Lesson 3 Flashcards
What is the primary way that drugs are eliminated?
Which mechanism allows for reabsorption of drugs in the intestine ?
What kind of molecules must can be secreted into the urine?(ionized or non ionized? Choose one)
What is the effect of lipophilic drugs on renal excretion and reabsorption?
Elimination of drugs occurs primarily through renal mechanism
Secretion into bile also possible, but allows for re-absorption in the intestine
Secretion into the urine requires ionized or hydrophilic molecules, but:
1.Most drugs are not small molecules that are highly ionized at body pH
2.Most drugs are poorly ionized and lipophilic
=> This decreases renal excretion and facilitates renal tubular reabsorption
3.Many drugs are highly protein bound, and therefore not efficiently filtered in the kidney
4.Most drugs would have a long duration of action if termination of their effects depended only on renal excretion
Inactivation versus elimination of the active drug
What is the primary way that drugs are eliminated?
Which mechanism allows for reabsorption of drugs in the intestine ?
What kind of molecules must can be secreted into the urine?(ionized or non ionized? Choose one)
What is the effect of lipophilic drugs on renal excretion and reabsorption?
Elimination of drugs occurs primarily through renal mechanism
Secretion into bile also possible, but allows for re-absorption in the intestine
Secretion into the urine requires ionized or hydrophilic molecules, but the problem with most drugs is that :
1.Most drugs are not small molecules that are highly ionized at body pH
2.Most drugs are poorly ionized and lipophilic
=> This decreases renal excretion (decreases excretion because drug is hydrophobic or lipophilic but drug has to be hydrophilic before it can be excreted cuz urine is made up primarily of water molecules) and facilitates renal tubular reabsorption
3.Many drugs are highly protein bound, and therefore not efficiently filtered in the kidney
4.Most drugs would have a long duration of action if termination of their effects depended only on renal excretion
Inactivation versus elimination of the active drug
The solution to all the above problems is drug metabolism
Why are hydrophilic molecules ionized?
Charge and Water Interaction**:
- Ionized = Charged: Ionized drugs have gained or lost an electron, resulting in a charge.
- Hydrophilic = Water-loving: Hydrophilic substances are attracted to water molecules due to their polarity and ability to form hydrogen bonds.
Mnemonic: “Ions love water”. This simple phrase reminds you that ionized drugs, being charged, have an affinity for water.
-
Contrasting Properties:
- Hydrophobic = Water-repelling: Hydrophobic substances are repelled by water and tend to be non-polar.
- Ionized drugs are not hydrophobic: Since they interact favorably with water, ionized drugs are hydrophilic.
Why is drug metabolism important?
Why are most metabolic products less pharmacologically active?
Which drugs are exceptions to this statement that most metabolic products less pharmacologically active?
Why is drug metabolism so important? Elimination of drugs and chemicals by the kidney is often compromised because the drug/chemical is too nonpolar, lipophilic and readily “reabsorbed” from tubular fluid. Metabolism can convert the drug to a more hydrophilic compound reducing reabsorption.
Most metabolic products ar else pharmacologically active because through metabolism, the products become more hydrophilic for renal excretion but for a drug to be very pharmacologically active, it must be hydrophobic or lipophilic
Most metabolic products are less pharmacologically active
Important exceptions:
Where the metabolite is more active - 3 examples
1.(Prodrugs, e.g. Erythromycin-succinate (less irritation of GI) –> Erythromycin, enaliprilat -> enalapril, codeine) : Erythromycin succinate is a pro drug and these pro drugs are only active when metabolized that’s why they are exceptions.
2.Where the metabolite is toxic (acetaminophen): cuz we’re saying metabolism makes the metabolite less pharmacologically active than the parent drug but in cases where metabolites are toxic, the metabolites are more pharmacologically active than the parent drugs. Just not in a good way
3.Where the metabolite is carcinogenic: cuz we’re saying metabolism makes the metabolite less pharmacologically active than the parent drug but in cases where metabolites are carcinogenic, the metabolites are more pharmacologically active than the parent drugs. Just not in a good way
What is the relationship between the bio transformation of drugs and normal biochemical processes occurring in the body
Close relationship between the biotransformation of drugs and normal biochemical processes occurring in the body:
-Metabolism of drugs involves many pathways associated with the synthesis of endogenous substrates such as steroid hormones, cholesterol and bile acids
-Many of the enzymes involved in drug metabolism are principally designed for the metabolism of endogenous compounds
-These enzymes metabolize drugs only because the drugs resemble the natural compound
Certainly! Let’s break it down:
- Enzymes Shared Between Drugs and Body: The same enzymes in your body that help break down drugs also help make and regulate natural substances like hormones (like estrogen or testosterone), cholesterol (which is important for cell function), and bile acids (which help digest fats).
- Competition for Enzymes: When you take a drug, it can compete with these natural substances for the enzymes that break them down. This competition can affect how well the drug works and how your body handles these natural substances.
- Effects on Natural Processes: Some drugs can change how these natural substances are made or broken down in your body. For example, medications like statins can lower cholesterol levels by affecting how your body produces cholesterol.
- Why It Matters: Understanding how drugs interact with these natural processes helps doctors predict how drugs might affect you. It also helps them manage any side effects or interactions between different drugs you might be taking.
In essence, drug metabolism isn’t just about how drugs break down in your body—it’s also about how they interact with and affect the natural processes that keep your body running smoothly.
State three example of drugs with more active metabolites
Erythromycin succinate – gram+ antibiotic; pH sensitive (so it needs enteric coating), nonpolar, esterified (succinic acid, proprionic acid); converted by cell esterases
Enaliprilat - ACE-Inhibitor; prodrug; esterase converts to Enalapril (active)
Codeine – O-demethylation (or oxygen demethylation) to morphine – more active analgesic than codeine; CYP2D6 metabolic enzyme; deficient in 10% caucasians, 2% in asians; reduced analgesia for same dosage
Difference between ionotropic and chronotropic drugs
In the body, what drug is metabolized to acetaminophen?
What enzymes are involved in this process?
What process does acetaminophen go through to be metabolized ?
What toxic metabolite is produced from this reaction?
How do these metabolites lead to liver damage?
Focus**: Ionotropic drugs focus on changing the force or strength of heart contractions, affecting myocardial contractility.
- Focus: Chronotropic drugs focus on altering the rate of heart contractions, affecting heart rate.
heart cells.
- Examples:
- Positive Ionotropic Agents: Digoxin (increases contraction force by affecting calcium levels), Dopamine (increases force at moderate doses).
- Negative Ionotropic Agents: Beta-blockers like Metoprolol (decrease contraction force by blocking beta-adrenergic receptors), Verapamil (reduces force by blocking calcium channels).
- Definition: Chronotropic drugs influence the rate or speed of heart contractions by affecting electrical signals that regulate heart rhythm.
-
Examples:
- Positive Chronotropic Agents: Epinephrine (increases heart rate and contraction strength via beta-adrenergic stimulation), Atropine (increases heart rate by blocking muscarinic receptors).
- Negative Chronotropic Agents: Beta-blockers like Atenolol (decrease heart rate by blocking beta-adrenergic receptors), Diltiazem (slows heart rate by blocking calcium channels).
- Ionotropic Drugs: Used to optimize cardiac output in conditions like heart failure or shock, where enhancing or reducing contraction strength is beneficial.
