-
and toxicityCHUNG S. YANG, JOHN F. BRADY,2 AND JUN-YAN
HONGLaboratory for Cancer Research, Department of Chemical Biology
and Pharmacognosy, College of Pharmacy,Rutgers University,
Piscataway, New Jersey 08855-0789, USA
Dietary effects on cytochromes P450, xenobiotic metabolism,
non, o/o,/nnne. n7l7Icnl en Th
ABSTRACT The levels and activities of cytochromeP450 enzymes are
influenced by a variety of factors, in-cluding the diet. In this
article, the effects of selected non-nutritive dietary chemicals,
macronutrients, micronutri-ents, and ethanol on cytochromes P450
and xenobioticmetabolism are reviewed in the light of our current
un-derstanding of the multiplicity and substrate specificityof
cytochrome P450 enzymes. Although the mechanismsof action of
several dietary chemicals on specific cyto-chrome P450 isozymes
have been established, those formacro- and micronutrients are
largely unknown. It isknown, however, that specific nutrients may
have variedeffects on different cytochrome P450 forms and thus
mayaffect the metabolism of various drugs differently. Nutri-tional
deficiencies generally cause lowered rates of xeno-biotic
metabolism. In certain cases, such as thiamin de-ficiency and mild
riboflavin deficiency, however, enhancedrates of metabolism of
xenobiotics were observed. Theeffects of dietary modulation of
xenobiotic metabolism onchemical toxicity and carcinogenicity are
discussed.-Yang, C. S.; Brady, J. F.; Hong, J. -Y. Dietary effects
oncytochromes P450, xenobiotic metabolism, and toxicity.FASEBJ. 6:
737-744; 1992.
Key Words: diet . nutrition cytochromes P450 . enzyme
regula-tion xenobiotics drug metabolism . toxicity .
carcinogens
THE CLOSE RELATIONSHIP BETWEEN DIET and xenobiotic me-tabolism
may be traced back to prehistoric days in animal-plant warfare
during evolution (1). Plants synthesized chem-icals for
self-protection and animals had to developxenobiotic-metabolizing
enzymes such as cytochrome P450(P450)3 for the detoxication of
these chemicals. The evolu-tion of the large number of P450 2 genes
400 million yearsago may correspond to the advance of animals onto
landwhere they encountered new terrestrial plants and
phyto-chemicals. The work of many investigators in the past 30years
has clearly established that various dietary factors havemarked
effects on the metabolism of drugs, environmentalchemicals, and
certain endogenous substrates. This topic hasbeen reviewed
extensively (2-9). However, only recently havewe begun to
understand some of these effects at the molecu-lar level. Dietary
influences on xenobiotic metabolism mayalter the therapeutic
effects of drugs and the toxicity or car-cinogenicity of
environmental chemicals. In this article, wereview the mechanisms
by which dietary chemicals andnutritional status affect the levels
and activities of P450 en-zymes, xenobiotic metabolism, as well as
chemical toxicityand carcinogenicity.
Because of space limitations, we have chosen to useselected
examples to illustrate key concepts rather than toconduct an
exhaustive review on this topic. Review articlesare cited instead
of original papers.
MODULATION OF P450 LEVELS AND XENOBIOTICMETABOLISM BY
NONNUTRITIVE DIETARYCHEMICALS
Dietary chemicals may affect the levels and activities of
P450isozymes at different steps as shown in Fig. 1. Dietary
chem-icals may affect the levels of P450 species by altering the
ratesof: 1) the transcription of specific P450 genes, 2) the
degra-dation of specific mRNA, 3) the translation process, and4)
P450 degradation through protein turnover or by suicideinhibition.
Many dietary chemicals are substrates of theP450-dependent
monooxygenase system. They or their me-tabolites may inhibit or
enhance the activities of this systemby binding to P450 species or
to NADPH:P450 reductase, byaffecting the interaction between these
enzymes, or by affect-ing key steps in the catalytic cycle.
Induction of P450-dependent activities
The effect of diet on P450-dependent monooxygenase activi-ties
was clearly demonstrated in the pioneering work ofWattenberg (10),
who discovered that rats on commercial ratchow had 68-fold higher
intestinal benzo(a)pyrene (BP)hydroxylase activity than those on a
purified diet. By addingdry vegetable powder to the purified diet,
Brussels sprouts,cabbage, turnips, and other vegetables were found
to be in-ducers of intestinal BP hydroxylase in rats. The effects
ofcruciferous vegetables and their components on drug metab-olism
have since been studied extensively. Three autolyticproducts of
indolylmethyl glucosinolate (glucobrassicin):indole-3-carbinol,
indole-3-acetonitrile, and indole-3-carboxy-aldehyde, isolated from
Brussels sprouts, were found to in-duce intestinal and hepatic BP
hydroxylase in rats. Thestructures of some of the nonnutritive
dietary compoundsare illustrated in Fig. 2. Indole-3-carbinol is
the most potentinducing agent among these indoles. Its induction of
BPhydroxylase and ethoxyresorufin dealkylase activities is
be-lieved to be mainly due to the induction of P450 1A1 in
theintestine and P450s IA1 and 1A2 in the liver (11).
