0-Hydroxybutyric acid dehydrogenase specificity

A /3-hydroxybutyric acid dehydrogenase that uses DPN has been known for many years. It is specific for i/-/3-hydroxybutyrate, and does not act with the CoA derivative. The original description of the CoA-requiring system included a specific requirement for D-hydroxy acids. Later a racemase that converts ii-/3-hydroxy CoA compounds to d-was reported, but it appears that the racemase is the resultant of an D-dehydrogenase together with the previously described D-enzyme. Racemization requires DPN. ... 

Dichlorophenoxy)butyric acid is converted in the presence of ATP into dichlorophenoxybutyryl coenzyme A. This acyl-CoA is converted by the electron acceptor flavine adenine dinucleotide (FAD) into dichlorophenoxycrotonyl-CoA. One carbon atom of the unsaturated bond is hydroxilated and dichlorophenoxy- -hydroxybutyric acid-CoA is formed. In certain plants possessing specific /3-oxidase enzyme systems, -ketobutyric acid-CoA is formed from this intermediate compound by the mediation of NAD and NADH in a reaction catalysed by -hydroxyacyl-CoA dehydrogenase. This compound is decomposed by hydrolysis into 2,4-D and acetyl-CoA. 

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. 

Hydroxybutyrate is activated to 3-hydroxybutyryl-CoA and oxidized via a specific 3-hydroxybutyryl-CoA dehydrogenase as in the /3-oxidation pathway to acetoacetyl-CoA. This is then converted to acetyl-CoA. which is oxidized in the citric acid cycle. 

The pH optimum for the lactate-to-pyruvate (L—>P) reaction is 8.8 to 9.8, and an assay mixture, optimized for LD-1 at 37 °C, contains NAD% 9mmol/L, and L-lactate, 80mmol/L. For the P —> L assay, at 37 °C, the pH optimum is 7.4 to 7.8, NADH 300fJ.mol/L, and pyruvate 0.85mmol/L. The optimal pH varies with the predominant isoenzymes in the sample and depends on the temperature and on substrate and buffer concentrations. The specificity of the enzyme extends from L-lactate to various related 2-hydroxyacids and 2-oxo-acids. The catalytic oxidation of 2-hydroxybutyrate, the next higher homologue of lactate, to 2-oxobutyrate is referred to as 2-hydroxybutyrate dehydrogenase (HBD) activity. LD does not act on n-lactate, and only NAD serves as a coenzyme. 

The enzyme hydroxyacyl dehydrogenase described above is specific for the L-isomer. Apparently, some mammalian tissue can also oxidize the D-isomer, but it is not clear what enzymic mechanism is responsible for that reaction. Although the presence of an enzyme that specifically catalyzes the oxidation of the D-hy-droxyacyl ester to yield the keto acid has been proposed by some, others believe that the D-hydroxyacyl is transformed to the L-hydroxy acid by enzymes with racemase activity—namely, crotonase and another racemase. The equilibrium of the enzyme reaction is modified by the presence of magnesium in the medium. The modification of the equilibrium probably results from the complexion of magnesium with the keto acid. Eliminating the product favors the formation of the hydroxyacyl. Hydroxybutyrate can also be oxidized by an enzyme found in the mitochondria of many tissues, such as brain, kidney, heart, and liver. Hydroxybutyrate dehydrogenase has been isolated, solubilized, purified from beef heart, and demonstrated to require lecithin for activity. 

Ketogenesis required three specific reactions which have nothing directly to do with fatty acid degradation (Krebs t aJL., 1971). These reactions are the HMG-CoA synthase reaction, the HMG-CoA lyase reaction, and the 3-hydroxybutyrate dehydrogenase reaction ... 

Acetoacetate Metabolism. An active deacylase in liver is responsible for the formation of free acetoacetate from its CoA derivative. The j8-hydroxybutyric dehydrogenase mentioned above and a decarboxylase are capable of converting acetoacetate into the other ketone bodies, /3-hydroxybutyrate, and acetone. liver does not contain a mechanism for activating acetoacetate. Heart muscle has been found to contain a specific thiophorase that forms acetoacetyl CoA at the expense of suc-cinyl CoA. Acetoacetate is thus used by peripheral tissues by activation through transfer, then reaction with either the enzymes of fatty acid synthesis or jS-ketothiolase and the enzymes that use acetyl CoA. 

Yeon YJ, Park H-Y, Yoo YJ (2013) Enzymatic reduction of levulinic acid by engineering the substrate specificity of 3-hydroxybutyrate dehydrogenase. Bioresour Technol 137 377-380 Zanghellini AL (2012) Fermentation route for the production of levulinic acid, levulinate esters, valerolactone, and derivatives thereof. World patent No 2012030860A1... 

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