- Chronotropic Drugs: Employed to manage heart rate abnormalities such as bradycardia (slow heart rate) or tachycardia (fast heart rate), adjusting heart rhythm as needed
Phenacetin: 1887, analgesic, antipyretic, negative inotropic.
Present in APC headache mix: (aspirin+phenacetin+caffeine)
Use today: “cutting” cocaine due to its analgesic properties, adulterant; chronic use leads to renal papillary necrosis due to toxic metabolites
Ethyl ester de-ethylation to acetaminophen (CYP2A13, CYP1A2)
Acetaminophen more potent than phenacetin
Phenacetin & acetaminophen conjugated with glucuronic acid or sulfate for elimination
Phenacetic metabolized by monooxygenase hydroxylation to toxic metabolites –
NAPQI = N-acetyl-(1,4) benzoquinone imine and epoxides
Acetaminophen also conjugated and hydroxylated (CYP2E1, CYP2A6, CYP1A2)
Hydroxylation leads to toxic metabolite imine and epoxide
Detoxification: conjugation with Hepatic cell glutathione (GSH)
If Overdose – deplete hepatic GSH; metabolites mitochondrial dysfunction, oxidative damage to proteins, liver cell necrosis and hepatic failure
Therapy: N-acetyl cysteine or methionine
Phenacetin, historically used as an analgesic and antipyretic, is notable for its negative inotropic effects on the heart. Originally a component of the APC headache mixture (Aspirin, Phenacetin, Caffeine), it’s now primarily associated with illicit practices such as “cutting” cocaine due to its analgesic properties. Chronic use of phenacetin can lead to renal papillary necrosis due to toxic metabolites.
In the body, phenacetin is metabolized into acetaminophen (paracetamol) through de-ethylation processes primarily involving CYP2A13 and CYP1A2 enzymes. Acetaminophen is more potent than phenacetin and is further metabolized through hydroxylation by enzymes like CYP2E1, CYP2A6, and CYP1A2. This metabolic pathway can produce toxic metabolites such as N-acetyl-(1,4) benzoquinone imine (NAPQI) and epoxides.
Toxic metabolites like NAPQI and epoxides can lead to liver damage by depleting hepatic cell glutathione (GSH), causing mitochondrial dysfunction, oxidative stress, and liver cell necrosis. Treatment for acetaminophen overdose typically involves administering N-acetylcysteine or methionine to replenish GSH levels and mitigate liver damage.
Understanding the metabolic pathways and toxicity of phenacetin and acetaminophen underscores the importance of safe usage and monitoring to prevent severe adverse effects like hepatic failure.
What are the two main phases of drug metabolism
Which of the two phases combines with endogenous substrates to make drugs more soluble ?
Phase I Reactions:
-Convert parent compound into a more polar (=hydrophilic) metabolite by adding or unmasking(exposing. Any reaction that removes a functional group or breaks bonds in a drug molecule, exposes rhat drug molecule and makes it more polar) functional groups (-OH, -SH, -NH2, -COOH, etc.) on a drug molecule
-Often these metabolites are rendered inactive by the conversion
-The reaction product may be sufficiently polar to be excreted readily
Phase II Reactions:
-Conjugation with endogenous substrate to further increase aqueous solubility.
-Conjugation with glucoronide, sulfate, acetate, amino acid
Phase I usually precedes phase II reactions
some drugs only need to undergo phase1 reaction and that’s enough for them. Others require both reactions.
What organ is the principal site for drug metabolism?
What is the first pass effect
Liver is principal site of drug metabolism:
Other sites include the gut(midozalam,cyclosporine,L dopa), lungs(metabolizes inhaled drugs example is propanolol and albuterol and inhaled corticosteroids ) , skin(metabolizes topical drugs, Topical corticosteroids, nitroglycerin (transdermal), local anesthetics.) and kidneys(vitamin D and penicillin or beta lactam antibiotics or cephalothin are metabolized by the kidneys)
For orally administered compounds, there is the
“First Pass Effect”
Intestinal metabolism
Liver metabolism
Enterohepatic recycling
Gut microorganisms - glucuronidases
The first-pass effect, also known as first-pass metabolism, refers to the phenomenon where a drug undergoes metabolism in the liver or intestines before reaching systemic circulation. Here’s a concise explanation:
- Definition: The first-pass effect describes the metabolism of a drug that occurs when it is absorbed from the gastrointestinal tract (via oral administration) or enters the liver (via portal circulation) before reaching the systemic circulation.so the liver metabolizes the drugs before they get into systemic circulation. This makes the amount of the drug available, smaller than it was when it was taken in. So it passed through mouth and through intestines to the liver then to systemic circulation.
- Mechanism: When a drug is absorbed orally, it travels through the portal vein to the liver first. In the liver, enzymes metabolize some of the drug before it can enter the general bloodstream and reach other organs or tissues.
- Significance: The extent of first-pass metabolism can significantly affect the bioavailability of a drug, which is the fraction of the administered dose that reaches systemic circulation unchanged and is available for therapeutic action.
- Examples: Drugs that undergo extensive first-pass metabolism include propranolol (a beta-blocker), lidocaine (a local anesthetic), and nitroglycerin (used for angina). For example, nitroglycerin undergoes rapid metabolism in the liver, which limits its oral bioavailability, so it is often administered sublingually or transdermally.
- Clinical Considerations: Understanding the first-pass effect is crucial for dosing and route of administration decisions in pharmacotherapy. Drugs that undergo extensive first-pass metabolism may require higher oral doses or alternative routes (such as sublingual, transdermal, or intravenous) to achieve therapeutic concentrations.
In summary, the first-pass effect highlights how drugs are metabolized in the liver or intestines before entering systemic circulation, impacting their bioavailability and therapeutic effectiveness.
Liver metabolism
Enterohepatic recycling
Gut microorganisms - glucuronidases
So from mouth to stomach to intestines where it is primarily absorbed (more absorbed in the duodenum and jejunum of the small intestine cuz of the plenty villi, increased blood supply) then it moves through the enterocytes(intestinal epithelial cells) into portal circulation and passes through the portal vein and into the liver for metabolism before it gets into the systemic circulation and loves to target organs.
Phase I and II - Summary:
Products are generally more water soluble
These reactions products are ready for (renal) excretion
There are many complementary, sequential and competing pathways
Phase I and Phase II metabolism are a coupled interactive system interfacing with endogenous metabolic pathways(endogenous because the body already has its own metabolic pathways and the drugs just take advantage of these pathways to act)
What is the end result of hydrophilic drugs in the body?
What is the end result of lipophilic drugs in the body?
Hydrophilic drugs (drugs that like water) absorbed in GI tract, pass through liver, get excreted by kidney.
B. Lipophilic (hydrophobic) drug without metabolism, absorbed into blood, may get excreted into tubular fluid in kidney but reabsorbed – no elimination.
Lipophilic drugs are absorbed through the gastrointestinal tract and enter the bloodstream.
• Due to their affinity for fats, these drugs tend to distribute well into fatty tissues and other lipid-rich areas of the body.
C. Lipophilic drug is slowly metabolized by liver enzymes. The liver tries to
Metabolize it. In this case, the hydrophilic metabolite formed from the liver enzymes trying to do some metabolism on the lipophilic drug is the only one eliminated, then the rest of the unchanged drug recirculates.
D. Lipophilic drug rapidly metabolized by liver enzymes, nearly complete elimination by kidney.
State the three main phase 1 metabolism reactions
Which of the reactions often converts prodrugs to their active forms or deactivating drugs
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Which enzyme is most commonly involved in the hydroxylation of drugs during Phase I metabolism?
