Enzymes lactic acid dehydrogenase

Some examples follow that illustrate the remarkable specificity of this kind of redox system. One of the last steps in the metabolic breakdown of glucose (glycolysis Section 20-10A) is the reduction of 2-oxopropanoic (pyruvic) acid to L-2-hydroxypropanoic (lactic) acid. The reverse process is oxidation of l-lactic acid. The enzyme lactic acid dehydrogenase catalyses this reaction, and it functions only with the L-enantiomer of lactic acid ... 

The NAD -NADH coenzyme system is involved in a large number of biological oxidation-reductions. Another reaction similar to the ethanol-acetaldehyde conversion is the oxidation of lactic acid to pyruvic acid by NAD and the enzyme lactic acid dehydrogenase ... 

An example using NADH for measurement is the determination of the enzyme lactic acid dehydrogenase (LDH), which is important in confirming myocardial infarction (heart attack). NAD" is required in the LDH-catalyzed oxidation of lactic acid to pyruvic acid ... 

Enantiomers often behave differently in a biological setting because these properties usually involve a reaction with another chiral molecule. For example, the enzyme lactic acid dehydrogenase will oxidize ( + )-lactic acid to pyruvic acid, but it will not oxidize ( —)-lactic acid (eq. 5.2). 

The cross-axis CPC (coil planet centrifuge), with column holders at the off-center position on the rotary shaft, enables retention of the stationary phase of aqueous-aqueous polymer-phase systems such as PEG 1000-potassium phosphate and PEG 8000-dex-tran T500 [3,4], Since the last decade, various types of cross-axis CPC (types XL, XLL, XLLL, and L) have been developed for performing CCC with highly viscous aqueous polymer-phase systems. The separation and purification of protein samples, including lactic acid dehydrogenase [5], recombinant enzymes, profiUn-actin complex, and so on, were achieved using these cross-axis CPCs [6]. 

Gault, M. H., Cohen, M. W., Kahana, L. M., Leelin, F. T., Meakins, ]. F., and Aronovitch, M., Serum enzymes in patients with carcinoma of the lung. Lactic acid dehydrogenase, phosphohexose isomeiase, alkaline phosphatase and glutamic ox-alacetic transaminase. Can. Med. Assoc. J. 96, 87-94 (1967). 

What we call anaerobic glycolysis also involves the same ten-step conversion of glucose to pyruvic acid or pyruvate. However, the pyruvic acid is instead further converted to lactic acid or lactate in a single step catalyzed by an enzyme called lactate dehydrogenase (and sometimes called lactic acid dehydrogenase). 

Although cross-axis CPCs yield less efficient separations than the type-J multilayer CPC, they provide more stable retention of the stationary phase and are therefore useful for large-scale preparative separations with polar solvent systems. These are especially useful for the purification of proteins with aqueous-aqueous polymer phase systems composed of PEG and potassium phosphate. The crossaxis CPC has been used for the purification of various enzymes including choline esterase, ketosteroid isomerase, purine nucleoside phosphorylase, lactic acid dehydrogenase, uridine phosphorylase (see Proteins Cross-Axis Coil Planet Centrifuge Separation, p. 1935). 

Lactic acid dehydrogenase and malic acid dehydrogenase are also increased with a maximum for both enzymes after two years and a decrease thereafter. 

Enzyme catalyzed reductions of carbonyl groups are more often than not com pletely stereoselective Pyruvic acid for example is converted exclusively to (5) (+) lactic acid by the lactate dehydrogenase NADH system (Section 15 11) The enantiomer... 

Nature uses enantioselective transfer hydrogenation to reduce metabolites, for example pyruvate to give (S)-lactic acid and 2-ketoglutarate to give (S)-2-hydroxy -glutarate. The reaction is reversible and the equilibrium position depends on the concentration of the species. The enzyme catalysts are named dehydrogenases, and they employ a soluble cofactor or hydride acceptor called NAD(P) in its oxi-... 

