Acetoacetic acid, activation

In extrahepatic tissues, acetoacetate is activated to acetoacetyl-CoA by succinyl-CoA-acetoacetate CoA transferase. CoA is transferred from succinyl-CoA to form acetoacetyl-CoA (Figure 22-8). The acetoacetyl-CoA is split to acetyl-CoA by thiolase and oxidized in the citric acid cycle. If the blood level is raised, oxidation of ketone bodies increases until, at a concentration of approximately 12 mmol/L, they saturate the oxidative machinery. When this occurs, a large proportion of the oxygen consumption may be accounted for by the oxidation of ketone bodies. 

In several subsequent publications, this promising multicomponent synthetic approach was used for the synthesis of certain types of biologically active heterocyclic compounds. For instance. Boros and co-authors [35] reported application of the three-component heterocyclization between bicyclic aminoazole 2, acetoacetic acid derivatives 3, and aldehyde 4 to obtain compound 5 being aza-analog of known [36] agonist of the calcetonine receptor (Scheme 4). 

Addition of ethyl acetate to a specimen having a transaminase activity of 47 units was responsible for the following increases in enzyme activity 10 mg/100 ml, 60 units 20 mg/100 ml, 77 units 40 mg/100 ml, 107 units and 80 mg/100 ml, 150 units. Transaminase activity in these specimens determined by another method ranged from 32 to 34 units (C7). Thus, when serum from patients with ketosis is assayed for aspartate aminotransferase activity by the diazo method, false elevations of activity may be recorded due to reaction of acetoacetic acid. In Table 11 are shown some values obtained by the diazo method and by an ultraviolet NADH NAD aspartate aminotransferase technique (B12). Examination of the medical records of these patients indicated that they were either diabetics who were in ketosis or individuals who were eating very poorly and had some degree of starvation ketosis. Similar elevations have been observed in patients receiving p-aminosalicylic acid (G6). 

In extraliepatic tissues, d-/3-hydroxybutyrate is oxidized to acetoacetate by o-/3-hydroxybutyrate dehydrogenase (Fig. 17-19). The acetoacetate is activated to its coenzyme A ester by transfer of CoA from suc-cinyl-CoA, an intermediate of the citric acid cycle (see Fig. 16-7), in a reaction catalyzed by P-ketoacyl-CoA transferase. The acetoacetyl-CoA is then cleaved by thiolase to yield two acetyl-CoAs, which enter the citric acid cycle. Thus the ketone bodies are used as fuels. 

Gas-phase decarboxylation of /i-ketocarboxylic acids XCOCH2COOH (X = H, OH, and CH3) has also been the subject of theoretical studies.42 Ah initio calculations reveal that decarboxylation via a six-membered (rather than four-membered) ring transition state is favoured. Activation barriers of 23.8, 23.3 and 28.5 kcal mol-1 have been calculated for decarboxylation of 3-oxopropanoic acid, acetoacetic acid, and malonic acid, respectively. Only marginal effects of solvent on the energy barriers and on the geometries of the reactants and transition structures are predicted. The activation energy predicted for reaction of malonic acid agrees well with the experimental value and rate constants have been predicted for decarboxylation of 3-oxopropanoic acid and acetoacetic acid in the gas phase. 

Structures 64 and 65 were proposed in [7] for the product from the condensation of isatin 7 with acetoacetic acid (a P-keto acid). The first must clearly be preferred, since the CH2 group must be more active than CH3 under the conditions of the Pfitzinger reaction. Actually in [21] the structure of the product 64 was proved by its oxidation to the tricarboxylic acid 66, which was also synthesized from the keto dicarboxylic acid 67 and isatin 7. 

Qualitative data on the cyclizations of pyrazinium 36, quinoxalinium 37, 1,2,4-triazinium 38, and pteridinium 40 salts with CH-active acetamides show that their reactivities follow the same order as has been established for diaddition reactions with simple nucleophiles (Section III,B,1) (86KGS1380). For example, the anilide of acetoacetic acid readily forms cycloadducts with quinoxalinium salts 37, but no cyclization products with pteridinium cations 40. By enhancing the NH-activity of the amide group, the cyclization with pteridinium salts could be achieved. Thus, the N-(pyridin-2-yl)-substituted amide of acetoacetic acid is able to undergo the cyclization reaction with the pteridinium cation 40d to afford pyrrolo [2,3-g] pteridine 62 (Scheme 48) (86KGS420). 

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. 

Acetoacetate is converted into acetyl CoA in two steps. First, acetoacetate is activated by the transfer of CoA from succinyl CoA in a reaction catalyzed by a specific CoA transferase. Second, acetoacetyl CoA is cleaved by thiolase to yield two molecules of acetyl CoA, which can then enter the citric acid cycle (Figure 22.22). The liver has acetoacetate available to supply to other organs because it lacks this particular CoA transferase. 3-Hydroxybutyrate requires an additional step to yield acetyl CoA. It is first oxidized to produce acetoacetate, which is processed as heretofore described, as well as NADH for use in oxidative phosphorylation. 

C. Leucine but none of the other amino acids listed is a branched-chain amino acid. The muscle has a very active branched-chain amino acid metabolic pathway and uses that pathway to provide energy for its own use. The products of leucine metabolism are acetyl-CoA and acetoacetate, which are used in the tricarboxylic acid cycle. Acetoacetate is activated by succinyl-CoA and cleaved to two molecules of acetyl-CoA in the P-ketothiolase reaction. The other branched-chain amino acids, valine, and isoleucine, yield succinyl-CoA and acetyl-CoA as products of their catabolism. 