- A) Esterase
- B) Alcohol dehydrogenase
- C) Cytochrome P450
- D) UDP-glucuronosyltransferase
**
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In the context of Phase I metabolism, what is the primary outcome of N-oxidation reactions?
- A) Addition of a hydroxyl group to a drug molecule
- B) Conversion of a drug molecule into a more polar form by adding oxygen to nitrogen
- C) Hydrolysis of ester or amide bonds
- D) Reduction of nitro groups to amino groups
**
-
Which of the following drugs is primarily metabolized through Phase I hydrolysis reactions rather than oxidation or reduction?
- A) Acetaminophen
- B) Diazepam
- C) Propranolol
- D) Aspirin
*
-
Which Phase I metabolic reaction is responsible for the conversion of codeine to morphine?
- A) Hydrolysis
- B) Oxidation
- C) Reduction
- D) Dealkylation
-
Which of the following statements about dealkylation reactions in Phase I metabolism is TRUE?
- A) Dealkylation reactions typically result in the addition of an oxygen atom to the drug molecule.
- B) Dealkylation involves the removal of alkyl groups from the drug molecule, which can result in the formation of a more active or toxic metabolite.
- C) Dealkylation is a form of hydrolysis that breaks ester bonds in the drug molecule.
- D) Dealkylation reactions are primarily mediated by UDP-glucuronosyltransferases.
Phase I Reactions:
Oxidation(OILRIG. For charged species,Oxidation is loss and reduction is gain)
Reduction
Hydrolytic cleavage(breaking bonds using water molecules. Breaks chemical bonds through the addition of water, often converting prodrugs to their active forms or deactivating drugs)
The above are the three main phase 1 reactions
Phase one reactions include oxidation, reduction, and hydrolysis, which introduce or unmask a functional group on the drug. The other options— acetylation, methylation, and sulphate conjuration-are phase two reactions,
Alkylation (Methylation)(adding an alkyl or methyl group)
Dealkylation(removing an alkyl group)
Ring cyclization
N-carboxylation
Dimerization
Transamidation
Isomerization
Decarboxylation
More interested in top 5
Here’s a clearer breakdown:
Phase I reactions generally involve modifications to the drug molecule itself to introduce or expose functional groups. These include:
1. Oxidation
2. Reduction
3. Hydrolysis
Main ones are the above.
8. Hydroxylation
Phase II reactions typically involve conjugation reactions where an endogenous substrate is added to the drug molecule to increase its solubility. These include:
1. Glucuronidation
2. Sulfation (Sulphate Conjugation)
3. Acetylation
4. Methylation
5. Amino Acid Conjugation
6. Glutathione Conjugation
- Methylation and Acetylation: These are phase II reactions as they involve the addition of a methyl group (methylation) or an acetyl group (acetylation) to the drug or its metabolites, increasing solubility for excretion.
- Reduction and Sulphate Conjugation: Reduction is a phase I reaction, while sulphate conjugation is a phase II reaction.
If you have specific sources that describe alkylation (including methylation) as phase I, they might be discussing specific biochemical contexts or exceptions. In general pharmacology, methylation and acetylation are recognized as phase II reactions.
Alcohol dehydrogenase (ADH) is an enzyme involved in Phase I reactions of drug metabolism. It primarily catalyzes the oxidation of alcohols into aldehydes. For example, ADH converts ethanol (an alcohol) into acetaldehyde (an aldehyde). This reaction is an oxidation process, which is one of the main types of Phase I metabolic reactions
Certainly! Here’s a summary:
-
Hydroxylation:
- What: Addition of a hydroxyl group (-OH) to a molecule.
- Enzyme: Cytochrome P450 (CYP) enzymes.
- Type: Oxidation reaction.
- Purpose: Increases polarity, making the molecule more water-soluble and preparing it for further metabolism.
-
Hydrolysis:
- What: Breakdown of a molecule by adding water, splitting it into smaller molecules.
- Enzyme: Hydrolases (e.g., esterases, amidases).
- Type: Decomposition reaction.
- Purpose: Cleaves bonds in complex molecules, facilitating their excretion or further metabolism.
Here are five challenging MCQs related to Phase I metabolism reactions:
-
Which enzyme is most commonly involved in the hydroxylation of drugs during Phase I metabolism?
- A) Esterase
- B) Alcohol dehydrogenase
- C) Cytochrome P450
- D) UDP-glucuronosyltransferase
-
In the context of Phase I metabolism, what is the primary outcome of N-oxidation reactions?
- A) Addition of a hydroxyl group to a drug molecule
- B) Conversion of a drug molecule into a more polar form by adding oxygen to nitrogen
- C) Hydrolysis of ester or amide bonds
- D) Reduction of nitro groups to amino groups
-
Which of the following drugs is primarily metabolized through Phase I hydrolysis reactions rather than oxidation or reduction?
- A) Acetaminophen
- B) Diazepam
- C) Propranolol
- D) Aspirin
-
Which Phase I metabolic reaction is responsible for the conversion of codeine to morphine?
- A) Hydrolysis
- B) Oxidation
- C) Reduction
- D) Dealkylation
-
Which of the following statements about dealkylation reactions in Phase I metabolism is TRUE?
- A) Dealkylation reactions typically result in the addition of an oxygen atom to the drug molecule.
- B) Dealkylation involves the removal of alkyl groups from the drug molecule, which can result in the formation of a more active or toxic metabolite.
- C) Dealkylation is a form of hydrolysis that breaks ester bonds in the drug molecule.
- D) Dealkylation reactions are primarily mediated by UDP-glucuronosyltransferases.
These questions are designed to test a deeper understanding of the specific mechanisms and outcomes of Phase I drug metabolism reactions.
State the two types of oxidation reactions
State the difference between deimination and deamination
Two types of oxidation reactions:
Oxygen is incorporated into the drug molecule (e.g. hydroxylation, epoxidations, sulfoxidations)
Oxidation causes the loss of part of the drug molecule
(e.g. oxidative deimination, dealkylation)
Microsomal Mixed Function Oxidases (MFOs)
-“Microsomes”
form in vitro after cell homogenization and fractionation of ER
Rough ER microsomes are primarily associated with protein synthesis
Smooth ER (are usually associated with steroid hormones) microsomes contain a class of oxidative enzymes called cytochrome P450 enzymes
-“Mixed Function Oxidases” or “Monooxygenases”
These enzymes require a reducing agent (NADPH) and molecular oxygen
(one oxygen atom appearing in the product and the other in the form of water)
Deimination and deamination are related terms in biochemistry, but they refer to different processes:
Deimination: Also known as citrullination, it’s the conversion of an arginine amino acid residue in a protein to a citrulline residue. This process is catalyzed by enzymes called peptidylarginine deiminases (PADs) and plays a role in various cellular processes, including protein function regulation and immune responses.
Deamination: Refers to the removal of an amino group (-NH2) from a molecule, resulting in the formation of a keto or aldehyde group. In biochemistry, deamination reactions are crucial for the breakdown of amino acids, nucleotides, and other biomolecules. Enzymes like deaminases and amidases catalyze these reactions.
Let me know if you’d like more details or have specific questions about these processes!
What kind of reactions do MFOs catalyze?
Mixed function oxidases consists of which 2 enzymes?
Why are MFOs called mixed function?
What by products are formed in the CYP450 catalytic cycle?
