Membranes mitochondrial, 3 - Hydroxybutyrate dehydrogenase

Ketone body synthesis occurs only in the mitochondrial matrix. The reactions responsible for the formation of ketone bodies are shown in Figure 24.28. The first reaction—the condensation of two molecules of acetyl-CoA to form acetoacetyl-CoA—is catalyzed by thiolase, which is also known as acetoacetyl-CoA thiolase or acetyl-CoA acetyltransferase. This is the same enzyme that carries out the thiolase reaction in /3-oxidation, but here it runs in reverse. The second reaction adds another molecule of acetyl-CoA to give (i-hydroxy-(i-methyl-glutaryl-CoA, commonly abbreviated HMG-CoA. These two mitochondrial matrix reactions are analogous to the first two steps in cholesterol biosynthesis, a cytosolic process, as we shall see in Chapter 25. HMG-CoA is converted to acetoacetate and acetyl-CoA by the action of HMG-CoA lyase in a mixed aldol-Claisen ester cleavage reaction. This reaction is mechanistically similar to the reverse of the citrate synthase reaction in the TCA cycle. A membrane-bound enzyme, /3-hydroxybutyrate dehydrogenase, then can reduce acetoacetate to /3-hydroxybutyrate. 

The penetrability of various thiol reagents through the inner mitochondrial membrane has been tested p-chloromercuribenzoate is a penetrant and inhibited d(—)-j8-hydroxybutyrate dehydrogenase in intact mitochondria on the inside of the mitochondrial membrane. 2-Chloromercuri-4-nitrophenol inhibited the ATPase of intact bovine heart mitochondria, and p-chloromercuribenzoate and mersalyl inhibited the liver microsomal calcium pump. While mersalyl had no effect on the Mg -dependent ATPase, adenylate cyclose from rat liver plasma was inhibited ... 

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-Ck)A, condenses with another molecule of acetyl-CoA, yielding /3-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.

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