By substituting (S)-(-)-l-amino-2-(dimethylmethoxymethyl)pyrrolidine (S)-(83) for (S)-(4), Enders has developed an efficient and enantioselective Hantzsch synthesis (Scheme 4). In this synthesis, the more-hindered hydrazone formed from (83) was condensed with an acetoacetic acid ester. Deprotonation of the hydrazone so-formed (the major tautomer present was an enehydrazine) followed by addition of an arylidene malonate derivative yielded (85), which could be closed with mild acid to yield optically active... 

Reactions using highly acidic active methylene compounds (pAa = 9-13) comprise nearly all the early examples of imine condensation reactions, some of which date back to the turn of the century. Reviews by Layer and Harada have summarized many of these reactions and include examples using diethyl malonate, ethyl cyanoacetate, ethyl malonamide, acetoacetic acid, benzoylacetic esters and nitroalkanes. Conditions of these reactions vary they have been performed both in protic and aptotic solvents, neat, and with and without catalysts. Elevated temperatures are generally required. Reactions with malonates have useful applications for the synthesis of 3-amino acids. For example, hydrobenzamide (87), a trimeric form of the benzaldehyde-ammonia Schiff base, and malonic acid condense with concomitant decarboxylation to produce p-phenylalanine (88) in high yield (equation 14). This is one of the few examples of a Mannich reaction in which a primary Mannich base is produced in a direct manner but is apparently limited to aromatic imines. 

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... 

Another path of manufacture of practical catalysts is using immobilized chiral metal complexes. Thus, the complex [Rh-BESIAP] was occluded in an elastomeric t5T)e polydimethylsiloxane membrane, which gave a re-generable active membrane-catalyst with the same enantioselectivity as the homogeneous catalyst in the hydrogenation of acetoacetic acid ester into methyl (7 )-(-)-3-hydroxybutyrate, that can be pol5mierized into polyester (Scheme 7.17.). 

Vinyl polymerization using metallocomplexes commonly proceeds by a radical pathway and rarely involves an ionic mechanism. For instance, metal chelates in combination with promoters (usually halogenated hydrocarbons) are known as initiators of homo- and copolymerization of vinylacetate. Similar polymer-bound systems are also known [3]. The polymerization mechanism is not well understood, but it is believed to be not exclusively radical or cationic (as polymerization proceeds in water). The macrochelate of Cu with a polymeric ether of acetoacetic acid effectively catalyzes acrylonitrile polymerization. Meanwhile, this monomer is used as an indicator for the radical mechanism of polymerization. Mixed-ligand manganese complexes bound to carboxylated (co)polymers have been used for emulsion polymerization of a series of vinyl monomers. Macromolecular complexes of Cu(N03)2 and Fe(N03)3 with diaminocellulose in combination with CCI4 are active in polymerization of MMA, etc. 

Acetoacetic acid, CHgCOCHgCOOH, is an aliphatic -keto acid. Derivatives are formed by reduction, activation of the carboxylic group, and decarboxylation. 

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 oxidation of the l( —) hydroxy fatty acyl-CoA is catalyzed by ketoacyl thiolases. An enzyme has been isolated from beef and sheep liver mitochondria that catalyzes such a reaction on fatty ketoacyl with chain lengths of four to eighteen carbons. The enzyme molecule contains SH groups that are essential for activity. The cell probably contains more than one thiolase. A thiolase specific for acetoacetic acid has been found and is discussed in the section on ketosis. 

Two observations suggest that ketosis in diabetes results from the combination of the accelerated hydroxymethylglutarate shunt and the increased rate of deacylation. Mitochondrial deacylation is the major pathway for converting acetoacetyl CoA to acetoacetic acid in both normal and alloxan-diabetic livers, and a marked increase in activity of acetoacetyl CoA deacylase and a moderate increase in the hydroxymethylglutarate cleavage enzymes is observed in alloxan-diabetic rats. 

The activity of hydroxymethylglutarate CoA reductase, which produces mevalonic acid—a precursor of cholesterol—was unchanged in diabetic rats. The observations made on diabetic rats contrast with those made in fasted animals in which ketosis is likely to result from activation of the hydroxymethylglutarate CoA shunt pathway, probably due to decreased activity of the hydroxymethylglutarate CoA reductase. In the presence of NADH and a specific mitochondrial dehydrogenase, acetoacetic acid is reduced to yield j8-hydroxybutyric acid, one of the ketone bodies that is excreted in the urine in ketosis. In fact, D-jS-hydroxy-butyric acid represents 50-75% of the blood content of ketone bodies. Therefore, hydroxybutyric acid metabolism assumes a particular importance. 

The conversion of acetoacetic acid to hydroxybutyric acid restitutes the 4-carbon compounds to the general metabolism because the liver contains enzymes that will activate jS-hydroxybutyric acid to yield j8-hydroxybutyric CoA, whereas acetoacetic acid cannot be acylated by the liver. 

Krebs Cycle and Fatty Acid Oxidation. A possible role of Krebs cycle intermediates in supporting fatty acid oxidation is now apparent. Complete oxidation to CO2 requires oxalacetate to introduce acetyl CoA into the citric acid cycle. But even the formation of acetoacetate requires the continued generation of ATP to support the activation of fatty acids. The transfer of electrons from fatty acid to oxygen is coupled with phosphate esterification, so that fatty acid oxidation has the theoretical capacity to be self-supporting. In the crude systems that contain all of the essential factors for fatty acid oxidation, fatty acid activation must compete with other reactions for the available ATP, and maximum rates of oxidation occur only when additional ATP is generated through operation of the Krebs cycle. 

Approximately the same dilution of activity was found for both the and the labels, indicating that the acetoacetic acid is derived as a 4-carbon atom unit from phenylalanine. 

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