Yes, Cytochrome P450 enzymes are a type of mixed-function oxidase (MFO).
Mixed-function oxidases are enzymes that catalyze oxidation reactions, using one substrate as the electron donor (usually NADPH or NADH which give two electrons and one of these electron reduces the ferric (Fe+++) in the CYP450 to ferrous (Fe++) ) and another substrate as the electron acceptor(CYP450) while the other electron enables the activation of molecular oxygen). This means they oxidize one molecule while reducing another.
In the context of mixed-function oxidases (also known as monooxygenases), such as the cytochrome P450 (CYP450) enzyme system, NADPH indeed undergoes oxidation as it loses electrons, and the electron acceptor (typically cytochrome P450) is reduced. Here is how it works:
Oxidation and Reduction in the Cytochrome P450 System:
1. Role of NADPH: • NADPH (Nicotinamide Adenine Dinucleotide Phosphate) serves as an electron donor. • When NADPH donates electrons, it is oxidized to NADP⁺.
Cytochrome P450 enzymes specifically catalyze monooxygenase reactions, which involve the insertion of one oxygen atom into the substrate, resulting in a hydroxylated product. The general reaction is:
Substrate + NADPH + H+ + O2 → Hydroxylated product + NADP+ + H2O
The “mixed-function” designation refers to the ability of these enzymes to perform multiple functions, including:
1. Oxidation
2. Reduction
3. Electron transfer
MFO consists of two enzymes:
1.A Flavoprotein, NADPH-cytochrome c reductase
One mole of this enzyme contains one mole each of flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD)
Enzyme is also called NADPH-cytochrome P450 reductase
2.Cytochrome P450
named based on its light absorption at 450 nm when complexed with carbon monoxide
is a hemoprotein containing an iron atom which can alternate between the ferrous (Fe++) and ferric (Fe+++) states; “has REDOX properties”
Electron acceptor
Serves as terminal oxidase
Its relative abundance compared to NADPH-cytochrome P450 reductase makes it the rate-limiting step in the oxidation reactions
What is the CP450 enzyme and why is it called 450?
Which organelle contains CP450
These enzymes contain a heme group (iron-porphyrin complex) and are responsible for catalyzing oxidation reactions, often involving the transfer of electrons. The name “P450” comes from the characteristic absorbance peak at 450 nm in the presence of carbon monoxide. Cytochrome P450 gets its name from its unique spectral properties. The “P450” designation refers to the enzyme’s ability to absorb light at a specific wavelength:
- “P” stands for “pigment” (referring to the heme group, a colored component of the enzyme)
- “450” represents the wavelength (in nanometers) at which the enzyme absorbs light, specifically the Soret band (a characteristic peak in the absorption spectrum of heme proteins)
In the presence of carbon monoxide (CO), the enzyme’s absorption peak shifts to 450 nm, which is why it’s called P450. This spectral property allows researchers to detect and quantify the enzyme’s activity.
The “Cytochrome” part of the name refers to the enzyme’s classification as a hemoprotein, containing a heme group (a porphyrin ring with an iron atom at its center). Cytochromes are a family of proteins that play crucial roles in various cellular processes, including electron transport and oxidation reactions.
Microsomes, which are vesicles derived from the smooth ER, contain Cytochrome P450 enzymes and are often used in vitro to study drug metabolism and toxicity.
How many families and sub families of cytochrome P450 genes do humans have ?
Which cytochrome is responsible for both drug and steroid metabolism
Humans have 18 families of cytochrome P450 genes and 43 subfamilies:
—CYP1 drug metabolism (3 subfamilies, 3 genes, 1 pseudogene)
-CYP2 drug and steroid metabolism (13 subfamilies, 16 genes, 16 pseudogenes)
-CYP3 drug metabolism (1 subfamily, 4 genes, 2 pseudogenes)
-CYP4 arachidonic acid or fatty acid metabolism (5 subfamilies, 11 genes, 10 pseudogenes)
-CYP5 Thromboxane A2 synthase (1 subfamily, 1 gene)
—CYP7A bile acid biosynthesis 7-alpha hydroxylase of steroid nucleus (1 subfamily member)
—CYP7B brain specific form of 7-alpha hydroxylase (1 subfamily member)
—CYP8A prostacyclin synthase (1 subfamily member)
—CYP8B bile acid biosynthesis (1 subfamily member)
—CYP11 steroid biosynthesis (2 subfamilies, 3 genes)
CYP17 steroid biosynthesis (1 subfamily, 1 gene) 17-alpha hydroxylase
—CYP19 steroid biosynthesis (1 subfamily, 1 gene) aromatase forms estrogen
—CYP20 Unknown function (1 subfamily, 1 gene)
—CYP21 steroid biosynthesis (1 subfamily, 1 gene, 1 pseudogene)
—CYP24 vitamin D degradation (1 subfamily, 1 gene)
—CYP26A retinoic acid hydroxylase important in development (1 subfamily member)
—CYP26B probable retinoic acid hydroxylase (1 subfamily member)
—CYP26C probabvle retinoic acid hydroxylase (1 subfamily member)
—CYP27A bile acid biosynthesis (1 subfamily member)
—CYP27B Vitamin D3 1-alpha hydroxylase activates vitamin D3 (1 subfamily member)
—CYP27C Unknown function (1 subfamily member)
—CYP39 7 alpha hydroxylation of 24 hydroxy cholesterol (1 subfamily member)
—CYP46 cholesterol 24-hydroxylase (1 subfamily member)
—CYP51 cholesterol biosynthesis (1 subfamily, 1 gene, 3 pseudogenes) lanosterol 14-alpha demethylase
State five inhibitors and five inducers of Cytochrome P450 enzymes
Here are 15 challenging MCQs based on Cytochrome P450 (CYP450) inhibitors and inducers:
-
Which of the following drugs is a known CYP450 inhibitor that can increase the plasma concentration of drugs metabolized by CYP450 enzymes?
- A) Carbamazepine
- B) Rifampin
- C) Ketoconazole
- D) Phenobarbital
**
-
Which CYP450 inducer is most likely to decrease the effectiveness of oral contraceptives by increasing their clearance?
- A) St. John’s Wort
- B) Cimetidine
- C) Erythromycin
- D) Fluconazole
**
-
Which of the following drugs would be least affected by the CYP450 inhibitor, grapefruit juice?
- A) Diazepam
- B) Lovastatin
- C) Warfarin
- D) Carbamazepine
**
-
Which CYP450 inhibitor is also known to affect the metabolism of drugs metabolized by CYP3A4?
- A) Isoniazid
- B) Omeprazole
- C) Metronidazole
- D) Sodium valproate
**
-
Which of the following drugs is both a CYP450 inducer and can lead to decreased levels of drugs metabolized by CYP3A4?
- A) Rifampin
- B) Amiodarone
- C) Ciprofloxacin
- D) Ketoconazole
**
-
Which drug is a CYP450 inducer and can reduce the effectiveness of certain anticoagulants, such as warfarin?
- A) Phenobarbital
- B) Fluconazole
- C) Erythromycin
- D) Sulfonamides
**
-
Which CYP450 inhibitor is known to interact with warfarin and increase its anticoagulant effect?
- A) Fluconazole
- B) St. John’s Wort
- C) Carbamazepine
- D) Rifampin
*
-
Chronic alcohol consumption has what effect on CYP450 enzymes compared to acute alcohol consumption?
