Malate dehydrogenase and malic enzyme
In T. brucei there are three different isoenzymes of MDH. In the bloodstream form 90 % of all MDH is cytosolic. The remaining 10% is present in the mitochondrion. In procyclics there is a mitochondrial MDH (10%), a cytosolic MDH (5%), identical to the major cytosolic MDH of the bloodform and a glycosomal MDH (85%) (Uttaro and Opperdoes, unpublished). In both life-cycle stages the cytosolic and mitochondrial isoenzymes, in conjunction with aspartate aminotransferase, must have a function in the transport of reducing equivalents from cytosol to mitochondrion and vice versa. In the procyclic stage the cytosolic pyruvate kinase is almost completely inactive because of the absence of its allosteric regulator fructose 2,6-bisphosphate. So all the PEP generated in the glycolytic pathway is forced back into the glycosome where PEPCK and MDH function in concert in the reoxidation of glycosomal NADH and the formation of one mole of glycosomal ATP per mole of NAD regenerated. Part of the glycosomal malate so produced is converted in the cytosol to pyruvate by malic enzyme which is mainly cytosolic (Opperdoes and Cottem, 1982).
In an isoenzyme study of 29 stocks of T. evansi from Egypt, Sudan, and Indonesia a perfect correlation was found between the presence of the type VII banding pattern of malic enzyme with a high resistance to suramin (Boid et al., 1989). However, the function of malic enzyme in this bloodstream trypanosome that is so closely related to T. brucei, is not known.