Detoxification Enzymes

Alternative Medicine Review ◆ Volume 3, Number 3 ◆ 1998 87

The Detoxification Enzyme Systemsby DeAnn J. Liska, Ph.D.

AbstractThe human body is exposed to a wide array of xenobiotics in one’s lifetime, from

food components to environmental toxins to pharmaceuticals, and has developedcomplex enzymatic mechanisms to detoxify these substances. These mechanismsexhibit significant individual variability, and are affected by environment, lifestyle, andgenetic influences. The scientific literature suggests an association between impaireddetoxification and certain diseases, including cancer, Parkinson’s disease, fibromyalgia,and chronic fatigue/immune dysfunction syndrome. Data regarding these hepaticdetoxification enzyme systems and the body’s mechanisms of regulating them suggeststhe ability to efficiently detoxify and remove xenobiotics can affect these and otherchronic disease processes. This article reviews the myriad detoxification enzymesystems; their regulatory mechanisms; and the dietary, lifestyle, and genetic factorsinfluencing their activities; as well as laboratory tests available to assess their functioning.(Altern Med Rev 1998;3(3):187-198)

IntroductionWe are exposed to a great number of xenobiotics during the course of our lifetime,

including a variety of pharmaceuticals and food components. Many of these compounds showlittle relationship to previously encountered compounds or metabolites, and yet our bodies arecapable of managing environmental exposure by detoxifying them. To accomplish this task, ourbodies have evolved complex systems of detoxification enzymes. These enzyme systems gen-erally function adequately to minimize the potential of damage from xenobiotics. However,much literature suggests an association between impaired detoxification and disease, such ascancer, Parkinson’s disease, fibromyalgia, and chronic fatigue/immune dysfunction syndrome.Therefore, accumulated data suggests an individual’s ability to remove toxins from the bodymay play a role in etiology or exacerbation of a range of chronic conditions and diseases.

The detoxification systems are highly complex, show a great amount of individual vari-ability, and are extremely responsive to an individual’s environment, lifestyle, and genetic unique-ness. This review of the detoxification systems is meant to whet the appetite for a more in-depthlook at detoxification and, as such, it may raise more questions than it answers.

Discovery of Detoxification ReactionsThe hypothesis that xenobiotics consumed by animals are transformed to water-soluble

substances and excreted through the urine was first put forth in the late 18th century. For years,

DeAnn Liska, Ph.D. — Associate Director of Research, HealthComm International, Inc.Correspondence address: P.O. Box 1729, Gig Harbor, WA 98335. e-mail: [email protected]

Copyright©2001 Thorne Research, Inc. All Rights Reserved. No Reprints Without Written Permission

Alternative Medicine Review ◆ Volume 3, Number 3 ◆ 1998

scientists collected urine from various animals,purifying and then chemically characterizingthe compounds present in an attempt to un-derstand how the body managed to removevarious xenobiotics. Hippuric acid, discoveredin 1773, was one of the first metabolites iden-tified in these early studies and, from chemi-cal characterization, was proposed to resultfrom the conjugation of glycine with benzoicacid (Figure 1). However, it was not until 1842that this hypothesis was officially tested. Kelleris attributed with performing the first challengetest, in which he took a dose of benzoic acid,collected his urine, and showed a direct rela-tionship between ingestion of benzoic acid andthe hippuric acid subsequently excreted.1

For more than 100 years after this ob-servation, research continued in the identifi-cation of various metabolites, and a variety ofconjugation reactions were identified. During

this time, glucuronic acid, sulfate, gly-cine, glutamine, taurine, ornithine, andglutathione were identified as conjugat-ing substances (Table 1). Although theconjugation reactions solved the puzzleof how a non-water-soluble compoundcan be converted to a substance that couldbe excreted in urine, it raised another

question. In all these cases of conjugation, thexenobiotic is required to have the ability toreact with the conjugating moiety, i.e., to havean active center or “functional” group to reactwith, and bind, the conjugating moiety. Whathappens with compounds that do not have areactive site?

