Phase 1 and 2 metabolic conversions and CYP enzymes

Question 1

what are CYP enzymes

Answer 1

• contain a heme group
• membrane bound and associated with p450 reductase
• large lipophilic cavity
• no specificity

Question 2

p450 reductase role

Answer 2

NADPH binds to p450 and electrons are shuttled through

Question 3

6 phase 1 cyp mediated reactions

Answer 3

hydroxylation, dealkylation, dehydrogenation, epoxidation, epoxidation and dihydroxylation, monohydroxylation

Question 4

where can hydroxylation take place

Answer 4

at any carbon

Question 5

MOA of hydroxylation

Answer 5

1. drug enters enzyme
2. Fe5+ is very reactive and takes e- from =O to form fe4+
3. radical formed, picks up OH from cyp 450
4. Fe3+ is reformed

Question 6

where can dealkylation take place

Answer 6

at any carbon next to a heteroatom

Question 7

MOA of dealkylation

Answer 7

1. hydroxylation takes place
2. OH bond of hydroxylation breaks, forming =O
3. =O breaks the bond with the rest of the molecule

Question 8

where can dehydrogenation take place

Answer 8

alkanes

Question 9

MOA of dehydrogenation

Answer 9

1. radical formed by hydroxylation
2. radical forms double bond
3. elimination reaction reduces Fe4+ and removes one group

Question 10

where can epoxidation take place

Answer 10

alkenes

Question 11

epoxidation MOA

Answer 11

1. double bond breaks to form a radical and breaks double bond in Fe5+=O
2. radical reduces Fe4+ x2
3. 2nd bond formed between radical and oxygen

Question 12

where can epoxidation and dihydroxylation of aromatics take place

Answer 12

on aromatic rings with no substituents

Question 13

MOA of aromatic epoxidation and monohydroxylation

Answer 13

1. pi bond forms covalent bond with O
2. reduces Fe5+ and forms C radical
3. radical binds to O to form epoxide and reduce Fe4+
4. epoxide hydrolase breaks epoxide

Question 14

where can monohydroxylation take place

Answer 14

on aromatic rings with 2 unsubstituted carbons

Question 15

monohydroxylation MOA

Answer 15

1. hydroxylation forms a radical
2. radical reduces Fe4+ and forms bond to O
3. Fe3+ breaks bond with O causing shift in benzene
4. =O finds free H+ to form OH

Question 16

phase 1 non-cyp metabolism

Answer 16

epoxide hydrolase and ethanol metabolism

Question 17

why are epoxides dangerous

Answer 17

covalently bond to N7 guanine in bases causing mutations and caner (apurinic DNA)

Question 18

epoxide hydrolase MOA

Answer 18

catalyses nucleophilic addition on presence of water (-O to 2x -OH)

Question 19

ethanol metabolism MOA

Answer 19

ethanol - alcohol dehydrogenase -> ethanal - aldehyde dehydrogenase -> ethanoic acid

Question 20

what is alcohol aversion therapy

Answer 20

disulfiram - inhibits aldehyde dehydrogenase leading to an increase in ethanal = increase in side effects

Question 21

other drugs that inhibit alcohol dehydrogenase

Answer 21

metronidazole, nitrofurantoin

Question 22

phase 2 metabolism reactions

Answer 22

glucuronidation, sulphation and glutathione conjugation

Question 23

what is glucuronidation

Answer 23

metabolite is conjugated with glucuronic acid

Question 24

MOA of glucuronidation

Answer 24

UDP + glucuronic acid cleaved by UDP-glucuronyl transferase and glucuronic acid is attached to an electron rich atom in the drug (N,O,S)

Question 25

what is sulphation

Answer 25

addition of SO3- to a drug, catalysed by sulfotransferase, requires PAPS

Question 26

what is PAPS

Answer 26

SO4 2- + ATP –> PAPS + ADD + PPI (3’phosphoadenosine-5’phosphosulfate)

Question 27

which molecules can undergo sulphation

Answer 27

usually phenols, sometimes alcohols

Question 28

what is glutathione conjugation

Answer 28

addition of glutathione to the molecule to neutralise it

Question 29

glutathione structure

Answer 29

gly-cys-glu

Question 30

MOA of glutathione conjugation

Answer 30

S in Cys is a strong nucleophile, with the help of glutathione-s-transferase, it is conjugated to the molecule

Question 31

factors affecting drug metabolism

Answer 31

• drug metabolising enzyme polymorphisms
• drug - drug interactions
• age
• gender
• disease

Question 32

how do enzyme polymorphisms affect drug metabolism

Answer 32

• SNIP - single nucleotide polymorphism can change the protein structure of the enzyme causing, no, increased or decreased enzyme activity

Question 33

how do drug-drug interactions affect drug metabolism

Answer 33

they can inhibit or induce the effects of other drugs

Question 34

how do drug-drug interactions inhibit metabolism

Answer 34

• CYP450 inhibition!
• by competitive/non-competitive inhibition, formation of inactive complexes or cyp450 destruction

Question 35

how do drug-drug interactions induce metabolism

Answer 35

induce the enzyme to work faster

Question 36

how does age affect drug metabolism

Answer 36

very young - not fully metabolically competent, virtually no phase 2 and limited phase 1
very old - decreased action, diminished enzyme induction and usually drug-drug interactions

Question 37

how does gender affect metabolism

Answer 37

first observed in rats, males metabolise faster - less significant in humans but thought to be due to sex hormones

Question 38

how does disease affect metabolism

Answer 38

endocrine, liver, infections can all alter metabolism

Question 39

what is alcoholic liver disease

Answer 39

prolonged alcohol exposure leading to inflammation - results in cirrhosis and fibrosis

Question 40

alcoholic liver disease effects on drug metabolism

Answer 40

acute exposure - inhibits
chronic exposure - induces
cirrhosis - inhibits

Question 41

what is cirrhosis

Answer 41

liver tissue is replaced by fibrous tissue resulting in metabolism inhibition (esp. CYP450)