Isoenzymes – significance MCQs With Answer

Izoenzymes (isoenzymes) are enzyme variants with similar catalytic functions but different amino acid sequences, tissue distribution, and kinetic properties. For B. Pharm students, understanding isoenzymes is essential for clinical diagnostics, pharmacology, and drug development — especially in interpreting enzyme assays, predicting drug metabolism, and recognizing genetic polymorphisms in CYP isoenzymes. Key topics include tissue-specific markers (CK, LDH, AST), isoform kinetics (Km/Vmax differences), electrophoretic separation, and clinical applications like myocardial infarction diagnosis and personalized therapy. Mastery of isoenzyme significance helps with lab interpretation, therapeutic monitoring, and pharmacogenomics. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What defines isoenzymes (isozymes)?

• Enzymes from different organisms that catalyze different reactions
• Different enzymes that catalyze the same reaction but differ in structure and regulation
• Same enzyme with identical sequence found in multiple tissues
• Enzymes that are only active under anaerobic conditions

Correct Answer: Different enzymes that catalyze the same reaction but differ in structure and regulation

Q2. Which feature commonly differs among isoenzymes?

• Substrate stoichiometry
• Tissue distribution and kinetic properties
• The elemental composition of substrates
• The directionality of the catalyzed reaction

Correct Answer: Tissue distribution and kinetic properties

Q3. Which isoenzyme family is most relevant to drug metabolism and pharmacogenomics?

• Lactate dehydrogenase (LDH)
• Cytochrome P450 (CYP) isoenzymes
• Creatine kinase (CK)
• Hexokinase isoforms

Correct Answer: Cytochrome P450 (CYP) isoenzymes

Q4. CK-MB is an isoenzyme important in diagnosing which condition?

• Chronic liver disease
• Acute myocardial infarction
• Pancreatitis
• Kidney failure

Correct Answer: Acute myocardial infarction

Q5. LDH isoenzymes are numbered LDH1 to LDH5. LDH1 is most abundant in which tissue?

• Liver
• Heart and red blood cells
• Skeletal muscle
• Lung tissue

Correct Answer: Heart and red blood cells

Q6. The term “LDH flip” refers to which diagnostic pattern?

• Decreasing LDH5 with increasing LDH4 in hepatic disease
• LDH1 activity becoming higher than LDH2, often seen after myocardial infarction
• Total LDH falling below normal in hemolysis
• Simultaneous rise of all LDH isoenzymes in infection

Correct Answer: LDH1 activity becoming higher than LDH2, often seen after myocardial infarction

Q7. Which laboratory technique is commonly used to separate isoenzymes based on charge differences?

• Mass spectrometry without separation
• Isoelectric focusing or electrophoresis
• Polymerase chain reaction (PCR)
• Immunoprecipitation without gels

Correct Answer: Isoelectric focusing or electrophoresis

Q8. Aspartate aminotransferase (AST) exists in which isoenzyme forms relevant to clinical testing?

• Only a single cytosolic form
• Cytosolic and mitochondrial isoenzymes
• Nuclear and extracellular isoforms
• Lysosomal and peroxisomal isoenzymes

Correct Answer: Cytosolic and mitochondrial isoenzymes

Q9. Why are isoenzyme kinetic parameters like Km important in pharmacology?

• Km defines enzyme solubility in blood
• Different Km values affect substrate affinity and drug metabolism rates
• Km predicts the color change in assays
• Km indicates enzyme molecular weight

Correct Answer: Different Km values affect substrate affinity and drug metabolism rates

Q10. Genetic polymorphisms in which CYP isoenzyme are classically associated with poor metabolizer phenotypes?

• CYP3A4
• CYP2D6
• CYP1A2
• CYP4A11

Correct Answer: CYP2D6

Q11. Which property can distinguish isoenzymes encoded by different genes?

• Identical post-translational modifications in all tissues
• Different amino acid sequences and regulatory elements
• Uniform expression across all cell types
• Exact same Km and Vmax in vivo

Correct Answer: Different amino acid sequences and regulatory elements

Q12. In clinical interpretation, an isolated rise in ALT is most suggestive of which organ injury?

• Cardiac muscle
• Liver hepatocellular injury
• Skeletal muscle injury
• Pancreatic inflammation

Correct Answer: Liver hepatocellular injury

Q13. Which statement about fetal and adult isoenzymes is true?

