EXAM 5 - Drug Metabolism 3

Question 1

What 3 enzymes are involved with oxidation phase 1 reactions?

Answer 1

• alcohol dehydrogenase
• aldehyde dehydrogenase
• peroxidase-dependent co-oxidation
• Cytochrome P450

Question 2

Describe alcohol dehydrogenase oxidation reaction.

Answer 2

• cytosolic
• highest levels in the liver
• alcohol –> aldehyde

Question 3

Describe aldehyde dehydrogenase oxidation reactions.

Answer 3

• some people have inactive ALDH2 –> in the body ALDH2 converts ADH to acetic acid –> ADH builds up in the body –> flushing syndrome
• alcohol –> aldehydes and carboxylic acid

Question 4

Describe the mechanism of prostaglandin H-synthase during acetaminophen metabolism.

Answer 4

• reduces peroxide to alcohol
• oxidizes acetaminophen –> creates free radical (can damage the kidney)

Question 5

Describe Prostaglandin H-synthase enzyme of phase 1 oxidation.

Answer 5

Peroxidase-dependent Co-oxidation
* Metabolizes arachindonic acid

Two types of PHS: Cox-1 and Cox-2
* each have two components: cyclooxygenase and peroxidase

Question 6

What is the function of the cyclooxygenase component of Cox-1 and Cox-2?

Answer 6

Metabolizes arachidonic acid and two molecules of oxygen to form PGG2.

Question 7

What is the function of the peroxidase component of Cox-1 and Cox-2?

Answer 7

Reduces peroxide to alcohol PGH2.
* Oxidation of something (like acetaminophen)
* oxidation can cause toxicity

Question 8

What was the thinking process behind developing Vioxx as an anticancer drug?

Answer 8

The peroxidase reactions cause oxidation of compounds that produce toxic substances/free radicals –> may cause cancer.
* if a drug is designed to block the production of reactive compounds. –> prevent cancer

Question 9

Define cytochrome.

Answer 9

Red, heme-containing proteins.
* when bound to carbon monoxide, it absorbs light at 450 nm.

Question 10

What enzyme or class of enzymes is responsible for the majority of drug metabolism?

Answer 10

CYP450 enzymes.

Question 11

Explain the nomenclature of CYP450s.

Answer 11

Question 11

Explain the nomenclature of CYP450s.

Answer 11

Question 12

How many CYP genes/pseudogenes do humans have?

Answer 12

> 50 CYP genes
>50 pseudogenes

Question 13

List the types of reactions catalyzed by P450s.

Answer 13

• Hydroxylation of an aliphatic or aromatic carbon
• epoxidation of a double bond
• heteroatom (S-, N-, I-) oxidation and N-hydroxylation
• heteroatom (O-, S-, N-, and Si-) dealkylation
• oxidative group transfer [heteroatom (N, S, or halogen) is replaced with oxygen]
• cleavage of esters
• dehydrogenation

Question 14

Identify this type of reaction catalyzed by P450s.

Answer 14

Hydroxylation of an aliphatic or aromatic carbon.

Question 15

Identify this type of reaction catalyzed by P450s.

Answer 15

Epoxidation of a double bond.

Question 16

Identify this type of reaction catalyzed by P450s.

Answer 16

Heteroatom (S, N, and I) oxidation and N-hydroxylation.

Question 17

Identify this type of reaction catalyzed by P450s.

Answer 17

Heteroatom (O, S, N, and Si) dealkylation
* methanol is oxidized to formaldehyde

Question 18

Identify this type of reaction catalyzed by P450s.

Answer 18

Oxidative group transfer
* heteroatom (N, S, or halogen) is replaced with oxygen

Question 19

Identify this type of reaction catalyzed by P450s.

Answer 19

Cleavage of esters
* instead of alcohol and acid (esterase)
* you have aldehyde and acid (P450)

Question 20

Identify this type of reaction catalyzed by P450s.

Answer 20

Dehydrogenation

Question 21

What is located at the center of cytochrome P450 compounds?

Answer 21

Heme group (Fe3+ with 4 surrounding N molecules)
* heme held in the catalytic site through hydrogen bonding by surrounding carboxylic acid groups

Question 22

What amino acid is attached to the Fe center of CYP450s? What are they used for?

Answer 22

Cysteine forms covalent bond with Fe3+
* holds heme group inside P450

Question 23

Explain the catalytic mechanism cycle of cytochrome p450s.

Answer 23

• When Fe3+ is reduced to Fe2+, the P450 an bind ligands such as O2 and CO.
• starts with heme group w/ Fe3+ center –> substrate binds catalytic site near the Fe3+
• electron comes in and reduces Fe3+ to Fe2+ (electron comes from another enzyme (NADPH-CYP450 reductase)
• Fe2+ can now bind to oxygen (or CO)
• another electron comes in to reduce oxygen to peroxide
• water leaves
• substrate RH gets oxidized (hydroxylation step) –> ROH
• product ROH leaves

Question 24

What is the overall reaction equation of CYP450?

Answer 24

Question 25

Describe the relation between NADPH-CYP450 reductase enzymes and CYP450 enzymes?

Answer 25

1 NADPH-CYP450 reductase enzyme gives enough electrons for 10-20 CYP450 enzymes.
* in the ER membrane - facing the cytosol