- A) Chronic alcohol increases CYP450 activity, while acute alcohol decreases it.
- B) Acute alcohol increases CYP450 activity, while chronic alcohol decreases it.
- C) Both chronic and acute alcohol decrease CYP450 activity.
- D) Both chronic and acute alcohol increase CYP450 activity.
*
-
Which CYP450 inhibitor is also used to treat fungal infections and has significant drug-drug interactions due to its inhibition of CYP3A4?
- A) Metronidazole
- B) Erythromycin
- C) Fluconazole
- D) Omeprazole
**
-
Which of the following substances is a CYP450 inducer that could potentially increase the metabolism of drugs such as cyclosporine A?
- A) Griseofulvin
- B) Fluconazole
- C) Cimetidine
- D) Erythromycin
-
Which of the following drugs, when used in combination with CYP450 inhibitors, could potentially lead to drug toxicity due to decreased metabolism?
- A) Carbamazepine
- B) Phenobarbital
- C) Rifampin
- D) Warfarin
**
-
Which CYP450 inducer is also known for causing reduced therapeutic effects of antiepileptic drugs due to increased clearance?
- A) Phenobarbital
- B) Cimetidine
- C) Omeprazole
- D) Metronidazole
**
-
Which CYP450 inhibitor is frequently used in the treatment of peptic ulcers and can interact with drugs metabolized by CYP450 enzymes?
- A) St. John’s Wort
- B) Omeprazole
- C) Rifampin
- D) Carbamazepine
**
-
Which of the following drugs is a potent CYP450 inducer that can lead to increased clearance of medications like oral contraceptives?
- A) Ketoconazole
- B) Griseofulvin
- C) Erythromycin
- D) Fluconazole
-
Which CYP450 inhibitor could result in a significant increase in the plasma concentration of drugs that are substrates for CYP3A4 when consumed concurrently?
- A) Rifampin
- B) Phenobarbital
- C) Grapefruit juice
- D) St. John’s Wort
СУР450 INHIBITORS
* DECREASE CYP450 ACTIVITY. This makes DRUG CLEARANCE SLOWER +
DRUGS HAVE GREATER EFFECT *
S Sodium valproate
T TICLODIPINE
I ISONIAZID
C CIMETIDINE
K KETOCONAZOLE
F FLUCONAZOLE
A ALCOHOL (ACUTE / BINGE),AMIODARONE
C—CIPROFLOXACIN,CHLORAMPHENICOL
E ERYTHROMYCIN
S SULFONAMIDES-antibiotic such as sulfadiazine
GROUP GRAPEFRUIT JUICE
C CRANBERRY juice
O OMEPRAZOLE
M METRONIDAZOLE
Mnemonic is inhibit STICK FACES GROUP. COM
СУР450 INDUCERS
* INCREASE CYP 45O ACTIVITY
DRUG CLEARANCE FASTER +
DRUGS HAVE LESSER EFFECT *
B BARBITUATES
R RIFAMPIN
A ALCOHOL (CHRONIC)
C CARBAMAZEPINES
S St JOHN’S WORT
G GRISEOFULVIN
P PHENYTOIN
P PHENOBARBITAL
S SULFONYLUREAS
mnemonic is: induce the BRACS GPPS.
BS CRAP GPS
Here are 15 challenging MCQs based on Cytochrome P450 (CYP450) inhibitors and inducers:
-
Which of the following drugs is a known CYP450 inhibitor that can increase the plasma concentration of drugs metabolized by CYP450 enzymes?
- A) Carbamazepine
- B) Rifampin
- C) Ketoconazole
- D) Phenobarbital
-
Which CYP450 inducer is most likely to decrease the effectiveness of oral contraceptives by increasing their clearance?
- A) St. John’s Wort
- B) Cimetidine
- C) Erythromycin
- D) Fluconazole
-
Which of the following drugs would be least affected by the CYP450 inhibitor, grapefruit juice?
- A) Diazepam
- B) Lovastatin
- C) Warfarin
- D) Carbamazepine
Carbamazepine: Primarily metabolized by CYP3A4, but carbamazepine is also an inducer of CYP3A4. This means it can actually increase the activity of CYP3A4, potentially reducing the impact of grapefruit juice on its metabolism.
-
Which CYP450 inhibitor is also known to affect the metabolism of drugs metabolized by CYP3A4?
- A) Isoniazid
- B) Omeprazole
- C) Metronidazole
- D) Sodium valproate
-
Which of the following drugs is both a CYP450 inducer and can lead to decreased levels of drugs metabolized by CYP3A4?
- A) Rifampin
- B) Amiodarone
- C) Ciprofloxacin
- D) Ketoconazole
-
Which drug is a CYP450 inducer and can reduce the effectiveness of certain anticoagulants, such as warfarin?
- A) Phenobarbital
- B) Fluconazole
- C) Erythromycin
- D) Sulfonamides
-
Which CYP450 inhibitor is known to interact with warfarin and increase its anticoagulant effect?
- A) Fluconazole
- B) St. John’s Wort
- C) Carbamazepine
- D) Rifampin
-
Chronic alcohol consumption has what effect on CYP450 enzymes compared to acute alcohol consumption?
- A) Chronic alcohol increases CYP450 activity, while acute alcohol decreases it.
- B) Acute alcohol increases CYP450 activity, while chronic alcohol decreases it.
- C) Both chronic and acute alcohol decrease CYP450 activity.
- D) Both chronic and acute alcohol increase CYP450 activity.
-
Which CYP450 inhibitor is also used to treat fungal infections and has significant drug-drug interactions due to its inhibition of CYP3A4?
- A) Metronidazole
- B) Erythromycin
- C) Fluconazole
- D) Omeprazole
-
Which of the following substances is a CYP450 inducer that could potentially increase the metabolism of drugs such as cyclosporine A?
- A) Griseofulvin
- B) Fluconazole
- C) Cimetidine
- D) Erythromycin
-
Which of the following drugs, when used in combination with CYP450 inhibitors, could potentially lead to drug toxicity due to decreased metabolism?
- A) Carbamazepine
- B) Phenobarbital
- C) Rifampin
- D) Warfarin
-
Which CYP450 inducer is also known for causing reduced therapeutic effects of antiepileptic drugs due to increased clearance?
- A) Phenobarbital
- B) Cimetidine
- C) Omeprazole
- D) Metronidazole
-
Which CYP450 inhibitor is frequently used in the treatment of peptic ulcers and can interact with drugs metabolized by CYP450 enzymes?
- A) St. John’s Wort
- B) Omeprazole
- C) Rifampin
- D) Carbamazepine
-
Which of the following drugs is a potent CYP450 inducer that can lead to increased clearance of medications like oral contraceptives?
- A) Ketoconazole
- B) Griseofulvin
- C) Erythromycin
- D) Fluconazole
-
Which CYP450 inhibitor could result in a significant increase in the plasma concentration of drugs that are substrates for CYP3A4 when consumed concurrently?
- A) Rifampin
- B) Phenobarbital
- C) Grapefruit juice
- D) St. John’s Wort
These questions are designed to assess a deeper understanding of how various substances influence CYP450 enzyme activity and the implications for drug interactions and metabolism.
Ethanol induces which cytochrome enzyme?
What about barbiturate? What cytochrome enzyme does it induce?
What cytochrome enzyme does cigarette smoke and charred food induce?
