Recent investigations on the crude pigeon liver extracts have shown the presence of an enzyme which catalyzes the oxidative decarboxylation of malate to pyruvate and carbon dioxide. The name malic enzyme has been given to this enzyme to distinguish it from malic dehydrogenase. Under aerobic conditions the addition of ferrous sulfate resulted in inhibition of the action of malic enzyme. Subsequent investigation showed that the inhibition obaerved was not due to ferrous ions, but due to ferric ions which were contaminating the ferrous sulfate. It was also shown that under aerobic conditions ferrous ions were rapidly oxidized to ferric ions. Investigation of this oxidation of ferrous ions proved the reaction to be non-enzymatic, and to require the presence of malate at a pH of 5.0. The presence of pyruvate also caused oxidation of ferrous ions to ferric ions, but at a much slower rate. The oxidation of ferrous to ferric ions was inhibited by the addition of glutathione, It is thermodynamically possible that this observed inhibition was due to the oxidation of ferrous ions to ferric ions by the -SH group of glutathione. Three possible sites of inhibition exist. The conversion of DPN to TPN, the reduction of pyruvate to lactate, and the oxidative decarboxylation of malate. The first two reactions showed no inhibition in the presence of ferric ions while the latter was inhibited. Varying concentrations of malate indicated that the reaction is not one of competition for malate by the ferric ions and the malic enzyme. The gradual increasing of the enzyme concentration in a reaction flask inhibited with ferric ions reveals a point where the inhibition is overcome. Increasing the enzyme concentration beyond this point resulted in a constant increase in carbon dioxide evolution per unit enzyme increase up to the point where the availability of the substrate effected the rate of reaction. The addition of glutathione to the reaction inhibited with low concentrations of ferric ions caused a decrease in the observed inhibition. It is speculated that the affected enzyme possesses functional groups which are oxidized by ferric ions, inactivating the enzyme and limiting the reaction. In the presence of glutathione, the ferric ions would be reduced to ferrous ions causing a decrease in inhibition.



College and Department

Physical and Mathematical Sciences; Chemistry and Biochemistry



Date Submitted


Document Type





Malate, Pyruvates



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Chemistry Commons