PHASE I AND PHASE II METABOLISM

Question 1

What is metabolism

Answer 1

Converting to a less pharmacologically active substance

• May end up being more active
• May end up being toxic/carcinogenic

Question 2

Phase I metabolism

Answer 2

• Convert compount to more polar hydrophilic by adding or unmasking a functional group (-OH, -SH, -NH2, -COOH)
• If sufficiently polar they may be excreted in the urine
• May need to undergo phase 2 metabolism

Question 3

Phase II metabolism

Answer 3

• Conjugation with endogenous substance = Increase aqueous solubility
• Conjugation with glucuronice, sulfate, acetate, amino acid

Question 4

Phase I reactions

Answer 4

• Oxidation, reduction, hydrolytic cleavage, alkylation, dealkylation, ring cyclization, n-carboxylation, dimerization, transamidation, isomerisation, decarboxylation

Question 5

Cytochrome P450

Answer 5

• Absorbs light at 450nm when complexed with CO
• Hemoprotein containing iron atom alternating between Fe2+ and Fe3+
• Electron acceptor
• 18 families and 43 subfamilies
• Catalyses reactions to split molecular oxygen where one is introduced into drug and the other leaves as water
• CYP3A4 is responsible for most drugs metabolism

Question 6

Flavin containing monooxygenase system

Answer 6

• Mainly in liver but may be in gut + lungs
• Located in SER (smooth endoplasmic reticulum)
• Oxidises compounds containing sulfur and nitrogen
• Use NADH and NADPH as cofactors
  1. Drug-H+O2+NADPH+H+>Drug-OH+NADP+H2O

Question 7

Structures prone to reduction

Answer 7

By reductase:
- Nitro 
- Azo
- Carbonyl
By hydrolysis (by esterases and peptidases):
- Esters
- Amide

Question 8

Structures prone to oxidation

Answer 8

• N-methyl
• Aromatic rings
• Terminal positions of alkyl chains
• Least hindered position of alicyclic rings

Question 9

Epoxide formation

Answer 9

For benzene, two routes:

1. Hydride shift where para position to R is favoured = more hydrophilic/more capable of being conjugated in phase 2
2. Epoxide hydrolase converts to a diol = more soluble in water and gets excreted in urine
   - Epoxides are alkylating agents making them toxic
   - Neighbouring carbons to O are very electrophilic allowing nucleobases and nucleophilic bases to react
   - Breaking these bonds are difficult
   - Can lead to mutations-cancers/other diseases

Question 10

NAPQI

Answer 10

Unwanted product formed by metabolism of paracetamol

• Conjugated ketone / alpha beta unsaturated ketone
• very electrophilic
• reacts strongly with nucleophiles
• can alkylate nucleophiles
• N7 of guanine can react
• Rearomatisation of that ring system makes C-N bond stronger and more difficult to remove

Question 11

Glucuronidation

Answer 11

• By UDP-glucuronosyltransferase
• On -OH, -SH, -NH2, -COOH groups
• Products often excreted in bile (may enter enterohepatic circulation)
• Can occur with amines, amides and sulfonamides

Question 12

Sulfoconjugation

Answer 12

• Facilitated by sulfotransferases
• For phenols, alcohols, amines and thiols
• Sulfation and glucuronidation are competing pathways

Question 13

Acetylation

Answer 13

• Facilitated by N-acetyltransferases
• requires cofactor acetly-coA
• Common reaction for aromatic amines and sulfonamides
• Takes place in liver
• acetyl-sulfonamide may be less soluble than parent compound causing renal toxicity due to precipitation in kidneys

Question 14

Amino acid conjugation

Answer 14

• ATP dependent acetyl coA synthetase and CoASH forms active AMP-drug coA-drug
• This then reacts with amino acid by N-acetylation

Question 15

Glutathione

Answer 15

• Conjugated by glutathione-S-transferase
• Removes potentially toxic compounds
• Conjugated compound attacked by peptidase forming cysteine
• This can be further acetylated for form N-acetylcysteine conjugate
• Can react with NAPQI, Epoxodes, Alkyl halides, alpha beta unsaturated groups