Induction of P450 enzymes:
PPAR (peroxisome proliferator activated receptor) ligands (e.g.clofibrate) induce P450 enzymes
CYP1 family are induced by aromatic hydrocarbons
(cigarette smoke; charred food)
CYP2E enzymes induced by ethanol
CYP2B enzymes induced 40-50 fold by barbiturates
How can polymorphism cause differences in drug metabolism
Which of the following is an example of a cytochrome P450 enzyme that shows significant genetic polymorphism?
• A) CYP3A4
• B) CYP2E1
• C) CYP2D6
• D) CYP1A2
Polymorphisms cause differences in drug metabolism:
CYP2C19 has a polymorphism that changes the enzyme’s ability to metabolize mephenytoin (a marker drug). In Caucasians, the polymorphism for the poor metabolizer phenotype is only seen in 3% of the population. However, it is seen in 20% of the asian population.
=> It is important to be aware of a person’s race when drugs are given that are metabolized differently by different populations
Which of the following is an example of a cytochrome P450 enzyme that shows significant genetic polymorphism?
• A) CYP3A4
• B) CYP2E1
• C) CYP2D6
• D) CYP1A2
Answer: C
Why: Got it! Let’s use the specific example of CYP2C19 and mephenytoin to illustrate how polymorphisms can change drug metabolism:
- CYP2C19 Enzyme: This enzyme is responsible for metabolizing various drugs, including mephenytoin, an anticonvulsant.
- Polymorphisms: Genetic variations in the CYP2C19 gene can lead to different versions of the enzyme that work at different speeds.
-
Poor Metabolizers (PM):
- Genetic Variant: Some people have a version of the CYP2C19 gene that produces an enzyme with little to no activity.
- Effect on Mephenytoin: These individuals cannot effectively metabolize mephenytoin. The drug stays in their body longer, increasing the risk of side effects.
-
Extensive Metabolizers (EM):
- Genetic Variant: These people have the “normal” version of the CYP2C19 gene, producing an enzyme with standard activity.
- Effect on Mephenytoin: They metabolize mephenytoin at a normal rate, achieving the desired therapeutic effect without excessive side effects.
-
Ultra-Rapid Metabolizers (UM):
- Genetic Variant: Some individuals have a version of the CYP2C19 gene that produces an enzyme with higher-than-normal activity.
- Effect on Mephenytoin: They metabolize mephenytoin very quickly, which may lead to lower drug levels in the body and reduced effectiveness.
- Dose Adjustment: Knowing a patient’s CYP2C19 genotype helps doctors adjust the dose of mephenytoin. For example, a poor metabolizer might need a lower dose to avoid side effects, while an ultra-rapid metabolizer might need a higher dose for the drug to be effective.
Polymorphisms in the CYP2C19 gene create different versions of the enzyme with varying abilities to metabolize drugs like mephenytoin. This affects how much of the drug is needed and how effective or safe it is for the patient. Understanding these differences helps tailor drug therapy to individual needs.
Which is a chief CP450 inhibitor ?
Which enzyme is NOT primarily involved in the metabolism of acetaminophen?
• A) CYP2E1 • B) UGTs (UDP-glucuronosyltransferases) • C) SULTs (sulfotransferases) • D) CYP3A4
Here are 10 MCQs based on the information provided about Cytochrome P450 enzymes and drug interactions:
-
Which of the following drugs induces CYP2B and leads to increased metabolism of other drugs?
- A) Ketoconazole
- B) Grapefruit juice
- C) Barbiturates
- D) Erythromycin
-
Which enzyme is inhibited by ketoconazole, leading to reduced metabolism of other drugs?
- A) CYP2B
- B) CYP3A4
- C) CYP1A2
- D) CYP2D6
-
Grapefruit juice is known to inhibit which of the following Cytochrome P450 enzymes?
- A) CYP2B
- B) CYP3A4
- C) CYP2D6
- D) CYP2E1
-
Which drug listed is a substrate for CYP3A4?
- A) Acetaminophen
- B) Codeine
- C) Diazepam
- D) Warfarin
-
Which of the following drugs would have its metabolism reduced by grapefruit juice?
- A) Lovastatin
- B) Lidocaine
- C) Taxol
- D) All of the above
-
What effect does barbiturate use have on CYP2B and related drug metabolism?
- A) Inhibition of CYP2B, leading to decreased metabolism of other drugs
- B) Induction of CYP2B, leading to increased metabolism of other drugs
- C) No effect on CYP2B
- D) Induction of CYP3A4, leading to increased metabolism of other drugs
-
Which of the following drugs is NOT a substrate for CYP3A4?
- A) Cyclosporin A
- B) Diazepam
- C) Acetaminophen
- D) Warfarin
-
How does grapefruit juice affect the concentration of drugs metabolized by CYP3A4?
- A) Decreases the concentration by enhancing drug metabolism
- B) Increases the concentration by inhibiting CYP3A4
- C) Has no effect on drug concentrations
- D) Decreases the concentration by inducing CYP3A4
-
Which drug listed is used as an immunosuppressant and is a substrate for CYP3A4?
- A) Codeine
- B) Erythromycin
- C) Cyclosporin A
- D) Taxol
-
If a patient consumes grapefruit juice while taking a CYP3A4 substrate, what is the likely outcome?
- A) Decreased drug efficacy due to increased metabolism
- B) Increased drug toxicity due to reduced metabolism
- C) No change in drug levels
- D) Enhanced drug metabolism due to CYP3A4 induction
Which of the following drugs is most likely to have a 12-fold increase in concentration due to grapefruit juice consumption?
• A) Lidocaine
• B) Warfarin
• C) Codeine
• D) Taxol
Which of the following drug interactions would be least affected by grapefruit juice consumption?
• A) Lovastatin
• B) Cyclosporin A
• C) Erythromycin
• D) Diazepam
2. A patient on warfarin begins taking a medication that induces CYP2B. What is the expected effect on warfarin therapy? • A) Increased anticoagulant effect due to enhanced metabolism • B) Decreased anticoagulant effect due to increased metabolism • C) No effect on warfarin metabolism • D) Increased bleeding risk due to decreased metabolism 3. Which enzyme’s activity would be most impacted by ketoconazole, leading to increased plasma levels of drugs metabolized by this enzyme? • A) CYP2E1 • B) CYP2D6 • C) CYP3A4 • D) CYP1A2 4. In the context of drug metabolism, which of the following is a key mechanism by which barbiturates alter drug metabolism? • A) Inhibition of CYP2B leading to decreased metabolism • B) Induction of CYP2B leading to increased metabolism of other drugs • C) Inhibition of CYP3A4 leading to decreased drug metabolism • D) Induction of CYP3A4 leading to decreased drug metabolism 5. Which of the following statements about CYP3A4 substrates is TRUE? • A) All CYP3A4 substrates are affected similarly by grapefruit juice. • B) CYP3A4 substrates generally exhibit increased metabolism when CYP3A4 is inhibited. • C) CYP3A4 substrates can have significantly increased plasma concentrations when CYP3A4 activity is inhibited by substances such as grapefruit juice. • D) Grapefruit juice exclusively affects CYP3A4 substrates by decreasing their metabolism and does not affect their plasma concentrations.