It was demonstrated that after an oral dose of indole-3-carbinol
to rats, P450 lAl mRNA was elevated severalfold
To whom correspondence should be addressed, at: Laboratoryfor
Cancer Research, Department of Chemical Biology and Phar-macognosy,
College of Pharmacy, Rutgers University, Piscataway,NJ 08855-0789,
USA.
2Present address: Chemistry Department, University of
Califor-nia, Davis, CA95616, USA.
3Abbreviations: P450(s), cytochromes P450; BP, benzo(a)pyrene;Ah
receptor, aromatic hydrocarbon receptor; RXM, acid reactionproducts
of indole-3-carbinol; NDMA, N-nitrosodimethylamine;NNK,
4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone; BHA, butyl-ated
hydroxyanisole; B HT, butylated hydroxytoluene.
-
P450 gene Inactivation of P450 enzymes
degradation P450 mRNA
Substrates
c)OO\PS
Flavone Tange,otin Nob,I.bn
Quercebn Kaenpfeoi
Figure 2. Structures of some dietary or related compounds.
738 Vol. 6 lanuarv 1992 The FASEBJournal YANC FT At
II III I III
transcription
translation
degradation .( P450 Reductase
Inactivation Inhibiton Enhancement
Figure 1. Possible sites for dietary effects on P450 enzymes.
TheP450 2E1 gene is used in the illustration. In addition to
affectingthe rates of transcription and translation, dietary
chemicals mayaffect the rates of degradation of P450 mRNA and
protein. Dietarychemicals may serve as substrates. They may also
interact withP450 enzymes and NADPH:P450 reductase, causing
inactivationof P450, or inhibition or enhancement of the
monooxygenase ac-tivities.
When diallyl sulfide was given orally to rats, microsomal
N-nitrosodimethylamine (NDMA) demethylase (indicative ofP450 2E1
activity) decreased markedly in several hours. Thiswas followed by
a lowering of the immunodetectable P4502El protein level in
microsomes (15). The inactivation ofP450 2E1 was also demonstrated
in vitro when diallyl sul-fone, an oxidative metabolite of diallyl
sulfide, was incubatedwith microsomes in the presence of an
NADPH-generatingsystem (15). The inactivation was time,
concentration, andNADPH-dependent, following a typical suicide
inhibitionpattern. It is believed that diallyl sulfone is converted
byP450 2E1 to a reactive intermediate that modifies the hememoiety
of P450 2E1 (20). Phenethyl isothiocyanate, occurringas a
glucusinolate in a variety of cruciferous vegetables,
alsoinactivates P450 2E1 by a suicide mechanism (21).
Psoralens,which are found in edible plants such as figs, celery,
parsley,and parsnip, were shown to decrease 7-ethoxycoumarin andBP
hydroxylase activities when added to human livermicrosomal
incubations in the presence of NADPH (22). Itwas suggested that
methoxsalen (8-methoxypsoralen), ber-gapten (5-methoxypsoralen),
and psoralen are suicide inhi-bitors of P450 enzymes; P450 lAl or
1A2 may be involved.
in the liver and colon, and P450 1A2 mRNA was also ele-vated in
the liver (11). The induction of P450 IAI involves thebinding of
the inducer to the Ah receptor. Nevertheless,indole-3-carbinol has
only low binding affinity to the Ahreceptor, whereas
indolo[3,2-b]carbazole has a much higheraffinity (12). It is
suggested that under the acidic conditionsin the stomach,
indole-3-carbinol can be converted toindolo[3,2-b]carbazole or
other acid reaction products(RXM) (13) that bind to the Ah receptor
and thereby increasethe transcription of the P450 1A1 gene. This
suggestion issupported by the observation that the inductive effect
wasfound when indole-3-carbinol was given to rats orally but
notwhen given intraperitoneally (13). Acid-treated
indole-3-carbinol was much more effective than untreated
indole-3-carbinol in inducing ethoxyresorufin activity in primary
cul-tures of rat hepatocytes (14). Certain flavones may also
in-duce P450 IA1 by binding to the Ah receptor and subse-quently
activating the P450 1A1 gene.
