Lactate keto acid

Pyruvic acid is the simplest homologue of the a-keto acid, whose established procedures for synthesis are the dehydrative decarboxylation of tartaric acid and the hydrolysis of acetyl cyanide. On the other hand, vapor-phase contact oxidation of alkyl lactates to corresponding alkyl pyruvates using V2C - and MoOa-baseds mixed oxide catalysts has also been known [1-4]. Recently we found that pyruvic acid is obtained directly from a vapor-phase oxidative-dehydrogenation of lactic acid over iron phosphate catalysts with a P/Fe atomic ratio of 1.2 at a temperature around 230°C [5]. 

Adam, W., Lazarus, M., Saha-Moller, C.R. and Schreier, P. (1998) Quantitative transformation of racemic 2-hydroxy acids into (R)-2-hydroxy acids by enantioselective oxidation with glycolate oxidase and subsequent reduction of 2-keto acids with D-lactate dehydrogenase. Tetrahedron Asymmetry, 9 (2), 351-355. 

The alanine cycle accomplishes the same thing as the Cori cycle, except with an add-on feature (Fig. 17-11). Under conditions under which muscle is degrading protein (fasting, starvation, exhaustion), muscle must get rid of excess carbon waste (lactate and pyruvate) but also nitrogen waste from the metabolism of amino acids. Muscle (and other tissues) removes amino groups from amino acids by transamination with a 2-keto acid such as pyruvate (oxaloacetate is the other common 2-keto acid). 

Gluconeogenic precursors are molecules that can be used to produce a net synthesis of glucose. They include all the intermediates of glycolysis and the citric acid cycle. Glycerol, lactate, and the o-keto acids obtained form the deamination of glucogenic amino acids are the most important gluconeogenic precursors. 

R)-2-Hydroxy-4-phenylbutyric acid was produced continuously in an enzyme membrane reactor by enzymatic reductive animation of the a-keto acid with d-lactate dehydrogenase coupled with formate dehydrogenase (FDH) for regeneration of NADH. Reactor performance data matched a kinetic reactor model (Schmidt, 1992). 

Enzyme-catalyzed syntheses of enantiomeric pure hydroxy acids use lactate dehydrogenases (LDH E.C. 1.1.1.27) or hydroxyisocaproate dehydrogenases (HicDH). Both kinds of enzymes are available as D- or L-specific catalysts. L-specific [5, 6] LDHs as well as D-specific [7-10] LDHs favorably catalyze the reduction of pyruvate, HicDHs (and mandelate dehydrogenase, too) convert keto acids with longer aliphatic or aromatic side chains. These enzymes can be isolated from Lactobacillus strains [11-14]. 

Although the utility of transaminases has been widely examined, one such limitation is the fact that the equilibrium constant for the reaction is near unity. Therefore, a shift in this equilibrium is necessary for the reaction to be synthetically useful. A number of approaches to shift the equilibrium can be found in the literature.53 124135 Another method to shift the equilibrium is a modification of that previously described. Aspartate, when used as the amino donor, is converted into oxaloacetate (32) (Scheme 19.21). Because 32 is unstable, it decomposes to pyruvate (33) and thus favors product formation. However, because pyruvate is itself an a-keto acid, it must be removed, or it will serve as a substrate and be transaminated into alanine, which could potentially cause downstream processing problems. This is accomplished by including the alsS gene encoding for the enzyme acetolactate synthase (E.C. 4.1.3.18), which condenses two moles of pyruvate to form (S)-aceto-lactate (34). The (S)-acetolactate undergoes decarboxylation either spontaneously or by the enzyme acetolactate decarboxylase (E.C. 4.1.1.5) to the final by-product, UU-acetoin (35), which is meta-bolically inert. This process, for example, can be used for the production of both l- and d-2-aminobutyrate (36 and 37, respectively) (Scheme 19.21).8132 136 137... 

Fig. 8 Synthesis of amino acids by a multienzyme system consisting of leucine dehydrogenase (LeuDH) catalyzing the reductive amination of the corresponding keto acid, L-lactate dehydrogenase (l-LDH), and lactate for the regeneration of NADH and urease for the in situ generation of ammonia. The coenzyme NAD+ was covalently bond to dextran, enzymes and dextran-coupled NAD+ were...

L-Lactate dehydrogenase (l-LDH, EC 1.1.1.27) catalyzes the reduction of pyruvate to (S)-lactate with a simultaneous oxidation of NADH. l-LDH is found in all higher organisms. There are two kinds of l-LDHs enzymes from one group are activated by fructose 1,6-diphosphate while the other group stays independent [71]. l-LDH is highly selective for pyruvate, short-chain 2-keto acids and phenylpyruvic acid [80]. All bacterial NAD+-dependent LDHs form lactate from pyruvate in vivo, and there is no evidence at all that they catalyze the other direction as well. The equilibrium constant lies far on the direction of lactate formation, and thus the reaction catalyzed by bacterial LDHs can be considered almost irreversible. LDHs from some lacto-bacilli like Lactobacillus fermentum or L. cellobiosus show no or just poor reaction with lactate [71], whereas mammalian LDHs can be considered as reversible [71]. Well characterized l-LDHs are summarized in Table 2. 

The two-enzyme system was also used to convert L-lactate into D-lactate with a yield better than 97%. L-Lactate is oxidized by L-lactate dehydrogenase to give pymvate. The keto acid is reduced in an electrochemical system at the cathode to racemic lactate and NADH is oxidized to NAD at the anode. The continuous... 

