Xenos

Xenobiotic Compound that is foreign to body

Stranger

As part of normal metabolism, body produces

toxins which have to be eliminated.

Humans are constantly exposed to

exogenous & endogenous toxins.

Biotransformation is the process whereby a

substance is changed from one chemical to

another by a chemical reaction in body.

Definition:

Detoxification is the process by which toxic

substances are converted into more soluble &

less toxic substances, which are mainly

eliminated through urine & bile.

Takes place in the “liver”.

The compounds to be detoxified include

Drugs – antibiotics, cardiac drugs, steroids, etc.

Carcinogens – food dyes, preservatives,

artificial sweeteners, alcohols, chemicals &

cosmetics etc.

Many endogenous & Xenobiotics are lipophilic.

They can easily cross lipid bilayers & transported by

lipoproteins.

Metabolism of endogenous compounds & xenobiotics

allows organisms to convert lipophilic compounds to

more water soluble forms which facilitates excretion

Many xenobiotic compounds contain aromatic rings &

heterocyclic ring structures, that we are unable to

degrade or recycle.

These structures are hydrophobic

Metabolism of foreign compounds occurs as

a results phase-I & phase-II reactions.

Phase-I reactions are

Oxidation

Reduction

Hydrolysis

Phase II is conjugation reactions of phase I

compounds.

Direct conjugation can also occur.

The cytochrome P450 is involved.

Approximately 50% of the drugs are

metabolized by isoforms of cytochrome P 450

Use of O2 that one atom of oxygen enters R-

OH and one atom enters water

This dual fate of Oxygen accounts for former

naming of mono-oxygenases as mixed

function oxidases

Conjugating agent Active form

Glucuronic acid UDP-glucuronic acid

Sulfate PAPS

Cysteine Glutathione

Acetic acid AcetylCoA

1.Oxidation

2.Reduction

3.Hydrolysis

Phase I metabolism adds functional group on

the molecule & increases water solubility.

Phase II metabolism adds an endogenous

substrate to produce a water-soluble

conjugate & is easily excreted.

Phase I activity is located in microsomal

fraction of the cell.

Phase II activity is located in the cytosolic

fraction of the cell.

Liver

Kidneys

Gastrointestinal tract

Skin

Testes

Ovaries

Adrenals

Placenta

Detoxification by oxidation:

Foreign substances - alcohols, aldehydes,

amines, anilides, aromatic hydrocarbons &

certain drugs are destroyed in the body by

oxidation.

It occurs predominantly in liver & kidney.

CYT P450, ADH, Aldehyde DH & mono amine

oxidase are involved.

Oxidation of Alcohols:

These are oxidized to corresponding acids.

CH3-CH2-OHEthyl alcohol

NAD+ NADH + H+

CH3-CHOAcetaldehyde

Benzyl alcohol Benzoic acid

Ethanol Acetic acid

Methyl alcohol Formic acid

Oxidation of Aldehyde:

Oxidized to corresponding acids.

CH3-CHOAcetaldehyde

NAD+

-CHO -COOH

Benzaldehyde Benzoic acid

NADH + H+

CH3-COOHAcetic acid

NAD+ NADH + H+

Detoxification of Amines:

These are oxidized to corresponding acids.

Benzylamine Benzoic acid + urea

Chloral Trichloroacetic acid

Aromatic hydrocarbons:

These are oxidized to phenols or phenolic

compounds, which are then conjugated with

sulphuric & glutamic acids.

Anilides:

These are oxidized to corresponding phenols

Acetanilide Aminophenol

Drugs:

Chloral is partly oxidized to trichloroacetic

acid, which excreted as its salts.

Sulfur compounds:

Organic sulfur is oxidized to sulfuric acid.

Certain aldehydes e.g., chloral, undergoes

reduction to form corresponding alcohol,

which is then conjugated with D-glucuronic

acid & excreted as corresponding

glucuronides.

Chloral Trichloroethanol + D-glucuronicacid (excreted as corresponding

glucuronides)

OHI

-NO2

INO2

O2N-

OHI

-NH2

INO2

O2N-H2O

Picric acid Picramic acid

Aromatic nitro compounds, e.g., p-

nitrobenzaldehyde is reduced to

corresponding amines & excreted after

conjugation

3. Hydrolysis

The hydrolysis of bonds - ester, glycoside

& amide.

Usually occurs in liver.

Acetanilide

Atropine

Procaine

Aspirine Salicylic acid + Aspirin acid

Aniline + Acetic acid

Tropic acid + tropine

PABA + Diethylaminoethanol

Definition:

It is a process by which the foreign molecules

or its metabolites are coupled with a

conjugating agent & converted to soluble,

nontoxic derivatives & easily excreted in urine.

Features:

Conjugating agents are available in the body &

some of them are synthesized in the body.

D-glucuronic acid

Certain amino acids as glycine, cysteine.

Conjugation occurs mainly in liver & some

extent in kidney

Conjugation produces less toxic, more

soluble compounds which are excreted..

