Enzymes lactic dehydrogenase

Again, the enzyme lactic dehydrogenase frequently shows elevated serum activity in muscular dystrophy, though it is cleared from serum with remarkable speed in dogs the intravenous injection of purified lactic dehydrogenase immediately raises its serum activity to 10 times the normal value, but after only 1 hour the greater part of it has already been removed from the serum (W18). 

Write the spontaneous reaction that occurs and calculate the AG of the reaction when the enzyme lactic dehydrogenase is added to a solution containing pyruvate, lactate, NAD, and NADH at the following concentration ratios (a) lactaie/pyruvate — 1, NAD /NADH= 1, (b) lactate/pyruvate = 159, NAD /NADH = 159, and (c) lactate/pyruvate = 1000, NADVNADH = 1000. ------------------------------------... 

Tubular damage causes enzymuria. Urinary excretion of the lysosomal enzyme N-acetyl-j3-D-glucosidase (NAG), the cytoplasmic enzyme lactic dehydrogenase (LDH), and the brush-border enzyme gamma glutamyl transpeptidase (GGT), caused by contrast media, has been studied in rats with drug-in-... 

A mitigating factor is whether conversion is selective to yield lactic acid or lactate, as in cancer cells, or selective to yield carbon dioxide and water, as in normal cells. This depends on the absence or presence of, say, inhibitors for the enzyme lactic dehydrogenase, as set forth in Chapter 3. That is, absence of inhibitors for lactate dehydrogenase would favor cancer cell metabolism.)... 

Catalysis by flavoenzymes has been reviewed and various analogues of FAD have been prepared e.g. P -adenosine-P -riboflavin triphosphate and flavin-nicotinamide dinucleotide ) which show little enzymic activity. The kinetic constants of the interaction between nicotinamide-4-methyl-5-acetylimidazole dinucleotide (39) and lactic dehydrogenase suggest the presence of an anionic group near the adenine residue at the coenzyme binding site of the enzyme. ... 

The generalization that the same dehydrogenase has the same stereospecificity, no matter what the source of the enzyme, has been tested now particularly well for malic and lactic dehydrogenases. In fact, one can venture a guess, that pyridine nucleotide dehydrogenases which oxidize a-hydroxycarb oxylic acids at the a-position, all have A stereospecificity for the pyridine nucleotide, regardless of their stereo-specificity for the substrate. Biellman and Rosenheimer 88> have assembled the data. One can add liver malic enzyme 90> to their list. 

Biochemical assays have been employed to measure changes in enzyme levels (e.g., aspartate aminotransferase, lactic dehydrogenase) as an indication of exposure to 1,2-dibromoethane in humans and animals (Albano et al. 1984 Botti et al. 1989 Letz et al. 1984 Van lersel et al. 1988). Decreased sperm counts per ejaculate and increased numbers of sperm with abnormal morphology have also been identified in workers following exposure to 1,2-dibromoethane (Ratcliffe et al. 1987 Wyrobek 1984). In general, these techniques are nonspecific for 1,2-dibromoethane exposure (see Chapter 2). There are no data to indicate whether a biomarker, if available, would be preferred over chemical analysis for monitoring exposure to 1,2-dibromoethane. 

Enzymes activities are particularly sensitive to the anticoagulant used in collecting the specimen. Heparin inhibits acid phosphatase (W16) and muramidase (Z5). Amylase activity is inhibited by oxalate or citrate (MIO), and lactic dehydrogenase and acid phosphatase lose activity in oxalate (C2). Alkaline phosphatase is stable in oxalate, oxalate-fluoride, or heparin, but 25 mAf citrate inhibits 50% of the activity, and as little as 50 mlf EDTA is completely inhibitory (B19). Leucine aminopeptidase is inhibited by EDTA, as is creatine phosphokinase (F3). Amylase activity has been reported to be only 83% of that in serum when oxalate or citrate-plasma is used (MIO). Heparin plasma appears to have no inhibitory effect. Despite the fact that clotting factor V is not stable in oxalate or EDTA, these are often used as anticoagulants to obtain plasma for prothrombin determinations (Z2, Z4). 