In his landmark 1947 monograph,Detoxification Mechanisms, R.T. Williamsdefined the field of detoxification. Williamsproposed that these non-reactive compoundscould be biotransformed in two phases:functionalization, which uses oxygen to forma reactive site, and conjugation, which resultsin addition of a water-soluble group to the re-active site.2 These two steps, functionalization

and conjugation, are termed Phase I andPhase II detoxification, respectively. Theresult is the biotransformation of a lipo-philic compound, not able to be excretedin urine, to a water-soluble compound ableto be removed in urine (Figure 2). There-fore, detoxification is not one reaction, butrather a process that involves multiplereactions and multiple players.

Today, the challenge to understanddetoxification continues. The question ofhow the body can handle such a widerange of compounds it has never beforeseen has led to considerable study in anattempt to understand the protein structureand regulation of various enzymesinvolved in detoxification. It is now

known one mechanism the body uses is abattery of enzymes, each with broadspecificity, to manage this challenge.Currently, over 10 families of Phase I enzymes

GlycineSulfateGlucuronic AcidOrnithineMercapturic AcidMethylationAcetylationCyanide detoxificationGlutamineGlucoside (1) plants

(2) insects

Keller (1842)Baumann (1876)Jaffe (1874)Jaffe (1877)Jaffe (1879); Baumann and Preusse (1879)His (1887)Cohn (1893)Lang (1894)Thierfelder and Sherwin (1914)Miller (1938)Myers and Smith (1953)

Conjugation Author and Date

Table 1. The discovery of the major conjugation reactions

COOH

Benzoic Acid

Glycine

Hippuric Acid

CO-NH-CH2-COOH

Figure 1. Glycination of benzoic acid


Alternative Medicine Review ◆ Volume 3, Number 3 ◆ 1998 Page 189

Detoxification

have been described, which include at least35 different genes. Phase II reactions areequally complex, and involve multiple genefamilies as well.

Enzyme Systems Involved inDetoxification

The Phase I System: The Phase Idetoxification system, composed mainly of thecytochrome P450 supergene family of en-zymes, is generally the first enzymatic defenseagainst foreign compounds. Most pharmaceu-ticals are metabolized through Phase Ibiotransformation. In a typical Phase I reac-tion, a cytochrome P450 enzyme (CypP450)uses oxygen and, as a cofactor, NADH, to adda reactive group, such as a hydroxyl radical.As a consequence of this step in detoxifica-tion, reactive molecules, which may be more

toxic than the parent molecule, are produced.If these reactive molecules are not furthermetabolized by Phase II conjugation, they maycause damage to proteins, RNA, and DNAwithin the cell.4 Several studies have shownevidence of associations between inducedPhase I and/or decreased Phase II activities andan increased risk of disease, such as cancer,systemic lupus erythematosus, and Parkinson’sdisease.5-10 Compromised Phase I and/or PhaseII activity has also been implicated in adversedrug responses.5,11,12

As stated, at least 10 families of PhaseI activities have been described in humans(Table 2). The major P450 enzymes involvedin metabolism of drugs or exogenous toxinsare the Cyp3A4, Cyp1A1, Cyp1A2, Cyp2D6,and the Cyp2C enzymes (Figure 3). Theamount of each of these enzymes present inthe liver reflects their importance in drug

Endotoxinsend products of metabolismbacterial endotoxins

Exotoxinsdrugs (prescription, OTC, recreational, etc.)chemicals

•agricultural•food additives•household•pollutants/contaminants

microbialPHASE I

[cytochrome P450 enzymes]PHASE II

[conjugation pathways]

Bile

Feces/stools

Serum

Kidneys

Urine

sulfationglucuronidationglutathione conjugation*acetylationamino acid conjugation

glycinetaurineglutamineornithinearginine

methylation

*N-acetylcysteine, cysteine, methionine are precursors

Antioxidant/ProtectiveNutrients/Plant Derivatives

carotenes (vit A)ascorbic acid (vit C)tocopherols (vit E)seleniumcopperzincmanganesecoenzyme Q10thiols (found in garlic, onions, & cruciferous vegetables)bioflavonoidssilymarinoligomeric proanthocyanidins (OPCs)

