Lab 7

🧠 DETAILED MULTI-PARAGRAPH SUMMARY

Drug metabolism refers to the enzymatic modification of drug molecules, primarily occurring in the liver, where intracellular enzymes transform drugs into more water-soluble compounds for excretion. The rate of drug metabolism is critical: if metabolism is too fast, drugs may become ineffective, while slow metabolism can lead to toxicity and adverse effects. Liver function plays a major role in determining drug clearance, and diseases affecting the liver can alter pharmacokinetics by influencing absorption, enzyme activity, blood flow, and excretion pathways.

Drug metabolism occurs in two major phases. Phase I reactions (oxidation, reduction, hydrolysis) introduce or expose polar functional groups such as –OH or –NH₂, often mediated by the cytochrome P450 (CYP450) enzyme system. These reactions may inactivate drugs, activate prodrugs, or leave activity unchanged. Phase II reactions involve conjugation with endogenous molecules (e.g., glucuronic acid, amino acids), making the compounds more water-soluble and typically inactive, allowing for easier elimination via urine or bile.

The cytochrome P450 enzyme system is a large family of heme-containing monooxygenases located in the smooth endoplasmic reticulum (SER) of hepatocytes. These enzymes metabolise approximately 75% of commonly used drugs. Their activity can be altered by drugs or environmental chemicals through induction (increased enzyme activity) or inhibition (reduced activity). These changes are central to drug-drug interactions, where one drug modifies the metabolism of another, potentially leading to antagonism, where the effect of one drug is reduced or abolished.

A key example is phenobarbitone, a known inducer of CYP450 enzymes, particularly CYP2E1. When administered, it increases the metabolic rate of certain drugs, such as aminopyrine, reducing their therapeutic effectiveness. Aminopyrine metabolism produces formaldehyde as a by-product, which can be measured experimentally. This forms the basis of the practical: assessing enzyme activity by quantifying formaldehyde production.

In the laboratory experiment, liver microsomal fractions (post-mitochondrial supernatants) from control and phenobarbitone-treated rats are used. Aminopyrine metabolism is measured by detecting formaldehyde using the Nash reagent, which produces a yellow colour measurable at 412 nm via spectrophotometry. A standard curve of known formaldehyde concentrations allows quantification of unknown samples. Increased absorbance corresponds to higher enzyme activity, demonstrating enzyme induction in treated samples.

Overall, this practical illustrates how enzyme activity can be measured in vitro and how drug metabolism can be altered by enzyme induction. It highlights the clinical importance of understanding drug interactions, variability in metabolism, and the biochemical basis of pharmacokinetics.

📌 BULLET POINT SUMMARY

• Drug metabolism = enzymatic modification of drugs (mainly in liver)

• Too fast metabolism → ineffective drug

• Too slow metabolism → toxicity

• Two phases:

  • Phase I: oxidation, reduction, hydrolysis (CYP450)

  • Phase II: conjugation → increased solubility → excretion

• CYP450 enzymes:

  • Located in smooth ER

  • Heme proteins

  • Metabolize ~75% of drugs

• Key enzymes: CYP3A4, CYP2D6, CYP2C9, CYP1A2, etc.

• Enzyme activity can be:

  • Induced → increased metabolism

  • Inhibited → decreased metabolism

• Drug interactions can lead to antagonism

• Example:

  • Phenobarbitone induces CYP enzymes

  • Increases metabolism of aminopyrine

• Aminopyrine metabolism → produces formaldehyde

• Formaldehyde detected using Nash reagent

• Yellow colour measured at 412 nm

• Standard curve used to calculate concentration

• Higher absorbance = higher enzyme activity

• Experiment compares:

  • Control liver vs treated liver

• Treated liver → higher metabolism

✏ FILL-IN-THE-BLANK SUMMARY

Drug metabolism is the _Enzymatic_____ modification of drug molecules and occurs mainly in the __liver___. It involves two phases: Phase I and Phase II. Phase I reactions include __oxdation____, __reduction_____ and ___hydrolysis_____, and are largely mediated by the _CYP450_____ enzyme system.