• Fetal isoenzymes always have lower activity than adult forms
• Fetal and adult isoenzymes can differ in oxygen affinity and regulation
• They are indistinguishable by electrophoresis
• Only adults express isoenzymes

Correct Answer: Fetal and adult isoenzymes can differ in oxygen affinity and regulation

Q14. Which isoenzyme of creatine kinase predominates in skeletal muscle?

• CK-MB
• CK-BB
• CK-MM
• CK-AB

Correct Answer: CK-MM

Q15. Isoenzymes are useful as tissue markers because:

• They always circulate at constant plasma levels
• They show tissue-specific expression enabling organ-specific diagnosis
• They are resistant to all forms of degradation
• They bind non-specifically to all drugs

Correct Answer: They show tissue-specific expression enabling organ-specific diagnosis

Q16. Which method provides high specificity for isoenzyme identification using antibody binding?

• Enzyme-linked immunosorbent assay (ELISA) with isoform-specific antibodies
• pH titration curves
• Simple colorimetric total enzyme assays without separation
• Light microscopy of tissue sections only

Correct Answer: Enzyme-linked immunosorbent assay (ELISA) with isoform-specific antibodies

Q17. Isoenzymes can differ in subunit composition. LDH is a tetramer composed of which subunits?

• Two alpha and two beta tubulins
• Combinations of M (muscle) and H (heart) subunits
• Four identical gamma subunits only
• Alpha, beta, gamma, and delta unique subunits

Correct Answer: Combinations of M (muscle) and H (heart) subunits

Q18. Which LDH isoenzyme predominates in liver and skeletal muscle?

• LDH1
• LDH3
• LDH5
• LDH2

Correct Answer: LDH5

Q19. A patient has elevated CK-MB but normal troponin. This scenario most likely suggests:

• Definite acute myocardial infarction regardless of context
• Possible muscle injury or assay interference; troponin is more specific for MI
• Renal failure as the primary cause
• Hepatic necrosis

Correct Answer: Possible muscle injury or assay interference; troponin is more specific for MI

Q20. Isoenzymes may arise by which molecular mechanisms?

• Alternative splicing, gene duplication, and post-translational modification
• Only from viral infection
• Spontaneous amino acid racemization in tissues
• Direct conversion of DNA into proteins without transcription

Correct Answer: Alternative splicing, gene duplication, and post-translational modification

Q21. Why are CYP3A4 and CYP3A5 grouped together yet behave differently?

• They are identical enzymes with no functional differences
• They belong to the same subfamily but differ in expression and substrate specificity
• One is mitochondrial while the other is peroxisomal
• They only metabolize carbohydrates

Correct Answer: They belong to the same subfamily but differ in expression and substrate specificity

Q22. Inhibitors of a specific isoenzyme may cause:

• Enhanced activity of that isoenzyme in all tissues
• Reduced metabolism of drugs that are substrates of that isoenzyme leading to drug toxicity
• No clinical consequences because other isoenzymes always compensate fully
• Immediate gene mutation to produce a new isoenzyme

Correct Answer: Reduced metabolism of drugs that are substrates of that isoenzyme leading to drug toxicity

Q23. Which laboratory finding supports acute myocardial damage when LDH isoenzyme distribution is evaluated?

• LDH5 markedly higher than LDH1
• LDH1 greater than LDH2 (LDH flip)
• Uniform decrease in all LDH isoenzymes
• LDH3 exclusively elevated

Correct Answer: LDH1 greater than LDH2 (LDH flip)

Q24. The main clinical advantage of measuring isoenzymes rather than total enzyme activity is:

• Isoenzyme assays are always cheaper
• They can localize tissue origin of injury
• Total activity provides tissue-specific information already
• Isoenzyme assays require no sample processing

Correct Answer: They can localize tissue origin of injury

Q25. Which is true about isoenzyme nomenclature?

• Isoenzymes are named only by molecular weight
• Nomenclature can be based on composition (e.g., CK-MB), numbering (LDH1–LDH5), or gene names (CYP2D6)
• There is a universal single naming system across all enzyme families
• Isoenzymes are always named after discoverers’ initials

Correct Answer: Nomenclature can be based on composition (e.g., CK-MB), numbering (LDH1–LDH5), or gene names (CYP2D6)

Q26. Which isoenzyme pattern suggests hemolysis rather than myocardial infarct?