Superoxide

nonpolar ∴lipid-soluble

lipid-soluble (nonpolar)toxins stored in adiposetissue contribute toincreased/mobilized toxinload with weight loss

Toxins

riboflavin (vit B2)niacin (vit B3)pyridoxine (vit B6)folic acidvitamin B12glutathionebranched-chain amino acidsflavonoidsphospholipids

Enzymes, Cofactors &Other Nutrients Used

oxidationreductionhydrolysishydrationdehalogenation

Reactions

intermediarymetabolites

excretoryderivativesToxins

Free Radicals

Secondarytissue damage

Reactive OxygenIntermediates

( (more polar

more water-soluble( ( polar ∴water-soluble( (

Figure 2. Liver detoxification pathways and supportive nutrients



metabolism.11,13 Most information on the PhaseI activities has been derived from studies withdrug metabolism; however, Phase I activitiesare also involved in detoxifying endogenousmolecules, such as steroids.

The Phase II System: Phase II conju-gation reactions generally follow Phase I acti-vation, resulting in a xenobiotic that has beentransformed into a water-soluble compoundthat can be excreted through urine or bile. Sev-eral types of conjugation reactions are presentin the body, including glucuronidation,sulfation, and glutathione and amino acid con-jugation (Table 3). These reactions requirecofactors which must be replenished throughdietary sources.

Much is known about the role of PhaseI enzyme systems in metabolism ofpharmaceuticals as well as their activation byenvironmental toxins and specific food

components. However, the role of Phase Idetoxification in clinical practice has receivedless consideration. The contribution of thePhase II system has received lesser attentionboth in academic research circles and inclinical practice. And, little is currently knownabout the role of the detoxification systems inmetabolism of endogenous compounds.

Is There a Phase III?: Recently,antiporter activity (p-glycoprotein or multi-drug resistance) has been defined as the PhaseIII detoxification system.14 Antiporter activityis an important factor in the first pass metabo-lism of pharmaceuticals and other xenobiotics.The antiporter is an energy-dependent effluxpump, which pumps xenobiotics out of a cell,thereby decreasing the intracellular concentra-tion of xenobiotics.15

Antiporter activity in the intestine ap-pears to be co-regulated with intestinal Phase

Cyp3A4,5

Cyp2C8,9,18

Cyp1A2

Cyp2E1

Cyp2A6

Cyp2D6

Cyp2B6

28.8 ± 10.4

18.2 ± 6.7

12.7 ± 6.2

6.6 ± 2.9

4.0 ± 3.2

1.5 ± 1.3

0.2 ± 0.3

Cyc losporinN ifed ip ineTestosterone

R-mephenytoinTolbutamideS-warfarin

Phenace tinC a ffe ineA fla toxin B1

E thanolC arbon te trachlorideD ime thylnitrosamine

CoumarinD ime thylnitrosamine

DebrisoquineSparte ineBufura lol

Cyc lophosfamide

Troleandomyc inKe toconazoleG estodene

Sulphahaphenazole

E llip tic ineFura fyllineα-Naphthoflavone

D ie thyld ithiocarbama teD ia llyl sulfide

Me thoxa len

Quinid ineA jma lic ineYohimb ine

Sulphahaphenazole

Induc ib le , (e .g ., Rifamp ic inDexame thasone)

Induc ib le , e .g ., Rifamp ic inG ene tic polymorphism(20% poor me tabolizers)

Induc ib le , e .g ., SmokingOmeprazoleG ene tic polymorphism(12% poor me tabolizers)

Induc ib le , e .g ., E thanol,Isoniaz idG ene tic polymorphism?