A second example of transcriptional regulation is the in-duction
of P450 2B1 by diallyl sulfide. Diallyl sulfide, a com-ponent of
garlic oil, has been shown to induce rat hepaticP450 2B1 based on
the increase of immunodetectable proteinand pentoxyresorufin
dealkylase activity (15). Recent studiesin our laboratory indicate
that this induction is accompaniedby the elevation of 2B1 mRNA
levels. Nuclear run-on experi-ments indicate that after
administration of diallyl sulfide, thetranscriptional rate of the
P450 2B1 gene was increasedmarkedly, and was observable at 4 h
after treatment (16). Inprimary culture of rat hepatocytes, P450
2Bl was not in-duced by diallyl sulfide but by its metabolite
diallyl sulfone(unpublished results).
Posttranscriptional mechanisms may also be involved inthe
induction of P450s by dietary factors. In the induction ofP450 2E1
by fasting, elevation of P450 2E1 mRNA was ob-served (17) but
transcriptional activation could not be con-vincingly demonstrated
by nuclear run-on experiments (un-published results). Stabilization
of the mRNA may play arole in this induction. In the induction of
P450 2E1 byethanol and acetone, elevation of the mRNA was not
ob-served (18, 19). Protein stabilization or increased
translationefficiency may be involved in this induction.
Enhancement of monooxygenase activities
Flavone, tangeretin, and nobiletin were found to increase
BPhydroxylase activity and aflatoxin B, activation in humanliver
microsomes (4). The metabolism of antipyrine andzoxazolamine was
also stimulated, but that of hexobarbital,coumarin, and
7-ethoxycoumarin was not. The stimulatoryeffect was P450
isozyme-specific. With purified P450 iso-zymes in a reconstituted
system, the effect was observed withrabbit P450s 3A6 and 1A2 but
not with P450s 2B4, 2C3, or1AI. It was proposed that these
flavonoids stimulate themonooxygenase activity by enhancing the
interactions be-
JCH.OK cxcIndoI.3.c,anoI Indo{.5.b)ca.bae1.
CH,CH.N #{149}C - S
DM oISd Phneth1 SotSocyaSai.
OCH.
cX0 cOOk
-
fllrrAPv crrrrc rmi vrMr,o,cyr,r .ACTAPCI ICkA 739
tween P450 and NADPH:P450 reductase and facilitating theflow of
electrons to P450. Administration of flavone to neo-natal rats also
increased the rate of zoxazolamine metabo-lism several-fold
(4).
Inhibition of monooxygenase activities
Dietary compounds can bind to the active sites of P450 en-zymes,
serving as substrates or competitive inhibitors. Forexample,
diallyl sulfide and phenethyl isothiocyanate arecompetitive
inhibitors of P450 2El-catalyzed reactions (20, 21).In addition,
phenethyl isothiocyanate also competitively in-hibits the
metabolism of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK),
a potent tobacco carcinogen, in lung micro-somes (23). In this
case, a purely competitive mode of inhibi-tion was not observed,
possibly due to the inactivation of theenzyme during the
incubation, and P450 2El was not involved.
Many dietary flavonoids have been shown to inhibit
mono-oxygenase activities. For example, quercetin,
kaempferol,morin, and chrysin inhibited BP hydroxylase activity in
hu-man liver microsomes (4). Quercetin, kaempferol, andnaringenin
inhibited nifedipen and filodipen oxidation cata-lyzed by P450 3A4
in human microsomes (24). It appearsthat flavones having free
hydroxy groups on the A ring areinhibitors, whereas flavonoids
containing no hydroxy groups(flavone, tangeretin, and nobiletin)
are activators (stimula-tors) of selected monooxygenase
activities.
EFFECTS OF MACRONUTRIENTS ON XENOBIOTICMETABOLISM
Compared with the effects of nonnutritive dietary chemicals,the
effects of macronutrients on drug metabolism are moredifficult to
understand. In the former case, the actions of acompound or its
metabolites can be studied, but in the lattercase we have to deal
with a nutritional state or status. There-fore, despite extensive
investigations, our understanding ofthe latter subject is mostly at
the descriptive level. Only re-cently have researchers begun to
look at specific changes onselected enzymes and the molecular
mechanisms involved.Many previously seemingly contradictory results
may be in-terpretable based on our new knowledge on P450 isozymes.A
dietary manipulation of nutritional status may increasethe levels
of a certain group of P450s but decrease those ofothers. Therefore,
depending on the substrates used in thestudy, opposite effects on
drug metabolism may be observed.