• Massive isolated rise in CK-MB only
• Prominent rise in LDH1 due to red blood cell release without cardiac symptoms
• Exclusive rise in ALT with normal cardiac enzymes
• Isolated rise in CK-BB

Correct Answer: Prominent rise in LDH1 due to red blood cell release without cardiac symptoms

Q27. Which statement about isoenzyme regulation is correct?

• All isoenzymes are constitutively expressed at constant levels
• Expression can be tissue-specific and regulated by developmental or hormonal signals
• Isoenzyme expression cannot change in disease states
• Isoenzymes do not undergo post-translational modifications

Correct Answer: Expression can be tissue-specific and regulated by developmental or hormonal signals

Q28. A pharmacologist studying drug interactions should pay attention to which isoenzyme property?

• Isoenzyme color under UV light
• Substrate specificity and sensitivity to inhibitors/inducers
• Gene length only
• Protein tertiary structure only, ignoring kinetics

Correct Answer: Substrate specificity and sensitivity to inhibitors/inducers

Q29. Which isoenzyme measurement is now considered more specific than CK-MB for myocardial injury?

• C-reactive protein
• Cardiac troponin I or T
• Total LDH only
• Serum amylase

Correct Answer: Cardiac troponin I or T

Q30. In drug development, why is isoenzyme profiling important?

• To ensure the drug is a perfect substrate for all isoenzymes
• To predict metabolism, identify primary metabolizing isoenzymes, and anticipate interactions and variability
• Only to determine drug color for packaging
• To replace clinical trials entirely

Correct Answer: To predict metabolism, identify primary metabolizing isoenzymes, and anticipate interactions and variability

Q31. Which isoenzyme is primarily measured to evaluate prostate cancer metastasis? (Trick: think tissue-specific enzymes)

• Prostate-specific antigen is not an isoenzyme but a tissue marker; alkaline phosphatase isoenzymes can indicate bone metastasis
• CK-MM is the specific marker for prostate cancer
• LDH5 is specific to prostate cancer
• AST mitochondrial isoenzyme indicates prostate cancer

Correct Answer: Prostate-specific antigen is not an isoenzyme but a tissue marker; alkaline phosphatase isoenzymes can indicate bone metastasis

Q32. Which statement distinguishes isoenzymes from allozymes?

• Allozymes and isoenzymes are identical concepts with no distinction
• Isoenzymes are different enzyme forms in the same species often from different genes; allozymes are allelic variants of the same gene
• Allozymes refer to proteins in different cellular compartments only
• Isoenzymes are always membrane-bound while allozymes are cytosolic

Correct Answer: Isoenzymes are different enzyme forms in the same species often from different genes; allozymes are allelic variants of the same gene

Q33. Which post-translational modification can create functionally distinct isoenzyme forms?

• Glycosylation and phosphorylation altering activity and localization
• Changing the genetic code directly in the protein
• Converting amino acids into nucleotides
• Removing the enzyme’s active site permanently in all tissues

Correct Answer: Glycosylation and phosphorylation altering activity and localization

Q34. Which of the following is a limitation when using total enzyme activity instead of isoenzyme profiling?

• Total activity always pinpoints the injured tissue
• Total enzyme assays may mask isoenzyme-specific changes and their tissue origins
• Total activity is never affected by hemolysis
• Total activity gives kinetic parameters directly

Correct Answer: Total enzyme assays may mask isoenzyme-specific changes and their tissue origins

Q35. Which isoenzyme is commonly elevated in patients with brain injury?

• CK-BB
• CK-MB
• LDH5
• ALT

Correct Answer: CK-BB

Q36. Which analytical technique can quantify isoenzyme expression at the mRNA level?

• Western blot
• Real-time PCR (qPCR) for specific isoenzyme transcripts
• Enzyme activity colorimetric assay only
• ELISA that measures protein mass only

Correct Answer: Real-time PCR (qPCR) for specific isoenzyme transcripts

Q37. A drug is metabolized primarily by CYP2C19. A patient with a loss-of-function CYP2C19 allele will likely experience:

• Increased metabolism and subtherapeutic levels
• Decreased metabolism and potential drug accumulation/toxicity
• No effect because CYP isoenzymes are redundant
• Immediate drug excretion without metabolism

Correct Answer: Decreased metabolism and potential drug accumulation/toxicity

Q38. Which isoenzyme measurement assists in differentiating skeletal muscle injury from myocardial injury?

