0-Hydroxybutyric acid oxidation

Ascorbic acid is involved in carnitine biosynthesis. Carnitine (y-amino-P-hydroxybutyric acid, trimethylbetaine) (30) is a component of heart muscle, skeletal tissue, Uver and other tissues. It is involved in the transport of fatty acids into mitochondria, where they are oxidized to provide energy for the ceU and animal. It is synthesized in animals from lysine and methionine by two hydroxylases, both containing ferrous iron and L-ascorbic acid. Ascorbic acid donates electrons to the enzymes involved in the metabohsm of L-tyrosine, cholesterol, and histamine (128). 

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. 

Senior, A.E. Shenatt, H.S.A. (1968). Biochemical effects of the hypoglycaemic compound pent-4-enoic acid and related non-hypoglycemic fatty acids. Oxidative phosphorylation and mitochondrial oxidation of pyruvate, 3-hydroxybutyrate and tricarboxylic acid-cycle intermediates. Biochem. J. 110,499-509. 

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-... 

The ketone bodies (acetoacetate, 3-hydroxybutyrate, and acetone) are formed in hepatic mitochondria when there is a high rate of fatty acid oxidation. The pathway of ketogenesis involves synthesis and breakdown of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) by two key enzymes, HMG-CoA synthase and HMG-GoA lyase. 

Allen and DuBois136 have calculated that the R. Q. of 0.707, obtainable when fat is completely oxidized, would be lowered to 0.669 if the /3-hydroxybutyric acid were not metabolized. If more than one molecule of the ketone bodies is produced from one molecule of the fatty acid, as has been suggested,83 the R. Q. would be further lowered. This may be further complicated by a resulting upset in acid-base balance. 

In some poorly controlled diabetic patients the high rate of fatty acid oxidation decreases the mitochondrial NADVNADH concentration ratio so that the 3-hydroxybutyrate/acetoacetate concentration ratio can rise to as high as 15 in the blood. Since a test for ketone bodies in the urine (using Clinistix or similar material) detects only acetoacetate this can result in a serious underestimate of the concentration of ketone bodies in the urine. 

When male F-344 rats were injected with NNN-2 -14c, 75-95% of the dose was excreted in the 48 hr urine. In one experiment, the urine was collected in vessels containing DNP reagent. However, the DNPs of 4-hydroxy-l-(3-pyridyl)-l-butanone and 4-hy-droxy-4-C3-pyridy1)butanal were not detected. Since this was likely due to further oxidation in vivo, methods were developed for isolation of their probable oxidation products. This resulted in identification of the lactone, 5- C3-pyridyl)—tetrahydrofuran-2-one (1-2%), the keto acid, 4-(3-pyridyl).-4-oxobutyric acid (1-2%) and the hydroxy acid, 4-(3-pyridyl)-4-hydroxybutyric acid (26-40%) as urinary metabolites. These metabolites resulted. 

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. 

T12. Fatty Acid Oxidation in Uncontrolled Diabetes When the acetyl-CoA produced during /3 oxidation in the liver exceeds the capacity of the citric acid cycle, the excess acetyl-CoA forms ketone bodies—acetone, acetoacetate, and D-/3-hydroxybutyrate. This occurs in severe, uncontrolled diabetes because the tissues cannot use glucose, they oxidize large amounts of fatty acids instead. Although acetyl-CoA is not toxic, the mitochondrion must divert the acetyl-CoA to ketone bodies. What problem would arise if acetyl-CoA were not converted to ketone bodies How does the diversion to ketone bodies solve the problem ... 

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.]... 

Liver mitochondria can convert acetyl CoA derived from fatty acid oxidation into the ketone bodies, acetoacetate and (3-hydroxybutyrate. (Acetone, a nonmetabolizable ketone body, is produced spontaneously from acetoacetate in the blood.) Peripheral tissues possessing mitochondria can oxidize 3-hydroxybutyrate to acetoacetate, which can be reconverted to acetyl CoA, thus producing energy for the cell. 

Oxidation of thioethers derived from the natural chirality pool , the readily available lactic acid and 3-hydroxybutyric acid, has been used in molar-scale preparation of enantiomerically pure sulfoxides methyl ( )-2-(phenylsulfinyl)acrylate and (K)-isopropenyl p-tolyl sulfoxide [107]. 

