Decarboxylation, acetoacetic acid oxidative

Dihydroxyphenylpyruvate was not oxidized to homogentisate, thus it cannot be an intermediate in the oxidation of p-hydroxyphenyl-pyruvate. The results suggest that the hydroxylation shift of the side chain and decarboxylation of the p-hydroxyphenylpyruvate are simultaneous processes. Additional evidence that 2,5-dihydroxyphenylpyruvate is not the intermediate was obtained by experiments in which the relative rates of oxidation of this compound and of p-hydroxyphenylpyruvate were compared in homogenates of rat liver, where the reaction proceeded to formation of acetoacetic acid. Oxidation of p-hydroxyphenylpyruvate proceeded much more rapidly. Other analogs of p-hydroxyphenylpyruvate were found to be inactive as substrates. 

Under metabolic conditions associated with a high rate of fatty acid oxidation, the liver produces considerable quantities of acetoacetate and d(—)-3-liydroxyl)utyrate (P-hydroxybutyrate). Acetoacetate continually undergoes spontaneous decarboxylation to yield acetone. These three substances are collectively known as the ketone bodies (also called acetone bodies or [incorrectly ] ketones ) (Figure 22-5). Acetoacetate and 3-hydroxybu-... 

Synthesis of the ketone bodies HMG CoA is cleaved to produce acetoacetate and acetyl CoA, as shown in Figure 16.23, Acetoacetate can be reduced to form 3-hydroxybutyrate with NADH as the hydrogen donor. Acetoacetate can also spontaneously decarboxylate in the blood to form acetone—a volatile, biologically non-metabolized compound that can be released in the breath. [Note The equilibrium between acetoacetate and 3-hydroxybutyrate is determined by the NADVNADH ratio. Because this ratio is high during fatty acid oxidation, 3-hydroxy-butyrate synthesis is favored.]... 

Foods frequently contain saturated and unsaturated aliphatic ketones with between 3 and 17 carbon atoms in the molecule. These ketones are formed by several different mechanisms. Frequently occurring aliphatic ketones are methylketones. The most common methylketone is acetone (propanone, 8-46, = 0). Acetone is present, usually in small quantities, in all biological substrates, where it arises by decarboxylation of acetoacetic (3-oxobutanoic) acid. Acetoacetic acid is formed as an intermediate during degradation of fatty acids by -oxidation. Acetone in the skins of apples, for example, is produced from pyruvic acid via citramalic acid (Figure 8.16). The relatively large amount of acetone is generated by acetone-butanol fermentation (see Section 8.2.2.1.1). Many other saturated and unsaturated methylketones occur as odour-active components of essential oils. For example, a component of cinnamon and star anise essential oils is heptane-2-one, also known as methyl pentyl ketone (8-46, = 4). 

Five enzyme steps have been demonstrated to be required in the conversion of tyrosine to fumaric and acetoacetic acid. These consist of a transamination to p-hydroxyphenylpyruvate, a simultaneous oxidation, migration of the side chain and decarboxylation to form homogentisic acid, oxidation of the latter to maleylacetoacetic acid, isomerization of this compound to fumarylacetoacetic acid, and hydrolysis of this acid to fumaric acid and acetoacetic acid. 

There are two major products of )8-oxidation. Complete operation of the cycle yields acetyl-CoA which can be fed into the TCA cycle as depicted in Figure 3.20. In some tissues, however, notably liver and the rumen epithelial cells of ruminant animals, acetoacetate accumulates. This compound, with its reduction product, )8-hydroxybutyrate, and its decarboxylation product, acetone, make up a group of metabolites known as the ketone bodies. Free acetoacetic acid may accumulate in liver in two ways. The CoA derivative may be enzymatically hydrolysed to the free acid and CoA and the liver tissue lacks the thiokinase to reconvert the acid into its thiolester. Alternatively acetoacetyl-CoA may be converted into hydroxymethyl-glutaryl-CoA (HMG-CoA) which is subsequently cleaved to free... 

