Lecture 8

Question 1

Phase II Biotransformation

Answer 1

• Conjugation reactions:
  – Glucuronidation.
  – Sulfation.
  – Glutathione conjugation.
  – Methylation.
  – Acylation.
  – Phosphate conjugation.
• Requires a polar group.
• Enzymes located in ER (UDP-GT) or cytoplasm.

Question 2

Glucuronidation & Glucosidation

Answer 2

• Conjugation with glucuronic acid or glucose.
• Most important and common type of Phase II
  biotransformation.
• Reactive intermediates can be formed from glucose.
• High capacity, low affinity reactions.
• Ability to react with wide range of functional groups.

Question 3

Glucuronidation

Answer 3

• UDP-GTs are inducible.
• Subject to inter-individual variation.
• Products filtered into urine are efficiently excreted.
• Products excreted into bile are susceptible to enterohepatic
  circulation:
  – Intestinal flora have -glucuronidase activity.
  – Glucuronides can be cleaved by acid or base.
  – Cleaved aglycones can be reabsorbed.

Question 4

Sulfate Conjugation

Answer 4

• Biotransforms xenobiotics as well as endogenous compounds.
• Occurs in vertebrates, invertebrates, fungi, bacteria.
• High affinity and low capacity.
• Enzyme catalyzes the transfer of sulfonate, not sulfate (i.e., SO3-,
  not SO4-).
• Generally is a detoxification mechanism but has been implicated
  in the formation of toxic intermediates.

Question 5

Acetylation

Answer 5

• Major route of biotransformation for xenobiotics with
  aromatic amines (R-NH2) or a hydrazines (R-NH-NH2).
• Cytosolic N-acetyltransferases:
  – Present in liver and many other tissues.
  – Wide species variability.
  – Humans, rats, and hamsters express two N-acetyltransferases (NAT-1
  and NAT-2).
  – Fast and slow acetylation phenotypes in humans.
• Can also have O-acetylation.

Question 6

Methylation

Answer 6

• Common but minor pathway.
• Makes substrates slightly less water soluble and masks
  available functional groups for conjugation.
• Wide variety of acceptor substrates:
  – Proteins, lipids, phospholipids and nucleic acids.
  – Xenobiotics.
  – Metals.

Question 7

Glutathione Conjugation

Answer 7

• Substrates share physicochemical properties:
  – Hydrophobic.
  – Contain an electrophilic atom.
  – React nonenzymatically with glutathione, but glutathione transferases
  (GSTs) increase the rate of reaction.
• Protective effect; concentration of glutathione is high (10 mM).
• Some GST substrates are also inducers.
• GSTs are major determinants of differential drug-induced toxicity.