Figure 3 Synthesis of ketone bodies. In the mitochondria of hepatocytes, acetyl-CoA derived from ,3-oxidation is converted to ketone bodies, primarily acetoacetate and ,3-hydroxybutyrate, rather than entering the tricarboxylic acid cycle. Two molecules of acetyl-CoA condense in a reversal of the last ,3-oxidation reaction (3-oxoacyl-CoA thiolase). The product, acetoacetyl-CoA, condenses with another molecule of acetyl-CoA, yielding ^-hydroxy, ,3-methyl-glutaryl-CoA (HMG-CoA), a reaction catalysed by HMG-CoA synthase. Cleavage of HMG-CoA by HMG-CoA lyase yields acetoacetate, regenerating one molecule of acetyl-CoA. Acetoacetate is reversibly reduced to ,3-hydroxybutyrate via the NAD-dependent enzyme ,3-hydroxybutyrate dehydrogenase. These ketone bodies can traverse the inner mitochondrial membrane, eventually reaching the bloodstream for ultimate use by the brain and other tissues.

Phytanic acid Cholesterol

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