimetidine showed no inhibitory effect toward
studies revealed. Therefore, the use of this substance is
NAPQI formation. According to some authors, cimeti-
not suggested in the case of acetaminophen poisoning.
dine is useful in the treatment of acetaminophen over-
Disulfiram is the most potent inhibitor of NAPQI for-
dose because of its cytochrome P450 inhibitory effect
mation in human liver microsomes. 4-Methylpyrazole,
(22, 23). However, cimetidine inhibits CYP2C9 enzyme
which can also be administered intravenously, might
(24), which is not involved in the metabolism of acet-
also be a useful tool in managing acetaminophen over-
aminophen. The fact that cimetidine is not useful in
managing acetaminophen overdose has also beenproved in clinical investigations (6, 25). Therefore, al-
though in some countries suggested, the administra-tion of cimetidine seems to be ineffective in the case of
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INFORMED CONSENT FOR LASER IN-SITU KERATOMILEUSIS (LASIK) FOR THE CORRECTION OF MYOPIA (NEARSIGHTEDNESS), FARSIGHTEDNESS (HYPEROPIA), ASTIGMATISM, OR MIXED ASTIGMATISM Introduction This information is being provided to you so that you can make an informed decision about the use of adevice known as a microkeratome, combined with the use of a device known as an excimer laser, to reshape