Phase II reactions involve _conjugation_________ of drugs with endogenous molecules, making them more _water____ soluble and easier to _execrete_____. The most important enzyme system in drug metabolism is the ____P450______, which is located in the ___smooth____ endoplasmic reticulum.

Some drugs can alter enzyme activity through __induction________ or inhibition__________. Induction increases enzyme activity, while inhibition __reduces________ it. These changes can result in drug-drug __interaction________.

Phenobarbitone is an example of an enzyme _inducer_______ that increases the activity of __CYP450_______ enzymes. Aminopyrine is metabolised by these enzymes to produce __formaldehyde________, which can be measured experimentally.

The detection of formaldehyde uses the ___NASH_______ reagent, which produces a _____yellow_____ colour. This is measured using a __spectrophmetror________ at ____412______ nm.

A ____stanard______ curve is used to determine the concentration of formaldehyde in unknown samples. Higher absorbance indicates ____higher______ enzyme activity.

✅ Answers

enzymatic
liver
oxidation
reduction
hydrolysis
cytochrome P450
conjugation
water
excrete
cytochrome P450 system
smooth
induction
inhibition
decreases
interactions
inducer
CYP450
formaldehyde
Nash
yellow
spectrophotometer
412
standard
higher

📝 40 EXAM-STYLE MCQs

Questions

1. Drug metabolism primarily occurs in the:
   A. Kidney
   B. Liver
   C. Brain
   D. Lung

2. Phase I reactions include:
   A. Conjugation
   B. Oxidation
   C. Excretion
   D. Filtration

3. Phase II reactions involve:
   A. Hydrolysis
   B. Oxidation
   C. Conjugation
   D. Reduction

4. CYP450 enzymes are located in:
   A. Nucleus
   B. Mitochondria
   C. Smooth ER
   D. Ribosomes

5. CYP450 enzymes are:
   A. Lipids
   B. Heme proteins
   C. Carbohydrates
   D. DNA

6. Enzyme induction results in:
   A. Decreased metabolism
   B. Increased metabolism
   C. No change
   D. Toxicity only

7. Enzyme inhibition results in:
   A. Increased metabolism
   B. Decreased metabolism
   C. Faster excretion
   D. Activation

8. Phenobarbitone is a:
   A. Inhibitor
   B. Inducer
   C. Substrate
   D. Antagonist

9. Aminopyrine metabolism produces:
   A. Glucose
   B. Ammonia
   C. Formaldehyde
   D. CO₂

10. Formaldehyde is detected using:
    A. Benedict’s reagent
    B. Nash reagent
    C. Biuret reagent
    D. Iodine

11. Nash reagent produces:
    A. Blue colour
    B. Green colour
    C. Yellow colour
    D. Red colour

12. Absorbance is measured at:
    A. 260 nm
    B. 340 nm
    C. 412 nm
    D. 600 nm

13. Phase I reactions make drugs:
    A. Less polar
    B. More polar
    C. Insoluble
    D. Toxic

14. Phase II reactions make drugs:
    A. Less soluble
    B. More soluble
    C. Active
    D. Lipid soluble

15. Drug antagonism means:
    A. Increased effect
    B. Decreased effect
    C. No interaction
    D. Toxicity

16. Microsomes are derived from:
    A. Nucleus
    B. ER
    C. Golgi
    D. Lysosome

17. Mixed function oxidases are also called:
    A. Kinases
    B. Monooxygenases
    C. Hydrolases
    D. Ligases

18. CYP450 enzymes metabolize approximately:
    A. 25% of drugs
    B. 50%
    C. 75%
    D. 100%

19. Increased metabolism leads to:
    A. Toxicity
    B. Reduced drug effect
    C. No effect
    D. Accumulation

20. Reduced metabolism leads to:
    A. Decreased toxicity
    B. Increased toxicity
    C. No change
    D. Faster clearance

21. Liver disease affects:
    A. Only absorption
    B. Only metabolism
    C. Multiple pharmacokinetic processes
    D. Only excretion

