Enzyme Induction and Enzyme Inhibition

Introduction

PART 1: ENZYME INDUCTION

Definition

Mechanism

1. Aryl Hydrocarbon Receptor (AHR) — CYP1A Induction

• Polycyclic aromatic hydrocarbons (e.g., benzo[a]pyrene from tobacco smoke, TCDD/dioxin) bind to the cytoplasmic AHR receptor.
• The inducer–AHR complex translocates to the nucleus, dimerizes with a nuclear protein Arnt, and activates regulatory (response) elements on CYP1A genes, inducing their transcription.
• Also triggered by cruciferous vegetables and the proton pump inhibitor omeprazole.

2. Pregnane X Receptor (PXR) — CYP3A4 Induction

• PXR is a member of the steroid-retinoid-thyroid hormone nuclear receptor superfamily.
• Ligands include rifampin, dexamethasone, phenobarbital, St. John's wort (hyperforin), mifepristone, atorvastatin.
• Activated PXR forms a heterodimer with Retinoid X Receptor (RXR), which binds to response elements upstream of CYP3A4 gene, stimulating transcription.

3. Constitutive Androstane Receptor (CAR) — CYP2B6, CYP2C9, CYP3A4 Induction

• CAR is activated by the phenobarbital class of inducers.
• Like PXR, it heterodimerizes with RXR and binds promoter response elements to induce gene expression.

4. Peroxisome Proliferator-Activated Receptor α (PPAR-α) — CYP4A Induction

• Activated by lipid-lowering drugs (fenofibrate, gemfibrozil).
• Mediates induction of CYP4A enzymes involved in fatty acid (e.g., arachidonic acid) metabolism.
• Also heterodimerizes with RXR.

5. Substrate Stabilization (Post-translational Mechanism)

• Some inducers act by decreasing enzyme degradation rather than increasing synthesis.
• Examples: ethanol → CYP2E1 induction; troleandomycin/clotrimazole → CYP3A; isosafrole → CYP1A2.

Key Inducers and Their Targets

Clinical Consequences

• Reduced drug efficacy: Faster metabolism reduces plasma levels of co-administered drugs (e.g., rifampin reduces oral contraceptive levels → contraceptive failure).
• Drug tolerance: Enhanced metabolism of the inducing drug itself.
• Increased prodrug activation: Enhanced conversion of prodrugs to active metabolites increases efficacy.
• Increased toxicity: If metabolites are toxic (e.g., acetaminophen with CYP2E1 induction by ethanol → more toxic NAPQI formed).
• Onset: Induction takes days to weeks as new protein must be synthesized; effects wane similarly after drug withdrawal.

PART 2: ENZYME INHIBITION

Definition

Types and Mechanisms

1. Competitive (Reversible) Inhibition

• The inhibitor competes with the substrate for the active site of the enzyme.
• Imidazole-containing drugs (cimetidine, ketoconazole) bind tightly to the heme iron of CYP enzymes, reducing metabolism of co-administered substrates (e.g., testosterone, warfarin).
• Effect is reversible and depends on relative concentrations.

2. Quasi-irreversible Inhibition (Metabolite-Intermediate Complexes)

• The drug is metabolized by a CYP enzyme to a reactive intermediate that forms a stable complex with the heme iron, rendering it catalytically inactive.
• Macrolide antibiotics (erythromycin, troleandomycin) are metabolized by CYP3A to nitroso intermediates that bind heme and inhibit the enzyme.
• Proadifen (SKF-525-A, used in research) also acts this way — quasi-irreversible inactivation.

3. Mechanism-Based (Irreversible/Suicide) Inhibition

• The inhibitor is converted by the enzyme to a reactive intermediate that:
  • Covalently binds the CYP apoprotein, or
  • Binds the heme moiety, or
  • Causes the heme to fragment and irreversibly modify the protein.
• This permanently destroys enzyme activity; recovery requires new enzyme synthesis.
• Examples:
  • Chloramphenicol → metabolized by CYP2B1 to a species that modifies the P450 protein.
  • Secobarbital → inactivates CYP2B1 by modifying both heme and protein.
  • Grapefruit furanocoumarins → irreversibly inactivate CYP3A4 in intestinal wall (first-pass effect increases drug bioavailability).
  • Other suicide inhibitors: ethinyl estradiol, norethindrone, spironolactone, ritonavir, ticlopidine, clopidogrel, selegiline.

4. Non-competitive / Allosteric Inhibition

• Inhibitor binds to a site other than the active site, altering enzyme conformation and reducing activity regardless of substrate concentration.

Key Inhibitors and Their Targets

Clinical Consequences

• Increased drug toxicity: Reduced clearance elevates plasma drug concentrations — e.g., erythromycin inhibiting CYP3A4 increases cyclosporine levels → nephrotoxicity.
• Prolonged drug action: Beneficial in some cases (e.g., ritonavir boosting other HIV drugs).
• Rapid onset: Competitive inhibition occurs immediately upon co-administration (contrast with induction which requires days).
• Drug-drug interactions: A common mechanism of clinically significant interactions involving narrow-therapeutic-index drugs (warfarin, digoxin, cyclosporine).

Comparison Summary

• Katzung's Basic and Clinical Pharmacology, 16th Edition — Enzyme Induction and Enzyme Inhibition, Chapter 4
• Miller's Anesthesia, 10th Edition — Biotransformation in the Liver
• Sleisenger and Fordtran's Gastrointestinal and Liver Disease — Genetic and Environmental Determinants of CYP Enzymes

• ✅ Clear definitions of both induction and inhibition
• ✅ Mechanism explained with nuclear receptors (PXR, CAR) — this is the detail that earns marks
• ✅ Consequences/clinical significance for both
• ✅ Examples with drug names
• ✅ Introduction and conclusion