PHASE I and II METABOLISM

Metabolism

plays a central role in the elimination of drugs and other foreign compounds xenobiotics from the body

Liver

Main site of metabolism and detoxification of endo/exogenous compounds. Rich in almost all of drug-metabolizing enzymes

PO drugs

pass thru the liver before being further distributed in body compartments

First-Pass Effect

Metabolism before reaching systemic circulation. Limit the BA of orally administered drugs

Extrahepatic Metabolism

Intestine, kidney, lungs, adrenal glands, placenta, brain, skin. Substrate specific.

Oxidation

Reduction

Hydrolysis

polar functional groups are introduced into the molecule or unmasked by:

1. Oxygen is

incorporated into the

drug molecule

2. Oxidation causes the

   loss of part of the

   drug molecule

Two types of oxidation reactions:

hydroxylation

Oxygen is incorporated into the drug molecule example

oxidative

deamination,

dealkylation

Oxidation causes the loss of part of the drug molecule example

Flavoprotein (NADPH–Cytochrome P450 reductase)
Cytochrome P450 (CYP450).

What are the two main enzyme components involved in the Phase I oxidation system?

It transports electrons from NADPH to Cytochrome P450.

What is the primary role of flavoprotein in Phase I metabolism?

Flavin Mononucleotide (FMN) and Flavin Adenine Dinucleotide (FAD).

What cofactors are found in flavoprotein?

NADPH.

What is the source of electrons used in Phase I oxidation?

It accepts electrons and catalyzes the oxidation of drugs.

What is the primary role of Cytochrome P450?

Iron (Fe)

What metal ion is found in Cytochrome P450?

Ferrous (Fe²⁺) and Ferric (Fe³⁺).

Between which oxidation states does the iron in Cytochrome P450 alternate?

NADPH → Flavoprotein (FMN & FAD) → Cytochrome P450 → Drug Oxidation.

Arrange the flow of electron transfer during Phase I metabolism.

CYP3A4

Responsible for metabolism of over 50% of prescription drugs metabolized by the liver

CYP3A4.

Which CYP isozyme metabolizes the largest number of drugs?

APAP
Lidocaine
Lovastatin
Erythromycin
Quinidine
Verapamil

Enumerate the substrates of CYP3A4.

Codeine
Dextromethorphan
Oxycodone
Paroxetine

Enumerate the substrates of CYP2D6

Taxol

Retinoic Acid Derivatives

Enumerate the substrates of CYP2C8.

Losartan
Ibuprofen
Phenytoin
Celecoxib
Diclofenac
Tolbutamide

Enumerate the substrates of CYP2C9.

microsomes

Vesicle-like structures formed in vitro after cell homogenization and fractionation of the endoplasmic reticulum (ER).

Rough microsomes

_______ are primarily associated with protein synthesis

Smooth microsomes

______ contain a class of oxidative enzymes called (Mixed Function Oxydase) MFOs

NADPH (reducing agent)

Molecular oxygen (O₂)

What are the two requirements of the Mixed Function Oxidase (MFO) system?

Cytochrome P450

What enzyme system plays the major role in Phase I oxidation?

NADPH.

What is the source of electrons during the oxidative pathway?

arene oxide

What intermediate is formed during aromatic hydroxylation?

Arenol (phenol).

What does the arene oxide rapidly rearrange into?

Reactive Oxygen Species (ROS).

What potentially harmful species can be formed from the arene oxide intermediate?

• Propranolol

• Phenobarbital

• Amphetamine

• Phenytoin

• Atorvastatin

Name at least 5 examples of phenyl-containing drugs that undergo aromatic hydroxylation in humans.

Clonidine (Catapres)
Probenecid (Benemid)

Which drugs are resistant to aromatic hydroxylation due to deactivating groups, and what are those groups?

• Protriptyline (Vivactil)

• Cyproheptadine (Periactin)

Which two closely related tricyclic drugs undergo olefinic epoxidation specifically at their 10, 11 position double bonds?

• Glucuronidation

• Sulfation

• Glycine/Amino acid conjugation

• Glutathione (GSH) conjugation

• Acetylation

• Methylation

What are the major Phase II reactions?

• Methylation

• Acetylation

Which Phase II reactions do NOT significantly increase water solubility?

UDP-glucuronic acid (UDPGA).

What activated coenzyme is used in glucuronidation?

UDP-glucuronyltransferase (UGT).

Which enzyme catalyzes glucuronidation?

Liver

Where are UDP-glucuronyltransferases found?

• Phenols

• Alcohols

• Enols

Which functional groups undergo glucuronidation?

• Morphine

• Acetaminophen

• Propranolol

Give examples of drugs that undergo glucuronidation.

Immature glucuronidation enzymes → drug accumulation → toxicity.

Why are neonates at risk during glucuronidation?

Neonatal hyperbilirubinemia

Gray Baby Syndrome

Which diseases are associated with deficient glucuronidation?

Produce water-soluble inactive metabolites.

What is the goal of sulfation?

PAPS (3'-Phosphoadenosine-5'-phosphosulfate).

What activated coenzyme is used in sulfation?

Sulfotransferases.

Which enzyme performs sulfation?

• Thyroxine

• Steroids

• Heparin

Which endogenous compounds undergo sulfation?

O-glucuronide conjugate.

In adults, what is the major metabolite of acetaminophen?

O-sulfate conjugate.

In infants (3–9 years), what is the major acetaminophen metabolite?

• Glycine

• Glutamine

Which amino acids are commonly used in conjugation?

Aromatic and arylalkyl carboxylic acids.

Which substrates usually undergo glycine conjugation?

Glutathione—a tripeptide:

Glutamate

Cysteine

Glycine

What is GSH?

Glutathione S-transferase (GST).

Which enzyme catalyzes GSH conjugation?

• Benzyl chloride

• Allyl chloride

• Methyl iodide

Which toxic chemicals are detoxified by GSH?

Acetyl-CoA.

Which coenzyme provides the acetyl group?

N-acetyltransferase.

Which enzyme catalyzes acetylation?

• Sulfonamides

• Hydrazines

• Hydrazides

Which drugs commonly undergo acetylation?

S-adenosylmethionine (SAM).

Which coenzyme is used in methylation?

Catechol-O-methyltransferase (COMT).

Which enzyme methylates catecholamines?

• Age

• Genetics

• Sex

What factors affect metabolism?

• Phenobarbital

• Cabbage

• Cauliflower

Name examples of enzyme inducers.

• Chloramphenicol

• Isoniazid

• Disulfiram

• Grapefruit

Name examples of enzyme inhibitors.

Cats and Pigs

Which animals lack glucuronyltransferase?

• Hypothyroidism

• Viral Infection

• Cancer

Which diseases impair drug metabolism?