Methylsuccinic acid has been prepared by the pyrolysis of tartaric acid from 1,2-dibromopropane or allyl halides by the action of potassium cyanide followed by hydrolysis by reduction of itaconic, citraconic, and mesaconic acids by hydrolysis of ketovalerolactonecarboxylic acid by decarboxylation of 1,1,2-propane tricarboxylic acid by oxidation of /3-methylcyclo-hexanone by fusion of gamboge with alkali by hydrog. nation and condensation of sodium lactate over nickel oxide from acetoacetic ester by successive alkylation with a methyl halide and a monohaloacetic ester by hydrolysis of oi-methyl-o -oxalosuccinic ester or a-methyl-a -acetosuccinic ester by action of hot, concentrated potassium hydroxide upon methyl-succinaldehyde dioxime from the ammonium salt of a-methyl-butyric acid by oxidation with. hydrogen peroxide from /9-methyllevulinic acid by oxidation with dilute nitric acid or hypobromite from /J-methyladipic acid and from the decomposition products of glyceric acid and pyruvic acid. The method described above is a modification of that of Higginbotham and Lapworth. ... 

Step 3 of Figure 29.12 Oxidation and Decarboxylation (2K,3S)-lsocitrate, a secondary alcohol, is oxidized by NAD+ in step 3 to give the ketone oxalosuccinate, which loses C02 to givea-ketoglutarate. Catalyzed by isocitrate dehydrogenase, the decarboxylation is a typical reaction of a /3-keto acid, just like that in the acetoacetic ester synthesis (Section 22.7). The enzyme requires a divalent cation as cofactor, presumably to polarize the ketone carbonyl group. 

Figure 9-4. Metabolism of the branched-chain amino acids. The first two reactions, transamination and oxidative decarboxylation, are catalyzed by the same enzyme in all cases. Details are provided only for isoleucine. Further metabolism of isoleucine and valine follows a common pathway to propionyl CoA. Subsequent steps in the leucine degradative pathway diverge to yield acetoacetate. An intermediate in the pathway is 3-hydroxy-3-methylglutaryl CoA (HMG-CoA), which is a precursor for cytosolic cholesterol biosynthesis.

Individuals with either type of diabetes are unable to take up glucose efficiently from the blood recall that insulin triggers the movement of GLUT4 glucose transporters to the plasma membrane of muscle and adipose tissue (see Fig. 12-8). Another characteristic metabolic change in diabetes is excessive but incomplete oxidation of fatty acids in the liver. The acetyl-CoA produced by JS oxidation cannot be completely oxidized by the citric acid cycle, because the high [NADH]/[NAD+] ratio produced by JS oxidation inhibits the cycle (recall that three steps convert NAD+ to NADH). Accumulation of acetyl-CoA leads to overproduction of the ketone bodies acetoacetate and /3-hydroxybutyrate, which cannot be used by extrahepatic tissues as fast as they are made in the liver. In addition to /3-hydroxybutyrate and acetoacetate, the blood of diabetics also contains acetone, which results from the spontaneous decarboxylation of acetoacetate ... 

Ketone body formation, which occurs within the matrix of liver mitochondria, begins with the condensation of two acetyl-CoAs to form acetoacetyl-CoA. Then acetoacetyl-CoA condenses with another acetyl-CoA to form /3-hydroxy-/3-methylglutaryl-CoA (HMG-CoA). In the next reaction, HMG-CoA is cleaved to form acetoacetate and acetyl-CoA. Acetoacetate is then reduced to form /3-hydroxybutyrate. Acetone is formed by the spontaneous decarboxylation of acetoacetate when the latter molecule s concentration is high. (This condition, referred to as ketosis, occurs in uncontrolled diabetes, a metabolic disease discussed in Special Interest Box 16.3, and during starvation. In both of these conditions there is a heavy reliance on fat stores and /3-oxidation of fatty acids to supply energy.)... 