P450s and drug interactions:
Barbiturates induce CYP2B => increased metabolism of other drugs
Antifungals (e.g. ketoconazole) inhibit fungal CYP51 and unintentionally also human CYP3A4
=> reduced metabolism of other drugs
Grapefruit juice contains a CYP3A4 inhibitor =>12 fold increase in some drug concentrations. Grapefruit juice is a Chief CP450 inhibitor
CYP3A4 Substrates: • Acetominophen (Tylenol) • Codeine (narcotic) • Cyclosporin A (immunosuppressant), • Diazepam (Valium) • Erythromycin (Antibiotic) • Lidocaine (local anaesthetic), • Lovastatin (HMGCoA reductase inhibitor), • Taxol (cancer drug), • Warfarin (anticoagulant).
Which enzyme is NOT primarily involved in the metabolism of acetaminophen?
• A) CYP2E1 • B) UGTs (UDP-glucuronosyltransferases) • C) SULTs (sulfotransferases) • D) CYP3A4
Answer: D
Here are 10 MCQs based on the information provided about Cytochrome P450 enzymes and drug interactions:
-
Which of the following drugs induces CYP2B and leads to increased metabolism of other drugs?
- A) Ketoconazole
- B) Grapefruit juice
- C) Barbiturates
- D) Erythromycin
-
Which enzyme is inhibited by ketoconazole, leading to reduced metabolism of other drugs?
- A) CYP2B
- B) CYP3A4
- C) CYP1A2
- D) CYP2D6
-
Grapefruit juice is known to inhibit which of the following Cytochrome P450 enzymes?
- A) CYP2B
- B) CYP3A4
- C) CYP2D6
- D) CYP2E1
-
Which drug listed is a substrate for CYP3A4?
- A) Acetaminophen
- B) Codeine
- C) Diazepam
- D) Warfarin
-
Which of the following drugs would have its metabolism reduced by grapefruit juice?
- A) Lovastatin
- B) Lidocaine
- C) Taxol
- D) All of the above
-
What effect does barbiturate use have on CYP2B and related drug metabolism?
- A) Inhibition of CYP2B, leading to decreased metabolism of other drugs
- B) Induction of CYP2B, leading to increased metabolism of other drugs
- C) No effect on CYP2B
- D) Induction of CYP3A4, leading to increased metabolism of other drugs
-
Which of the following drugs is NOT a substrate for CYP3A4?
- A) Cyclosporin A
- B) Diazepam
- C) Acetaminophen
- D) Warfarin
-
How does grapefruit juice affect the concentration of drugs metabolized by CYP3A4?
- A) Decreases the concentration by enhancing drug metabolism
- B) Increases the concentration by inhibiting CYP3A4
- C) Has no effect on drug concentrations
- D) Decreases the concentration by inducing CYP3A4
-
Which drug listed is used as an immunosuppressant and is a substrate for CYP3A4?
- A) Codeine
- B) Erythromycin
- C) Cyclosporin A
- D) Taxol
-
If a patient consumes grapefruit juice while taking a CYP3A4 substrate, what is the likely outcome?
- A) Decreased drug efficacy due to increased metabolism
- B) Increased drug toxicity due to reduced metabolism
- C) No change in drug levels
- D) Enhanced drug metabolism due to CYP3A4 induction
Nn Which of the following drugs is most likely to have a 12-fold increase in concentration due to grapefruit juice consumption?
• A) Lidocaine
• B) Warfarin
• C) Codeine
• D) Taxol
Answer: B
Which of the following drug interactions would be least affected by grapefruit juice consumption?
• A) Lovastatin
• B) Cyclosporin A
• C) Erythromycin
• D) Diazepam
Answer: C
2. A patient on warfarin begins taking a medication that induces CYP2B. What is the expected effect on warfarin therapy?
• A) Increased anticoagulant effect due to enhanced metabolism
• B) Decreased anticoagulant effect due to increased metabolism
• C) No effect on warfarin metabolism
• D) Increased bleeding risk due to decreased metabolism
Answer: B
3. Which enzyme’s activity would be most impacted by ketoconazole, leading to increased plasma levels of drugs metabolized by this enzyme?
• A) CYP2E1
• B) CYP2D6
• C) CYP3A4
• D) CYP1A2
Answer: C
4. In the context of drug metabolism, which of the following is a key mechanism by which barbiturates alter drug metabolism?
• A) Inhibition of CYP2B leading to decreased metabolism
• B) Induction of CYP2B leading to increased metabolism of other drugs
• C) Inhibition of CYP3A4 leading to decreased drug metabolism
• D) Induction of CYP3A4 leading to decreased drug metabolism
Answer: B
5. Which of the following statements about CYP3A4 substrates is TRUE?
• A) All CYP3A4 substrates are affected similarly by grapefruit juice.
• B) CYP3A4 substrates generally exhibit increased metabolism when CYP3A4 is inhibited.
• C) CYP3A4 substrates can have significantly increased plasma concentrations when CYP3A4 activity is inhibited by substances such as grapefruit juice.
• D) Grapefruit juice exclusively affects CYP3A4 substrates by decreasing their metabolism and does not affect their plasma concentrations.
Answer: C
These questions cover the basics of Cytochrome P450 interactions, including induction, inhibition, and the impact of substances like grapefruit juice on drug metabolism.
State four things needed for drug oxidation to occur
Drug oxidation requires:
1.Cytochrome P450(this enzyme usually or mainly catalyzes oxidation reactions even though it can catalyze other reactions. In this, a substrate either gains oxygen atoms or loses hydrogen atoms)
2.Cytochrome P450 reductase: Cytochrome P450 Reductase:
• Function: This enzyme transfers electrons from NADPH to the CYP450 enzyme. It is essential for the catalytic cycle of CYP450, enabling the reduction of the heme iron.
• Mechanism: Cytochrome P450 reductase receives electrons from NADPH and sequentially transfers them to the CYP450 enzyme.
3.NADPH(NADPH (nicotinamide adenine dinucleotide phosphate)**: This serves as the electron donor for the reduction of cytochrome P450 reductase, which in turn supplies electrons to CYP450 enzymes.)
4.Molecular oxygen: Molecular oxygen is required for the oxidation reactions catalyzed by CYP450 enzymes. It acts as the oxidizing agent, with one oxygen atom incorporated into the substrate and the other reduced to water.
Explain how the oxidation cycle occurs
Which form of oxidized CP450 enzyme combines with the drug to form an iron cytochrome P450 complex?
Fe2+ ferrous iron or Fe3+ ferric iron
1. Which step in the cytochrome P450 catalytic cycle directly involves the reduction of molecular oxygen?
A) The combination of oxidized cytochrome P450 with a drug substrate
B) The donation of an electron from NADPH to cytochrome P450 reductase
C) The formation of the “activated oxygen”-cytochrome P450-substrate complex
D) The transfer of “activated” oxygen to the drug substrate
E) The dissociation of the oxidized product from cytochrome P450
**
2. What role does NADPH play in the cytochrome P450 enzyme system?
A) It donates electrons to reduce molecular oxygen
B) It binds directly to the substrate to facilitate oxidation
C) It combines with cytochrome P450 to form a binary complex
D) It is oxidized by cytochrome P450 reductase during the reaction
E) It transfers “activated” oxygen to the drug substrate
**
3. In the cytochrome P450 catalytic cycle, what is the oxidation state of iron after the initial combination of oxidized cytochrome P450 with a drug substrate?
A) Fe2+
B) Fe3+
C) Fe4+
D) Fe5+
E) The oxidation state remains unchanged
4. Which of the following statements best explains the significance of the “activated oxygen” in the cytochrome P450 cycle?
