Phase I Metabolism (Non-CYP450)

Question 1

Alcohol oxidation

Answer 1

Substrate: primary/ secondary alcohol
Enzyme: alcohol dehydrogenase
Cofactor: NAD+
product: aldehyde
byproduct: H+

Alcohol dehydrogenase catalyses the oxidation of ethanol to acetaldehyde, using NAD+ as a cofactor (which accepts an e- from the substrate/ ethanol and becomes reduced to NADH). A free H+ is released from the reaction as a byproduct.

Acetaldehyde- contributes to hangover symptoms (headaches, nausea)

Question 2

Alcohol oxidation (second reaction system)

Answer 2

Clears acetaldehyde toxicity by dehydrogenating acetaldehyde into acetic acid.

Enzyme: aldehyde dehydrogenase
Cofactors: NAD+ (accepts electron from the substrate/ acetaldehyde and becomes reduced to NADH), water
Product: acetic acid

Exploited therapeutically to treat alcohol dependency- disulfiram inhibits aldehyde dehydrogenase thus induces nausea upon consumption of alcohol

Question 3

N-oxidation

Answer 3

enzyme: flavin monooxygenase (FMO)
cofactors: [O], NADPH

substrate: tertiary amine
product: N-oxide

Question 4

Which drugs undergo FMO-mediated N-oxidation?

Answer 4

nicotine
Metabolite: nicotine-1-oxide

Question 5

Oxidative deamination

Answer 5

substrate: any of primary, secondary or tertiary amines
enzyme: monoamine oxidase
cofactor: O2

Sumatriptan - prodrug. Metabolite: active, indole-acetic acid derivative; migraine headache treatments (changing a tertiary amine group (whole) for a carboxylic acid group)

Question 6

Aldehyde oxidation

Answer 6

enzyme: aldehyde oxidase
cofactors: at first just H2O, then just O2
byproduct: H2O2

Question 7

What drugs undergo aldehyde oxidation?

Answer 7

Benzaldehyde, which is oxidised to an intermediate by aldehyde oxidase, using water. The intermediate is then oxidated to benzoic acid, catalysed by aldehyde oxidase again and requiring O2 this time. Results in a hydrogen peroxide byproduct (H2O2).

Question 8

Xanthine oxidation

Answer 8

Oxidation of hypoxanthine to xanthine (H2O, O2; product: xanthine, H2O2) and that of xanthine to uric acid (H2O. 02; product: uric acid, H2O2)

Uric acid at high levels will crystallise in the joints (especially common in big toe) - gout

Xanthine oxidase inhibitors given as treatment.

Question 9

What are potential problems of inhibiting xanthine oxidases?

Answer 9

Xanthine oxidases are also known (in rare cases) to catalyse the conversion of 6-mercaptopurine (highly cytotoxic drug given as anticancer) to 6-thioxanthine and 6-thioxanthine then to 6-thiouric acid (which is inactive). If xanthine oxidase is inhibited to treat gout, then nothing to convert 6-mercaptopurine, which instead builds up in the blood.

Question 10

Azo reduction

Answer 10

-N=N-
Cofactor: H+

Prontosil (prodrug) metabolism by colonic bacteria (‘intestinal microbe reductase’), split at its azo group in two: an inactive form, and an active bacteriostatic sulphanilamide.

Question 11

Nitro reduction

Answer 11

-NO2
Cofactor: H+

Nitrobenzene, which is reduced to aniline by the intestinal microflora nitroreductase enzyme. Gut microbiota involved in metabolism of drugs

Can also be CYP450 mediated, such as clonazepine (nitro group reduced to amino group by CYP450). This metabolises clonazepine to an inactive metabolite called 7-amino clonazepine (functionalised for Phase II Metabolism)

Question 12

Sulfoxide reduction

Answer 12

S=O
Cofactor: H+
enzyme: methionine sulfoxide reductase

Sulindac reduced to sulindac sulfide by methionine sulfoxide reductase.

Sulindac (prodrug) - NSAID for chronic inflammatory conditions

Question 13

Quinone and epoxide reduction

Answer 13

Vitamin K epoxide reduced to Vitamin K (epoxide reductase). Vitamin K reduced to Vitamin K hydroquinone (Vitamin K quinone reductase). Vitamin K hydroquinone goes on to produce clotting factors, therefore high levels of Vitamin K -> increased clotting factors.

Warfarin taken because it inhibits epoxide reductase and Vitamin K quinone reductase; Vitamin K epoxide cannot be reduced, so controlled clotting factors.

Question 14

Halogenated hydrocarbons

Answer 14

Eg, carbon tetrachloride.

Reduced by CYP450 and cofactor NADPH to a trichloromethyl radical that’s hepatotoxic and hepatocarcinogenic. This can be further metabolised to a chloroform molecule.

Carbon tetrachloride - in vivo animal studies as ‘archetypal toxin’, was used as WWI chemical weapon

Question 15

Ester hydrolysis

Answer 15

Breaks an ester bond with a molecule of water (hydroxide ion). Forms alcohol and carboxylic acid as products.

Cofactor: water !!!!!!!!!!!!!!!!!!!!!

Activation - heroin, or dimorphine. Carboxyl esterase 2 hydrolyses it in the presence of water to morphine (active form) and two acetic acid groups.

Inactivation - cocaine. Carboxyl esterase hydrolyses it in the presence of water to ecgonic methyl ester and benzoic acid.

Question 16

Amide hydrolysis

Answer 16

Cleavage of an amide bond within a given drug. Amide group hydrolysed to a carboxylic acid product, and an ammonia byproduct.

Procainamide - metabolised by carboxyl esterase and buturylcholinesterase in the presence of water. To product: P-aminobenzoic acid and byproduct: diethyl aminoethyl amine.

Question 17

Hydrolysis via alkaline phosphatase

Answer 17

Cofactor: water

Phosphenytoin (prodrug, hydrophilic) hydrolysed by alkaline phosphatase in presence of water and phosphoric acid to N-hydroxymethyl-phenytoin, which is then spontaneously converted to phenytoin (active, anti-epileptic) - which is lipophilic and is thus well-absorbed by tissues.

Question 18

Lactone hydrolysis

Answer 18

Cofactor: water
By paraoxanase enzymes (synthesised in the liver but transported out and into the plasma)

Lovastatin Lactone (prodrug, more lipophilic) is hydrolysed by paraoxanase enzymes in the presence of water to lovastatin (lactone group hydrolysed) - lovastatin (active, statin).