Induc ib le , e .g ., PyrazoleG ene tic polymorphism

Non-induc ib leG ene tic polymorphism(10% poor me tabolizers)

P450-Isozyme

% of Tota lP450 Substra tes Se lec tive Inhib itors

Sources of Variab ility(Induc ib ility)

Da ta from Vermeulen (1996) and re ferences there in.4

Summary (Incomp le te) of re la tive contents, substra tes, inhib itors, and sources of variab ility

Table 2. Cytochromes P450 in human liver



Detoxification

I Cyp3A4 enzyme.16 This observation suggeststhe antiporter may support and promote detoxi-fication. Possibly, its function of pumping non-metabolized xenobiotics out of the cell andback into the intestinal lumen may allow moreopportunities for Phase I activity to metabo-lize the xenobiotic before it is taken into cir-culation (Figure 4).

Two genes encoding antiporter activ-ity have been described: the multi-drug resis-tance gene 1 (MDR1) and multi-drug resis-tance gene 2 (MDR2).15 The MDR1 gene prod-uct is responsible for drug resistance of manycancer cells, and is normally found in epithe-lial cells in the liver, kidney, pancreas, smalland large intestine, brain, and testes. MDR2activity is expressed primarily in the liver, andmay play a role similar to that of intestinalMDR1 for liver detoxification enzymes; how-ever, its function is currently undefined.

Regulation of DetoxificationActivities

Since the detoxification systems func-tion to help in the management of exposure toexogenous compounds, the body has devel-oped several mechanisms to regulate detoxi-fication activity (Table 4). Specific detoxifi-cation pathways may be induced or inhibiteddepending on the presence of various dietaryor xenobiotic compounds, the age and sex ofthe individual, genetics, and lifestyle habits,such as smoking.5,13,17-20 Furthermore, diseasecan also influence activity of the enzymes. Insome disease states, detoxification activitiesappear to be induced or up-regulated, whereas,in other conditions these activities may be in-hibited from acting or not produced at highlevels.12,13

CYP1A2ClozapineImipramineCaffeineAcetaminophenPhenacetinCYP2E1

ChlorzoxazoneEnfluraneHalothane

CYP2C19DiazepamCitalopramOmeprazoleProguanil

CYP2C9DiclofenacIbuprofenLosartanPhenytoinTolbutamide

CYP3A4/-3A5AmitriptylineCarbamazepineClarithromycinCyclosporineLignocaineMidazolamNifedipineParoxetineTerfenadine

CYP2D6AmitriptylineCodeineHaloperidolImipramineMetoprololNortriptylineOndansetronPropafenone

Figure 3. Major detoxification activities in drug metabolism. Reprinted from Iarovici (1997) with permission.11



InductionWhen the body is confronted with a

high xenobiotic load, the Phase I and/or PhaseII enzymes involved in detoxifying thiscompound can be induced, leading to moreenzyme being present and a faster rate ofxenobiotic detoxification. Inducers can bemono-functional or multi-functional (Figure5). A mono-functional inducer affects only oneenzyme or one phase of detoxification,whereas, a multi-functional inducer affectsmultiple activities.17

Mono-functional inducers, such aspolycyclic hydrocarbons from cigarette smokeand aryl amines from charbroiled meats, re-sult in dramatic induction of the Cyp1A1 andCyp1A2 enzymes, leading to a substantial in-crease in Phase I activity, with little or no in-duction of Phase II enzymes.21-25 Similarly, glu-cocorticoids and anti-convulsants induce

Cyp3A4 activity, andethanol, acetone, andisoniazid induceCyp2E1.13,16,17,26,27 In-duction of these activi-ties without co-induc-tion of Phase II activi-ties may lead to an un-coupling of the Phase Iand Phase II balance ofactivity and, therefore,a higher level of reac-tive intermediates,which can cause dam-age to DNA, RNA, andproteins.17,24, 28