Protein
Diets with low protein content or lower quality of
proteinusually result in lower rates of xenobiotic metabolism thana
normal diet. This effect was observed in human volunteersconcerning
the metabolism of antipyrine and theophylline(2, 4) and in animals
with a variety of xenobiotics (25). TheP450 enzymes may be affected
because protein synthesis isretarded under protein deficiency
conditions. However, theeffect on xenobiotic metabolism may be
observed even withoutcommon signs of nutritional protein
deficiency. It is possiblethat dietary protein level influences the
physiological state,such as hormone levels, which affect the level
of P450 enzymes.
Lipid and carbohydrate
In comparison to a fat-free diet, feeding a 3-10% corn oildiet
to rats caused an increase in the microsomal metabolismof a variety
of substrates, including aminopyrine, ethyl-
morphine, hexobarbital, heptachlor, BP, NDMA, and aniline(8, 26,
27). Generally, fat levels are important in affectingP450 levels,
and those rich in polyunsaturated fatty acids aremore effective
than those rich in saturated and monounsatu-rated fatty acids.
Dietary lipids are also essential in produc-ing an optimal
induction of P450 enzymes by inducers suchas phenobarbital, and the
effect is observable at the mRNAlevel (27). Although most studies
suggested that the factorsresponsible for the increased drug
metabolism were lipids,the lipid-to-carbohydrate ratio may be
important. Othercomponents or contaminants in oils such as vitamin
E, cho-lesterol, and lipid peroxides may have also been
contributingfactors in certain experiments (26).
Recent results from our laboratory indicate that, in com-parison
to a fat-free diet, a 20% corn oil diet produced atwofold higher
constitutive level of P450 2E1 but did notaffect the maximal
acetone-inducible level of this enzyme(27). When diets containing
different amounts of corn oilwere fed to rats, those on the higher
fat diet had higher P4502E1 levels and higher blood acetone levels
(27), consistentwith the hypothesis that ketone bodies and ketosis
are keyfactors for the regulation of P450 2E1 (28). Menhaden oiland
corn oil were more effective than olive oil and lard inmaintaining
high levels of P450 2E1, suggesting that factorsother than ketosis
were involved. The total concentration ofmicrosomal P450 was higher
in rats on a diet with higherlipid content; specifically, P450s 3A
and 2A1 were higher,whereas 2B1 and 2C11 were not affected
(29).
Carbohydrates are usually used as variable caloric sourcesfor
isocaloric diets in studying the effects of dietary proteinsor
lipids on xenobiotic metabolism. The higher P450 levelsobserved
with higher protein or lipid diets have usually beenattributed to
the protein or lipids. However, metabolic glu-cose deprivation,
seen in cases of fasting and diabetes,caused induction of P450 2E1,
possibly due to the ketoticconditions produced. Sucrose, glucose,
or fructose, whengiven in drinking water to mice maintained on a
rodent chow,were reported to decrease drug metabolism rates in vivo
andin vitro (30). The high sugar intake from the drinking watermay
have reduced the dietary intake of protein (or othernutrients) that
affected monooxygenase activities. Alterna-tively, a high dietary
or blood glucose level may inhibit thesynthesis of P450s due to
inhibition of y-aminolevulinic acidsynthesis (31).
Fasting and dietary restriction
During fasting, microsomal aminopyrine N-demethylaseand
hexobarbital hydroxylase activities were decreased, butaniline
p-hydroxylase and p-nitroanisole O-demethylase ac-tivities were
increased in male rats (32). The induction ofP450 2E1 by fasting
(17) can account for the increased ani-line hydroxylase and NDMA
demethylase activities. Fastingfor 2 or 3 days caused a 50%
decrease in the level of themale-specific P450 2C11 (33), and this
may account for thepreviously observed decrease in aminopyrine
demethylaseactivity. During fasting, the mRNA for P450 2E1 was
sig-nificantly elevated, a situation similar to diabetes (34),
butsuch elevation was not observed during the induction ofP450 2E1
by acetone pretreatment (19). The results Suggestthat there are
differences in the mechanisms of P450 2E1 in-duction under these
conditions.
In nutritional studies with experimental animals, pairfeeding is
frequently used to allow the animals in the controlgroup eating a
quantity of diet equivalent to that consumedby animals in the
experimental group. If the pair-fed animalsconsume this limited
amount of food in 2 h, usually early in
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740 Vol. 6 Ianuarv 1992 Th cAcIR In, ,n.I VAklt CT Al
the morning, this will create a fasting situation of 22 h
inwhich increases in P450 2E1 and its activities can be ob-served
(35). The practice of overnight fasting before an ex-periment may
also cause an increase in the P450 2E1 level.
