 In tumor cells the citric acid cycle is truncated due to an inhibition of the enzyme aconitase (EC 4.2.1.3) by high concentrations of reactive oxygen species (ROS) Aconitase catalyzes the conversion of citrate to isocitrate.On the other hand, tumor cells over express phosphate dependent glutaminase and NAD(P)-dependent malate decarboxylase, which in combination with the remaining reaction steps of the citric acid cycle from α-ketoglutarate to citrate impart the possibility of a new energy producing pathway, the degradation of the amino acid glutamine to glutamate, aspartate, pyruvate CO2, lactate and citrate.Besides glycolysis in tumor cells glutaminolysis is another main pillar for energy production.