Acetyl-CoA. There is a key metabolite of energy metabolism. It is produced in mitochondria by decarboxylation of pyruvate, beta oxidation of fatty acids, or hydrolysis of acetoacetate. In condensation reaction with oxaloacetate, acetyl-CoA yields citrate, which is the first intermediate in the tricarboxylic acid chain. Acetyl-CoA is also used for the synthesis of acetylcholine and the acetylation of several low molecular weight compounds and proteins. In liver and adipose tissue, acetyl-CoA is used for the synthesis of the fatty acids chain. 

The question arises as to whether acetone is a primary product of the oxidation of isovalerate or is formed secondarily by decarboxylation of acetoacetate. The experimental results of Zabin and Bloch are in accord with a reaction mechanism in which isovaleric acid is oxidized initially at the carbon 2-position to yield a 3-carbon and a 2-carbon fragment. This conclusion is based on the high absolute C concentrations found in the methyl carbons of acetone and also in comparison to that in the methyl or methylene carbons of acetoacetate. Secondly, the C C ratios were significantly greater in the acetone fractions than in the corresponding carbon atoms of acetoacetate. This finding can be explained only if acetone is formed directly from isovaleric acid, but is at variance with the assumption that acetone arose exclusively by decarboxylation of acetoacetate. The authors also point out that it is possible that the postulated 3-carbon intermediate is not acetone, but that acetone is formed in a side reaction from a more labile 3-carbon precursor. 

The authors point out that other possibilities for the catabolism of glutarate are (1) a direct splitting of the molecule into a 2- and 3-carbon fragment (2) /3-oxidation to acetone dicarboxylic acid followed by decarboxylation to acetoacetate or (3) direct decarboxylation to butyrate. 

The shortcomings of the latter scheme are that no examples are known of the decarboxylation by biochemical reactions of a saturated dicar-boxylic acid without a prior oxidation step in the molecule, and, secondly, it is difficult to understand how, if acetate were the primary product and it condensed to form acetoacetate, the resulting acetoacetate would also not be labeled to some extent in the carbonyl carbon. 

The first steps of the [ C]squalene synthesis resemble the pathway selected for the synthesis on [3 - C]coenzyme QIO 13061 (Figure 6.84). In this case alkylation with solanesyl bromide (3011 and subsequent hydrolysis and decarboxylation of the ester function converted ethyl [3- C]acetoacetate into [2- C]solanesylacetone 302. Chain extension of 302 in a Homer-Wadsworth-Emmons reaction, reduction of the resulting a,/3-unsatu-rated ester 303 and coupling of the resulting alcohol ([3- C]decaprenol, 3041 to 2-methyl-5,6-dimethoxy-1,4-hydroquinone (3051 provided the coenzyme in an overall radiochemical yield of 8% after oxidation. Methodologically comparable pathways have also been published for the synthesis of [5- C]farnesylacetic acid and [3 - C]menaquinone . 

In the metabolism of L-leucine, the isovaleryl-CoA produced by the oxidative decarboxylation step is further metabolized by a series of enzyme-catalysed steps to acetoacetate and acetyl-CoA and thence into the tricarboxylic acid cycle. Specific enzyme deficiencies at every stage of this metabolic pathway are known and are described in Section 10.3. In contrast, only one disorder of L-isoleucine metabolism subsequent to the oxidative decarboxylation step has been recognized (Section 10.4), and no disorders of the L-valine pathway from isobutyryl-CoA have been described. This may be due to their relative rarity but possibly also to greater difficulty in their detection. The metabolism of valine and leucine is, however, of particular interest in the organic acidurias, since both are major precursors of propionyl-CoA and methylmalonyl-CoA, defects in the metabolism of which lead to propionic acidaemia and methylmalonic aciduria (Chapter 11). 

---