Under the catalysis of HMG-coa synthetase, acetocoa produced by fatty acid oxidation is synthesized into ketone. There are enzymes that produce ketone bodies in the liver, but there are no enzymes that use ketone bodies. Ketone bodies are used in extrahepatic tissues, such as myocardium, skeletal muscle and brain, which means that ketone bodies are a main form of lipid energy output by the liver and provide energy for important tissues of the body when hungry. The key enzymes catalyzed by succinyl-CoA thiotransferase and acetylacetylthiokinase.