22. Phase II reactions usually produce:
    A. Active drugs
    B. Inactive drugs
    C. Toxic drugs
    D. Gases

23. Standard curves relate:
    A. Time vs concentration
    B. Absorbance vs concentration
    C. pH vs temperature
    D. Volume vs mass

24. Spectrophotometry measures:
    A. Weight
    B. Absorbance
    C. Pressure
    D. Temperature

25. Formaldehyde concentration is proportional to:
    A. Time
    B. Absorbance
    C. Temperature
    D. Volume

26. Control sample lacks:
    A. Liver
    B. Substrate
    C. Enzyme
    D. Water

27. Treated sample contains:
    A. No enzyme
    B. Induced enzyme
    C. Inhibited enzyme
    D. No substrate

28. Aminopyrine is:
    A. Antibiotic
    B. Analgesic
    C. Hormone
    D. Vitamin

29. CYP2E1 metabolizes:
    A. Glucose
    B. Aminopyrine
    C. DNA
    D. RNA

30. Induction increases:
    A. Drug half-life
    B. Drug metabolism
    C. Toxicity always
    D. Absorption

31. Inhibition increases:
    A. Clearance
    B. Drug levels
    C. Excretion
    D. Binding

32. Nash reagent detects:
    A. Proteins
    B. Lipids
    C. Formaldehyde
    D. Sugars

33. Yellow colour intensity reflects:
    A. Enzyme inhibition
    B. Enzyme activity
    C. pH
    D. Temperature

34. Liver microsomes are used to study:
    A. DNA replication
    B. Drug metabolism
    C. Protein synthesis
    D. Cell division

35. ZnSO₄ is used to:
    A. Increase reaction
    B. Precipitate proteins
    C. Add colour
    D. Heat solution

36. Ba(OH)₂ is used to:
    A. Neutralize
    B. Precipitate impurities
    C. Add substrate
    D. Measure absorbance

37. Drug metabolism mainly occurs in:
    A. Blood
    B. Liver
    C. Skin
    D. Brain

38. Enzyme induction is:
    A. Rare
    B. Clinically important
    C. Irrelevant
    D. Impossible

39. Drug interactions occur due to:
    A. Absorption only
    B. Enzyme changes
    C. Temperature
    D. Pressure

40. Higher absorbance indicates:
    A. Lower metabolism
    B. Higher metabolism
    C. No metabolism
    D. Error

✅ MCQ ANSWERS

1. B

2. B

3. C

4. C

5. B

6. B

7. B

8. B

9. C

10. B

11. C

12. C

13. B

14. B

15. B

16. B

17. B

18. C

19. B

20. B

21. C

22. B

23. B

24. B

25. B

26. B

27. B

28. B

29. B

30. B

31. B

32. C

33. B

34. B

35. B

36. B

37. B

38. B

39. B

40. B

Abstract MCQs — Drug Metabolism

1. Clinical Interpretation

A patient rapidly metabolises a prescribed drug. Which outcome is MOST likely?

A. Increased toxicity
B. Reduced therapeutic effectiveness
C. Drug accumulation in tissues
D. Complete inhibition of excretion

Answer: B

2. Pharmacokinetic Reasoning

Why is the liver considered the primary organ of drug metabolism?

A. It stores all administered drugs
B. It contains high concentrations of intracellular metabolising enzymes
C. It directly excretes all drugs into urine
D. It prevents drugs from entering circulation

Answer: B

3. Phase I Logic

What is the primary purpose of Phase I metabolism?

A. Eliminate drugs immediately in urine
B. Introduce or expose polar functional groups
C. Bind drugs permanently to proteins
D. Prevent drugs entering hepatocytes

Answer: B

4. Functional Consequences

Which statement BEST describes the possible outcomes of Phase I reactions?

A. Drugs are always inactivated
B. Drugs are always activated
C. Drugs may be activated, inactivated, or unchanged
D. Drugs become immediately excreted without modification

Answer: C

5. Abstract Biochemistry

Why does increasing drug polarity generally enhance excretion?

A. Polar molecules diffuse more easily through lipid membranes
B. Water-soluble molecules are more readily eliminated in urine
C. Nonpolar compounds are filtered faster by kidneys
D. Polarity prevents hepatic metabolism

Answer: B

6. Mechanistic Thinking

Which cellular structure is MOST associated with CYP450 enzymes?

