In diabetic ketoacidosis (DKA), ketogenesis is activated due to high glucagon:insulin ratio. Acetyl-CoA derived from unchecked beta-oxidation is channeled into ketone body synthesis in hepatic mitochondria. The committed step in ketogenesis is catalyzed by HMG-CoA synthase (mitochondrial). Why is ketogenesis limited to the LIVER (and not muscle or adipose tissue) despite these tissues also performing beta-oxidation?

• A Only liver mitochondria express the mitochondrial form of HMG-CoA synthase (HMGCS2) ✓
• B Only liver has sufficient carnitine to import fatty acids for beta-oxidation during fasting
• C Muscle and adipose tissue lack HMG-CoA lyase, the enzyme releasing acetoacetate from HMG-CoA
• D Only hepatocytes express the VLDL secretion pathway needed to export ketone bodies

Explanation

The mitochondrial isoform of HMG-CoA synthase (HMGCS2) is expressed at high levels exclusively in hepatocytes and, to a lesser extent, intestinal epithelium. This enzyme catalyzes the condensation of acetyl-CoA with acetoacetyl-CoA to form HMG-CoA, the committed step of ketogenesis. Muscle, adipose tissue, and other organs perform beta-oxidation but do not express significant HMGCS2 and therefore cannot synthesize ketone bodies — acetyl-CoA enters the TCA cycle or is used for local lipid synthesis. HMGCS2 expression is transcriptionally regulated by PPARalpha (activated by fasting/glucagon) and FOXA2. This tissue-specific expression of HMGCS2 is the biochemical basis for the liver's unique role as the organ of ketone body production.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

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Written and medically reviewed by the StethoPrep medical team.