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Phase 2 Conjugation: Glucuronidation and Why Some Drugs Last Longer | Anesthesia Foundational Concepts

Imagine trying to catch a very slippery fish with your bare hands. It keeps slithering away. Now imagine someone glues a heavy, easy-to-grip handle onto the fish. Suddenly you can catch it.

That is conjugation. Fat-soluble drug molecules are the slippery fish — they slip through the kidneys, get reabsorbed, and stick around in the body. Phase II enzymes glue a polar handle onto each drug molecule, usually glucuronic acid, and the resulting compound is water-loving enough for the kidneys to grab and flush out.

This episode covers how that works, why almost every drug needs chemical alteration before it can leave the body, the difference between Phase I and Phase II and the role of glucuronidation. Then it follows conjugation to the bedside, where two stories anchor almost everything anesthesiology needs to know.

Key Takeaways

• Phase II conjugation attaches a large polar group to a lipid-soluble drug so the kidneys can excrete it.

• Glucuronidation is the busiest pathway.

• Morphine is the famous exception.

• Morphine-6-glucuronide (M6G) is roughly twice as potent as morphine at the μ-opioid receptor and accounts for about 10 percent of metabolites.

• Acetaminophen overdose works through the same machinery. Glucuronidation and sulfation saturate, more drug shunts through CYP2E1 to NAPQI, glutathione depletes, and NAPQI binds hepatocyte proteins. N-acetylcysteine replenishes glutathione so conjugation of NAPQI can resume.

• Phase II is relatively preserved in liver disease.

• Newborns have immature UGT capacity. That shortfall is behind neonatal jaundice.

Timestamps

00:00 — The slippery fish

01:17 — Two fates, two phases

02:00 — Phase I versus Phase II

02:30 — Glucuronidation and the other tags

03:17 — Immature at birth: jaundice and gray baby

03:46 — The exception: morphine-6-glucuronide

04:55 — From Theory to Practice

05:06 — Morphine and renal failure

06:09 — Acetaminophen and NAPQI

07:19 — One habit of mind

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