Drug Biotransformation I and II Flashcards
What is biotransformation?
A metabolic conversion of endogenous or exogenous substances
Its purpose is to deactivate or improve elimination of exogenous substances and xenobiotics
Can also activate a pro-drug as well as produce actie metabolites
What factors influence metabolism?
Genetic
Physiologic
Pharmacodynamic
Environmental
What is the difference between Phase I and Phase II metabolism?
Phase I metabolism deals with the biotransformation of core reactions by adding, exposing, or modifying functional groups.
Phase I Rxns: oxidation, reductions, hydroxlation, hydrolysis.
Phase I reactions may provide new sites for Phase II metabolism.
Phase II metabolism is made up of conjugation (addition) reactions.
There are only a limited number of donor molecules in Phase II: glucouronidation, sulfation, methylation, acetylation, etc.
Functional Groups in Phase II are masked by addition of auxiliary groups.
What type of enzymes are in Phase I metabolism?
Oxygenases
What type of enzymes are in Phase II metabolism?
Transferases
List the Pase I enzymes and their reactions.
Cytochrome P450s (P450 or CYP) - C and O oxidation, dealkylation, others
Flavin-containing monooxygenases (FMO) - N, S, and P oxidation
Epoxide hydrolases (mEH, sEH) - Hydrolysis of epoxides
List the Phase II enzymes and their reactions.
Sulfotransferases (SULT) - addition of sulfate
UDP-glucouronosyltransferases (UGT) - addtion of glucouronic acid
Glutathione-S-transferases (GST) - addition of glutathione
N-acetyltransferases (NAT) - addition of acetyl group
Methyltransferases (MT) - addtion of methyl group
What are the additional metabolism enzymes and their reactions?
Alcohol dehydrogenases - reduction of alcohols
Aldehyde dehydrogenases - reduction of aldehydes
NADPH-quinone oxidoreductases (NOO) - reduction of quinones
What are the characteristics of the Phase I superfamily Cytochrome P450 (CYP450)?
Bound to smooth ER
Major metabolic enzyme class
Binds substrate, then molecular oxygen
What are the characteristics of the Phase I superfamily Flavin Monooxygenases (FMO)?
Bound to smooth ER
Minor metabolic enzyme class
Binds/activates molecular oxygen, then binds substrate
What are the characteristics of the Phase I superfamily Hydrolases?
Bound to smooth ER and free in cytosol
Minor metabolic enzyme class
True or false: Phase I metabolism must come before Phase II reactions.
False
Where are the CYPs found?
Found in liver and extrahepatic tissue
• Bound to smooth endoplasmic reticulum
– Microsomal fraction: homogenized tissue contains smooth ER
CYPs have an absolute requirement for what?
P450 has absolute requirement for NADPH & molecular O2
• Molecular oxygen is consumed
– Oxidation – 1 oxygen atom incorporated into substrate
– Reduction – 1 oxygen atom is reduced to H2O
- Endogenous: steroids, fatty acids, prostaglandins, bile acids
- Exogenous: drugs, carcinogens, toxins
What are CYP450 Isoforms?
- Different amino acid sequence within the same enzyme class
- Results in distinct structure and functions
Nomenclature based on sequence homology not function
- CYP|FAMILY (number) | SUBFAMILY (letter) | GENE(number)
- Example: CYP1A1
Explain CYP450 Isoform specificity.
Substrates predicted by membrane binding CYP
- Lipophilic substrates bound tightly to ER
- Hydrophilic substrates bound weakly to ER
Specificity based on active site structure
- Wide range of specificity between isoforms
- May be highly constrained, restricting to specific 3D shape
- Flexibility or large lipophilic pockets accept diverse structures
– CYP3A4 is capable of metabolizing nearly 50% of drugs
What are the most common CYP Isoforms?
3A4/3A5 and 2D6
Types of Interactions with CYP450 Isoforms:
Isoforms can be induced/inhibited/shared
• Common drug-drug interaction
• Induction by drug activating gene transcription – Increased expression of enzyme
– Carbamazepine induces CYP 1A2, 3A4, 2C8/9, 2D6
• Inhibition by drug binding to active site of CYP enzyme
– Faster onset and recovery (if reversible)
– Erythromycin inhibits CYP 1A2 and 3A4
• Shared when two different substrates compete for binding to pharmacophore (CYP enzyme)
– Donepezil & tramadol are substrates for CYP 2D6
What are common CYP inducers?