The oxidation of homoserine by Cr(VI) has been compared with that of simple alcohols and 4-hydroxybutyric acid (HBA). The formation of CrOj during the oxidation was taken as evidence for the intermediacy of Cr(II). Whilst the rate law for homoserine has a first- and a second-order term, the rate laws for alcohols and HBA display only the second-order term. The second-order rate constants for HBA and homoserine are similar (suggesting that the ammo group of homoserine does not participate in binding to the chromium in this pathway), and about 10 times lower than for the alcohols, accounted for in terms of carboxylate binding to Cr(VI) in the intermediate ester (2), lowering the rate. The additional first-order term seen only for homoserine must arise from involvement of the amino group and this additional pathway is proposed to proceed via a tricyclic intermediate (l).13... 

Excess acetate (C2) can be converted to the mobile ketone body energy source aceto-acetate (C4) and thence its reduced form hydroxybutyrate (C,) for transport throughout the body. Excess acetate can be carboxylated (via acetylCoA carboxylase) to form malonylCoA (C3), the donor for further C2 additions (with C02 elimination) in the anabolic synthesis of long chain fatty acids. Fatty acids are components of the phospholipids of cellular membranes and are also stored as triacylglycerols (triglycerides) for subsequent hydrolysis and catabolic fatty acid oxidation to yield reduced coenzymes and thence ATP (see Chapter 2). 

D-Lactate cytochrome c reductase can oxidize D-2-hydroxymonocar-boxylic acids, but only D-lactate and D-2-hydroxybutyrate are oxidized at appreciable rates. The enzyme exhibits a similar high specificity for electron acceptors. It reacts with cytochrome c and phenazine methosul-fate as electron acceptors, but not with ferricyanide, methylene blue, 2,6-dichloroindophenol, and menadione 308, 312, 313). With D-lactate as substrate and at Fmax with respect to acceptor, phenazine methosulfate is reduced at 30° eight times as fast as cytochrome c 308). The values at 30° and pH 7.5 are D-lactate, 0.29 mM n-2-hydroxybutyrate,... 

Disposition in the Body. Readily absorbed after oral administration. The major metabolite is free bromide ion hydrolysis to an active metabolite, 2-bromo-2-ethylbutyramide, also occurs followed by oxidation to 2-bromo-2-ethyl-3-hydroxybutyramide other metabolites include 2-ethylbutyrylurea and 2-ethyl-2-hydroxybutyric acid. Carbromal is excreted in the urine mainly as bromide ion and partly as 2-ethyl-2-hydroxybutyric acid, with very little as unchanged drug. Peak bromide excretion is attained after about 48 hours. 

The hypothesis of the existence of the branched-chain parasaccharinic acid now depended solely on the identity of the reduction product, hydroxyhexanoic acid. To support his previous contention that the barium salt of this acid is, indeed, barium 2-ethyl-4-hydroxybutyrate, Kiliani also prepared the calcium salt and found that it, too, closely resembled the corresponding salt of 2-ethyl-4-hydroxybutyric acid. As emphasized by Kiliani, neither the reductions with hydriodic acid and red phosphorus nor the oxidations with nitric acid proceed in good yield to single products. Accordingly, Kihani remained firm in his conviction that his preparation, although it was apparently a mixture, nevertheless contained the branched-... 

During prolonged starvation or when carbohydrate metabolism is severely impaired, as in uncontrolled diabetes mel-iitus (see Chapter 25), the formation of acetyl-CoA exceeds the supply of oxaioacetate. The abundance of acetyl-CoA results from excessive mobilization of fatty acids from adipose tissue and excessive degradation of the fatty acids by p-oxidation in the liver. The resulting acetyl-CoA excess is diverted to an alternative pathway in the mitochondria and forms acetoacetic acid, P-hydroxybutyric acid, and acetone—three compounds known collectively as ketone bodies (Figure 26-9). The presence of ketone bodies is a frequent finding in severe, uncontrolled diabetes melUtus. 

Acetoacetate and 6-hydroxybutyrate are products of normal metabolism of fatty acid oxidation and serve as metabolic fuels in extrahepatic tissues. Their level in blood depends on the rates of production and utilization. Oxidation increases as their plasma level increases. Some extra-hepatic tissues (e.g., muscle) oxidize them in preference to glucose and fatty acid. Normally, the serum concentration of ketone bodies is less than 0.3 mM/L. 

THE KETONE BODIES Most of the acetyl-CoA produced during fatty acid oxidation is used by the citric acid cycle or in isoprenoid synthesis (Section 12.3). Under normal conditions, fatty acid metabolism is so carefully regulated that only small amounts of excess acetyl-CoA are produced. In a process called ketogenesis, acetyl-CoA molecules are converted to acetoacetate, /3-hydroxybutyrate, and acetone, a group of molecules called the ketone bodies (Figure 12.7). 

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