The multi-functional inducers in-clude many of the fla-vonoid moleculesfound in fruits and veg-etables. For example,ellagic acid found inred grape skin has beenshown to induce sev-

eral Phase II enzymes while decreasing PhaseI activity.29-31 Garlic oil, rosemary, soy, cab-bage, and brussels sprouts all contain com-pounds that can induce several Phase II en-zyme activities.24,29,32-35 Commonly, the glu-tathione S-transferase and glucuronyl trans-ferases are induced by multi-functional induc-ers. In general, this increase in Phase II sup-ports better detoxification in an individual andhelps to promote and maintain a healthy bal-ance between Phase I and Phase II activities.The enhancement of Phase II activity has beenproposed to explain, at least in part, the abil-ity of fruits and vegetables to protect againstmany cancers.17,24,25,28,29

InhibitionPhase I and Phase II enzyme activities

can also be inhibited. Inhibition can occur bycompetition between two or more compounds

H2O

Glutathione

Glucuronic acid(UDPGA)b

Sulfuric acid(PAPS)b

Methyl Group(SAM)b

Acetic acid(Acetyl-CoA)b

Amino acids(Acetyl-CoA,taurine, glycine)

Epoxide hydrolase

Glutathione transferases

Glucuronyl transferases

Sulfotransferase

N- and O- methyl transferases

N-acetyl transferases

Amino acid transferases

MicrosomesCytosol

Microsomes

Microsomes

Cytosol

CytosolMicrosomes

Cytosol

Microsomes

Epoxides

Electrophiles

Phenols, thiols, amines,Carboxylic acids

Phenols, thiols, amines

Phenols, amines

Amines

Carboxylic acids

Reaction Enzyme Localizationa Substrates

a Microsome refers to membrane-associated activities but these activities may be localized to the cellular membraneor to internal membranes; cytosol refers to soluble activities present in the cytosolic portion of the cellb Abbreviations in brackets are co-substrates: UDPGA = urindine - 3', 5' - diphosphoglucuronic acid; PAPS = 3' -phosphoadenosine 5' - phosphosulfate; SAM = S - adenosylmethionine; CoA = coenzyme A.

Data from Vermeulen.4

Table 3. Major phase II detoxification activities in humans



Detoxification

for the same detoxifying enzyme. Increasedtoxic load may lead to inhibition of detoxifi-cation of a number of compounds by simplyoverwhelming the systems and competing fordetoxification enzyme activities. Moreover,some compounds selectively inhibit only onedetoxifying activity; for example, quinidine,which competitively inhibits Cyp2D6 activ-ity.13 Cimetidine is an example of a compoundthat can bind directly to the heme iron of thecytochrome P450 reactive site to inhibit all cy-tochrome-dependent Phase I enzyme activi-ties.13 Much has been written recently aboutgrapefruit juice, which contains high amountsof the flavonoid naringenin and its ability toinhibit first pass metabolism of many drugsthat are detoxified through theCyp3A4 enzyme-antiporter systemin the intestine.36-39

A common mechanism of in-hibition for some Phase II enzymesis the depletion of necessary cofac-tors. In humans, sulfation is particu-larly susceptible to inhibition due tocompromised cofactor status. Serumsulfate concentration is a balancebetween absorption of inorganic sul-fate and its production from cysteine,and sulfate elimination by urinaryexcretion and incorporation into lowmolecular weight substrates ofsulfation. In humans, serum sulfatelevels vary dramatically throughout24 hours, and are decreased in indi-viduals who are fasting or ingestinghigh levels of substances that aremetabolized by sulfation, such as ac-etaminophen.40-42 Humans excreteapproximately 20-25 millimoles of sulfate in24 hours; therefore, sulfate reserves must bemaintained through dietary intake of sulfur-containing amino acids or inorganic sulfate,both of which have been shown to support in-creased serum sulfate levels.40

PolymorphismsGenetic differences in the ability of an

individual to metabolize xenobiotics are re-lated to the presence of different versions ofthe gene encoding that activity, or genetic poly-morphism. Cyp2D6 is the classic example ofthe influence of genetic polymorphism on phe-notype. Several varieties of the Cyp2D6 geneexist in the population; some encode an en-zyme with a lower activity than others. Indi-viduals who receive two versions of the geneencoding slower Cyp2D6 activity do notdetoxify substances through the Cyp2D6 path-way as fast as those who receive genes encod-ing faster acting enzymes.5,7,11,19,43 These indi-viduals have been termed “slow metabolizers.”