A. Rough endoplasmic reticulum
B. Nucleus
C. Smooth endoplasmic reticulum
D. Golgi apparatus

Answer: C

7. Enzyme Modulation

If a drug inhibits CYP450 activity, what is the MOST likely consequence for another drug metabolised by the same enzyme?

A. Faster metabolism
B. Reduced plasma concentration
C. Slower metabolism and possible toxicity
D. Increased renal filtration only

Answer: C

8. Induction Concept

Phenobarbitone increases CYP450 activity. This is an example of:

A. Antagonism
B. Hydrolysis
C. Induction
D. Conjugation

Answer: C

9. Drug Interaction Logic

Why can enzyme induction reduce the effectiveness of certain drugs?

A. Drugs become less water soluble
B. Drugs are metabolised and cleared more rapidly
C. Liver blood flow decreases
D. Excretion pathways become blocked

Answer: B

10. Experimental Interpretation

In the practical, increased formaldehyde production indicates:

A. Reduced enzyme activity
B. Greater aminopyrine metabolism
C. Lower CYP450 expression
D. Decreased spectrophotometer sensitivity

Answer: B

11. Abstract Experimental Design

Why is formaldehyde measured instead of aminopyrine directly?

A. Formaldehyde is easier to quantify spectrophotometrically
B. Aminopyrine cannot be metabolised in vitro
C. Formaldehyde inhibits CYP450 enzymes
D. Aminopyrine has no absorbance properties

Answer: A

12. Clinical Pharmacology

A patient with severe liver disease may require lower drug doses because:

A. Drug metabolism may be impaired, increasing toxicity risk
B. Liver disease always increases excretion
C. CYP450 enzymes become permanently activated
D. Drugs cannot enter circulation

Answer: A

13. Antagonism

Drug antagonism in metabolism refers to:

A. One drug increasing another drug’s effect
B. One drug reducing or abolishing another drug’s effect
C. Complete absence of metabolism
D. Conjugation failure

Answer: B

14. Phase II Understanding

Phase II metabolism MOST commonly results in compounds that are:

A. More lipid soluble and active
B. More water soluble and inactive
C. Unable to leave hepatocytes
D. Resistant to renal excretion

Answer: B

15. Conceptual Physiology

Why are CYP450 enzymes considered “mixed-function oxidases”?

A. They catalyse only reduction reactions
B. They participate in oxidation reactions involving oxygen transfer
C. They only metabolise endogenous compounds
D. They function independently of electron transfer

Answer: B

16. Experimental Logic

What is the role of the Nash reagent in the practical?

A. To induce CYP450 enzymes
B. To metabolise aminopyrine
C. To detect formaldehyde by producing a yellow colour
D. To inhibit liver enzymes

Answer: C

17. Spectrophotometric Reasoning

Why does higher absorbance at 412 nm indicate greater enzyme activity?

A. More yellow product formed from formaldehyde detection
B. Reduced formaldehyde concentration
C. Lower CYP450 activity
D. Increased protein precipitation

Answer: A

18. Abstract Drug Metabolism

A prodrug is BEST described as:

A. A drug immediately excreted unchanged
B. A drug activated through metabolism
C. A permanently inactive compound
D. A drug resistant to enzymatic modification

Answer: B

19. Systems Thinking

Why are drug-drug interactions involving CYP450 enzymes clinically significant?

A. CYP450 enzymes metabolise a large proportion of commonly used drugs
B. CYP450 enzymes only metabolise toxic compounds
C. Drug metabolism occurs exclusively in kidneys
D. CYP450 enzymes are unaffected by environmental chemicals

Answer: A

20. Integrative Interpretation

A phenobarbitone-treated rat shows much greater absorbance at 412 nm compared with controls.

Which conclusion is MOST valid?

A. Aminopyrine metabolism decreased
B. CYP450 enzyme induction increased metabolic activity
C. Nash reagent failed
D. Formaldehyde production was inhibited

Answer: B