Lactate dehydrogenase reaction

FIGURE 19.30 (a) Pyruvate reduction to ethanol in yeast provides a means for regenerating NAD consumed in the glyceraldehyde-3-P dehydrogenase reaction, (b) In oxygen-depleted muscle, NAD is regenerated in the lactate dehydrogenase reaction. 

FIGURE 14-19 Alternative paths from pyruvate to phospho-enolpyruvate. The path that predominates depends on the glucogenic precursor (lactate or pyruvate). The path on the right predominates when lactate is the precursor, because cytosolic NADH is generated in the lactate dehydrogenase reaction and does not have to be shuttled out of the mitochondrion (see text). The relative importance of the two pathways depends on the availability of lactate and the cytosolic requirements for NADH by gluconeogenesis. 

Lactate consumption The direction of the lactate dehydrogenase reaction depends on the relative intracellular concentrations of pyruvate and lactate, and on the ratio of NADH/NAD+ in the cell. For example, in liver and heart, the ratio of NADH/NAD+ is lower than in exercising muscle. These tissues oxidize lactate (obtained from the blood) to pyruvate. In the liver, pyruvate is either converted to glucose by gluconeogenesis or oxidized in the TCA cycle. Heart muscle exclusively oxidizes lactate to CO2 and H20 via the citric acid cycle. 

Figure 9-7 (A) Effect of glucose and glucose 6-phosphate concentrations on reaction rate of yeast hexokinase at equilibrium. Reaction mixtures contain 1-2.2 mM ATP, and 25.6 mM ADP at pH 6.5. From Fromm et al.51 (B) Effect of lactate and pyruvate concentrations on equilibrium reaction rates of rabbit muscle lactate dehydrogenase. Reaction mixtures contained 1.7 mM NAD+, and 30 - 46 pM NADH in Tris-nitrate buffer, pH 7.9, 25°C. From Silverstein and Boyer.53...

Karpov, A.R. and Andreeva, A.P. (1992) The character of kinetics of lactate dehydrogenase reaction in some species of the Gadidae family (In Russian). Voprosy Ikhtiologii 32,144-149. 

Figure 2.14. Modified scheme of lactate dehydrogenase reaction. Nuclei participating in the formation of the transition state are marked by (Deng and Callender, 1999). Reproduced with permission. ...

This dimeric enzyme (subunit 35000) catalyses a reaction similar to the lactate dehydrogenase reaction, and the subunit structures of the enzymes are strikingly similar [83-85] (Fig. 20). Crystallographic [85] and other [86] evidence suggests that the reaction mechanisms are similar. The 4-pro-R hydrogen of NADH is transferred to the Re side of the oxaloacetate to give L-malate [87],... 

The activity of enzymes whose reactions do not produce a convenient spectroscopic change can be monitored by coupling to a suitable andllary reaction for example, the activity of alanine aminotransferase can be monitored by coupling to the lactate dehydrogenase reaction ... 

Figure 19. Calculated contributions to the free energy functions, for the hydride-transfer step (1 in Fig. 18.) of the lactate dehydrogenase reaction in a reference solvent cage (w, left) and in 1 he protein active site p, right). The Agf s are given as functions of the corresponding energy gaps (the Ac s) which are taken as the...

The results of stopped-flow studies of the lactate dehydrogenase reaction have proved to be more difficult to interpret than those of alcohol dehydrogenase. The dissociation velocity constants for the binary NADH compounds of the pig heart and skeletal muscle lactate dehydrogenases are much larger than that for liver alcohol dehydrogenase, and also larger than the maximum specific rates of lactate oxidation at pH 6.0-7.0 (Table VII). Some earlier step must therefore be rate-limiting. 

Lactate Dehydrogenase Reaction Isoenzymes of Lactate Dehydrogenase... 

For example, in the case of the lactate dehydrogenase reaction the mechanism is... 

Fig. 22.9. Lactate dehydrogenase reaction. Pyruvate, which may be produced by glycolysis, is reduced to lactate. The reaction, which occurs in the cytosol, requires NADH and is catalyzed by lactate dehydrogenase. This reaction is readily reversible.

Another consequence of the very high NADH/NAD ratio is that the balance in the lactate dehydrogenase reaction is shifted toward lactate, resulting in a lacticacido-sis (see Fig. 25.6, circle 6). The elevation of blood lactate may decrease excretion of nric acid (see Fig. 25.6, circle 7) by the kidney. Consequently patients with gout (which results from precipitated uric acid crystals in the joints) are advised not to drink excessive amounts of ethanol. Increased degradation of purines also may contribnte to hyperuricemia. 

The MTT is reduced to formazan by the NADH, which is generated in the NAD-dependent lactate dehydrogenase reaction. The presence of detergents such as Triton X-100 surfactant (alkylaryl polyether alcohol) accelerates the reduction of the tetrazol-ium salt manifold. After an additional incubation inhibition zones become visible. The antibacterial compounds appear as clear spots on the colored formazan background. 

Accumulation of compounds related to the mitochondrial pathway can be detected in one or more body fluids of most patients [1, 2, 15]. Special attention has to be paid to the lactate concentration. Excess of lactate and alanine will be produced after reduction or transamination of accumulated pyruvate (see Fig. 27.1). If there is a severe block in the pyruvate oxidation pathway, and the produced lactate can not adequately be removed by peripheral tissues, it accumulates in blood, urine and/or cerebrospinal fluid, dependent upon the affected tissue(s). A decreased activity of the respiratory chain will shift the equilibrium of the lactate dehydrogenase reaction to conversion of pyruvate to lactate (see also Sect. 1). Thus, patients with a respiratory chain defect should demonstrate an increased lactate/pyruvate ratio in blood, whereas pyruvate dehydrogenase deficiency should result in a normal lactate/pyruvate ratio. However, this tool for differential diagnosis is not helpful in all cases. Furthermore, some patients do not accumulate lactate in blood or urine. 

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