How can we measure levels of bioactivation? Select the sentence that is false

Question

BlackTom AI · Accepted Answer

Covalent binding (bioactivation) is typically assessed by trapping reactive metabolites (e.g., with glutathione), using radiolabels to quantify covalent associations, or by measuring plasma protein binding with standard separation methods.

1) Relevant knowledge points and concepts
- Bioactivation: enzymatic conversion of a drug to a reactive metabolite that can covalently bind macromolecules (proteins, DNA).
- Glutathione (GSH) trapping: GSH is a nucleophilic surrogate that forms stable adducts with electrophilic metabolites; these adducts are detected by LC/MS or HPLC.
- Covalent protein binding assays: commonly use a radiolabeled drug incubated with liver microsomes; after incubation free drug is removed (precipitation/wash or dialysis) and radioactivity associated with protein is measured to quantify covalent binding (pmol equiv/mg protein). Plasma protein binding (non-covalent) is measured by equilibrium dialysis or ultrafiltration, not by the covalent-binding protocol.
- Safety threshold: a commonly used informal cutoff for microsomal covalent binding is <50 pmol equiv/mg protein as low risk.

2) Step-by-step reasoning showing why option b is false
- Option b states “The most direct way to measure protein binding is by digesting radiolabeled drugs with liver microsomes and quantifying the level of radiation left after washout the free drug.”
- First, the phrase “digesting radiolabeled drugs with liver microsomes” is incorrect: microsomes contain metabolic enzymes that metabolize drugs, not a digestion step; you incubate radiolabeled drug with microsomes to allow bioactivation, not “digest” the drug.
- Second, “the most direct way to measure protein binding” conflates two different assays: non‑covalent plasma protein binding (best measured by equilibrium dialysis/ultrafiltration) versus covalent protein binding/covalent adduct formation (measured by radiolabel covalent binding assays). Therefore the statement is inaccurate and misleading.

3) Calculations/intermediate steps
- No numeric calculation required. Note that covalent binding readout is typically reported as pmol equivalent radioactivity per mg microsomal protein; the cited safety cutoff (~50 pmol equiv/mg) arises from empirical comparisons.

4) Why the correct answer (b) is correct as the false statement
- Because b misstates the experimental procedure (“digesting…with liver microsomes”), incorrectly labels the “most direct” method for protein binding, and mixes assay types (covalent binding vs. plasma protein binding). Thus b is the false sentence.

5) Why the other options are not false
- a: True — GSH trapping in microsomal assays with LC/MS detection is standard for identifying non‑radiolabeled reactive metabolites.
- c: True — GSH serves as a small‑molecule surrogate nucleophile that mimics protein thiols and forms detectable adducts.
- d: Largely true in context — if radiolabeling the drug is impractical, labeled cofactors or microsomal components can be used to assess covalent binding, and empirical cutoffs (e.g., <50 pmol equiv/mg) are used to flag low risk.

6) Conclusion
- Statement b is false for the reasons above. This confirms the provided answer: b.

How can we measure levels of bioactivation? Select the sentence that is false单项选择题

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