L10 – non-P450 mediaed phase I metabolism

Question 1

What are the two main types of non-P450 Phase I metabolism reactions?

Answer 1

Oxidation reactions and hydrolysis reactions.

Question 2

Name the non-P450 enzymes involved in oxidation reactions.

Answer 2

Flavin-linked monooxygenases (FMOs), prostaglandin H-synthase-dependent co-oxidation, amine oxidases (like monoamine oxidase), and oxidoreductases (e.g. alcohol dehydrogenase, aldehyde dehydrogenase).

Question 3

Name the non-P450 enzymes involved in hydrolysis reactions.

Answer 3

Esterases and epoxide hydrolases.

Question 4

What are FMOs and how are they similar/different from CYP450s?

Answer 4

Flavin-linked monooxygenases are a multienzyme family in the ER. They are similar to CYP450s in function but produce different metabolites, are a smaller family, use FAD and NADPH instead of heme iron, and are less inducible.

Question 5

What are FMOs responsible for oxidising?

Answer 5

Nucleophilic centres in molecules, particularly nitrogen (N), phosphorus (P), and sulfur (S) atoms.

Question 6

What co-factors do FMOs require?

Answer 6

NADPH (electron donor) and O₂ (for oxygenation).

Question 7

Break down the step-by-step mechanism of FMOs.

Answer 7

1. FMO-FAD is reduced by NADPH to form FMO-FADH₂. 2. FMO-FADH₂ reacts with O₂ to make FMO-FAD-OOH (active oxygen species). 3. The substrate is oxygenated (gets an -OH or other O-containing group). 4. H₂O and NADP⁺ are released, and FMO returns to original FAD form.

Question 8

Which FMO isoform is inducible and by what?

Answer 8

FMO5 is inducible and is induced by rifampicin in hepatocytes (through PXR receptor).

Question 9

What is N-deacetyl ketoconazole metabolised into by FMOs?

Answer 9

It undergoes N-hydroxylation.

Question 10

What is tamoxifen metabolised into by FMO1?

Answer 10

Tamoxifen-N-oxide.

Question 11

Where is FMO1 found in highest levels?

Answer 11

Kidney.

Question 12

Where is FMO3 found in high levels?

Answer 12

Liver.

Question 13

What does prostaglandin H-synthase (PHS) do in co-oxidation reactions?

Answer 13

Converts arachidonic acid → PGG₂ (via COX activity), converts PGG₂ → PGH₂ (via peroxidase activity), and also oxidises xenobiotics in the same cycle → co-oxidation.

Question 14

How is PHS different from CYP450 and FMOs?

Answer 14

PHS does not require NADPH or O₂ as a cofactor and works during physiological prostaglandin synthesis.

Question 15

What are the 3 enzymes involved in peroxidase co-oxidation?

Answer 15

Prostaglandin H-synthase (PHS), myeloperoxidase, and lactoperoxidase.

Question 16

What are amine oxidases and what do they do?

Answer 16

Amine oxidases oxidise monoamines, diamines, and polyamines. Example: Monoamine oxidase (MAO) – key role in neurotransmitter breakdown.

Question 17

Where is MAO found and what cofactor does it use?

Answer 17

MAO is located in mitochondria and uses FAD (flavin adenine dinucleotide) as a cofactor.

Question 18

Explain the reaction mechanism of monoamine oxidase (MAO).

Answer 18

Amine (R–CH₂–NH₂) is oxidised → imine (R–CH=NH). Imine is hydrolysed → aldehyde (R–CHO) + ammonia (NH₃). FADH₂ is oxidised → FAD + H₂O₂ (hydrogen peroxide).

Question 19

What are the two forms of MAO and where are they located?

Answer 19

MAO-A: intestine (e.g. metabolises propranolol); MAO-B: liver.

Question 20

What is the role of oxidoreductases?

Answer 20

Oxidoreductases detoxify or activate molecules via oxidation/reduction, including alcohol dehydrogenase, aldehyde dehydrogenase, and carbonyl reductase.

Question 21

Describe alcohol dehydrogenase’s function.

Answer 21

Alcohol dehydrogenase converts alcohols → aldehydes, uses NAD⁺ as oxidising agent, and detoxifies ethanol in the liver.

Question 22

What happens after ethanol is converted to acetaldehyde?

Answer 22

Acetaldehyde is reactive/toxic and is further metabolised by aldehyde dehydrogenase to acetate (safe).

Question 23

What reaction does aldehyde dehydrogenase catalyse?

