**11.7. Mechanisms of phase II reactions**

The phase II reactions generally involve coupling of drug/drug metabolite with an endogenous substance to enhance their removal from the body. They require participation of specific transferase enzymes and high energy activated endogenous substances.

Most of the conjugation reactions result in detoxification of the drug although in some cases conjugation reactions result in bioactivation of drugs. The following is a summary of the different types of phase II biotransformation reactions;

Glu - Cys - Gly + drug X

mercapturic acid which is easily excreted from the body

can undergo glucoronidation by a transferase as shown below:

UDPGA OH transferase

R

*11.7.2. Glucuronidation*

OH

Benzoyl group

O

C

+

Glutathione - S - transferase

SH SX

H2O

Glu - Cys - Gly

Cys - Gly

SX

Acetyl CoA

CoA

**Figure 9.** Glutathione conjugation reactions for a drug with a suitable nucleophilic centre leads to the formation of

This is the conjugation of a drug or xenobiotic with glucuronic acid. Many functional groups are subject to glucuronidation. The benzoyl group in morphine, (an analgesic) and the amine group in meprobamate (a sedative) can undergo glucuronidation. A drug with a benzoyl group

HOOC

Glucuronyl

H

OH H

O O

Glutamyl transpeptidase Glutamylamino

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N- Acetyl - Cys - SX (Mercapturic acid)

> Benzoyl glucuronide

C

O

+ UDP

R

acid

Amino acid

Introduction to Biochemical Pharmacology and Drug Discovery

SX

Acetyltransferase

Cysteinylglycinase

Gly Cys

**Figure 8.** The transformation pathways for drugs with quinine moieties to generate free radicals.

#### *11.7.1. Glutathione conjugation*

Glutathione-S-transferases catalyze the enzymatic conjugation of xenobiotics with the endog‐ enous tripeptide glutathione, glutamylcystenylglycine (GSH). The xenobiotics with suitable electrophilic centres such as the epoxides and nitro groups can be subjected to nucleophilic attack by glutathione (Figure 9). The final product, mercapturic acid is easily excreted from the body.

**Figure 9.** Glutathione conjugation reactions for a drug with a suitable nucleophilic centre leads to the formation of mercapturic acid which is easily excreted from the body

#### *11.7.2. Glucuronidation*

Most of the conjugation reactions result in detoxification of the drug although in some cases conjugation reactions result in bioactivation of drugs. The following is a summary of the

NADPH

1e

NADPH cytochrome P450 reductase

NAD(P)H O.

2-

OH .

**Figure 8.** The transformation pathways for drugs with quinine moieties to generate free radicals.

Glutathione-S-transferases catalyze the enzymatic conjugation of xenobiotics with the endog‐ enous tripeptide glutathione, glutamylcystenylglycine (GSH). The xenobiotics with suitable electrophilic centres such as the epoxides and nitro groups can be subjected to nucleophilic attack by glutathione (Figure 9). The final product, mercapturic acid is easily excreted from

H2O2

<sup>O</sup> <sup>O</sup> O2

OH

NADP+

OH

semiquinine

different types of phase II biotransformation reactions;

O

O

detoxification

2 e-

NAD(P)<sup>+</sup>

Quinone

490 Drug Discovery

Hydroquinone

*11.7.1. Glutathione conjugation*

the body.

This is the conjugation of a drug or xenobiotic with glucuronic acid. Many functional groups are subject to glucuronidation. The benzoyl group in morphine, (an analgesic) and the amine group in meprobamate (a sedative) can undergo glucuronidation. A drug with a benzoyl group can undergo glucoronidation by a transferase as shown below:

#### *11.7.3. Epoxide hydration*

A number of aromatic compounds are transformed by phase I reactions to form epoxide intermediates. The epoxides are reactive electrophilic species that can bind covalently to proteins and nucleic acids to bring about toxic effects. These epoxides are detoxified via the nucleophilic attack of water molecule on one of the electron deficient carbon atoms of the oxizane ring as shown below:

*11.7.5. Methylation*

**reactions**

is used as an anticancer agent (Figure 11)

**Figure 11.** Methylation reactions leading to the formation of methylthiopurine

and initiates hepatotoxicity leading to hepatic necrosis.

**12. Adverse drug reactions associated with drug biotransformation**

Many adverse drug reactions can be traced to an improper balance between bioactiva‐ tion and detoxification reactions. For example, when the analgesic acetaminophen is giv‐ en at normal therapeutic doses, it undergoes glucuronidation and sulfation reactions that terminate the action of the drug and hasten its elimination. However, some of the drug is bioactivated via Cyt P450 to form N-acetylbenzoquinimine, a reactive intermediate that can be detoxified by conjugation with glutathione (GSH). When excessive doses of the drug are given, glucuronidation and sulfation reactions become saturated and more acet‐ aminophen is bioactivated via Cyt P450. This imbalance leads to high concentrations of Nacetylbenzoqunonine which cannot be sufficiently eliminated by the limited concentrations of gluthathione. This metabolite binds covalently to cellular protein thiols

Most of the methyl transferases are cytosolic enzymes. They utilize S-adenosyl methionine (SAM) as the methyl donor. The final metabolite, thiopurine, has antineoplastic properties and

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The glucuronide conjugates can be excreted via the bile or urine.

#### *11.7.4. Acetylation*

Acetylation is achieved by cytosolic enzymes known as N-acetyl transferases which catalyze transfer of acetate from acetyl co-enzyme A to primary aromatic amine or hy‐ drazides (figure 10)

**Figure 10.** Acetylation reations leading to the formation of N– Acetylsulfanilamide, the final metabolite of the antimi‐ crobial agent sulfanilamide which is secreted from the body.
