3-Hydroxybutyrate acetoacetate

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. 

Fig.1.4.3 Reactions involved in the enzymatic measurement of 3-hydroxybutyrate, acetoacetate, lactate and pyruvate...

Somewhat surprisingly, within the mitochondria the ratio [NAD+]/[NADH] is 100 times lower than in the cytoplasm. Even though mitochondria are the site of oxidation of NADH to NAD+, the intense catabolic activity occurring in the (3 oxidation pathway and the citric acid cycle ensure extremely rapid production of NADH. Furthermore, the reduction state of NAD is apparently buffered by the low potential of the (3-hydroxybutyrate-acetoacetate couple (Chapter 18, Section C,2). Mitochondrial pyridine nucleotides also appear to be at equilibrium with glutamate dehydrogenase.169... 

Ketone bodies When the level of acetyl CoA from (3-oxidation increases in excess of that required for entry into the citric acid cycle, the acetyl CoA is converted into acetoacetate and D-3-hydroxybutyrate by a process known as ketogenesis. D-3-hydroxybutyrate, acetoacetate and its nonenzymic breakdown product acetone are referred to collectively as ketone bodies (Fig. 5). 

Normally, some acetoacetate is converted to 3-hydroxybutyrate. Acetoacetate and 3-... 

In fasting or diabetes, oxaloacetate is consumed to form glucose by the gluconeogenic pathway (Section 16,3.2) and hence is unavailable for condensation with acetyl CoA. Under these conditions, acetyl CoA is diverted to the formation of acetoacetate and d-3-hydroxybutyrate. Acetoacetate, d-3-hydroxyhutyrate, and acetone are often referred to as ketone bodies. Abnormally high levels of ketone bodies are present in the Wood of untreated diabetics (Section 22.3.6). 

Similar considerations apply to biological systems. The standard electrode potential of the 3-hydroxybutyrate/acetoacetate half-cell is —0.26 V and that of the lactate/pyruvate half-cell -0.19 Hence in the presence of the... 

The concentration and associated ratio of the ketone bodies, aceto-acetate and 3-hydroxybutyrate, may also be helpful [15, 18, 19]. Ketosis and keto-aciduria are observed in certain patients with a mitochondrial disorder. A non-physiological increase of ketone bodies postprandially may be another indicator of a mitochondrial defect (Saudubray et al). Increased 3-hydroxybutyrate/acetoacetate ratio may suggest a defect in the respiratory chain in liver tissue. 

A second case dscribed as one of acetoacetyl-CoA thiolase deficiency, has been reported by Robinson et al (1979) (Section 10.4.1), in which urinary metabolites included 3-hydroxybutyrate, acetoacetate, 2-methyl-3-hydroxy-butyrate, 2-methylacetoacetate and tiglylglycine. The authors postulated an enzyme deficiency of short-chain mitochondrial 3-keto acid-CoA thiolase, affecting both acetoacetyl-CoA and 2-methylacetoacetyl-CoA metabolism, ascribing a common enzyme to both metabolic pathways. The presentation of this case was very different from that of de Groot et al, (1977), and further study of 2-methylacetoacetyl-CoA thiolase activity by more direct enzyme assays may resolve the question of the distinction of these enzyme systems (see Section 12.3 below also). 

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-... 

Wooton-Gorges, S., et al., 2005. Detection of cerebral beta-hydroxybutyrate, acetoacetate and lactate on proton MR spectroscopy in children with diabetic ketoacidosis. Am J Neuroradiol, 26 pp. 1286-1291... 

Higher than normal intakes of MCTGs have become attractive to the athlete since the MCFAs that make them up can be absorbed faster, go into portal circulation, and enter the mitochondria without carnitine. It is well known that increased consumption of MCTGs can increase circulation of ketone bodies. Provided the liver has ample calories, the rapid metabohsm of MCFA can result in the production of ketone bodies (P-hydroxybutyrate, acetoacetate, and acetone) or an increase in free fatty adds (FFAs) in drculation. These ketones and FFAs can then be used by the muscles for energy. How well this works will be discussed in Section 3.4. 

Moreover, the significance of these assays is limited they provide no information on free coenzyme concentration nor on cell compart-mentation. It is preferable to estimate the concentrations of oxidized and reduced substrates (Hohorst et al., 1959, 1961 Borst, 1963 Krebs, 1967 Krebs et al., 1967 Williamson et al., 1967 Veech and Krebs, 1969 Krebs and Veech, 1969 for example, the lactate pyruvate ratio gives information on the free cytoplasmic NADHiNAD ratio. The 8-hydroxybutyrate acetoacetate ratio corresponds to the mitochondrial NADH NAD ratio. 

Ketone body synthesis occurs only in the mitochondrial matrix. The reactions responsible for the formation of ketone bodies are shown in Figure 24.28. The first reaction—the condensation of two molecules of acetyl-CoA to form acetoacetyl-CoA—is catalyzed by thiolase, which is also known as acetoacetyl-CoA thiolase or acetyl-CoA acetyltransferase. This is the same enzyme that carries out the thiolase reaction in /3-oxidation, but here it runs in reverse. The second reaction adds another molecule of acetyl-CoA to give (i-hydroxy-(i-methyl-glutaryl-CoA, commonly abbreviated HMG-CoA. These two mitochondrial matrix reactions are analogous to the first two steps in cholesterol biosynthesis, a cytosolic process, as we shall see in Chapter 25. HMG-CoA is converted to acetoacetate and acetyl-CoA by the action of HMG-CoA lyase in a mixed aldol-Claisen ester cleavage reaction. This reaction is mechanistically similar to the reverse of the citrate synthase reaction in the TCA cycle. A membrane-bound enzyme, /3-hydroxybutyrate dehydrogenase, then can reduce acetoacetate to /3-hydroxybutyrate. 

Acetoacetate and /3-hydroxybutyrate are transported through the blood from liver to target organs and tissues, where they are converted to acetyl-CoA (Figure 24.29). Ketone bodies are easily transportable forms of fatty acids that move through the circulatory system without the need for eomplexation with serum albumin and other fatty acid—binding proteins. 

Most of the acetyl-CoA formed by 3-oxidation in liver is converted to acetoacetate by the 3-hydroxy-3-methylglutaryl-CoA pathway (Guzman and Gelen, 1993). Acetoacetate is reversibly converted to D-3-hydroxybutyrate by D-3-hy-droxybutyrate dehydrogenase in the mitochondrial matrix in all tissues. 

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. 

In most cases, ketonemia is due to increased production of ketone bodies by the liver rather than to a deficiency in their utilization by extrahepatic tissues. While acetoacetate and d(—)-3-hydroxybutyrate are readily oxidized by extrahepatic tissues, acetone is difficult to oxidize in vivo and to a large extent is volatilized in the lungs. 

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