• CK-MM predominance suggests skeletal muscle; CK-MB elevation suggests cardiac origin
• LDH1 predominance always indicates liver disease
• ALT measurement alone differentiates muscle from heart
• CK-BB exclusively indicates myocardial injury

Correct Answer: CK-MM predominance suggests skeletal muscle; CK-MB elevation suggests cardiac origin

Q39. Why might two isoenzymes have different inhibitor sensitivities?

• Because inhibitors change the DNA sequence of isoenzymes
• Differences in active site conformation and surrounding residues affect binding
• Inhibitors only bind to non-enzymatic proteins
• All isoenzymes are equally sensitive to any inhibitor

Correct Answer: Differences in active site conformation and surrounding residues affect binding

Q40. Which LDH isoenzyme is typically elevated in hepatic necrosis?

• LDH1
• LDH2
• LDH3
• LDH5

Correct Answer: LDH5

Q41. In the context of personalized medicine, isoenzyme profiling helps to:

• Identify color changes in formulations
• Predict individual variability in drug response and guide dose adjustments
• Eliminate the need for therapeutic drug monitoring
• Ensure uniform response across all patients

Correct Answer: Predict individual variability in drug response and guide dose adjustments

Q42. Which characteristic is NOT typical of isoenzymes?

• Different kinetic parameters (Km/Vmax)
• Tissue-specific expression patterns
• Always identical amino acid sequences
• Distinct sensitivity to inhibitors or activators

Correct Answer: Always identical amino acid sequences

Q43. Electrophoretic separation of isoenzymes utilizes differences in which property?

• Intrinsic enzyme fluorescence only
• Charge and sometimes size leading to distinct migration patterns
• Magnetic susceptibility of isoenzymes
• Enzyme catalytic rate during electrophoresis

Correct Answer: Charge and sometimes size leading to distinct migration patterns

Q44. Which clinical scenario best emphasizes the need to measure isoenzymes instead of total enzymes?

• When trying to determine whether elevated transaminases are from muscle or liver origin
• When measuring blood glucose levels
• When assessing vitamin deficiencies
• When total enzyme level is within normal limits and no tissue localization is needed

Correct Answer: When trying to determine whether elevated transaminases are from muscle or liver origin

Q45. Which CYP isoenzyme family is responsible for metabolizing the largest proportion of marketed drugs?

• CYP1A only
• CYP3A family (e.g., CYP3A4/5)
• CYP7A exclusively
• CYP26 family only

Correct Answer: CYP3A family (e.g., CYP3A4/5)

Q46. Heat stability tests historically helped differentiate isoenzymes because:

• Some isoenzymes denature at different temperatures allowing selective inactivation
• All isoenzymes are equally heat-stable
• Heat stabilizes all isoenzymes making them more active
• Heat selectively converts all isoenzymes to a single form

Correct Answer: Some isoenzymes denature at different temperatures allowing selective inactivation

Q47. Which isoenzyme change is often seen in skeletal muscle injury but not in isolated hepatic injury?

• Rise in ALT only
• Marked increase in CK-MM
• Exclusive rise in LDH1 only
• Decrease in all LDH isoenzymes

Correct Answer: Marked increase in CK-MM

Q48. In research, recombinant expression of isoenzymes allows scientists to:

• Study isoenzyme-specific kinetics, inhibitor profiles, and structural differences
• Eliminate the need for sequencing genes
• Automatically predict patient outcomes without clinical data
• Produce identical proteins regardless of isoform

Correct Answer: Study isoenzyme-specific kinetics, inhibitor profiles, and structural differences

Q49. Which statement about isoenzyme-based assays and assay interference is correct?

• Isoenzyme assays are never affected by hemolysis or lipemia
• Interfering substances can cause false results; specific immunoassays often reduce cross-reactivity compared to activity assays
• Interference is only a theoretical problem and does not occur clinically
• Assay interference always results in decreased signals only

Correct Answer: Interfering substances can cause false results; specific immunoassays often reduce cross-reactivity compared to activity assays

Q50. Evolutionarily, isoenzymes provide organisms with which advantage?

• Complete redundancy with no specialization
• Functional diversification allowing adaptation to different tissues, developmental stages, or environmental conditions
• Faster mutation rates that destroy enzyme function
• Prevention of allosteric regulation

Correct Answer: Functional diversification allowing adaptation to different tissues, developmental stages, or environmental conditions

G S Sachin

I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.

Mail- Sachin@pharmacyfreak.com