Oxalacetate malic dehydrogenase reaction

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. 

Further evidence of like character in support of the participation of coenzyme I in reactions associated with the reduction of dehydroascorbic acid has been advanced by Waygood (1950). Cell-free extracts of wheat seedlings were found to contain a malic dehydrogenase enzyme, reducing coenzyme I, as well as ascorbic oxidase and peroxidase enzymes. When to such extracts malic acid, coenzyme I, and ascorbic acid were added, together with a fixative for the oxalacetate formed in the reaction, the system absorbed oxygen in excess of that required for the complete oxidation of ascorbic acid. In this system methylene blue could replace ascorbic acid. 

The former is called malic dehydrogenase. Although it was reported early that the enzyme is specific for DPN, in fact malic dehydrogenase from several sources has been found to react with TPN at 5-7 per cent of the rate of the DPN reaction. The affinities for DPN and TPN are quite similar, Kn equals about 10 at neutral pH. The equilibrium of the reaction at neutral pH values lies far to the side of malate and DPN, but, as discussed previously, at higher pH values the equilibrium of reactions in which H+ participates is shifted, and near pH 10 the oxidation of malate proceeds to a considerable extent and at a good rate. Even at low pH values, however, the oxidation of malate is readily carried out when coupled with an effective system for oxidizing DPNH or removing oxalacetate (as citrate formation). 

In chloroplasts oxalacetic acid could be reduceo to malic acid (by NADP-malate dehydrogenase) and this acid could be decarboxylated and will regenerate the substrate for PEPCase. Another reaction is also possible - oxalacetic acid to be directly decarboxylated by oxalacetate decarboxylase. In either case the decarboxylase reactions will strengthen the flow of CO2 to RuBPCase and will contribute to the better operation of Calvin cycle. 

---