EFFECT OF ALCOHOL CONSUMPTION ONXENOBIOTIC METABOLISM
Consumption of alcoholic beverages is widespread and canaccount
for a significant portion of caloric intake of certainindividuals.
Ethanol can affect the absorption, plasma pro-tein binding, blood
flow, and distribution of xenobiotics aswell as have profound
influence on phase I and phase IImetabolism of xenobiotics (36,
37).
The induction of P450 enzymes by ethanol has long beenpostulated
(36). The induction of P450 2E1 has subsequentlybeen demonstrated
in animals and humans (38, 39). Thisenzyme catalyzes the metabolism
of small molecules such asethanol, other alcohols, NDMA, acetone,
aniline, enflurane,ether, acetaminophen, benzene, chloroform,
carbon tetra-chloride, dihaloethanes, and other small halogenated
hydro-carbons (Fig. 3) (40, 41). It is understandable, therefore,
thatchronic consumption of ethanol will increase the rate of
themetabolism and the toxicity of the aforementioned com-pounds. On
the other hand, the acute effect of alcohol con-sumption is an
inhibition of P450 2E1 function due to thecompetitive inhibition by
ethanol.
As reviewed by Lieber (36), chronic administration ofethanol
increases the rate of the metabolic clearance of avariety of drugs
such as meprobamate, pentobarbital,aminopyrine, tolbutamide,
propranolol, rifampicin, andtestosterone. As a result of acute
ethanol consumption,however, impairment of the metabolism of drugs
has beenobserved with meprobamate, benzodiazepines, phenothia-zine,
barbiturate, morphine, methadone, warfarin, xylene,and other
compounds. The induction and inhibition of P4502E1 may account for
only a small portion of these inhibitoryactions. The induction of
other P450s such as P450 2B1 andpossible inhibition of the
metabolism of these drugs byethanol remain to be studied.
Chronic ethanol administration to rats caused a prolifera-tion
of the endoplasmic reticulum of the upper small intes-tine along
with increases in P450 content and NADPH:cyto-
SubstratesAcetone,AlcoholsAniline, PyridineBenzene, Phenol,
StyreneAlkanesCHCI3, Cd4Vinyl
chlorideDihaloethanesTrichloroethyleneN-NitrosodimethylamineN-NitrosodiethylamineAzoxymethanet-ButylhydroperoxideEthersEnflurane,
HalothaneChlorzoxazoneAcetaminophen
Figure 3. Dietary effectorsand substrates for P450 2E1.
chrome c reductase activity. In addition, increased
intestinalmicrosomal activities of benzphetamine demethylase,
7-ethoxycoumarin deethylase, BP hydroxylase, and anilinehydroxylase
have been observed (36). Chronic ethanol con-sumption was also
shown to increase P450 content and themetabolic activation of
N-nitrosopyrrolidine in rat esopha-gus (42). The extent to which
these effects are due to the in-duction of P450 2E1 or its activity
remains to be determined.The induction of P450 2E1 in kidney and
lung has also beenobserved (19, 43).
EFFECTS OF MICRONUTRIENTS ON XENOBIOTICMETABOLISM
The effects of vitamins, especially vitamin deficiencies, ondrug
metabolism have been investigated extensively (2, 5, 8,9).
Seemingly conflicting results in older literature may bemore
understandable with some new insights on this topic.1) Whereas a
general effect of all severe vitamin deficienciesis the decreased
metabolic functions and lowered levels ofP450-dependent metabolic
activities, mild deficiency in acertain nutrient may enhance
P450-dependent activities.Therefore, depending on the severity of
the deficiency, oppo-site effects on xenobiotic metabolism may be
observed.2) Deficiency in a single nutrient may produce varied
effectson the metabolism of different xenobiotics due to the
differ-ent effects on specific P450 isozymes. These points are
illus-trated in the cases of riboflavin and thiamin
deficiencies.
During the development of riboflavin deficiency, there wasa
gradual decrease in the activity of NADPH:P450 reductase,possibly
due to the lowered levels of cellular FAD and FMN.On the other
hand, during early stages of riboflavin deficiency,the levels of
some P450s may be increased, possibly to com-pensate for the
decreased NADPH:P450 reductase levels;some of the monooxygenase
activities such as NDMA de-methylase, aniline hydroxylase, and
aminopyrine demethyl-ase activities were elevated. When the
deficiency was pro-longed, the level of P450 enzymes decreased,
resulting indepressed levels of all P450-dependent activities using
sub-strateS such as BP, aminopyrine, ethylmorphine,
N-methyl-aniline, aniline, and acetanilide. Therefore, mild
deficiencyand severe deficiency had opposite effects on the
metabolismof certain drugs and carcinogens (8).
