Ketogenesis

Ketogenesis refers to the ability of some acetic acid bacteria to oxidize glycerol to dihydroxyacetone. This property can also be used to separate two important species of Acetobacter found in wines, A. aceti and A. pasteurianus (Swing, 1992), because A. aceti can oxidize glycerol but A. pasteurianus cannot. 

Transfer 10 mL into 18 x 150mm test tubes, cap, and sterilize by autoclaving at 121°C/250°F for 15min. 

Inoculate each tube with the bacterial isolate (0.1 mL of an actively growing culture) and incubate. 

Once growth is observed, evaluate glycerol oxidation by detecting the presence of dihydroxyacetone. 

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The central role of the mitochondrion is immediately apparent, since it acts as the focus of carbohydrate, hpid, and amino acid metabohsm. It contains the enzymes of the citric acid cycle, P-oxidation of fatty acids, and ketogenesis, as well as the respiratory chain and ATP synthase. 

KETOGENESIS OCCURS WHEN THERE IS A HIGH RATE OF FATTY ACID OXIDATION IN THE LIVER... 

Figure 22-7. Pathways of ketogenesis in the liver. (FFA, free fatty acids HMG, 3-hy-d roxy- 3-m et hy I g I uta ry I.)...

Figure 22-9. Regulation of ketogenesis. -(Dshow three crucial steps in the pathway of metabolism of free fatty adds (FFA) that determine the magnitude of ketogenesis. (CPT-I, carnitine palmitoyltransferase-l.)...

Theoretically, a fall in concentration of oxaloacetate, particularly within the mitochondria, could impair the ability of the citric acid cycle to metabolize acetyl-CoA and divert fatty acid oxidation toward ketogenesis. Such a fall may occur because of an increase in the [NADH]/[NAD+] ratio caused by increased P-oxida-tion affecting the equilibrium between oxaloacetate and malate and decreasing the concentration of oxaloacetate. However, pyruvate carboxylase, which catalyzes the conversion of pyruvate to oxaloacetate, is activated by acetyl-CoA. Consequently, when there are significant amounts of acetyl-CoA, there should be sufficient oxaloacetate to initiate the condensing reaction of the citric acid cycle. 

Inherited CPT-I deficiency affects only the fiver, resulting in reduced fatty acid oxidation and ketogenesis, witfi fiypoglycemia. CPT-II deficiency affects pri-... 

Inherited defects in the enzymes of (3-oxidation and ketogenesis also lead to nonketotic hypoglycemia, coma, and fatty hver. Defects are known in long- and short-chain 3-hydroxyacyl-CoA dehydrogenase (deficiency of the long-chain enzyme may be a cause of acute fetty liver of pr nancy). 3-Ketoacyl-CoA thiolase and HMG-CoA lyase deficiency also affect the degradation of leucine, a ketogenic amino acid (Chapter 30). 

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. 

Ketogenesis is regulated at three cmcial steps (1) control of free fatty acid mobihzation from adipose tissue (2) the activity of carnitine palmitoyltransferase-1 in hver, which determines the proportion of the fatty acid flux that is oxidized rather than esteri-fied and (3) partition of acetyl-CoA between the pathway of ketogenesis and the citric acid cycle. 

Mayes PA, Laker ME Regulation of ketogenesis in the liver. Biochem Soc Trans 1981 9 339. 

Three compounds acetoacetate, P-hydroxybutyrate, and acetone, are known as ketone bodies. They are suboxidized metabolic intermediates, chiefly those of fatty acids and of the carbon skeletons of the so-called ketogenic amino acids (leucine, isoleucine, lysine, phenylalanine, tyrosine, and tryptophan). The ketone body production, or ketogenesis, is effected in the hepatic mitochondria (in other tissues, ketogenesis is inoperative). Two pathways are possible for ketogenesis. The more active of the two is the hydroxymethyl glutarate cycle which is named after the key intermediate involved in this cycle. The other one is the deacylase cycle. In activity, this cycle is inferior to the former one. Acetyl-CoA is the starting compound for the biosynthesis of ketone bodies. 

The Deacylase Pathway for Ketogenesis is feasible after the formation of acetoace-tyl-CoA which is subject to hydrolysis to acetoacetate in the liver with the involvement of acetoacetyl-CoA hydrolase, or deacylase. 

The initial reactions in the first step, prior to the formation of P-hydroxy-p-methylglutaryl-CoA from acetyl-CoA, resemble those involved in ketogenesis with the only distinction that ketogenesis occurs in the mitochondria, while cho-lesterol biosynthesis is carried out extramitochondrially ... 

One suggestion to explain this discrepancy is that two pathways of fat catabolism are available and that ketone body formation is the resultant of only one type of breakdown.177 This latter type, also called the indirect fat utilization, 182 occurs in the liver the catabolism of fat in the muscle, called the -direct method, either involves no ketogenesis or the ketone bodies are immediately utilized and no accumulation occurs. 

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