Liver acetoacetate activation

While an active enzymatic mechanism produces acetoacetate from acetoacetyl-CoA in the liver, acetoacetate once formed cannot be reactivated directly except in the cytosol, where it is used in a much less active pathway as a precursor in cholesterol synthesis. This accounts for the net production of ketone bodies by the liver. 

Hepatic yd-oxidation, without oxidation of acetyl-CoA through the TCA cycle, produces a substantial amount of energy. At such a time, liver is actively engaged in gluconeogenesis so that mitochondrial oxaloacetate is depleted, TCA cycle activity is depressed, and acetyl-CoA levels rise. The last reaction in -oxidation is conversion of acetoacetyl-CoA to acetyl-CoA, with an equilibrium in favor of high levels of acetoacetyl-CoA. Thus, acetyl-CoA and acetoacetyl-CoA accumulate and form HMG-CoA cleavage of this last compound yields acetoacetate, which is reduced to jd-hydroxybutyrate. Acetone results from nonenzymatic decarboxylation of acetoacetate. Ketone body formation occurs exclusively in liver (see Chapter 18) and is prominent in starvation and diabetes owing to the... 

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. 

The rate of mitochondrial oxidations and ATP synthesis is continually adjusted to the needs of the cell (see reviews by Brand and Murphy 1987 Brown, 1992). Physical activity and the nutritional and endocrine states determine which substrates are oxidized by skeletal muscle. Insulin increases the utilization of glucose by promoting its uptake by muscle and by decreasing the availability of free long-chain fatty acids, and of acetoacetate and 3-hydroxybutyrate formed by fatty acid oxidation in the liver, secondary to decreased lipolysis in adipose tissue. Product inhibition of pyruvate dehydrogenase by NADH and acetyl-CoA formed by fatty acid oxidation decreases glucose oxidation in muscle. 

Acetoacetate picked up from the blood is activated in the mitochondria by succinyl CoA ace-toacetyl CoA transferase (common name thiophorase), an enzyme present only in extrahepatic tissues 3-hydroxybutyrate is first oxidized to acetoacetate. Because the liver lacks this enzyme, it carmot metabolize the ketone bodies. 

FIGURE 17-19 D-fj-Hydroxybutyrateasafuel. d-/3-Hydroxybutyrate, synthesized in the liver, passes into the blood and thus to other tissues, where it is converted in three steps to acetyl-CoA. It is first oxidized to acetoacetate, which is activated with coenzyme A donated from succinyl-CoA, then split by thiolase. The acetyl-CoA thus formed is used for energy production. 

Most tissues oxidize the acetyl-CoA produced during P-oxidation to C02 and water via the TCA cycle. During fasting, however, the liver utilizes the intermediates of the TCA cycle as gluconeogenic substrates. Under these conditions, the Ever converts acetyl-CoA to ketone bodies (acetoacetate and P-hydroxybutyrate) (Figure 32-5). Most other peripheral tissues can oxidize ketone bodies by the pathway shown in the figure. After entering the mitochondria, acetoacetate reacts with succinyl-CoA to form acetoacetyl-CoA, a reaction that is catalyzed by 3-oxoacid-CoA transferase. Alternatively, acetoacetyl-CoA is formed by direct activation of acetoacetate by the enzyme acetoacetyl-CoA synthetase. Acetoacetyl-CoA is then cleaved to form two molecules of acetyl-CoA by acetoacetyl-CoA thiolase.As noted earlier in... 

Acetoacetate can be activated by the transfer of CoA from succinyl CoA in a reaction catalyzed by a specific CoA transferase. Acetoacetyl CoA is then cleaved by thiolase to yield two molecules of acetyl CoA, which can then enter the citric acid cycle (Figure 22.20). The liver has acetoacetate available to supply to other organs because it lacks this particular CoA transferase. 

How does the liver meet its ovm energy needs a-Ketoacids derived from the degradation of amino acids are the liver s own fuel. In fact, the main role of glycolysis in the liver is to form building blocks for biosyntheses. Furthermore, the liver cannot use acetoacetate as a fuel, because it has little of the transferase needed for acetoacetate s activation to acetyl CoA. Thus, the liver eschews the fuels that it exports to muscle and the brain. 

The liver and to a lesser extent the kidneys, contain glucose 6-phosphatase, whereas muscle and the brain do not. Hence, muscle and the brain, in contrast with the liver, do not release glucose. Another key enzymatic difference is that the liver has little of the transferase needed to activate acetoacetate to acetoacetyl CoA. Consequently, acetoacetate and 3-hydroxybutyrate are exported by the liver for use by heart muscle, skeletal muscle, and the brain. 

The liver cannot use ketone bodies because it lacks the thiotransferase enzyme that activates acetoacetate. 

B. This compound is acetoacetate, which is synthesized in the liver when blood insulin levels are low. HMG CoA synthetase is the key regulatory enzyme for synthesis, not oxidation. Acetoacetate is transported to tissues, such as muscle, where it is activated in the mitochondrion by succinyl CoA (not ATP), cleaved to 2 acetyl CoA, and oxidized via the TCA cycle, which requires the vitamin thiamine as thiamine pyrophosphate, a cofactor for a-ketoglutarate dehydrogenase. Biotin is not required. 

