8. Ketone bodies are formed from ____________ in the liver and used for ________________.

Question

BlackTom AI · Accepted Answer

Ketone bodies are synthesized from acetyl-CoA in liver mitochondria and serve as an alternative energy source for extrahepatic tissues.

Relevant knowledge points:
- Ketogenesis occurs in the liver during fasting, prolonged exercise, or uncontrolled diabetes when fatty acid β-oxidation yields excess acetyl-CoA.
- Major ketone bodies: acetoacetate, β-hydroxybutyrate, and acetone.
- Extrahepatic tissues (brain during prolonged fasting, heart, skeletal muscle, kidney cortex) convert ketone bodies back to acetyl-CoA for entry into the TCA cycle.
- The liver cannot catabolize ketone bodies because it lacks succinyl-CoA:3-oxoacid CoA transferase (thiophorase).

Step-by-step biochemical reasoning and intermediate steps:
1) Two acetyl-CoA molecules condense: acetyl-CoA + acetyl-CoA → acetoacetyl-CoA (enzyme: thiolase).
2) Acetoacetyl-CoA + acetyl-CoA → HMG-CoA (enzyme: HMG-CoA synthase — rate-limiting for ketogenesis).
3) HMG-CoA → acetoacetate + acetyl-CoA (enzyme: HMG-CoA lyase).
4) Acetoacetate is reduced to β-hydroxybutyrate (enzyme: β-hydroxybutyrate dehydrogenase) or decarboxylates to acetone (spontaneously).

Utilization in extrahepatic tissues:
1) β-hydroxybutyrate → acetoacetate (β-hydroxybutyrate dehydrogenase).
2) Acetoacetate + succinyl-CoA → acetoacetyl-CoA + succinate (enzyme: succinyl-CoA:3-oxoacid CoA transferase, thiophorase) — this step is absent in liver.
3) Acetoacetyl-CoA → 2 acetyl-CoA (enzyme: thiolase), which enter the TCA cycle for ATP production.

Why the correct answer is correct:
The biochemical pathway clearly starts from acetyl-CoA and produces ketone bodies in the liver; those ketone bodies are transported to and oxidized by extrahepatic tissues for energy. The liver’s lack of thiophorase ensures ketones are exported rather than consumed there.

Why other plausible options are incorrect (brief):
- Ketones are not synthesized directly from pyruvate or glucose as primary substrates in fasting — glucose-derived pyruvate would be inadequate when oxaloacetate is low.
- They are not primarily used by the liver; the liver exports them because it cannot convert them back to acetyl-CoA.

Conclusion: This confirms the correct answer: B. acetyl-CoA / energy production in extrahepatic tissues.

8. Ketone bodies are formed from in the liver and used for ____.单项选择题

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8. Ketone bodies are formed from ____________ in the liver and used for ________________.单项选择题