In the above reactions under modest conditions, traces of pyruvate have been found (13), which indicates that a-keto or a-imino acids are intermediates in the pathways to a-hydroxy or a-amino acids. The reductive amination of a-keto acids has been demonstrated experimentally under alkaline conditions with FeS/H2S or Fe(OH)2 as catalyst and reducing agent (14). The formation of significant amounts of pyruvate from CO and FeS/nonylmercaptan at 250° C and 2000 bar has been reported (15). Remarkably, pyruvate is stable under these conditions and apparently not reduced to lactate. 

Heterogeneous catalytic hydrogenation of the methyl esters of a-keto acids over modified metal catalysts other than nickel have been studied using a cinchonidine-modified platinum catalyst. " Methyl pyruvate and methyl benzoylformate were hydrogenated to form methyl (R)-lactate and (R)-mandelate with high ee (81-84%). 

Stereospecific reductions of a-keto acids are well documented. l- and D-lactate dehydrogenases from common mammalian or bacterial sources are available for this purpose. By using a preselected l- or d-LDH, the preparation of a (2S)- or (2/ )-hydroxy acid of >99% ee can be virtually assured. The structural range of a-keto acids (29) that has been subjected to preparative-scale reductions to hydroxy acids... 

A further consequence of thiamine depletion during parenteral nutrition can be severe lactic acidosis (44). Six cases have been described from Japan with associated hypotension, Kussmaul s respiration, and clouding of consciousness, as well as abdominal pain not directly related to the underlying disease. During parenteral nutrition administration there was blockade of oxidative decarboxylation of alpha-keto acids such as pyruvate and alpha-ketoglutarate, resulting in pjruvate accumulation and massive lactate production. None of the patients responded to sodium bicarbonate or other conventional emergency treatments for shock and lactic acidosis. Thiamine replenishment with intravenous doses of 100 mg every 12 hours resolved the lactic acidosis and improved the clinical condition of three patients. 

Reductions catalyzed by lactate dehydrogenase formation of chiral a-hydroxy acids from ck--keto acids. 

Resistance to vancomycin is via a sensor histidine kinase (VanS) and a response regulator (VanR). VanH encodes a D-lactate dehydrogenase/a-keto acid reductase and generates D-lactate, which is the substrate for VanA, a D-Ala-D-Lac ligase. The result is cell wall precursors terminating in D-Ala-D-... 

It was discovered in 1958 that anaerobically grown yeast contains a form of lactate dehydrogenase which is different from the d- and L-lac-tate cytochrome c reductases of aerobic yeast (306, 319). The enzyme has been partially purified (320, 321), and shown to contain flavin (320-322). Gel filtration studies have suggested a molecular weight of about 100,000 (320, 321). Preparations of the enzyme oxidize several d-2-hydroxyacids to the respective keto acids in a reversible manner (320). For the forward reaction, ferricyanide, 2,6-dichloroindophenol, menadione, and methylene blue have been used as electron acceptors, and for the reverse reaction leucomethyl viologen and FMNHa are effective electron donors (320). A number of L-2-hydroxyacids and 2-keto acids have been shown to be competitive inhibitors. Oxalate, cyanide, o-phenanthro-line, and EDTA are also potent inhibitors (320, 321, 323, 324). The inhibition by metal chelators develops slowly and is reversed by addition of Zn, Co, Mn +, or Fe + (320, 323-326). Substrates prevent the inhibition by chelators at concentrations considerably lower than their respective Km values (327). It has been suggested that EDTA inactivation involves the removal of a metal, most probably Zn +, from the substrate binding site of the enzyme (325, 326, 328, 329). However, others have... 

Lactate dehydrogenase (LDH) catalyzes the interconversion of the hydroxy-acid lactate and the keto-acid pyruvate with the coenzyme nicotinamide adenine dinucleotide [48]. This enzyme plays a fundamental role in respiration, and multiple isozymes have evolved to enable eflRcient production of substrate appropriate for the microenvironment [49]. Two main subunits, referred to as heart and muscle (skeletal), are combined in the functional enzyme as a tetramer to accommodate aero-... 

The usefulness of a mutant dehydrogenase was demonstrated in a practical synthesis of 4-amino-2-hydroxy acids, which themselves are valuable as y-turn mimics for investigations into the secondary structure of peptides[146]. Chemoenzymatic synthesis of these compounds were achieved by lipase catalyzed hydrolysis of a a-keto esters to the corresponding a-keto acids followed by reduction employing a lactate dehydrogenase in one pot. Wild type lactate dehydrogenase from either Bacillus... 

Kinetic resolutions have a maximum yield of only 50%. Therefore, a second enzymatic process was added after completion of the glycolate oxidase-catalyzed kinetic resolution[131). By addition of D-lactate dehydrogenase (E.C. 1.1.1.28) together with formate dehydrogenase for NADH regeneration, enantiospecific reduction of the 2-keto acid was achieved. Overall, a quantitative transformation (deracemization) of the racemic 2-hydroxy acid into the corresponding (R)-2-hydroxy acid was achieved (Fig. 16.2-29). 

Lactate dehydrogenases use several 2-hydroxy and 2 keto acids and other analogs as substrates 75-79). With the rabbit muscle enzyme (77),... 

Lactate dehydrogenase reduces an a-keto acid to a hydroxy acid with NADH. The enzyme will, however, also oxidize glyoxalate (XXIII) to... 

Williams, J.C. McDermott, A.E. Dynamics of the flexible loop of triose-phosphate isomerase—The loop motion is not ligand-gated. Biochemistry 1995, 34. 8309-8319. Holbrook, J.J. Liljas, A. Steindel, S.J.. et al. Lactate dehpdrogenase. Enzymes 1975,11, 191-292. 3rd ed. Wilks, H.M. Halsall. D.J. Atkinson, T., et al. Designs for a broad substrate-specificity keto acid dehydrogenase. Biochemistry 1990, 29. 8587-8591. 

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