Inhibitors of lactic dehydrogenase have been reported in commercial preparations of NAD+ and NADH (B4, M6, S28). The concentration of inhibitory substances varied from lot to lot. In a serum lactic dehydrogenase study with NAD+ from 8 sources, activities were found to vary from 145 to 75 units (B4). Inhibitors of lactic dehydrogenase activity have also been observed in dialyzates in uremic patients (W8) and in human urine (G8). The purity of available substrate can also effect enzyme activity. Schwartz and Bodansky observed that, in 6 batches of fructose 6-phosphate, all weighed to a 0.5 mM concentration, the actual concentration varied from 0.13 mAf to 0.55 mM (S14). 

CS causes alkylation of sulfhydryl-containing enzymes and inhibits lactic dehydrogenase, glutamic dehydrogenase, pyruvic decarboxy-. lase, and alpha-glycerophosphate dehydrogenase.24,40 it reacts with a number of nucleophilic compounds, such as glutathione, plasma protein, and lipoic acid.24... 

Cucinell et aJL. 3 reversed the CS inhibition of lactic dehydrogenase in dogs by injecting sodium thiosulfate. Rats poisoned by CS at 80 mg/kg (more than the LD50) were saved by injections of thiosulfate. Cucinell e a l. speculated that the toxic effects of CS were caused by inhibition of sulfhydryl-containing enzymes. 

By using dideuterioethanol, CH3CD2OH, as substrate, they demonstrated that one atom of deuterium is transferred from substrate to coenzyme, and further that this atom of deuterium is transferred from the coenzyme, under the influence of lactic dehydrogenase, to pyruvate (Fisher, et al., 1953 Loewus et al., 1953a,b Vennesland and Westheimer, 1954). The converse experiment was also carried out, that is to say, the reactions were performed with normal substrate and coenzyme, but in D20 this resulted in transfer of ordinary hydrogen from substrate to coenzyme. The question they had set out to answer was then settled the oxidation-reductions proceed by direct transfer of hydrogen between substrate and coenzyme. These results have subsequently been confirmed in numerous investigations with other enzymic reactions that require NAD+ or NADP+. 

The mechanism of toxicity of ethylene glycol involves metabolism, but unlike previous examples, this does not involve metabolic activation to a reactive metabolite. Thus, ethylene glycol is metabolized by several oxidation steps eventually to yield oxalic acid (Fig. 7.84). The first step is catalyzed by the enzyme alcohol dehydrogenase, and herein lies the key to treatment of poisoning. The result of each of the metabolic steps is the production of NADH. The imbalance in the level of this in the body is adjusted by oxidation to NAD coupled to the production of lactate. There is thus an increase in the level of lactate, and lactic acidosis may result. Also, the intermediate metabolites of ethylene glycol have metabolic effects such as the inhibition of oxidative phosphorylation, glucose metabolism, Krebs cycle, protein synthesis, RNA synthesis, and DNA replication. 

NADH as an end product. This implicates oxidized malic acid, either pyruvic or oxaloacetic acid, as another end product. By adding commercial preparations of L-lactic dehydrogenase or malic dehydrogenase to the reaction mixture, Morenzoni (90) concluded that the end product was pyruvic acid. Attempts were then made to show whether two enzymes—malate carboxy lyase and the classic malic enzyme, malate oxidoreductase (decarboxylating), were involved or if the two activities were on the same enzyme. The preponderance of evidence indicated that only one enzyme is involved. This evidence came from temperature inactivation studies, heavy-metal inhibition studies, and ratio measurements of the two activities of partially purified preparations of Schiitz and Radlers malo-lactic enzyme (76, 90). This is not the first case of a single enzyme having two different activities (91). 

Zinc is essential for the function of more than 100 enzymes (e.g., thymidine kinase, carbonic anhydrase, lactic dehydrogenase, alkaline phosphatase) involved in a variety of metabolic activities in the body,... 

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... 

Enzymes that contain NAD or NADP are usually called dehydrogenases. They participate in many biochemical reactions of lipid, carbohydrate, and protein metabolism An example of an NAD-requmng system is lactic dehydrogenase which catalyzes the conversion of lactic acid to pyruvic... 

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 ... 

It is likely that at low zinc concentrations there is impairment of the activity of vital zinc metalloenzymes such as lactic dehydrogenase, alkaline phosphatase, carbonic anhydrase, carboxypeptidase, and of enzymes in which zinc acts as a cofactor. Injection experiments showed that radioactive 65Zn preferentially concentrated in healing tissues27. ... 

---