The Cyp2D6 enzyme is an important detoxi-fier of many narrow spectrum drugs, includ-ing antiarrhythmics, antidepressants, and an-tipsychotic drugs. Adverse side-effects occur-ring from these drugs may be reduced by de-creasing dosages in those individuals who areCyp2D6 slow metabolizers.11,43

Stomach

GutLumen

Feces

Enterocyte

Anti-PorterPortalVein

to

LiverCYP3A4

Figure 4. Phase III metabolism: The antiporter



Polymorphisms in the Phase II activ-ity of N-acetyltransferase can also lead to slowmetabolizers. Associations have been foundbetween slow metabolism through the N-acetyltransferase pathway and high risk ofsome types of cancer and Parkinson’s dis-ease.5,7,10 Polymorphisms in Cyp2D6 have alsobeen associated with a high risk of early onsetParkinson’s disease.5

Other FactorsSeveral other factors also influence the

expression and resultant activity of many ofthe detoxification enzymes. Like many other

proteins, detoxification activities are understrict developmental control. Phase I Cyp3Aenzymes and enzymes catalyzingglucuronidation, sulfation, and glutathione

conjugation are present in the human fetus. Bythe time of birth, these enzymes are capableof catalyzing most Phase I biotransformationreactions; however, the rate of these reactionsis generally slower than in adults. After twoweeks of life, Phase I and Phase II detoxifica-tion systems become produced more fully.13

Sex and age also affect the type,amount, and activity of the various detoxifi-cation enzymes. The Cyp3A family of detoxi-fication enzymes is particularly sensitive tohormones. For example, premenopausalwomen generally show 30-40 percent moreCyp3A4 activity than men or postmenopausalwomen.16,44,45 The Cyp3A4 enzyme appears tobe regulated, at least in part, by progesterone.44

Since Cyp3A4 is the major Phase I detoxifi-cation pathway for the anti-epileptic agentsphenobarbital and phenytoin, pregnantwomen, in whom this activity is increased,more readily eliminate these drugs and, there-

fore, may require a higher dose during preg-nancy.14

Disease and health status of the indi-vidual also influence detoxification activi-ties. Since the majority of detoxification oc-curs in the liver, it is not surprising that im-pairment of normal liver function due toalcoholic disease, fatty liver disease, bil-iary cirrhosis, and hepatocarcinomas canlead to lower detoxification activity in gen-eral.12,13 However, different enzyme sys-tems occur in different regions of the liver;Phase I activities are membrane associated,whereas the majority of Phase II activityoccurs in the cytosol.4 Therefore, theamount of decrease in detoxification activ-ity may vary from one isozyme to another.

Moreover, some conditions can lead toan increase or induction of activity. For ex-ample, Cyp2E1 catalyzes the oxidation of

ethyl alcohol to acetaldehyde, and in additiondetoxifies many of the small carbon-chainmolecules, including ketone bodies, that re-sult from gluconeogenesis and the breakdown

G ene tic polymorphismsAge and G enderD ie t and LifestyleEnvironmentD isease

Table 4. Factors influencing detoxification activity.

Mono-functionalinducer

Drug Intracellularreceptor mRNA P450(s)

Drug Intracellularreceptor mRNA

Multi-functionalinducer

P450(s) Glucuronyltransferase

Epoxidehydrolase

Glutathionetransferase

Figure 5. Mono-functional and Multi-functional induction. Reprinted from Park, Kitteringham, Pirmohamed, Tucker (1996) with permission.17



Detoxification

of fatty acids. Cyp2E1 is increased in insulin-dependent diabetes, in morbidly obese indi-viduals and, conversely, during starvation.46,47

The reason Cyp2E1 is induced in these condi-tions is not fully understood; however, it mayhave to do with the role Cyp2E1 plays in me-tabolism of products from gluconeogenesisand energy metabolism, and may influencehow these pathways become imbalanced dur-ing altered metabolism. The full range ofchanges in other detoxification systems in re-sponse to health status is not fully understood.

