1. Induction Increased transcription Increased protein synthesis
Enhanced stability of protein
Synthesis of enzyme with higher catalytic activity
Inducible forms of CYP: CYP1A1 (PAH), CYP2B, CYP3A4 (PB), CYP2E1 (EtOH)
Constitutive: CYP2A

2. Ah-locus mediated induction
Example:
Ah-locus mediated induction
AhR, receptor in cytoplasm, binds ligand: eg PAHs, TCDD, some PCBs
Bound AhR loses 2 heat-shock proteins (hsp90), becomes phosphorylated
Activated bound AhR migrates to nucleus, forms complex with Ah receptor nuclear translocation factor Arnt
AhR-Arnt complex binds to regulatory sequences in DNA (DRE, dioxin-responsive elements)
Transcription of CYP1A1 gene and other genes

3. Other “inducers” also interact with receptors
CAR, responds to phenobarbital-type inducers, regulates CYP2B, CYP3A4, CYP reductase, transferases (?)
PXR, CYP3A
PPARα, CYP4A
LXR, FXR control enzymes involved in bile acid and lipid metabolism

4. Phase II: Conjugation
Synthetic reaction of a xenobiotic (or of a Phase I metabolite of a xenobiotic) with an endogenous substance
Results in introduction of polar, ionizable groups to enhance water solubility and hence excretion

5. Major Phase II reactions
Glucuronidation
Sulfation
Conjugation with amino acids
Conjugation with glutathione
Methylation
Acetylation

6. Glucuronidation
Enzyme: glucuronyl transferase, or glucuronosyl transferase
Targets:
hydroxyl groups: Phenols, Alcohols, Dihydrodiols (ether glucuronides)
Carboxylic acids (ester glucuronides)
Amines (N-glucuronides)
Thiols (S-glucuronides)
Carbon (C-glucuronides, rare)

7. Reaction
Phenol
Phenyl glucuronide

8. Glucuronidation Conjugating moiety: glucuronic acid, a sugar
Co-factor: UDP-glucuronic acid (UDPGA), derived from glycogen synthesis
Located in endoplasmic reticulum
Multiple families of isoforms:UGT1, UGT2
UGT , UGT
Inducible

9. Uridine-5’-diphospho--D-glucuronic acid (UDPGA)

10. Glucuronidation Typical substrates:
Phenol
1-Naphthol
4-Hydroxybiphenyl
3-Hydroxybenzo[a]pyrene
Benzo[a]pyrene-7,8-dihydrodiol
2-Naphthylamine
Bilirubin
Steroids

11. Sulfation Sulfotransferase ST,15 isoforms (xx-ST) Targets
Hydroxyl groups (phenols, alcohols)
Amino groups
Thiols
Conjugating moiety: sulfuric acid, H2SO4
Co-factor: 3’phosphoadenosine 5’phosphosulfate (PAPS), formed from ATP + sulfate
Located in cytosol, Probably not inducible

12. Sulfation Typical substrates
Ethanol
Phenol
3-Hydroxybenzo[a]pyrene
Cholesterol
2-Naphthylamine
N-hydroxy-2-naphthylamine

13. Reaction
PAPS
PAP

14. Conjugation with amino acids
Amino acid transferases
Targets: carboxylic acids
Conjugating moieties: Glycine, glutamine, alanine, taurine, histidine, ornithine
Co-factor: Acetyl CoA (CoASH) and ATP
In cytosol

15. Reaction
Benzoic acid
Benzoyl-CoA
Hippuric acid

16. Conjugation with glutathione
Glutathione S-transferases (GST)
Targets: Epoxides, halogens
Conjugating moiety: Glutathione
Co-factor: None
Mainly in cytosol
Inducible
Multiple families of isoforms: GSTA, GSTM, GSTP, GSTT ()(αμπθ)

17. Glutathione A tripeptide Glutamic acid (Glu) Glycine (Gly)
Cysteine (Cys)

18. Reaction

19. Typical substrates Organic halides, e.g methyl iodide, benzyl chloride
Alkenes e.g. diethyl maleate
Epoxides

20. Mercapturic acid pathway