Conjugation occurs independently or it can

follow oxidation, reduction or hydroxylation

1.Methylation

2.Acetylation

3.Conjugation with Sulfuric acid

4.Conjugation with D-glucuronic acid

5.Conjugation with Amino acids

6.Conjugation with Glutamine

7.Conjugation with Glutathione

Detoxification by methylation is limited

Usual methyl donor is SAM (active methionine)

Methylation of heterocyclic N- atom compounds of the

pyrimidine & Quinoline types. e.g, Nicotin amide

-CONH2-CONH2

CH3NNicotinamide N Methyl Nicotinamide

SAM

Acetyl coA, takes part in conjugation reactions

Conjugation with acetic acid occurs only with

aromatic amino groups

Sulpha drugs are conjugated by acetylation &

excreted as acetylated derivatives.

SulfanilamideAcetyl CoA

PABAAcetyl CoA

Acetylated sulfanilamide

Acetyl derivative of PABA

• Acetylation is catalyzed by acetyl transferase.

PAPS is the sulfate group donor

Sulfuric acid is used for detoxification of phenolic &

hydroxyl groups

Substances like phenol, cresol, indole are formed in

the gut by the action of intestinal bacteria are

absorbed & transported to liver, where they are

conjugated with sulfate to form Ethereal sulfates,

which are excreted in urine, less toxic & more acidic.

Phenol

Active sulfate

OHI

O II

O-S-OI II

O

Phenyl sulfuric acid

It is most important & commonest

detoxification process.

D-glucuronic acid is participated in this

reaction, as its active form UDP-glucoronic

acid which is formed in uronic acid pathway

Bilirubin to form bilirubin diglucuronide

Aromatic acids, e.g. benzoic acid

Phenols & other secondary & tertiary aliphatic

alcohols.

Certain drugs like morphine, menthol,

pyramidon, acetalinide, sulfa pyridine, etc.

Antibiotics like chloramphenicol

Hormones:

Thyroid hormones & derivatives of steroids.

Glucuronic acid form two types of linkages

An ether linkage, e.g. in phenyl glucuronide

An ester linkage, e.g. in benzoyl glucuronide

Formation of glucuronides play an important

role in detoxification of exogenous &

endogenous compound & their excretion as

corresponding glucuronides.

Glycine combines with harmful substances -

aromatic carboxylic acids to form harmless

derivatives which are excreted in urine.

Ex. Benzyl Co A is conjugated with glycine to

form hippuric acid.

Benzyl CoA + Glycine Hippuric acid

Nicotinic acid + Glycine Nicotinuric acid

L-Cysteine: Aromatic compounds are

conjugated with L-cysteine in the presence of

acetic acid to form mercapturic acids.

Bromobenzene + L-cysteine + acetic acid

Bromophenyl mercapturic acidBromobenzene + L-cysteine + acetic acid

Naphthyl mercapturic acid

Glutamine conjugates phenyl acetic acid to

form phenylacetyl glutamine & excreted in

urine

Accounts for mousy odour of urine in PKU.

Phenyl acetic acid + Glutamine

Phenyl acetyl Glutamine

It is a tripeptide.

Potentially toxic electrophilic xenobiotics

(carcinogens) are conjugated to the

nucleophilic GSH

Glutathione S-transferase is involved.

Present in liver cytosol & other tissues.

Exhibit different substrate specifities

P 450 = absorption peak at 450 nm

The cytochrome P450 contain large group of enzymes.

CYPs are major enzymes involved in drug metabolism.

The most common reaction catalyzed by cytochrome

P450 is a monooxygenase reaction (Oxidation), e.g.

insertion of one atom of oxygen into an organic

substrate (RH) while the other oxygen atom is reduced

to water.

RH + O2 + 2H+ + 2e–→ ROH + H2O

Oxygen

NADPH

Embedded in lipid bilayer next to

cytochrome P450 oxido-reductase.

Monooxygenase is also called as mixed

function oxidase, is associated with

microsomes.

About 150 isoforms are present.

CYP denotes a cytochrome P450

CYP1A1

CYP=CytochromoP450

1=Family

A=Subfamily

1=is the first individual member of that

subfamily.

Involved in phase I of the metabolism.

Involved in the metabolism of many endogenous

compounds (e.g. steroids).

All are hemoproteins.

Exhibit broad substrate specificity.

Extremely versatile catalysts, catalyzing about 60

types of reactions.

They catalyze reactions involving introduction of one

atom of oxygen into the substrate & one into water.

Their hydroxylated products are more water soluble

& facilitating their excretion.

Present in liver & also in intestine, brain & lungs.

Located in smooth endoplasmic reticulum or in

mitochondria

In some cases their products are mutagenic or

carcinogenic

Many have a molecular mass of about 55kDa.

Their activities may be altered in diseased tissues

(e.g. cirrhosis), affecting drug metabolism.

Harper’s Biochemistry – 25th edition

Medical Biochemistry – AR Aroor

Text book of Biochemistry – DM Vasudevan