In contrast, an enzymic reduction utilizing NADH will be executed stereospecifically, with hydride attaching to one particular face of the planar carbonyl. Which face is attacked depends upon the individual enzyme involved. For example, reduction of pymvic acid to lactic acid in vertebrate muscle occurs via attack of hydride from the Re face (see Section 3.4.7), and produces the single enantiomer (S )-lactic acid. Hydride addition onto the alternative Si face is a feature of some microbial dehydrogenase enzymes. 

In the Korkes and Ochoa (11) mechanism proposed for the malo-lactic reaction (see top of next page), pyruvic acid is either a short-lived, fleeting intermediate, or it is bound to malic enzyme so that as soon as it is formed by the enzyme, it is converted to lactic acid by lactate dehydrogenase. [Both malic enzyme ( malic ) and malate dehydrogenase (de-... 

They stated further that, the new adaptive enzyme catalyzing Reaction 3 appears to be similar to the malic enzyme of pigeon liver, although strictly DPN (instead of TPN)-specific. The coenzyme specificity explains the ready occurrence of Reaction 1. Therefore, the authors showed that exogenous NAD was required for the overall reaction (malic acid -> lactic acid), but because this activity was measured manometrically, they never demonstrated the formation of reduced NAD. Similarly, they did not attempt to show that pyruvic acid was the intermediate between L-malic acid and lactic acid. Instead, the formation of pyruvic acid was inferred from the NAD requirement and because the malic acid dissimilation activity remained constant during purification while the lactate dehydrogenase activity decreased (14). In fact, attempts to show any appreciable amounts of pyruvic acid intermediate failed (22). 

The most confusing aspect of the pathway proposed by Ochoa and his group now rests with the NAD requirement. In proceeding from L-malic acid to L-lactic acid, there is no net change in oxidation state. Yet in whole cells or cell-free extracts, the malo-lactic fermentation will not proceed in the absence of NAD. Therefore, by the proposed mechanism, one is unable to demonstrate the appearance of reduced cofactor, and the NAD specificity cannot be explained as a redox requirement. However, in the time since this mechanism was proposed, an NAD dependent enzyme (glyceraldehyde-3-phosphate dehydrogenase) has been described which requires NAD in a non-redox capacity (29), and it is possible that the same is true for the enzyme causing the malic acid-lactic acid transformation. 

Dehydrogenases often act primarily to reduce a carbonyl compound rather than to dehydrogenate an alcohol. These enzymes may still be called dehydrogenases. For example, in the lactic acid fermentation lactate is formed by reduction of pyruvate but we still call the enzyme lactate dehydrogenase. In our bodies this enzyme functions in both directions. However, some enzymes that act mainly in the direction of reduction are called reductases. An example is aldose reductase, a member of a family of aldo-keto reductases71 73 which have (a / P)8-barrel structures.74 76... 

Some enzymes contain bound NAD+ which oxidizes a substrate alcohol to facilitate a reaction step and is then regenerated. For example, the malolactic enzyme found in some lactic acid bacteria and also in Ascaris decarboxylates L-malate to lactate (Eq. 15-12). This reaction is similar to those of isocitrate dehydrogenase,110-112 6-phosphogluconate dehydrogenase,113 and the malic enzyme (Eq. 13-45)114 which utilize free NAD+ to first dehydrogenate the substrate to a bound oxoacid whose (3 carbonyl group facilitates decarboxylation. Likewise, the bound NAD+ of the malolactic... 

Deficiency of pyruvate dehydrogenase is the most frequent cause of lactic acidemia ac Since this enzyme has several components (Fig. 15-15), a number of forms of the disease have been observed. Patients are benefitted somewhat by a high-fat, low-carbohydrate diet. Transient lactic acidemia may result from infections or from heart failure. One treatment is to administer dichloroacetate, which stimulates increased activity of pyruvate dehydrogenase, while action is also taken to correct the underlying illness.d Another problem arises if a lactate transporter is defective so that lactic acid accumulates in muscles.6... 

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