Phenobarbital - CYP 3A4/CYP 2C (CYP 2B6, 2D6, and 1A2)
Phenytoin/carbamazepine - CYP 3A4/CYP 1A2/CYP 2C
Rifampin - CYP 3A4/CYP 2C/CYP 2B6/2D6
St. John’s wort - CYP 3A /CYP 2C9/CYP 1A2
Cigarette smoke - CYP 1A1/2
Alcohol - 2E1 and CYP 2D6
Dietary substances indole‐3‐carbinol CYP 1A2
What is Reversible inhibition?
Interactions at heme‐iron complex
Lipophilic site on the apoprotein
Ex: norfloxacin, cimetidine, diltiazem, and voriconazole
What is the Metabolite‐P450 complexation?
Alkylamine oxidation to nitrosoalkane metabolite
High‐affinity to reduced P450 intermediates of CYP 2B, 2C, and 3A
Essentially an irreversible complexation requiring new synthesis
Must possess dimethylamino sugar and compound is lipophilic
Ex: erythromycin and clarithromycin
What is Mechanism‐based inhibition (“suicide inhibition”)?
• Parent drug is not inhibitor of CYP but functional groups on the drug are oxidized and irreversibly binds to the enzyme
Requires new enzyme synthesis
Ex: 17α‐ethinyl estradiol, norethindrone, chloramphenicol, cyclophosamide, and spironolactone
What are General CYP450 Rxns adding oxygen?
Aromatic hydroxylation
Aliphatic hydroxylation
N-Oxidation
Sulfoxidation
Aromatic hydroxylation
Aliphatic hydroxylation
N-Oxidation
Sulfoxidation
General CYP450 Rxns Cleaving Bonds
(Deamination)
General CYP450 Rxns Cleaving Bonds
(O-Dealkylation)
General CYP450 Rxns Cleaving Bonds
(N-Dealkylation)
General FMO N-oxidation reactions
General FMO N-oxidation reactions
General FMO S-oxidation reactions
Additional Phase I metabolism
Reductases
• Disulfides, sulfoxides, N-oxides, nitro, olefins, and carbonyls
Dehydrogenases
Protons
- Alcohol dehydrogenase
- Aldehyde dehydrogenase
Oxidases
• Monoamine and diamine oxidase
– 1°, 2°, and 3° amines:
– must have a proton on adjacent atom!
– Regulation of neurotransmitters
– Oxidation to imines, then hydrolysis to an aldehyde
– MAO vs. CYP deamination: MAO is selective methylene (CH2)
Mechanism: Non-CYP450 hydroxylases
Enzyme
• Molybdenum hydroxylases
– Xanthine oxidase (oxidoreductase)
GOUT
• Electron transfer system
– Molybdenum (one);
– Fe-S complex (two);
– FAD (one)
• Oxygen incorporated from water
– Other hydroxylases (CYP450) use molecular oxygen
• Catalytic species Mo(V)-OH
Mechanism: Non-CYP450 hydroxylases
Enzyme
Molybdenum hydroxylases
Different from aldehyde dehydrogenase
• Aldehydeoxidase
– Metabolizes: aldehydes and azaheterocycles
Aza-heterocycles are oxidized
2-hydroxy, 2-amino purine nucleosides are more efficiently metabolized
– Does not metabolize: Thia- and oxa-heterocyles
– Carbon alpha to nitrogen (generatesalactam)
• Known drug substrates:
– Zaleplon
– Brimonidine
– Thioguanine
• Known drug inhibitors:
– SERMs (Raloxifene) and estradiol
Phase II conjugation auxiliary groups:
Phase II conjugation auxiliary groups:
Phase II conjugation auxiliary groups:
What are the Glucuronic acid basics?
Glucuronidation
- Predominant phase II reaction
- Increases water solubility
- Large supply of glucuronic acid is needed
Glucuronic acid: Bioactivation
Activity modulation
- Most metabolites are inactive or less reactive than the parent substance
- Some metabolites have increased activity
– Morphine(3-O-gucuronide):
Majormetaboliteofmorphine
20-fold greater [plasma] > morphine
Antagonistatopiatereceptors
– Morphine(6-O-glucuronide):
μ-receptoragonist
2-fold greater [plasma] > morphine
650-foldgreateractivity>morphine
What are the basics of Sulfate Metabolism?
Minor phase II reaction
- Limited sulfate stores
- Can produce reactive electrophiles
Two types of sulfotransferases
• Cytosolic
– Phenolic steroids, neurotransmitters, xenobiotics
• Membranebound(golgiapparatus)
– Glycosaminoglycans, glycoproteins, tyrosinyl groups of peptides
– Not typical for xenobiotic metabolism
