Explore the system
Transform, then eliminate
Follow Phase I and Phase II reactions, and see how enzyme induction and inhibition change the model.
Sources and model limits
Original schematic. Positions show relationships; structures and concentrations are not to scale.
- Julien's Primer of Drug Action · 15 · 1 · Transform, then eliminate · PDF 15, 16, 17 · 1.8, 1.9
Course notes and existing practice
Learning objective
Describe metabolic transformation, CYP enzymes and conjugation without equating all metabolism with inactivation.
Chemical transformation
The liver is a major site of systemic drug metabolism, but metabolism also occurs elsewhere. Enzymes transform parent compounds into products with potentially different properties. Some products have little relevant activity; others remain active. Metabolism often helps make products easier to excrete, but the fate of a particular compound requires its own evidence.
Phase I and Phase II
The lecture introduces CYP enzymes and oxidation, reduction and hydrolysis as Phase I processes, and conjugation, including glucuronidation, as Phase II. These labels describe reaction categories, not a mandatory two-step conveyor belt for every drug. Conjugation joins a compound to another chemical group. This lesson does not use the deck's simplified electron-charge descriptions or antioxidant discussion as universal chemistry rules.
- Active metabolite
- A product of metabolism that retains or produces pharmacological activity.
- Conjugation
- A metabolic reaction that attaches another chemical group to a compound.
- Cytochrome P450 (CYP)
- An enzyme family important in the metabolism of many drugs.
Professor deck 04, PDF pages 32, 33, 34, 35, 36. Course material is paraphrased. Qualifications and excluded clinical generalizations are documented in the Pharmacokinetics editorial record. Source review does not establish clinical validation.
Examples and practice explanations are original study aids, not professor quotations, exam requirements or medication instructions.
Julien connection: a distinction to explain on your own
Phase I and phase II classify reactions; not every drug must pass through both in order. Before predicting an enzyme interaction, identify whether the enzyme clears an active parent or activates a prodrug. Inhibiting those two pathways can have opposite consequences for active exposure.
Book connection: see the Julien Chapters 1–4 · Midterm study section in the Library for worked examples and recall practice. The cited source uses consolidated PDF page numbers.