Answer 23

Aldehyde + NAD⁺ → Carboxylic acid (R–COOH) + NADH + H⁺.

Question 24

What enzyme is given to alcoholics to inhibit aldehyde dehydrogenase?

Answer 24

Disulfiram (Antabuse) leads to acetaldehyde buildup → makes drinking unpleasant.

Question 25

What is carbonyl reductase and what does it do?

Answer 25

Carbonyl reductase catalyses reduction of ketones (R–CO–R') to secondary alcohols and uses NADPH as reducing agent.

Question 26

What anticancer drug is metabolised by carbonyl reductase?

Answer 26

Doxorubicin is converted to doxorubicinol (less active metabolite).

Question 27

What is NAD(P)H quinone oxidoreductase (NQO1)?

Answer 27

NQO1 is an enzyme that detoxifies quinones, converts quinones to hydroquinones (non-toxic form), and bypasses the reactive intermediate (semiquinone).

Question 28

Why is avoiding semiquinone important in quinone detox?

Answer 28

Semiquinones produce free radicals → oxidative damage. NQO1 reduces the risk by going via 2e⁻ reduction (direct to hydroquinone).

Question 29

Where is quinone oxidoreductase expressed highly and why?

Answer 29

Quinone oxidoreductase is highly expressed in the skin to protect against UV-induced oxidative stress.

Question 30

What are esterase enzymes and what do they do?

Answer 30

Esterases cleave ester bonds in drugs, producing acid + alcohol, and are found in plasma and liver (cytosolic and microsomal).

Question 31

What are the 4 types of esterases?

Answer 31

1. Butyrylcholinesterase – e.g. breaks aspirin. 2. CES1 (Carboxylesterase 1) – prefers large acyl, small OH (in liver). 3. CES2 – prefers large OH, small acyl (in gut). 4. Paraoxonase (PON1) – handles statins, parathion, oxidised lipids.

Question 32

What is paraoxonase 1 (PON1)?

Answer 32

PON1 is a multi-functional enzyme that degrades oxidised lipids and breaks organophosphates (e.g. insecticides), found in liver and plasma.

Question 33

What do epoxide hydrolases do?

Answer 33

Epoxide hydrolases detoxify epoxides by adding water, making diols (non-toxic), and are especially important for benzo[a]pyrene metabolism.

Question 34

How many types of epoxide hydrolases are there?

Answer 34

There are two types: microsomal and cytosolic, differing in substrate specificity.

Question 35

Why is epoxide hydrolase important in benzo[a]pyrene metabolism?

Answer 35

It prevents formation of DNA-adduct-forming epoxides (carcinogenic).

Question 36

Which of the following best describes the cofactor requirements of Flavin-containing monooxygenases (FMOs)? A. Require NADPH and heme iron B. Require NADPH and FAD C. Require NADH and biotin D. Require ATP and Fe²⁺

Answer 36

B

Question 37

What is a key difference between FMOs and CYP450s? A. FMOs are mitochondrial while CYP450s are microsomal B. FMOs require heme iron while CYP450s do not C. FMOs are non-inducible (mostly), while many CYP450s are inducible D. FMOs perform conjugation while CYP450s perform hydrolysis

Answer 37

C

Question 38

Which enzyme is responsible for catalysing the oxidation of dopamine? A. Alcohol dehydrogenase B. Monoamine oxidase C. Epoxide hydrolase D. Paraoxonase 1

Answer 38

B

Question 39

In the MAO reaction, what is the immediate product of monoamine oxidation? A. Ammonia B. Imine C. Carboxylic acid D. Hydroxy acid

Answer 39

B

Question 40

Which reaction is catalysed by aldehyde dehydrogenase? A. Alcohol → Aldehyde B. Aldehyde → Carboxylic acid C. Ketone → Alcohol D. Ester → Acid + Alcohol

Answer 40

B

Question 41

What product is formed when doxorubicin is reduced by carbonyl reductase? A. Tamoxifen B. Acetaldehyde C. Doxorubicinol D. Glucuronide conjugate

Answer 41

C

Question 42

Which of the following enzymes contributes to detoxification by avoiding formation of a semiquinone intermediate? A. MAO B. FMO C. NAD(P)H quinone oxidoreductase D. CES2

Answer 42

C

Question 43

Which enzyme cleaves epoxides using water and helps prevent carcinogenic DNA adducts? A. CYP3A4 B. Aldehyde dehydrogenase C. Epoxide hydrolase D. Prostaglandin H synthase

Answer 43

C