Malic dehydrogenase

It has been observed that whereas the catalytic activity of malic dehydrogenase in water is not influenced by pressure, in reversed micelles it shows a bell-shaped dependence, suggesting regulation of the enzymatic activity by pressure application, which cannot be realized in aqueous solutions [180],... 

When linked enzyme assays are used, the exogenous added enzymes may also be contaminated with small traces of the primary enzyme whose activity is measured, thereby leading to falsely high activities In this instance it is also desirable to make certain that the added enzymes are free of any undesirable activity, i.e, pig heart malic dehydrogenase should be free of GOT activity when used for GOT assays (17),... 

Stereoview of the a-carbon models of the subunits of s-malic dehydrogenase (sMDH) and lactic dehydrogenase (LDH). 

The enzyme reagent is added, malic dehydrogenase glucose-6-dehydrogenase... 

MALEYLACETOACETATE ISOMERASE MALEYLPYRUVATE ISOMERASE MALEYLPYRUVATE ISOMERASE Malic dehydrogenase,... 

Soldiers on a 2-hour strenuous march experienced statistically significant increases in lactic dehydrogenase (p < 0.001), malic dehydrogenase... 

For quantitative results no completely satisfactory procedure is available. Enzymatic procedures (81) using L-malic dehydrogenase suffer from possible interference of tartaric acid (82). Malic acid can be fer-... 

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). 

When considering the mechanism of the malo-lactic fermentation, the possibility that malic acid may be converted first to oxaloacetic acid (by malic dehydrogenase) must be recognized. This acid could then be decarboxylated to pyruvic acid, and subsequent reaction would yield lactic acid. However, if this were the case, there then should be no situation where malic acid would be decarboxylated faster than oxaloacetic acid. This, however, was shown to occur at pH 6 (14). Similarly, Flesch and Holbach (15) report that malic dehydrogenase has an optimal pH of 10, but that the malo-lactic reaction proceeds at pH 5.6. Therefore, it would not seem likely that the cell would degrade malic acid by this mechanism hence, the oxaloacetic acid intermediate would not be available to the organism. 

These reactions were demonstrated by manometric measurement of the carbon dioxide evolved when resting cells were placed in contact with the substrates. The auhors (II) state The extracts contain lactic dehydrogenase but little or no malic dehydrogenase as tested spectrophotometri-cally with reduced DPN and pyruvate or oxaloacetate respectively. It was also reported (II, 14) that Reaction 1 results from the combination of the following two reactions ... 

Linked oxidation and decarboxylation. Metabolic pathways often make use of oxidation of a (3-hydroxy acid to a (3-oxoacid followed by decarboxylation in the active site of the same enzyme. An example is conversion of L-malate to pyruvate (Eq. 13-45). The Mg2+ or Mn2+-dependent decarboxylating malic dehydrogenase that catalyzes the reaction is usually called the malic enzyme. It is found in most organisms.237-240 While a concerted decarboxylation and dehydrogenation may sometimes occur,241-242 the enzymes of this group appear usually to operate with bound oxoacid intermediates as in Eq. 13-45. 

Malic dehydrogenase—Prepare this reagent just before use and store at 4°C. Dilute commercially available enzyme in 0.10 M potassium phosphate buffer, pH 7.5, in which 1 mg/ml bovine serum albumin has been dissolved. The final concentration of malic dehydrogenase should be 200 units/ml. Prepare a total of 20 ml of this reagent. 

R. K. Gerding and R. G. Wolfe, J. Biol. Chem., 244 1164 (1%9). Malic Dehydrogenase. VIII. Large Scale Purification and Properties of Supernatant Pig Heart Enzyme. 

Following are a set of assay conditions for marker enzymes of mitochondria (citrate synthetase, malic dehydrogenase, fumarase, and succinate dehydrogenase) and glyoxysomes (citrate synthetase, malate synthetase, and malic dehydrogenase). Some or all of these activities may be assayed across the density gradient. Their quantitative distribution is shown in Table 9-2. 

Both are abundant in skeletal muscle, myocardium, liver, and erythrocytes, so that hemolysis must be avoided, and in serum they may be assayed spectrophotometrically by their conversion of phosphate-buffered pyruvate to lactate (R6, W16) or oxalacetate to malate (S25) at the expense of added NADH2, when the rate of decrease of optical density at 340 m x thus measmes the serum activities of the respective enzymes. Recently, however, the reverse reaction has been found best for serum lactic dehydrogenase assay (A2a). In conventional spectrophotometric units the normal ranges are 100-600 units per ml for lactic dehydrogenase (W16) and 42-195 xmits per ml for malic dehydrogenase (S25) as before, one conventional spectrophotometric unit per ml = 0.48 pmoles/ minute/liter (W13). 

Glutamic-oxalacetate transaminase (GOT) is released into the blood stream as a result of myocardial infarction. The enzyme is assayed in serum by following the decrease in the absorbance of NADH in the malic dehydrogenase (MDH)-coupled reaction sequence shown below. 

A reaction mixture contained excess aspartate (i.e., lOO times its value), 0.1 ml of serum, 0.3 /rmole of NADH, and an excess of malic dehydrogenase in a total volume of 0.9 ml. The reaction was started by adding an excess of a-ketoglutarate in 0.1 ml. After a short lag, the absorbance decreased at a rate of 0.04 A unit/min. The cuvette had a light path of 1 cm. Calculate the concentration of GOT in the patient s serum (i.e., the specific activity of the serum in terms of enzyme units/ml). 

---