Lecture 7 Flashcards
Phase I
Hydrolysis/Reduction/Oxidation
Phase II
Conjugation
Biotransformation
a biochemical process that alters the chemical structure of substances to help the body excrete them
Extrahepatic microsomal and non-
microsomal enzymes
oxidation, conjugation, acetylation, sulfation, GSH, alcohol
& aldehyde dehydrogenase, hydrolysis, ox/red)
Hepatic microsomal enzymes
oxidation, conjugation
Hepatic non-microsomal enzymes
acetylation, sulfation, GSH, alcohol &
aldehyde dehydrogenase, hydrolysis,
ox/red
Microsomes
Artefactual membranous particles, not present in the living cell, derived from
pieces of the endoplasmic reticulum present in homogenates of tissues or cells. Microsomes
sediment from such homogenates when centrifuged at 100,000×g and higher. The microsomal
fraction obtained in this way is often used as a source of mono-oxygenase enzymes.
Phase I Biotransformation
- Introduction of a new functional group on the parent molecule.
- Sometimes followed by loss of the group and conversion into
another product. - Hydrophilicity increases slightly.
- May inactivate or activate parent drug.
- Many types of reactions involved.
- Multiple enzyme systems and cofactors.
Phase I Reactions: Hydrolysis
- Carboxyesterases, phosphatases, and peptidases:
– Hydrolysis of esters.
– Hydrolysis of phosphate groups.
– Hydrolysis of peptide bonds. - Epoxide hydrolase:
– H2O added to epoxides. - Can also be spontaneous (non-enzymatic).
Phase I Reactions: Reductions
- Azo reduction:
– -N=N- to two -NH2 groups. - Nitro reduction:
– -N=O to single -NH2 group. - Disulfide reduction (R-S-S-R’).
- Sulfoxide reduction
- Quinone reduction:
– Cytosolic flavoprotein NAD(P)H quinone oxidoreductase. - Two-electron reduction, no oxidative stress.
– Flavoprotein P450-reductase. - One-electron reduction, produces superoxide ions.
- Dehalogenation:
– Reductive (H replaces X).
– Oxidative (X and H replaced with =O).
– Dehydrodechlorination (2 X’s removed, form C=C)
Phase I Reactions: Oxidation-Reduction
- Alcohol dehydrogenase:
– Alcohols to aldehydes. - Aldehyde dehydrogenase:
– Aldehydes to carboxylic acids. - Aldehyde oxidase.
– Aldehydes to carboxylic acids
Phase I Reactions: CYP450 System
- CYP enzyme family is the major Phase I catalyst of drug and endogenous
compound oxidations in liver, kidney, GI tract, skin, lungs. - Oxidative reactions require the CYP heme protein, the CYP reductase enzyme,
NADPH, phosphatidylcholine, and O2:
NADPH + H+ + O2 + drug NADP+ + H2O + oxidized drug - CYPs are in close association with CYP reductase in a 10/1 ratio.
- The reductase serves as the electron source for the oxidative
reaction cycle. - Origin of enzyme family name: CO binds to the reduced Fe(II)
heme and absorbs at 450 nm.
Inhibition of P450
- Drug-drug interactions due to reduced rate of
biotransformation. - Competitive:
– S and I compete for active site. - Mechanism-based:
– Irreversible; covalent binding to active site.
– “Suicide inhibition”.
Induction of P450
- Induction can cause marked increases in P450 activity (>20-fold).
- Induction can increase tolerance to some drugs while enhancing
the toxicity of others. - Induction can decrease the therapeutic effect of drugs by
increasing the rate and pattern of metabolism. - Xenobiotics are known to induce enzymes that play a major in
their own biotransformation.
Phase I Reactions: Peroxidases
- Couple oxidation to reduction of H2O2 & lipid hydroperoxides.
- Prostaglandin H synthetase:
– Causes nephrotoxicity by activating aflatoxin B1, acetaminophen to
DNA-binding compounds. - Lactoperoxidase (mammary gland).
- Myeloperoxidase (bone marrow):
– Causes bone marrow suppression by activating benzene to DNA-
reactive compound.
