EXAM 5 - Cofactors for Phase 2 Metabolism 2

Question 1

Describe sulfation/sulfonation.

Answer 1

• catalyzed by sulfotransferases (STs)
• all cytosolic
• substrates are usually alcohols
• The nucleophilic site of the xenobiotic attacks the sulfur –> displaces phosphate group
• result: substrate linked to sulfate group
• products can go into urine OR bile for excretion

Question 2

Can sulfonation produce toxic products?

Answer 2

Yes. some products re more toxic than substrates.

Question 2

Describe acetylation.

Answer 2

Add acetate group
* catalyzed by N-acetyltransferases (only 2: NAT1 and NAT2)
* slight (if any) decrease in polarity
* Amines –> amides
* Hydrazines –> hydrazides

Question 3

Describe methylation.

Answer 3

• minor pathway
• catalyzed by methyltransferases (xMT) - NAMED BASED ON SUBSTRATE
• usually decreases water solubility
• cytosolic and microsomal
• O, N, and S methylation

Question 4

Describe amino acid conjugation.

Answer 4

• conjugated to carboxylic acids via amine group of AAs
• or hydroamines via carboxyl group of AAs such as serine or proline

Question 5

Describe the process of conjugating amino acids to carboxylic acids via amine group of AAs.

Answer 5

• amine (NH2) group of amino acid gets added on to substrate
• entire AA is added to electrophilic site of substrate

Question 6

Describe the process of conjugating amino acids to hydroxylamines via carboxyl group of AAs.

Answer 6

• Carboxyl end of serine is added onto the oxygen of the substrate
• entire AA is added to electrophilic site of substrate

Question 7

Describe glutathione conjugation.

Answer 7

• glutathione s-transferase (GSTs) use GSH as cofactor
• 95% cytosolic
• substrate characteristics: hydrophobic, contain electrophilic site, react nonenzymatically with GSH
• enzyme classes: A, P, M, S, T, K (dimers)
• each monomer is bound to glutathione
• GSH conjugates can be excreted in the bile or further metabolized to mercapturic acids in the kidneys

Question 8

Explain the enzyme mechanism of glutathione s-transferase.

Answer 8

GSH x enzyme-O- (with tyrosine in active site)
* tyrosine pulls proton off of glutathione to make hydrogen bond –> makes sulfur more nucleophilic (better nucleophile to attack the electrophilic site of drugs)
* since the sulfur is more nucleophilic, it is more likely to attack electrophilic sites on drugs –> whole molecule of glutathione is added to the drug.

Question 9

Explain how glutathione conjugates are metabolized to mercapturic acids in the kidneys.

Answer 9

In the liver
* normally GSTs will form glutathione conjugates in the liver that are excreted as feces

In the kidney
* y-glutamyltranspeptidase removes glutamic acid from GT conjugate
* aminopeptidase M removes glycine
* N-acetyltransferase adds acetyl group –> mercapturic acid
* excreted through urine