Rats fed a thiamin-deficient diet had higher concentra-tions of
P450, cytochrome b5, and NADPH:cytochrome creductase activity in
liver microsomes than those fed a dietsufficient in thiamin. The
deficient rats also had increasedrates in the metabolism of
acetaminophen, NDMA, amino-pyrine, ethylmorphine, zoxazolamine,
heptachlor, aniline,N-methylaniline, acetanilide, and BP, but not
in the metabo-lism of hexobarbital (8). Recent studies from our
laboratoryindicated that thiamin deficiency increased the hepatic
micro-somal P450 2E1 level (two- to fivefold) but not the P450
2Clllevel (44). This observation provides an enzymatic basis forthe
enhanced rate of in vivo metabolism of aniline, NDMA,and
acetaminophen, all of which are substrates for P450 2El.Thiamin
deficiency was shown to increase cytosolic glutathi-one
S-transferase activity moderately but not steroid isomer-ase
activity (44). The mechanisms of these effects on drug-metabolizing
enzymes remain to be elucidated.
Mechanistic information on the effects of other micro-nutrients
on xenobiotic metabolism is lacking. These effects,together with
the aforementioned effects on xenobiotic me-tabolism by riboflavin
and thiamin nutrition, are summa-rized in Table 1.
-
Vitamin A deficiency
Vitamin A-high doseNiacin deficiency
Riboflavin deficiencyMild
SevereThiamin deficiency
Vitamin C deficiency
Vitamin C - high dose
Vitamin E deficiency
Folic acid deficiencyAluminum - high dose
Magnesium deficiencyCopper deficiency
Iron deficiency
Iron-large doseSelenium deficiencyZinc deficiency I Metabolism
of pentobarbital, aminopyrine
Abbreviations and signs used: I, decrease; I, increase;
reductase, NADPH: P450 reductase activity. Original references can
befound in reviews (2, 3, 5, 8, 9).
TABLE 1. Effects of micronutrients on xenobiotic metabolism
DIETARY EFFECTS ON XENORIOTIC MFTAROI 1cM 741
Nutritional status Xcnobiotic metabolism and enzymes
P450Metabolism of aminopyrine, ethylmorphine, aniline, BP,
7-ethoxycoumarin
I Metabolism of aniline, 7-ethoxycoumarinMetabolism of
anesthetics
ReductaseI Metabolism of NDMA, aniline, aminopyrine
Metabolism of BP, aminopyrine, ethylmorphine, N-methylaniline,
aniline, acetanilide1 P450 2E1, reductase, cytochrome b5I NDMA,
acetaminophen, aniline, aminopyrine, ethylmorphine, zoxazolamine,
BP
P450, reductaseMonooxygenase activitiesMonooxygenase
activitiesMetabolism of codeine, ethylmorphine, BPInduction of P450
2B1 by barbiturates
I Hepatic P450I Metabolism of p-nitrophenole, ethylmorphine
Cadmium, cobalt, and otherheavy metals-high dose 1 P450 and
related activities
Calcium deficiency I Monooxygenase activities
I Monooxygenase activitiesI Metabolism of aniline, hexobarbitalI
Metabolism of BPI Metabolism of hexobarbital, aminopyrineI
Metabolism of anilineI NADPH-dependent lipid peroxidationI
Induction of P450 by phenobarbital
DIETARY EFFECTS ON P450TOXICITY
2E1 AND CHEMICAL
In this section, P450 2E1 is used as an example to illustratethe
dietary modulation of P450 enzymes and the effect of thismodulation
on toxicity and carcinogenicity of chemicals. Asillustrated in Fig.
3, diet may provide inducers, suppressors,inhibitors, and
substrates for P450 2El. In addition to affect-ing the hepatic
microsomal P450 2E1 level measured in vitro,a low fat/high
carbohydrate diet also resulted in a lower rateof enflurane
metabolism in rats than a high fat/low carbohy-drate diet (29).
Based on these results, it may be suggestedthat individuals on a
low fat diet may have, on average, lowerP450 2E1 levels than those
on a high fat diet. Frequent con-sumption of ethanol is known to
elevate the level of P450 2E1in humans, and is believed to be a key
mechanism for the en-hanced toxicity of acetaminophen, CCI,, and
other chemi-cals. Fasting can be an important factor in raising
P450 2E1levels in individuals undergoing weight reduction by
severefasting or a low carbohydrate diet. The induction of P4502E1
by fasting probably also contributes to the enhanced tox-icity of
acetaminophen, although other factors such asdecreased glutathione
levels are also important.