Isotopic experiments shed more light on the contradictory evidence. Weinhouse and Millington (905) incubated liver slices with tyrosine labeled with in the 8-position of the side chain, and found that the activity appeared in the methylene carbon of acetoacetate. Schepartz and Gurin (772, 773) incubated with liver slices phenylalanine labeled with in the carboxyl group or a-position, or in positions 1, 3, and 5 of the aromatic ring. They found that the a-carbon of the side chain became the carboxyl-carbon of acetoacetate either C-1 or C-3 of the ring became the terminal methyl... 

Ketone body production and utilization. Ketone bodies are produced in the liver from fatty acids derived from adipocyte lipolysis. They are released and used as fuel in peripheral tissues. The initial step in acetoacetate metabolism is activation to acetoacetyl-CoA by succinyl-CoA. HMG-CoA, /S-hydroxy-y3-methylglutaryl-CoA HB, /i-hydroxybutyrate. 

Their acute toxicity to mammals is low, but the two active substances may cause dermatitis. In rats fed daily on a diet containing quinomethionate a high cumulative toxicity was observed. A dietary level of 500 mg/kg for 90 days reduced body-weight, caused hypertrophy of the liver, and inhibited acetoacetate synthesis and the microsomal enzymes. It primarily inhibited the function of the HS-enzymes (pyruvate dehydrogenase, succinate dehydrogenase, malate dehydrogenase and a-ketoglutarate oxidase) (Carlson and DuBois, 1970). 

In muscle, most of the fatty acids undergoing beta oxidation are completely oxidized to C02 and water. In liver, however, there is another major fate for fatty acids this is the formation of ketone bodies, namely acetoacetate and b-hydroxybutyrate. The fatty acids must be transported into the mitochondrion for normal beta oxidation. This may be a limiting factor for beta oxidation in many tissues and ketone-body formation in the liver. The extramitochondrial fatty-acyl portion of fatty-acyl CoA can be transferred across the outer mitochondrial membrane to carnitine by carnitine palmitoyltransferase I (CPTI). This enzyme is located on the inner side of the outer mitochondrial membrane. The acylcarnitine is now located in mitochondrial intermembrane space. The fatty-acid portion of acylcarnitine is then transported across the inner mitochondrial membrane to coenzyme A to form fatty-acyl CoA in the mitochondrial matrix. This translocation is catalyzed by carnitine palmitoyltransferase II (CPTII Fig. 14.1), located on the inner side of the inner membrane. This later translocation is also facilitated by camitine-acylcamitine translocase, located in the inner mitochondrial membrane. The CPTI is inhibited by malonyl CoA, an intermediate of fatty-acid synthesis (see Chapter 15). This inhibition occurs in all tissues that oxidize fatty acids. The level of malonyl CoA varies among tissues and with various nutritional and hormonal conditions. The sensitivity of CPTI to malonyl CoA also varies among tissues and with nutritional and hormonal conditions, even within a given tissue. Thus, fatty-acid oxidation may be controlled by the activity and relative inhibition of CPTI. 

The liver also plays a central role in lipid metabolism. When excess fuel is available, the liver synthesizes fatty acids. These are used to produce triglycerides that are transported from the liver to adipose tissues by very low density lipoprotein (VLDL) complexes. In fact, VLDL complexes provide adipose tissue with its major source of fatty acids. This transport is particularly active when more calories are eaten than are burned During fasting or starvation conditions, however, the liver converts fatty acids to acetoacetate and other ketone bodies. The liver cannot use these ketone bodies because it lacks an enzyme for the conversion of acetoacetate to acetyl CoA. Therefore the ketone bodies produced by the liver are exported to other organs where they are oxidized to make ATP. 

The amino acids phenylalanine and its hydroxylated derivative, tyrosine, are both catabolised in the livers of animals to fumaric acid and acetoacetic acid via homogentisic acid. This is formed by the oxidation of 4-hydroxyphenylpyruvate, catalysed by the copper containing enzyme 4-hydroxyphenylpyruvate dioxygenase, which requires vitamin C for its activity. The complete sequence is shown in Figure 5.13. The dioxygenase is so called because both the atoms of the... 

In mitochondria, acetoacetate is activated to acetoacetyl CoA by succinyl CoA acetoacetate CoA transferase. As the name suggests, CoA is transferred from succinyl CoA, a TCA cycle intermediate, to acetoacetate. Although the liver produces ketone bodies, it does not use them, because this thiotransferase enzyme is not present in sufficient quantity. 

In normal liver, only relatively small amounts of ketone bodies are formed. Their concentration in the blood is 0.5-D.8 mg per 100 ml plasma. The acetoacetate produced by this physiological K. is degraded in the peripheral musculature. Coenzyme A from succi-nyl-CoA is transferred to the acetoacetate by aceto-acetate succinyl-CoA transferase. Direct activation of acetoacetate by coenzyme A and ATP can also occur (Fig, 2). The acetoacetyl-CoA produced in either case is thioclastically cleaved into two molecules of acetyl-CoA, consuming a CoA molecule in the process. In carbohydrate deficiency (starvation, ketone-mia in ruminants), or deficient carbohydrate utilization (diabetes mellitus), K. is greatly increased. The cause of this pathological K. is a disturbance of the equilibrium between the degradation of fatty acids to acetyl-CoA and its utilization in the tricarboxylic acid cycle. The several-fold increase in the oxidation of the fatty acids leads under these conditions to an increase in the intracellular acetyl-CoA concentration. This leads to the condensation of 2 molecules of... 

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