Acetoacetate, beta-hydroxybutyrate, and acetone collectively are called ketone bodies. The first two are synthesized from acetyl-CoA, in the mitochondria of liver cells; acetone is formed by spontaneous decarboxylation of acetoacetate. Ketone body synthesis (Ketogenesis) in liver is effectively irreversible because the enzyme that catalyzes the conversion of acetoacetate to acetoacetyl-CoA is not present in liver cells. Ketone bodies, unlike fatty acids and triglycerides, are water-soluble. They are exported from the liver, and are taken up by other tissues, notably brain and skeletal and cardiac muscle. Extrahepatic tissues utilize ketone bodies by converting the beta-hydroxybutyrate successively to acetoacetate, acetoacetatyl-CoA, finally to acetyl-CoA which is oxidized via the TCA cycle to yield energy. In a normal person, this pathway of ketone body synthesis and utilization is most active during extended periods of fasting. Under these conditions, mobilization and breakdown of stored fatty acids generates abundant acetyl-CoA acetyl-CoA in liver cells for synthesis of ketone bodies, and their utilization in other tissues minimizes the demand of these tissues for glucose. The levels of acetone in ketone bodies are much lower than those of acetoacetic acid and beta-hydroxybutyric acid. Acetone cannot be converted back to acetyl-CoA, and is excreted in urine, or breathed out through the lungs.

EXAMPLE: Ketone Body Metabolism References