The activity of P450 2E1 is also modulated by dietary
inhi-bitors and suppressors of this enzyme. The consequence ofthis
inhibition and suppression of P450 2E1 is the inhibitionof
metabolism and toxicity of certain xenobiotics. The inhi-
bition of enflurane metabolism by diallyl sulfide andphenethyl
isothiocyanate, which are competitive and suicideinhibitors of P450
2E1, has been demonstrated in rats (un-published results).
The inhibition of carcinogen metabolism in the liver canincrease
the carcinogen exposure to nonhepatic organs andthus may enhance
nonhepatic carcinogenesis. This effect hasbeen demonstrated with
ethanol, which inhibited hepatocar-cinogenesis of NDMA but enhanced
tumorigenesis in thenasal cavity (45). The dual effects of ethanol,
i.e., an acuteeffect of inhibition and a chronic effect of
enhancement ofNDMA-induced carcinogenesis, have also been observed
(46).
Knowing that many dietary factors can modulate P450 2E1,it may
be questioned whether a higher or lower level of thisenzyme is more
beneficial to health. A possible physiologicalfunction of P450 2E1
is in the initial step for the conversionof acetone to glucose
(47), but the rate of this metabolic path-way is rather low and its
physiological importance remainsto be established. Lowering P450
2E1 levels may thus de-crease the susceptibility to many toxic
chemicals, providedthe parent compounds are not toxic and can be
disposed ofby other metabolic pathways or by exhalation. One
exceptionis the dihaloethanes, which are activated by a
glutathione-dependent pathway and detoxified by P450
2E1-dependentmetabolism (41). A second aspect of this question is
thatP450 2E1 is known to be effective in causing lipid
peroxida-tion (48). It remains to be determined whether higher
levels
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742 Vol. 6 January 1992 The FASEB lournal YANC FT Al
of P450 2El can contribute to oxidative stress in vivo.
Theapplication of dietary-derived P450 2E1 inhibitors and
sup-pressors for the prevention of acetaminophen toxicity is be-ing
explored in our laboratory. Of particular importance
areapplications with alcoholics or patients taking isoniazidwhose
P450 2El levels are known to be elevated.
DIETARY EFFECTS ON DRUG METABOLISM,TOXICITY, AND
CARCINOGENESIS
Food and dietary components may affect the fate of a drug
ortoxicant by the following mechanisms: 1) altering the ratesof its
absorption and uptake, 2) reacting or tightly bindingwith the drug,
3) competing with the drug for binding toplasma proteins, and 4)
affecting phase I and phase IImetabolism. Interfering with
P450-dependent metabolismappears to be the most selective mechanism
by which dietarycomponents exert their effects on drug metabolism
and car-cinogenesis. For example, the observation that the
drinkingof 200 ml of grapefruit juice markedly inhibited the
oxida-tion of nifedipine and felodipine in human volunteers (49)may
be interpreted on the basis that grapefruit is rich inquercetin and
naringenin, which are effective inhibitors ofP450 3A4, the isozyme
responsible for the metabolism ofthese dihydropyridine drugs (24).
P450 3A4 is also importantin the activation of aflatoxin B, in
humans (50). It may bespeculated that grapefruit consumption would
inhibit thebioactivation of aflatoxin B, and could inhibit
hepatocar-cinogenesis in populations exposed to rather high
concentra-tions of this carcinogen.
Indoles and isothiocyanates are two major classes of com-pounds
that occur as glucosinolates in cruciferous vegeta-bles. The
actions of indole-3-carbinol and phenethyl isothio-cyanate may
account for some of the reported inhibitoryactions of cruciferous
vegetables against chemically inducedcarcinogenesis in animals and
for the association betweenthe frequent consumption of these
vegetables with lowercancer incidences at different organ sites
(51). The inductionof P450 1A1 in the intestine may help to
metabolize andeliminate dietary polyaromatic hydrocarbons in the
intes-tine, and thus may reduce the exposure of internal organs
tosuch carcinogens. In studies of aflatoxin B1-induced
hepato-carcinogenesis in the trout, the inhibitory action of the
acidreaction products of indole-3-carbinol (RXM) on the meta-bolic
activation of aflatoxin B, may be a major mechanismfor the
inhibition of carcinogenesis by indole-3-carbinol (52).On the other
hand, when indole-3-carbinol was given afterthe aflatoxin B,
treatment period, it enhanced carcinogenesisin the trout (53); the
mechanisms are not known.
