Xenobiotics

Dr.  Atif H. Khirelsied

Faculty of MedicineInternational University of Africa

Khartoum, Sudan

Introduction to XenobioticsIntroduction to Xenobiotics

H d i f i h i l• Humans are exposed to various foreign chemicals e.g., drugs, food additives, environmental pollutants

• Xenos = strange, foreign

• Knowledge of xenobiotic metabolism is essential to d t di funderstanding of:

– Pharmacology and therapeuticsD i t ti– Drug interactions

– Toxicology

Xenobiotics metabolismXenobiotics metabolism

• Occurs mainly in the ER of the liver.

• Involves 30 different enzymes.Involves 30 different enzymes.

• Occurs in two phases of reactions– Phase I reactions

– Phase II reactions

• The overall purpose of the two phases is to increase polarity (water solubility) andincrease polarity (water solubility) and enhance excretion.

Xenobiotics metabolismXenobiotics metabolism

• In some cases it may increase toxicity or carcinogenicity.g y

D ifi i i i i l d• Detoxification is inappropriately used term

Examples of phase I reactionsExamples of phase I reactions

• Hydroxylation

• DeaminationDeamination

• Dehalogenation

• Desulfuration

• EpioxidationEpioxidation

• Peroxygenation

• Reduction

Examples of phase II reactionsExamples of phase II reactions

• Conjugation with glucuronic acid

• Conjugation with glutathioneConjugation with glutathione

• Sulfation

• Acetylation

• MethylationMethylation

Examples of phase I reactions

• Hydroxylation

B PhenolBenzene Catechol

Aniline Parahydroxy aniline

Examples of phase I reactions

• OxidationOxidation

BenzaldehydeToluene Benzoic acidBenzaldehydeToluene Benzoic acid

Examples of phase I reactions• Oxidation

i idMuconic acid

Catechol

β Ketoadipic acidβ‐Ketoadipic acid

+

Succinic acidAcetyl‐CoA

Examples of phase I reactions• Oxidation

Salicylic acid Gentisic acid

Homogentisic acidPhenylacetic acid

Examples of phase I reactions• Concomitant oxidation‐reduction

p‐Nitrobenzaldehyde p‐Aminobenzoic acid

Examples of phase I reactions• Deamination

A i iArginine

Examples of phase I reactions• Decarboxylation

Cadaverine

iLysine 

Examples of phase I reactionsExamples of phase I reactions

• Hydroxylation

• DeaminationDeamination

• Dehalogenation

• Desulfuration

• EpioxidationEpioxidation

• Peroxygenation

• Reduction

Isoenzymes of cytochrome PIsoenzymes of cytochrome P450

• They are mono‐oxygenases found in the liver ER.

• They catalyze hydroxylation of reactions.

• Comprises 11 families of enzymes.p y

• Heme containing proteins that give peak absorbance at 450 nm.

The overall  reaction catalyzed by CYP450

• RH  +  O2 + NADPH+H → R‐OH + H2O + NADP2 2

• RH represents various substances including:• RH represents various substances including:– Pesticides– Petroleum products– Pollutants, e.g., polychlorinated biphenyls (PCB).– Endogenous substances e.g., steroid hormones, eicosanoids, fatty acids, retinoids

Cytochrome P NomenclatureCytochrome P450 Nomenclature

• CYP  = cytochrome P450• Arabic number = family (40% homology)Arabic number   family (40% homology)

• Letter = subfamily (55% homology)

• Arabic number  = Individual enzyme

• Example, CYP1A1 

Important features of cytochrome P450enzymes 

h h i1. They are hemoproteins

2. Highly versatile and diverse

3. Widely distributed across species, including bbacteria.

4. Highly concentrated in the SER of liver cells. 

Important features of cytochrome P450enzymes 

5. A large number of isoforms (about 150) discovered.

6 C i l f ili (40% i il i )6. Constitute several families (40% similarity) and subfamilies (55% similarity).

7 Th NADPH t d b t7. They use NADPH to reduce substances.

Important features of cytochrome P450enzymes 

8. Certain types of CYP450 exist in the mitochondria and use NADP‐linked flavoproteincalled adrenodoxin reductase, and iron‐sulfur containing protein adrenodoxin.g p

9 Th t i li id i l h h tid l h li9. They contain lipids, mainly phosphatidylcholine.

10.Most enzymes are inducible.

Important features of cytochrome P450enzymes 

11.Certain types of CYP450 are involved in the metabolism of carcinogens e.g. CYP1A1.

CYP1A1 metabolize polycyclic Aromatic Hydrocarbons (PAH)

Smokers have high levels of these enzymes

12.They are polymorphic proteins

Polymprohism of CYPPolymprohism of CYP450

• Polymorphic forms have different kinetic properties e.g., CYP2D6.p p g

CYP2D6 b li d b i i• CYP2D6 metabolizes debrisoquin(antihypertensive) and sparteine(antiarrythemic)– Extensive metabolizers (rapid clearance)Extensive metabolizers (rapid clearance)

– Poor metabolizers (slow clearance)

Phase II reactionsPhase II reactions

• Glucuronidation

• Uses UDP‐GlucuronateUses UDP Glucuronate– Enzyme gucuronyl transferase

S b i l d b i id ili– Substrates include: benzoic acid, aniline, meprobamate, phenol, steroids.

Phase II reactionsPhase II reactions• Sulfation

– Uses adenosine‐3‐phospho‐5‐pyrophosphate (PAPS) as sulfate donor( )

Substrates include: alcohols arylamides steroids– Substrates include: alcohols, arylamides, steroids, phenols.

Phase II reactions

• Sulfation

Phase II reactionsPhase II reactions

j i i h l hi ( )• Conjugation with glutathione (GSH)– Uses glutathione (γ‐glutamyl‐cysteine‐glycine).

– The reaction is catalyzed by glutathione‐S‐y y gtransferase.

– Glutathione has other important functions• Decomposition of H2O2p• Important intracellular reductant• Amino acid transport in the kidney

Phase II reactions• Conjugation with glutathione (GSH)• Conjugation with glutathione (GSH)

Phase II reactions

• Conjugation with glycine

Phase II reactionsPhase II reactions

• Acetylation– Uses acetyl‐CoA as acetate donor.y

The reaction is catalyzed by acetyl transferase– The reaction is catalyzed by acetyl transferase.

– Substrates include:• Isoniazid (anti‐tuberculosis)

Phase II reactionsPhase II reactions

• Acetylation

AC‐HN

Sulfonamide  N‐acetyl sulfonamide 

Phase II reactionsPhase II reactions

• Methylation– Uses S‐adenosyl‐methionine as methyl donor.y y

The reaction is catalyzed by methyl transferase– The reaction is catalyzed by methyl transferase.

– Substrates include:• Isoniazid (anti‐tuberculosis)