1. Metabolism - Biotransformation
Supplementary readings:
Casarett and Doull,Chapter 6
Timbrell, Chapters 4 and 5

2. Non-polar
(lipophilic)
Hydrophobic
XENOBIOTIC
Phase I Metabolism
Oxidation
Can accumulate in tissues
Solubility in lipids
INTERMEDIATE METABOLITE
Phase II Metabolism
Conjugation
Solubility in water
May be reactive/toxic
WATER-SOLUBLE METABOLITE
Lipophobic
Hydrophilic
(Polar)
ELIMINATION

3. What is a xenobiotic ?

4. Phase I reactions Chemical modification of xenobiotics
Introduces or uncovers polar functional groups that provide sites for Phase II metabolism
Major classes of reaction:
Oxidation
Reduction
Hydrolysis

5. Overview of oxidations, reductions, hydrolyses
Loss of electrons M M+ + e-
Gain of oxygen R + O RO

6. Oxidation reactions
Hydroxylation

7. Epoxidation

8. Overview of oxidations, reductions, hydrolyses
Gain of electrons M+ + e- M
Loss of oxygen RO R + O
Gain of hydrogen R + H RH

9. Reduction Cr6+ + 3 e- Cr3+ Nitro to amino group
Chromium VI to Chromium III
Cr e Cr3+

10. Hydrolysis Addition of water
Cleavage of R-O or R-N bond accompanied by addition of H2O
R’-O-R + H2O R’-O-H + R-OH
R’-N-R + H2O R’-N-H + R-OH
H H

11. Principal Phase I enzymes
Cytochrome P450
Flavin monooxygenase
Monoamine oxidase
Esterases
Amidases
Hydrolases
Reductases, dehydrogenases, oxidases

12. Flavin monooxygenase Flavoprotein Mixed-function amine oxidase
Located in smooth endoplasmic reticulum, in human, pig, rabbit liver, guinea-pig lung, human kidney
Uses NADPH as a source of reducing equivalents
Not inducible

13. Overall reaction
R-H + O2 + NADPH + H+
R-OH + H2O + NADP+

14. Monoamine oxidase Metabolizes endogenous monoamine neurotransmitters
Uses NADPH as a source of reducing equivalents
Found in the endoplasmic reticulum and in mitochondria, of nerve endings and liver

15. Esterases
Hydrolyse esters to carboxylic acid and alcohol functional groups
Non-specific esterases in plasma, more substrate-specific forms in liver cytosol

16. Amidases Hydrolyse amides to carboxylic acids and amines (or ammonia)
Found in plasma and in liver cytosol

17. Hydrolases
Hydrolyse ethers

18. Reductases, dehydrogenases, oxidases
In cytosol, endoplasmic reticulum, mitochondria

19. Cytochrome P450 Heme protein
Terminal oxidase of the mixed-function oxidase (MFO) electron-transfer system
Located in the smooth endoplasmic reticulum of all major organs and tissues
Uses NADPH as a source of reducing equivalents
Inducible