It has also been demonstrated that indole-3-carbinol,when
incorporated into the diet, increased the estradiol2-hydroxylase
activity in rats and humans (54), possibly dueto the induction of
P450s lAl and 1A2. Because 2-hydroxyl-ation converts estradiol to
nonuterotropic and antiestrogenicmetabolites, the increase in this
metabolic activity was sug-gested to reduce the incidence of
estrogen-related cancers.
Phenethyl isothiocyanate was a very potent inhibitor
ofNNK-induced lung tumorigenesis (55) and
N-nitrosometh-ylbenzylamine-induced esophageal carcinogenesis (56).
Theaction can be attributed largely to its inhibition of
carcino-gen activation (23). Probably via a similar mechanism,
di-allyl sulfide was also an effective inhibitor in both
carcino-genesis models (unpublished results; ref 57). Diallyl
sulfideand related organosulfur compounds may be partially
re-sponsible for the negative association between the consump-tion
of allium vegetables and incidence of gastric cancer (58).
The amount of diallyl sulfide derived from garlic is only
inquantities of 3-100 ftg/g. The estimated human intake
ofglucosinolates through consumption of cooked vegetables isabout
30 mg per day. It may be questioned whether suchsmall quantities of
dietary inhibitors can have a significanteffect in inhibiting
carcinogenesis. Two aspects may be perti-nent to this question: 1)
Many dietary compounds may becompetitive inhibitors of P450
enzymes; even when presentat low concentrations, they could
effectively inhibit themetabolism of low concentrations of
carcinogens. 2) Manydietary chemicals can inhibit carcinogen
activation. Al-though most of them are present only in small
concentra-tions, in combination their actions can be
significant.
The human diet is also known to contain mutagens, carcin-ogens,
and tumor promoters. The effects of these compoundson health have
to be considered in light of the capabilityof the body to detoxify
these dietary chemicals. Phase IImetabolism, which is usually not
considered in most muta-genesis assays in vitro, may be of
particular importance. Itis also important to consider the doses of
these chemicals re-quired to produce the possible harmful effects.
Dependingon the dose, a compound can have either beneficial or
harm-ful effects, and sometimes the effects depend on the
experi-mental model used. This point can be illustrated with
thecommonly used food additives, butylated hydroxyanisole(BHA) and
butylated hydroxytoluene (BHT). These antiox-idants may be present
at a total concentration of up to 0.02%in certain food items. When
added to the diet at a concentra-tion of 0.5%, BHA and BHT
inhibited carcinogenesis inseveral animal models (59). Induction of
phase II enzymesand inhibition of carcinogen activation have been
proposedas the mechanisms of inhibition (8, 59). However, with 1
or2% of BHT or BHA in the diet, respectively, tumor promo-tion
activity has been demonstrated in a two-stage urinarybladder
carcinogenesis model (60).
CONCLUDING REMARKS
Studies of the multiplicity and substrate specificity of
P450isozymes have contributed to our understanding of
dietaryeffects on xenobiotic metabolism. A dietary chemical may
in-crease the level of certain P450s and decrease the level
ofothers; thus the rates of the metabolism of certain drugs maybe
enhanced and those of others lowered. The distinction be-tween an
inhibitory effect after an acute dose and an induc-tion effect
after a treatment also helps to explain the diver-gent effects of
dietary chemicals on drug metabolism. Inaddition, a nutritional
deficiency may have different effectson the metabolism of a certain
drug; the rate may be en-hanced in mild deficiency but decreased in
severe deficiency.
Most of the studies reviewed herein were carried out usingliver
microsomes from rats and mice. These results can pro-vide us with
some basic understanding of the mechanisms bywhich a dietary factor
may affect drug metabolism. Cautionmust be applied when
extrapolating the information ob-tained from hepatic tissues to
nonhepatic tissues and fromanimals to humans. With the
understanding of human xeno-biotic metabolism as a goal,
researchers are faced with thefollowing challenges: 1) to further
elucidate the detailedmechanisms by which diet affects
xenobiotic-metabolizingenzymes, 2) to understand the basis for the
tissue and speciesspecificities of xenobiotic-metabolizing enzymes,
3) to furthercharacterize the catalytic properties of human
xenobiotic-metabolizing enzymes, and 4) to pursue well-planned
hu-man studies concerning the nutritional impact on drugmetabolism
and toxicity.
-
DIETARY EFFECTS ON XENOBIOTIC METABOLISM 743
This work was supported by National Institutes of Health
grantsES03938, CA46535, and CA37037, a grant from the American
In-stitute for Cancer Research, and NIEHS Center grant ES05022.The
authors wish to thank Ms. Dorothy Wong for her excellentsecretarial
assistance and Ms. Marie Leithauser for helping to pre-pare the
manuscript.
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