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Biotransformation of Toxicants
Lect 6
Metabolism (Biotransformation)
 Many xenobiotics undergo chemical transformation
(biotransformation; metabolism)
 Biotransformation is often mediated by enzymes
 Alteration of the parent molecule, or conjugation of
the parent molecule (or its metabolites) with
endogenous substances
 Example: Cholinesterase metabolize the local
anesthetic agent procaine and the muscle-paralyzing
agent succinylcholine.
Types of Biotransformation Reactions
 Nonsynthetic (Phase I) and synthetic (Phase II)
 Phase I reactions:
o Modification of the basic structure of the substrate
o No covalent binding of the substrate to an endogenous
compound
o Examples include hydrolysis, oxidation, and reduction
reactions
o Phase I enzymes are often membrane-bound (e.g.,
microsomal).
Oxidation
 Uses molecular oxygen (O2); one O with H, 2nd O
with substrate
 Involves several enzymatic steps
 The oxidative system is often known as the “mixed
function oxidase” system”.
 One subfamily of the mixed function oxidase system
is the group of enzymes known as Cytochrome P450 enzymes.
 Cytochrome P-450 is a heme-containing cytochrome
protein located in ER
Examples of Oxidation Reactions
 Deamination – replacement of an amine group
(NH2) with an oxygen (O) atom
 N-, O-, or S-Dealkylation – replacement of an alkyl
group (e.g., CH3) with a hydrogen atom. Typically,
the alkyl group in the parent molecule is bonded to
a N, O, or S atom.
 Aliphatic or aromatic hydroxylation – addition of a
hydroxyl group (OH) to a molecule
 N-oxidation – replacement of a hydrogen atom on
an amine with an oxygen
Continue
 S-oxidation – addition of an oxygen atom to a sulfur
atom
 Conversion of a hydroxyl group (alcohol) to a
carboxyl group (acid)
Reduction
 Azo reduction – reduction of an azo bond (N=N) to
two amines (NH2)
 Nitro reduction – reduction of a nitro group (NO2)
to an amine
Hydrolysis
 Addition of water (H2O) to an ester bond (CO-O-C)to
form an alcohol (C-OH) and a carboxylic acid (COOH)
 R-CO-O-C-R + H-O-H
ROH + R-COOH
Phase II Reactions
 Involve addition of a cofactor to a substrate to form
a new product.
 Phase II enzymes may be either microsomal or
cytosolic.
 Various factors can affect the availability of
cofactors. For example, fasting markedly reduces the
amount of glutathione available in the liver.
 Sulfation,Glucuronidation, Acetylation, Methylation,
Glutathione conjugation, Amino acid conjugation
and Mercapturic acid formation are examples of
Phase II Reactions.
Sulfation
 Replacement of a hydrogen atom (H) with a sulfate
(SO3)
 Uses the enzyme sulfotransferase
 Uses the cofactor called PAPS (phosphoadenosine
phosphosulfate)
 Produces a highly water-soluble sulfuric acid ester
Glucuronidation
 Replacement of a hydrogen atom with a glucuronic
acid
 Uses the enzyme UDP-glucuronosyl transferase
(UDP-GT)
 Uses the cofactor called
diphosphate glucuronic acid)
UDPGA
(uridine
Acetylation
 Replacement of a hydrogen atom with an acetyl
group
 Uses the enzyme acetyltransferase
 Uses the cofactor called acetylCoA (acetyl coenzyme
A)
Methylation
 Replacement of a hydrogen atom with a methyl
group
 Uses the enzyme methyltransferase
 Uses the cofactor
methionine)
called
SAM
 Common but relatively minor pathway
(S-adenosyl
Glutathione Conjugation
 Adds a glutathione molecule to the parent
compound, either by direct addition or by
replacement of an electrophilic substituent (e.g., a
halogen atom)
 Uses the enzyme glutathione transferase (GST)
 Uses the cofactor called glutathione (a tripeptide
madeup of glycine, cysteine, and glutamic acid)
 One of the major Phase II enzymatic pathways
 Amino acid conjugation; Adds an amino acid to the
parent compound.
 Mercapturic acid formation; Formed by cleavage of
the glycine and glutamic acid substituents from a
glutathione conjugate, followed by N-acetylation of
the resulting product
Significance of Biotransformation
 Major part of the pathway for elimination of many
xenobiotics
 Decrease or an increase (or no change) in toxicity
 Formation of reactive metabolites.
Metabolism of Acetaminophen
Acetaminophen
Phase II
Phase I
Liver
Liver
Glucuronidation
Sulfation
Toxic Quinoneimine
Covalent Binding
Non Toxic Conjugates
Macromolecules
Activity of Drug Metabolizing Enzymes