Role of the Intestine inDetoxification

Most literature on detoxification refersto liver enzymes, as theliver is the site of the ma-jority of detoxification ac-tivity for both endogenousand exogenous compounds.However, the first contactthe body makes with themajority of xenobiotics isthe gastrointestinal tract.Over the course of a life-time, the gastrointestinaltract processes more than25 tons of food, which rep-resents the largest load ofantigens and xenobioticsconfronting the humanbody.48 Furthermore, sincemost drugs are consumedorally, the gastrointestinaltract is also the first contactwith many drugs. It is not surprising, then, thatthe gastrointestinal tract has developed a com-plex set of physical and biochemical systemsto manage this load of exogenous compounds.

Several factors influence how much ofa chemical ends up in the system, requiringdetoxification by the liver. The gastrointestinaltract initially provides a physical barrier to

exogenous components. As previouslydiscussed, the gastrointestinal tract is thesecond major site in the body fordetoxification. Detoxification enzymes suchas Cyp3A4 and the antiporter activities havebeen found in high concentrations at the tip ofvilli in the intestine.16,49,50 Adequate first passmetabolism of xenobiotics by thegastrointestinal tract requires integrity of thegut mucosa. Compromised barrier function ofthe mucosa will easily allow xenobiotics totransit into the circulation without opportunityfor detoxification. Therefore, support forhealthy gut mucosa is instrumental indecreasing toxic load.

The gastrointestinal tract influencesdetoxification in several other ways. Gut mi-

croflora can produce compounds that eitherinduce or inhibit detoxification activities.Pathogenic bacteria can produce toxins thatcan enter circulation and increase toxic load.51-

54 Moreover, in a process called enterohepaticrecirculation, gut microflora also have the abil-ity to remove some conjugation moieties, suchas glucuronosyl side chains, converting the

EnzymeCyp1A2Cyp2C9Cyp2C19Cyp2D6Cyp2E1Cyp3A4

N-acetyl transferaseGlucuronyl transferaseSulfationGlycination

Probe DrugCaffeineTolbutamideMephenytoin, ProguanilSpateine, Dextromethorphan, DebrisoquineChlorzoxazoneErythromycin (breath test), Midazolam,

6β-hydroxycortisolSulphadimidine, Isoniazid, CaffeineOxazepam, AcetaminophenAcetaminophenBenzoic acid, Salicylate

Data from Park et al., 17 Patel et al.,57 and Levi et al.58

Table 5. Examples of in vivo probes for drug metabolizing enzymes.



xenobiotic to its original form, and allowingit to reenter circulation, leading to an increasedtoxic load.54-56

SummaryThe detoxification system in humans

is extensive, highly complex, and influencedby myriad regulatory mechanisms. This com-plexity of the detoxification systems and theindividual uniqueness shown in ability todetoxify substances suggests one test or typeof assay to fully assess detoxification statuswill not be possible. For some enzyme activi-ties, such as the Cyp2D6, which are primarilyinfluenced by genetics, determination of slowor fast metabolizers is possible with one geneanalysis. However, to understand the detoxi-fication profile of an individual, other ap-proaches are necessary.

The first analyses to identify detoxifi-cation pathways were challenge tests, in whicha known amount of a substance was ingestedand the amount of metabolite found in urineover a specified time period was collected andquantified (Table 5). This type of test takesinto account myriad factors influencing detoxi-fication activities and remains the standard to-day. The search for appropriate challenge sub-stances and better methods to interpret the in-fluence of the variety of detoxification activi-ties on overall health and well-being is thechallenge we face.

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Detoxification

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