Heart acetoacetate activating enzyme

Fatty acid oxidation can be terminated in either of two ways. Acetoacetyl CoA can either be cleaved to two molecules of acetyl CoA which condense with oxalacetate to form citrate, or it can be deacylated to acetoacetate by a deacylase specific for d-keto butyryl CoA. - In kidney and heart muscle there is no accumulation of acetoacetate, whereas in liver acetoacetate is formed in preference to citrate. The non-accumulation of acetoacetate in tissues other than liver probably is referable to the following circumstances. All tissues but liver contain activating enzymes which catalyze the conversion of acetoacetate to acetoacetyl CoA. Thus any acetoacetate formed by deacylation is thrust back as it were into the metabolic wheel. In liver deacylation is not opposed by this reactivation of acetoacetate. Hence acetoacetate accumulates only in liver. 

Both kidney and heart mitochondria contain enzymes which catalyze respectively the activation of acetate and acetoacetate by a... 

Beinert and Stansley " have investigated the nature of the reactions leading to asymmetrically-labeled acetoacetate with a soluble enzyme system from pig heart, viz. the acetoacetate activation and cleavage enzyme system. This system carries out the condensation-cleavage reaction... 

A variety of thiokinases probably exist, but only a few of them have been identified. Acetic acid and butyryl thiokinase have been purified from a variety of sources, including yeast, liver, and muscle. These two enzymes differ in their specificity for the substrate. Acetic thiokinase catalyzes only the oxidation of propionic, acetic, and acrylic acids, but butyryl thiokinase activates fatty acids of chain lengths ranging from 4-to 12-carbon units. A third thiokinase was also discovered. It acts on fatty acid chains with 5- to 22-carbon units and is found in the microsomes. This intracellular distribution is in striking contrast with the cellular location of all other enzymes involved in fatty acid oxidation, which are all in mitochondria. The palmityl enzyme, which is active in the presence of ATP and CoA, becomes inactive when incubated in the absence of CoA therefore, it has been proposed that the active form of the enzyme involves the formation of an enzyme-CoA complex. The heart, the skeletal muscle, and the kidney also contain a thiokinase that specifically activates acetoacetic acid. Acetoacetic acid thiokinase is absent in liver this observation is significant in the pathogenesis of ketosis. 

The liver is clearly well equipped to utilize free fatty acids and to interconvert acetoacetate and hydroxybutyrate, but the virtual absence of 3-Oxoacid-CoA transferase and lipoprotein lipase means that any significant uptake of ketone bodies and triglycerides is restricted to extra-hepatic tissues. Heart and kidney contain the necessary enzymes to deal with all four fuels and this may reflect their high metabolic activity. Page and Williamson (1971) have shown that normal human brain has the capacity to utilize ketone bodies. 

Acetoacetate Metabolism. An active deacylase in liver is responsible for the formation of free acetoacetate from its CoA derivative. The j8-hydroxybutyric dehydrogenase mentioned above and a decarboxylase are capable of converting acetoacetate into the other ketone bodies, /3-hydroxybutyrate, and acetone. liver does not contain a mechanism for activating acetoacetate. Heart muscle has been found to contain a specific thiophorase that forms acetoacetyl CoA at the expense of suc-cinyl CoA. Acetoacetate is thus used by peripheral tissues by activation through transfer, then reaction with either the enzymes of fatty acid synthesis or jS-ketothiolase and the enzymes that use acetyl CoA. 

Active acetoacetate is formed by replacement of the succinyl group in the thioester bond by acetoacetate. Crude preparations of the 8-keto acyl CoA cleavage enzyme of heart muscle are usually contaminated with the kinase which catalyzes the above replacement reaction. 

The oxidation of acetate, acetoacetate, and fatty acids by a nonmito-chondrial system from pig heart has been described. The heart system consists of three parts (1) a particulate nonmitochondrial fraction (2) a group of soluble enzymes and (3) a coenzyme concentrate, incompletely characterized, but which is known to contain di- and triphospho-pyridine nucleotides, coenzyme A, and ATP. The system reflects many of the properties of intact mitochondria and lends itself to a study of the reaction sequences in the activation and oxidation of fatty acids. 

---