A) It allows the enzyme to form a binary complex with the drug substrate
B) It acts as a reducing agent to regenerate cytochrome P450
C) It stabilizes the cytochrome P450-drug complex during the reaction
D) It provides the oxidative power necessary for drug substrate oxidation
E) It initiates the formation of NADPH
5. If the cytochrome P450 reductase is defective, which step in the P450 cycle would be directly impaired, leading to decreased drug metabolism?
A) Binding of the drug substrate to cytochrome P450
B) Initial electron donation to cytochrome P450
C) Formation of the binary complex
D) Transfer of “activated” oxygen to the drug substrate
E) Dissociation of the oxidized product
The cycle involves four steps:
NB : Fe2+ is ferrous iron and Fe3+ is ferric iron
1.Oxidized (Fe3+ or ferric iron) cytochrome P-450 combines with a drug substrate to form a binary complex.(Iron cytochrome P450 complex)
2.NADPH donates an electron to the cytochrome P-450 reductase, which in turn reduces the oxidized cytochrome P-450-drug complex (so the complex gains an electron since reduction has occurred)
3.A second electron is introduced from NADPH via the same cytochrome P-450 reductase, which serves to reduce molecular oxygen and form an “activated oxygen”-cytochrome P-450-substrate complex.
4.This complex in turn transfers “activated” oxygen to the drug substrate to form the oxidized product. The potent oxidizing properties of this activated oxygen permit oxidation of a large number of substrates.
so this continues like a positive feedback loop and keeps going on and on and on
- Formation of the Binary Complex:
• Oxidized (Fe³⁺ or ferric iron) cytochrome P450 binds to a drug substrate, forming a binary complex.
• At this stage, cytochrome P450 is in its oxidized state (Fe³⁺).- First Electron Donation:
• NADPH donates an electron to cytochrome P450 reductase, which then reduces the cytochrome P450-drug complex from Fe³⁺ to Fe²⁺ (ferrous iron). This is the reduction of the complex. - Second Electron Donation and Formation of Activated Oxygen:
• A second electron is donated by NADPH via cytochrome P450 reductase. This electron is used to reduce molecular oxygen (O₂) bound to the cytochrome P450.
• This results in the formation of an “activated oxygen”-cytochrome P450-substrate complex. The oxygen is partially reduced and now in a highly reactive state, making this part of the cycle more complicated as it involves both reduction (to activate oxygen) and oxidation (due to the subsequent steps). - Oxidation of the Substrate:
• The activated oxygen is transferred to the drug substrate, leading to its oxidation and the formation of the oxidized drug product.
- First Electron Donation:
Clarification on the Confusion:
• The term “oxidized cytochrome P450” refers to the Fe³⁺ state of the enzyme at the beginning of the cycle. • The “activated oxygen” complex forms after the reduction of the enzyme and molecular oxygen. This complex is what enables the oxidation of the drug substrate.
Corrected Understanding:
• Oxidation in this context refers to the final step where the substrate is oxidized. • Reduction occurs earlier when the cytochrome P450 is reduced from Fe³⁺ to Fe²⁺ and when oxygen is reduced to form the reactive “activated oxygen” species.
Given this explanation, the formation of the “activated oxygen” complex involves reduction of oxygen, not oxidation. The oxidation happens when this “activated oxygen” is transferred to the substrate.
No, CYP450 reductase does not transfer the activated oxygen itself. Instead, the transfer of activated oxygen is performed by the cytochrome P450 enzyme (CYP450) after it is activated. However, cytochrome P450 reductase plays a crucial supporting role in the process.
-
Cytochrome P450 System:
- The cytochrome P450 enzyme system is composed of two key proteins:
- Cytochrome P450 enzyme (CYP450)
- NADPH-cytochrome P450 reductase (CPR)
- The cytochrome P450 enzyme system is composed of two key proteins:
-
Function of CYP450 Reductase:
- CYP450 reductase is an electron transfer protein that shuttles electrons from NADPH (nicotinamide adenine dinucleotide phosphate) to the CYP450 enzyme.
- This transfer of electrons is essential for the activation of molecular oxygen (O₂) bound to the heme iron (Fe) of the CYP450 enzyme.
-
Steps in Oxygen Activation and Transfer:
- Substrate Binding: A drug (substrate) binds to the CYP450 enzyme.
-
Electron Transfer by CYP450 Reductase:
- CYP450 reductase transfers electrons from NADPH to the heme iron of the CYP450 enzyme in two sequential steps. This process reduces the heme iron from Fe³⁺ to Fe²⁺.
-
Oxygen Binding and Activation:
- Molecular oxygen (O₂) binds to the reduced Fe²⁺ form of CYP450.
- A second electron is transferred, leading to the formation of a highly reactive iron-oxo species (FeO³⁺), often referred to as Compound I.
-
Oxygen Transfer to the Substrate:
- The activated oxygen (Compound I) in the CYP450 enzyme transfers to the substrate, oxidizing it (e.g., hydroxylation, dealkylation).
While CYP450 reductase does not directly transfer activated oxygen, it is critical in providing the necessary electrons that enable the CYP450 enzyme to activate oxygen and subsequently transfer it to the substrate during the oxidation process. The actual transfer of activated oxygen to the drug substrate is carried out by the CYP450 enzyme, not by CYP450 reductase.
Here are five challenging MCQs based on the steps of the cytochrome P450 cycle you described:
1. Which step in the cytochrome P450 catalytic cycle directly involves the reduction of molecular oxygen?
A) The combination of oxidized cytochrome P450 with a drug substrate
B) The donation of an electron from NADPH to cytochrome P450 reductase
C) The formation of the “activated oxygen”-cytochrome P450-substrate complex
D) The transfer of “activated” oxygen to the drug substrate
E) The dissociation of the oxidized product from cytochrome P450
Answer: C) The formation of the “activated oxygen”-cytochrome P450-substrate complex
2. What role does NADPH play in the cytochrome P450 enzyme system?
A) It donates electrons to reduce molecular oxygen
B) It binds directly to the substrate to facilitate oxidation
C) It combines with cytochrome P450 to form a binary complex
D) It is oxidized by cytochrome P450 reductase during the reaction
E) It transfers “activated” oxygen to the drug substrate
Answer: A) It donates electrons to reduce molecular oxygen
3. In the cytochrome P450 catalytic cycle, what is the oxidation state of iron after the initial combination of oxidized cytochrome P450 with a drug substrate?
A) Fe2+
B) Fe3+
C) Fe4+
D) Fe5+
E) The oxidation state remains unchanged
Answer: B) Fe3+
4. Which of the following statements best explains the significance of the “activated oxygen” in the cytochrome P450 cycle?
A) It allows the enzyme to form a binary complex with the drug substrate
B) It acts as a reducing agent to regenerate cytochrome P450
C) It stabilizes the cytochrome P450-drug complex during the reaction
D) It provides the oxidative power necessary for drug substrate oxidation
E) It initiates the formation of NADPH
Answer: D) It provides the oxidative power necessary for drug substrate oxidation
5. If the cytochrome P450 reductase is defective, which step in the P450 cycle would be directly impaired, leading to decreased drug metabolism?
A) Binding of the drug substrate to cytochrome P450
B) Initial electron donation to cytochrome P450
C) Formation of the binary complex
D) Transfer of “activated” oxygen to the drug substrate
E) Dissociation of the oxidized product
Answer: B) Initial electron donation to cytochrome P450
These questions are designed to test a deep understanding of the cytochrome P450 catalytic cycle and its components.
